Food Engineering Reviews https://doi.org/10.1007/s12393-025-09415-8 Introduction Freezing and drying are the two oldest and most widely used techniques for extending the shelf life of foods by reducing microbial and enzymatic activity, thereby preserv- ing the quality of food products [1]. More importantly, they increase the availability of off-season products and broaden the product range available to consumers [2]. Freeze dry- ing (FD) combines the processes of freezing and drying. It begins with freezing the food products, followed by drying through sublimation after reducing the pressure to below the triple point level [3]. FD is often regarded as the ‘Gold Stan- dard’ of drying methods because it maintains superior food quality. This is achieved through low-temperature process- ing, which minimises the loss of heat-sensitive compounds, Mahsa Majzoobi mahsa.majzoobi@rmit.edu.au 1 Department of Food Technology and Nutrition, School of Science, Bundoora West Campus, RMIT University, PO Box 71, Melbourne, VIC 3083, Australia 2 RRI-PPP Grain Quality Testing Laboratory, International Rice Research Institute, Dhaka 1213, Bangladesh 3 School of Chemical Engineering, University of New South Wales, Sydney, NSW, Australia Abstract Freeze drying (FD) is a leading method for preserving food quality, particularly for heat-sensitive products; however, significant operational costs and slow throughput constrain its use. These drawbacks, along with growing environmental awareness and the need for sustainability, are putting pressure on the industry to enhance energy efficiency and the overall sustainability of FD practices. This review outlines the latest advances in improving the energy efficiency of freezing and freezing and FD processes, thereby reducing processing time and energy consumption. These improvements involve a combination of innovative technologies such as radiofrequency (RF), high-pressure (HP), magnetic field (MF), high-volt- age electric field (HVEF), microwave (MW), infrared radiation (IR), ultrasound (US), and instant controlled pressure drop (DIC). In particular, hybrid systems that integrate these technologies with FD have shown synergistic benefits in enhancing drying kinetics and reducing processing costs. This review also discusses the influence of alternative pretreatments on the efficiency of FD and the quality of the resulting dry products. Combining advanced technologies with freezing and FD processes significantly increased mass transfer rates, shortened processing times, reduced energy consumption, and pro- duced superior-quality foods. Recent trends also include the application of artificial intelligence (AI) and machine learning to model, monitor, and optimise FD processes, enabling data-driven decision-making and improved process control. These advances have the potential to revolutionise the food industry by allowing greater control over ice crystal nucleation rates and sizes, reducing production costs, and improving the overall quality of final products. In addition, novel pretreatments significantly improve mass and heat transfer, shorten processing time, minimise nutrient losses and improve the overall quality of the end products. These innovations have important implications for scaling FD in industrial applications and advancing sustainable food processing systems. Keywords  Freezing · Freeze drying · Ultrasound · High-pressure · Infrared radiation · Electrohydrodynamic Received: 5 March 2025 / Accepted: 16 June 2025 © The Author(s) 2025 Technological Innovations in Freeze Drying: Enhancing Efficiency, Sustainability, and Food Quality Abdulla Al Faruq1 · Asgar Farahnaky1 · Mina Dokouhaki1 · Husne Ara Khatun2 · Francisco J. Trujillo3 · Mahsa Majzoobi1 1 3 https://doi.org/10.1007/s12393-025-09415-8 http://crossmark.crossref.org/dialog/?doi=10.1007/s12393-025-09415-8&domain=pdf&date_stamp=2025-6-22 Food Engineering Reviews prevents degradation and oxidation reactions, preserves the natural colour and nutritional value of foods [4, 5]. Evidence suggests that food loss in developed countries is comparable to that in developing countries. Notably, over 40% of food loss in developing countries occurs during the post-harvest and processing stages, whereas in developed countries, over 40% of food loss occurs at the retail and con- sumer levels [6]. Therefore, adopting efficient processing and preservation techniques is crucial for enhancing food security and reducing postharvest losses. FD, in particular, plays a vital role in preserving high-value and perishable foods by significantly extending shelf life while retaining nutritional and sensory qualities. Additionally, there is a growing demand for high-quality dehydrated products with enhanced nutritional and organoleptic properties. Freezing and FD are considered as possible processes and preserva- tion techniques that can minimise the loss of food by remov- ing moisture and preserving the structure and nutritional value of the food [7]. However, the longer processing times, structural dete- rioration caused by ice crystal formation, reduced drying or freezing efficiency, and high energy consumption of conventional freezing (CF) and FD techniques limit their applicability in the food industry [8]. To address these limi- tations and reduce food loss across the supply chain, recent advances in FD technologies-such as microwave-assisted FD, infrared-assisted FD, high-pressure pretreatment, and instant controlled pressure drop (DIC) have been shown to accelerate drying rates, improve mass and heat trans- fer, and significantly enhance the retention of heat-sensi- tive nutrients. Similarly, hybrid FD systems incorporating high-voltage electric fields demonstrated reductions in dry- ing time of up to 30% while maintaining the integrity of polyphenols and flavonoids [7, 8]. These technologies not only improve product quality and reduce postharvest waste but also contribute to sustainable food production systems by lowering energy inputs and minimising environmental impact. Thus, novel technologies are needed to enhance the efficiency of freezing and FD processes, particularly in high-capacity industrial production settings, to combat food waste, achieve improved quality and quality control, reduce energy consumption, environmental impact, and production costs, and ensure safer operations. The FD process typically consists of three stages: (a) freezing, where most of the free water is transformed into ice crystals; (b) primary drying, where ice is sublimated under vacuum pressure; and (c) secondary drying, where unfrozen or bound water is removed from the food matrix. Because FD is performed under vacuum, it is also known as vac- uum-assisted freeze drying (VAFD). Despite the numerous unique advantages of FD, its industrial implementation is restricted by its costly operation, high energy consumption, long processing time, and significant maintenance expenses. Therefore, it is essential to find technological advancements that can enhance the performance of FD, thereby reduc- ing processing times, minimising costs, and maintaining or improving the quality of the dried products. In FD, freezing is the first step before drying by sublimation. Lowering the temperature effectively slows down chemical and biochem- ical reactions as well as microbial growth. Consequently, it delays the degradation of colour and texture, minimises nutrient loss, and reduces the occurrence of undesirable fla- vours [9]. During the initial stages of the freezing process, water molecules within the food undergo a phase transition, resulting in the formation of ice crystals. The crystallisation of ice involves two primary stages: nucleation and the sub- sequent growth of nuclei, which collectively influence the size and distribution of the ice crystals [10]. Slow freezing rates result in the formation of large ice crystals predomi- nantly in extracellular regions, whereas fast freezing rates lead to the generation of small crystals that are uniformly distributed throughout the tissue. Currently, the food indus- try freezes food via conventional freezing techniques such as cold air freezing, direct-contact freezing, and immersion freezing [11]. However, the long processing time and slow freezing rate are characteristics of CF processes that are of great concern in the food industry, as they contribute to the formation of large ice crystals, causing textural damage and deformation in food products. Despite these limitations, CF remains a popular option as it is simple and effective in extending the shelf life of products. Advanced freezing technologies (AFTs), includ- ing radiofrequency-assisted freezing (RFAF) [12, 13], high-pressure assisted freezing (HPAF) [14, 15], magnetic field-assisted freezing (MFAF) [16, 17], high-voltage elec- trostatic field-assisted freezing (HVEFAF) [18, 19] and dehydrofreezing (DF) [20, 21], offer promising alternatives to the constraints of conventional methods (Table 1). These technologies focus on rapid freezing and the use of cryopro- tectants to minimise the formation of ice crystals, thereby preserving the food’s quality. Similarly, numerous stud- ies have reported on advanced freeze drying technologies (AFDTs) processes such as microwave-assisted freeze dry- ing (MWAFD) [22, 23], infrared radiation-assisted freeze drying (IRAFD) [24, 25], ultrasound-assisted freeze drying (USAFD) [26–28], and instant controlled pressure drop- assisted freeze drying (FD-DIC) [29, 30], which improve the quality of dried foods compared to FD while improving drying rate, saving energy and minimising processing time (Table  2). Although novel technologies present numerous benefits, several factors often hinder their adoption in com- mercial and industrial settings. These include the require- ment for specialised knowledge, a thorough understanding of their operational processes, and the high costs associated 1 3 Food Engineering Reviews Sample Processing methods Conditions Key Findings Refer- enceAdvantages Disadvantages Rainbow trout fillet Radiofre- quency-assisted freezing (RFAF) Power: 2 kW, 27.12 MHz, 3 kV. Electrode gap: 2, 3 and 4 cm. The control sample had an ice crystal size of 52.02 ± 7.16 μm, while the RFAF sample had an ice crystal size ranging from 13.19 ± 2.25 to 31.14 ± 4.88 μm based on the electrode gap. The control sample had a drip loss of 4.13%, whereas the RFAF sample exhibited a drip loss that ranged from 2.75–3.71% µm based on the electrode gap. Did not decrease the overall freezing time compared to tradi- tional methods. [34] Shrimp Hot air drying (HAD) Vac- uum-assisted freeze drying (VAFD) High-pressure- assisted vacuum- freeze drying (HPP-VAFD) HAD: 50 °C for 22 h. VAFD: Frozen at -80 °C (3 h), Primary drying at -35 °C 10 Pa (3 h), secondary drying at 50 °C, 10 Pa (19 h). HPP-VAFD: 550 MPa (10 min) + VAFD. After 22 h of FD, HAD reduced the moisture content by 6.32%, VAFD by 43.87%, and HPP-VAFD by 32.47%. . [137] Shrimp and porcine liver High-pressure shift freezing (HPSF) Conventional freezing (CF) Immersion freezing (IF) HPSF: 100, 150, 200 MPa, -9, -15, -21 °C, 3 MPas− 1. FD: -20 °C. IF: 50% ethanol (v/v) at atmospheric pressure at -20 °C. HPSF significantly reduced phase transition times (shrimp: 2.97, 1.53, 1.1 min; porcine liver: 2.47, 1.22, 0.83 min at 100, 150, and 200 MPa) compared to CF (148 min for shrimp, 85 min for liver) and IF (5.9 and 5.5 min, respectively). Higher pressures in HPSF led to faster freezing and formation of numerous small ice crystals. High pressure had a significant impact on protein denaturation. [44] Live pacific white shrimp Static magnetic field-assisted freezing (SMFAF) IF SMF: Magnetic field strength: 60 mT, temperature: -35, − 30, -25 and − 20 ºC. IF: -35 ºC. The IF process required drying time of 466 s. SMF (− 35 ºC) process required drying time of 360 s. MF was not as effective at higher freezing temperatures [51] Tilapia fillets Alternating magnetic field assisted freez- ing (AMFAF) CF Magnetic field: 5 mT, Frequency: 50, 100, 150, 200, and 250 Hz. The control sample had freezing time, hardness, and fractal dimensions of 28 min, 275.73 g, and 1.881, respectively. The AMF-50 Hz sample had a freezing time of 25.5 min, a hard- ness of 291.57 g and a fractal dimension of 1.896. The AMF-100 Hz sample had a freezing time of 26.5 min, a hardness of 285.75 g and a fractal dimension of 1.893. The AMF-150 Hz sample had a freezing time of 28.5 min, a hardness of 283.98 g and a fractal dimension of 1.884. The AMF-200 Hz sample had a freezing time of 23 min, a hard- ness of 335.18 g and a fractal dimension of 1.916. The AMF-250 Hz sample had a freezing time of 24 min, a hard- ness of 300.92 g and a fractal dimension of 1.90. [17] Cherry CF SMFAF AMFAF SMFAF: Magnetic field intensities: 2, 10, 15, and 20 mT. AMFAF: Magnetic field intensities: 0.05, 0.28, 0.64, 1.26, and 1.74 mT, frequency: 50 Hz. The phase change time and average ice crystal area for FD were 15.50 min and 5538.5 ± 1506 µm2 The minimum phase change times and average minimum ice crystal area for SMF were 13.24 min and 1846.57 ± 104 µm2 at 20 mT. The minimum phase time for AMF was 11.69 min at 1.74 mT and 1220.5 ± 16 µm2 at 1.26 mT. [16] Korla fragrant pear SMFAF -18 °C, 2, 4, 6, 8, and 10 mT. CF process reached the freezing point at 2870 s. MFAF at 2 mT achieved the freezing point at 3200 s. MFAF at 4 mT achieved the freezing point at 2600 s. MFAF at 6 mT achieved the freezing point at 1920 s. MFAF at 8 mT achieved the freezing point at 1750 s. MFAF at 10 mT achieved the freezing point at 1650 s. Control group had higher hardness and fracturability than the SMFAF sample. [138] Table 1  Application of novel freezing to different food products 1 3 Food Engineering Reviews Regarding freezing, the application of RF can influence ice nucleation and cause initial crystals to break down into smaller pieces. RF waves create torque in water molecules by modifying their equilibrium relationships inside the ice clusters. As a result, RF can be used to control the size of ice crystals during the freezing process. A study by Hafezparast- Moadab et al. [34] compared the effects of conventional air- blast freezing and RFAF on the quality attributes of rainbow trout fillets-thawing. The results showed that RFAF induced less structural damage, less drip loss, and superior textural properties compared with air-blast freezing. The study also revealed that the control sample produced larger ice crys- tals (52.02 ± 7.16 μm) compared to the samples frozen by RFAF (13.19 ± 2.25 to 31.14 ± 4.88 μm), which were more uniformly distributed within RAAF-frozen fillets, result- ing in less cell disruption and improved moisture retention. RF waves can mobilise and rotate water molecules, which may result in a higher degree of nucleation. This may be one of the main reasons for the strong impact of this tech- nique in reducing the size of ice crystals. Similar findings were reported by Manzocco et al. [13], who investigated the microstructure and quality of meat after processing with low-voltage RF combined with cryogenic, slow freez- ing, and blast freezing techniques. The findings showed that the application of RF enhanced the preservation of the tissue microstructure and restricted myofibril denatur- ation, thereby promoting the retention of immobilised water within the intracellular environment. One of the major limitations of RFAF freezing is the risk of uneven temperature distribution. Interaction between electric fields and the product may cause localised overheat- ing, especially at corners and edges, which may lead to run- away heating. To solve this problem, research recommends several methods, including placing samples in a heating or cooling medium [35], using a similar dielectric material around the samples [36], and periodically moving or rotat- ing the samples [37]. with large-scale implementation. This manuscript presents a comprehensive review of innovative freezing and FD pro- cesses, highlighting novel pretreatments and their roles in enhancing the overall quality and efficiency of FD. Addi- tionally, several innovative pretreatments, including micro- wave blanching (MWB), radio frequency blanching (RFB), ultrasound (US), pulsed electric field (PEF), high-pressure processing (HPP), cold plasma (CP), and electrohydrody- namics (EHD), that influence freezing and FD processes are discussed. Advanced Freezing Technologies (AFTs) Radiofrequency-Assisted Freezing (RFAF) Radiofrequency (RF) technology is commonly used in the food industry for heating, but it has recently found appli- cations in freezing processes. RF heating is an alternative to conventional thermal treatment due to its rapid, uniform, and volumetric heating capabilities. Due to its longer wave- length than microwaves, RF can penetrate deeper into food materials, leading to a more uniform distribution of heat and temperature. It operates at frequencies between 300  kHz and 300  MHz, with wavelengths ranging from 1  mm to 1000 km [31]. The three most frequently utilised frequen- cies are 13.56 MHz ± 6.68 kHz, 27.12 MHz ± 160.00 kHz, and 40.68 MHz ± 20.00 kHz, which are applied in industrial, scientific, and medical applications [32]. In RF-assisted freezing systems, an RF generator creates an alternating electrical field between two electrodes where the food sam- ples are placed. This oscillating RF field induces polar mol- ecules, particularly water molecules in the food, to rotate and realign. The friction generated by the rotational move- ment of these molecules at very high frequencies helps to accelerate the freezing process by promoting the formation of smaller ice crystals [33]. This can improve the texture and quality of the frozen food, as smaller ice crystals are less damaging to cell structures [12, 13]. Sample Processing methods Conditions Key Findings Refer- enceAdvantages Disadvantages Apples Dehydrofreez- ing (DF) Partial drying at 45 °C, velocity 2 m/s, and humidity of 12%. Freezing at -30 °C. A raw sample with MC 700% (db) required a freezing time of 86 min. Dehydrating samples with MC 200% (db) required a freezing time of 46 min. Dehydrated samples with MC 100% (db) required a freezing time of 12 min. Dehydrated samples with MC 30% (db) required a freezing time of 10 min. DF was less effec- tive for products with naturally high-water con- tent, as ice crystal formation can still pose structural damage. [21] Carrot, sweet red bell pepper, DF Precooling at 0 °C, 70% RH for 15 h. Freezing at -20, -80, -196 °C. In bell peppers, DF increased firmness by ~ 52% compared to conventional methods, indicating less cellular damage. In car- rots, firmness improved by ~ 35%, showing better cell preserva- tion when drying preceded freezing. Excessive mois- ture loss during the pre-drying stage. [20] Table 1  (continued) 1 3 Food Engineering Reviews Sampe Drying method Drying condition Key findings Refer- encesAdvantages Disadvantages Pineapple Microwave- assisted freeze drying (MWAFD) MW power and frequency are 1 kW and 2450 MHz. MWAFD reduced energy consumption by 34.5% and drying time by 33.3% compared to pure vacuum freeze dry- ing (VAFD). Uneven heating created hotspots in the pineapple, caus- ing some areas to overheat while others stayed undried. [23] Barley grass MWAFD Freeze drying (FD) MW power levels (1, 1.5, and 2 W g− 1), pressure 50 Pa, temperature − 40 ºC. FD process required energy 1.17 kWhg− 1 and time 12.06 h The MWAFD process required a minimum energy of 0.61 kWhg− 1 and a time of 7 h at MW power level 1.5 Wg− 1 MWAFD dem- onstrated a more significant alteration in odour in compari- son to FD. [59] Sea cucumber FD HAD MWAFD FD: 50 Pa, -40 °C. HAD: 1.5 ms− 1 velocity, 20% relative humidity, temperature 60 °C. MWAFD: -20 °C, MW power: 1.6, 2, 2.3 Wg− 1. The processing time taken by FD, HAD, and MWAFD was 18 h, 8 h, and 12 h, respectively. Therefore, MWAFD required 40% less processing time than FD. Corona discharge occurs due to air ionisation under vacuum conditions, which limits the effectiveness of MW power. [139] Chinese yam Pulse-sprayed microwave- assisted freeze drying (PSMWAFD) Frequency 915 and 2450 MHz, Test pressure: 50, 100, 150, 200 and 250 Pa, MW power 150 W, freezing at -40 °C. Reduced drying time by 41.38% and total energy consumption by up to 34.4% compared with FD. The risks of corona discharge and the complexity of control systems are disadvantages of this method. [63] Potato chips FD Infrared radiation drying (IRD) Infrared radi- ation-assisted freeze drying (IRAFD) FD: -40 °C, 80 Pa IRD: Wavelength 3.0 μm, power 450 W, 70 °C IRAFD: FD + Temperature 60 °C, IR 2.3–14 μm and heat flux density is 0.703 Wcm− 2 Compared to FD, IRAFD saved 25–29% of drying time and 43–46% of the total energy consumption. [68] Kale yoghurt melts FD IRAFD IRAFD-Micro- wave vacuum drying (MVD) FD: -40 °C, 80 Pa IRAFD: FD + Temperature 60 °C, IR 2.3–14 μm and heat flux density is 0.703 Wcm− 2 IRAFD-MVD: IRFD + MW power 8 Wg− 1, pressure 11 kPa. IRAFD shortened the drying time by 9.54% compared with FD. IRAFD- MVD shortened the drying time by 31.08% compared with IRAFD. The application of IR led degradation in bioactive colorants such as chlorophylls and carotenoids. [25] Carrots Ultrasound- assisted freeze drying (USAFD) US power 0, 122.6, 178.7, and 229.8 W US durations 0, 5, 10, 15 s. After 20.34 h, the water content in FD was 25.69%, and in USAFD, 14.95%. Compared to FD, the sublimation time was reduced by 21.8%. [73] Flos Sophorae Immaturus USAFD US power 150, 300, 450, and 600 W. US durations 5, 10, 15 min. FD: -60 °C and 0.035 mbar. USAFD reduced drying time by 40% compared to control. Additionally, total flavonoid content increased by 29%, and rutin content increased by 27%. High power US or prolonged exposure runs the risk of nutri- ent loss and damage to chlorophyll. [75] Red bell pepper USAFD US amplitude 30%, 50% and 70%. Power 76 W, 90 W and 110 W. Time: 4, 8, 12, 16 and 24 h. USAFD shortened the FD time by 11.5%. Applying US continuously raises the product’s tem- perature, resulting in melting and struc- tural changes. [74] Jackfruit bulb chips Instant con- trolled pressure drop-assisted freeze drying (FD-DIC) HAD-DIC FD-DIC: -56 °C, 0.1 kPa, DIC at 3 kPa to 5 kPa, 60 °C for 2.5 h. HAD-DIC: Temperature 60 °C, velocity 1.2 ms− 1, and humidity 12%, DIC at 3 kPa to 5 kPa, 60 °C for 2.5 h. FD-DIC had the highest expansion ratio (119%) and better texture than HAD-DIC and FD. FD-DIC samples exhibited higher retention of phenolic compounds and carotenoids compared to HAD-DIC but were similar to FD. About 22% of phe- nolic compounds are lost during the DIC processing step. [29] Table 2  Application of innovative freeze drying to different food products 1 3 Food Engineering Reviews crystals, release latent heat, and form ice, causing the prod- uct temperature to increase to the freezing point [44]. In the HPIF, the pressure is applied in two stages. First, the prod- uct undergoes compression to moderate pressure, followed by freezing to maintain its structure [14]. During the first stage, the product remains unfrozen. However, the pressure is elevated in the second stage, inducing a phase transition [45]. HPSF emerged as the most promising technique among the high-pressure freezing methods discussed above, primar- ily due to the extent of supercooling achieved in the sample once the pressure is released. Fernández et al. [15] compared HPF and HPSF methods, examining the ice crystal distribu- tion and its impact on the microstructure of gelatin powder under varying pressure and temperature conditions. Their findings revealed that HPSF demonstrated superior super- cooling and nucleation rates compared to HPF, along with rapid freezing rates during pressure expansion. After releas- ing the pressure, the temperature decline resulted in a rapid phase transition period (5.9, 8.6, and 13.7 min in HPSF vs. 14.8, 14.1, and 23.1 min in HPF at 0.1, 50, and 100 MPa, respectively). This results in a uniform distribution of small ice crystals in the sample. These findings are consistent with those reported by Su et al. [44], who studied the ice crystal formation in shrimp muscle and porcine liver under HPSF at 100 MPa (-9 °C), 150 MPa (-15 °C), and 200 MPa (-21 °C), producing a large number of small and regular ice crystals that were homogeneously distributed throughout the sample when compared to CF at the same temperature. The authors underscored that the production of uniform ice crystal sizes is primarily due to the large temperature reduction (super- cooling of 6.5, 11.8, and 16.8  °C, respectively) generated by the HPSF method. Rather than producing high-quality food, HPAF offers significant advantages in energy effi- ciency compared to CF methods. Li et al. [46] compared and analysed energy consumption and performance at the same capacity of atmospheric quick-freezing and HPAF. The findings show that HPAF consumed 68,980 kJ, while atmospheric quick freezing consumed 280,260 kJ. High Pressure-Assisted Freezing (HPAF) High-pressure processing (HPP) comprises a pressure range from 100 to 800  MPa to inactivate many pathogenic and spoilage bacteria, yeasts, fungi, and viruses [38]. HPAF freezes food samples under high pressure [39]. Conse- quently, HPAF lowers the nucleation and growth rates of ice crystals, thereby reducing the structural damage caused by the volume expansion of ice crystals during crystallisa- tion [40]. The efficiency of the freezing process is signifi- cantly influenced by pressure, time, and temperature [41]. This process is the same as CF at atmospheric pressure (0.1  MPa), with the only difference being that it occurs under increased pressure. During the precooling phase, the item to be frozen is subjected to compression until it reaches the desired pressure level and is subsequently cooled under pressure. Once a specific level of supercooling is achieved, a phase transition occurs, initiating ice nucleation [42]. According to Fernández et al. [15] HPAF can be divided into three different categories based on the phase transition: high-pressure freezing (HPF), where the phase transition occurs under constant pressure; high-pressure shift freez- ing (HPSF), where the phase transition is caused by pres- sure unloading; and high-pressure induced freezing (HPIF), where an increase in pressure initiates a phase transition and continues at constant pressure. The HPF process is similar to CF but is conducted under constant high pressure [39]. The cooling process of the sample proceeded from the outermost layer to the innermost region. It is generally believed that ice crystals form only in the outer part of the product, which is in direct contact with the cooling medium [43]. In the HPSF, the product is placed in a high-pressure vessel, and the pressure is increased to the desired level, typically approximately 200  MPa. The product is cooled to -20 °C or the desired temperature for a short period before ice is formed [15]. Then, the pressure is released quickly, resulting in uniform supercooling of the product that causes a phase transition. Supercooling of food samples leads to immediate and homogeneous nucleation throughout the product. Fine ice nuclei grow into small ice Sampe Drying method Drying condition Key findings Refer- encesAdvantages Disadvantages Black mulberry HAD FD HAD-DIC FD-DIC HAD: 70 °C, 2.5 m/s. 9 h, FD: First drying at -55 °C, 0.01 kPa; Second drying at 25 °C. 48 h. HAD-DIC: 70 °C for 3 h; DIC at 80 °C for 5 min, followed by vacuum drying at 0. 1 MPa and 70 °C for 3 h. F-EPD: -55 °C, 0.01 kPa for 12 h; DIC 80 °C for 5 min; vacuum drying at 0. 1 MPa and 70 °C for 3 h. HAD required 3 h. FD required 12 h. HAD-DIC required 3 h. FD-DIC required 12 h. FD-DIC had the best hardness 29.46 N, crispness 30, rehydration ratio 1.65 g/g, better colour and an excellent overall sensory assessment score compared to FD, HAD-DIC, and HAD. Comparatively lower preservation of anthocyanins and antioxidant capacity relative to FD. [79] Table 2  (continued) 1 3 Food Engineering Reviews strength; stronger bonds become more tightly linked, while weaker ones may weaken or break when subjected to a mag- netic field. Consequently, the macromolecular clusters either decrease in size or release free monomeric molecules due to the magnetic field. This reduction in cluster size results in an increase in non-freezing water, which helps preserve prod- uct quality more effectively [50]. There are four common methodologies for applying MF during the freezing process at various intensities or frequen- cies: Static Magnetic Fields (SMF), Alternating Magnetic Fields (AMF), Oscillating Magnetic Fields (OMF), and Pulsed Magnetic Fields (PMF). Wei et al. [51] investigated the effect of SMF (60 mT) at different temperatures (-35 to -20 ºC) on the physicochemi- cal properties of Pacific white shrimp, particularly focusing on the − 35 ºC immersion freezing. The authors concluded that the SMF at -35 ºC resulted in the shortest total freezing time of approximately 380 s, which is about 22.7% less than that of the − 35 ºC immersion freezing (466 s). Additionally, the phase transition time was shortened by roughly 10.2%. The research also indicated that the ice crystal distribution in the shrimp muscle of the SMF-processed sample was fine and homogeneous. As the freezing temperature increased, the size and distri- bution range of the ice crystals in the shrimp muscle grew, while the magnetic field inhibited ice crystal growth. This inhibition likely occurs because the magnetic field reduces the size of water clusters by breaking hydrogen bonds within and between them. Smaller clusters mean more nucleation points and a higher ice nucleation rate, leading to quicker and finer crystallisation. The increase in ice nuclei diminishes the energy potential barrier needed for crystal- lisation, thereby accelerating the phase transition process, as evidenced by the reduction in phase transition time and the promotion of an even distribution of ice crystals. Ye et al. [17] investigated AMF during the freezing of tilapia fil- lets at frequencies of 50, 100, 150, 200, and 250 Hz with magnetic field intensity. They noticed that the influence of the MF on freezing time and texture profile does not change monotonically (200  Hz > 250  Hz > 50  Hz > 100  Hz > con- trol > 150 Hz) as the frequency increases. This non-linearity may be attributed to resonance effects between the magnetic field and the rotational and vibrational modes of water mol- ecules, which could be frequency-specific and influenced by the ionic content in the food matrix. However, the micro- structure was analysed based on the fractal dimension and different patterns compared to the pattern mentioned above (250 Hz > 200 Hz > 50 Hz > 100 Hz > 150 Hz > control). Tang et al. [16] studied the effects of SMF (2, 10, 15, and 20 mT) and AMF (0.05, 0.28, 0.64, 1.26, 1.74 mT at a frequency of 50  Hz) on the cherry freezing process and conducted an analysis. Their findings showed that the control group HPAF is an innovative technology that provides notable benefits, including quick freezing rates and reduced ice crys- tal size. Nevertheless, its technical drawbacks, particularly regarding equipment limitations and energy consumption, present significant obstacles to its broader implementation. Presently, most HPAF systems are limited to laboratory- scale setups, with only a handful of pilot-scale options available for industrial use. Additionally, the initial cost of HPAF equipment is significantly higher than that of CF systems, primarily due to the requirement for robust pres- sure vessels, hydraulic systems, and sophisticated control mechanisms [47]. Magnetic Field-Assisted Freezing (MFAF) Water molecules exhibit dipolar characteristics. Each molecule consists of one oxygen atom and two hydrogen atoms, resulting in a partial positive charge on the hydrogen side and a partial negative charge on the oxygen side [48]. When exposed to a magnetic field, these dipolar water mol- ecules experience a torque that tends to align them along the magnetic field lines. This alignment disrupts the usual hydrogen-bond network, resulting in changes to the water structure and dynamics. The magnetic field can induce the formation of ring electric currents from the linear and cyclic hydrogen-bonded chains of water molecules through pro- ton transfer. The electric current of the ring has the poten- tial to either weaken or strengthen the hydrogen bonds and induce reorientation of water molecules, consequently alter- ing the physical properties of water. This reorientation and bond disruption result in smaller and more uniform clusters of water molecules. These clusters more readily form ice nuclei, reducing the energy barrier for crystallisation and facilitating faster and more uniform ice formation [49]. Fig- ure 1 illustrates a speculative result of the MFAF process, where the hydrogen bonds between water molecules vary in Fig. 1  Microscopic diagram of water molecules during MFAF [50] 1 3 Food Engineering Reviews conventional methods. Thawing loss decreased from 5.7% with CF to 1.7% in an electric field of 10 kV/m and 2.4% in a magnetic field of 2 mT. Dehydrofreezing (DF) Dehydrofreezing (DF) is a freezing technique that reduces the moisture content of food to an appropriate level before freezing it in a dehydrated state. This is generally achieved using a pre-drying process that removes excess moisture from food. Partial dehydration can be achieved using air- drying and osmotic dehydration methods. Subsequently, the product undergoes freezing. DF, which combines partial dehydration with freezing, is more effective in maintaining the textural quality of agricultural products after freezing and thawing while minimising cell damage than freezing without partial pre-dehydration [20, 21, 55]. In plant cells, water is present in the cells in vacuoles cytosol, cytoplasm, and outside the cells as extracellular water. Water in the vacuole is crucial for maintaining the cell turgor pressure, contributing to the firmness and crispiness of the product. Water in the cytoplasm interacts with cytoskeletal proteins and solutes to stabilise the membrane [40]. Freezing usually begins in the extracellular space because of unbound water and a lower concentration of solutes than the intracellular content, resulting in a higher freezing point. Ice formation reduces the liquid water content within the system while increasing the concentration of extracellular solutes, caus- ing water to be transported out of the cell via osmosis [56]. As a result, substantial amounts of extracellular ice crystals form, leading to destruction and cell deformation. In con- trast, fast freezing generates smaller, uniformly distributed ice crystals inside and outside the cells, reducing cellular shrinkage and deformation. However, at ultralow tempera- tures, fast freezing may cause cell membranes to crack or puncture owing to the rapid expansion of water [20]. DF has been proposed as a solution to the structural dam- age caused by freezing food. For instance, Ben Haj Said et al. [21] investigated the effect of initial water content on the freezing characteristics and quality attributes of apples using DF, comparing two freezing conditions: high practi- cal freezing rate and low practical freezing rate. The authors noted that the freezing rate significantly impacted apples with higher water content; prolonged freezing resulted in more thaw exudate, suggesting increased damage to cell structure. Consequently, a fresh apple with high moisture content (700% db) requires longer freezing time (86 to 329 min, depending on the freezing rate) than apples with lower water content (100 and 30% db) (32 min), which ren- ders the effect of the freezing rate on thaw exudate water insignificant. Schudel et al. [20] reported similar findings and compared DF to CF processes for fruits and vegetables required a phase change time of 15.50 min, while the short- est phase change times for SMF (10 mT) and AMF (1.74 mT) were 13.24 min and 11.69 min, respectively. Regarding drip losses, SMF showed a minimum of 4.79% at 10 mT and AMF showed a minimum of 4.64% at 1.26 mT, whereas FD was at 10.39%. Additionally, the average ice crystal area for FD was 5538.5 ± 1506 µm2, while the minimum average ice crystal area for SMF was 1846.57 ± 104 µm2 at 20 mT and 1220.5 ± 16 µm2 at 1.26 mT. High-Voltage Electrostatic Field-Assisted Freezing (HVEFAF) High-voltage electric field (HVEF) is a non-thermal food processing method that has recently attracted significant interest because it minimises adverse effects on food qual- ity and nutrition while preserving the physical and sensory attributes of food compared to conventional thermal pro- cessing and preservation techniques [52]. The HVEF pro- cess relies on generating electrical wind through corona discharge. Corona discharges ionise air into a needle-plate electrode system. The ions formed in a small region around the needle electrodes are then accelerated by an electric field, with the resulting momentum transferring from the air ions to the neutral air molecules, propelling the bulk fluid towards the surface [53]. This process can be categorised into two main types: static electric field (SEF), which occurs when the intensity or direction of the electric fields remains constant over time, and high-voltage electrical discharge (HVED), which occurs when the applied voltage surpasses the discharge onset criteria, resulting in discharge plasma by ionising a fluid surrounding a high-voltage electrode [54]. High-voltage electrostatic field-assisted freezing (HVE- FAF) is an innovative technique that uses high-voltage elec- tric fields to improve the freezing process of food. In this process, the food is placed between two electrodes, where the DC power generates an HVEF [53]. This can cause the polarisation and realignment of water molecules during freezing, decrease supercooling, and promote the initial growth of ice nuclei. Jia et al. [19] investigated the effects of HVEF on the freezing behaviour and quality maintenance of pork tenderloin. The results showed that the total area of ice crystals in the cross-sections of pork tenderloin treated with 10 kV HVEF was much smaller than that without HVEF. Hu et al. [18] explored the impacts of different high-voltage electric fields (10 kV/m, 30 kV/m, 50 kV/m) and magnetic fields (2 mT, 4 mT, 6 mT, and 8 mT) on the freezing pro- cess of pork tenderloin. They evaluated factors such as the freezing duration, ice crystal dimensions, thawing loss, and various physicochemical attributes. The study revealed that optimal conditions, using a magnetic field of 2 mT and an electric field of 30 kV/m, resulted in a 40.04% and 37.81% reduction in freezing time, respectively, compared to 1 3 Food Engineering Reviews shortened the total drying time, as elevated temperatures enhance the speed at which water evaporates from the sample. Although increasing the MW power could further accelerate the drying process, it did not have a pronounced effect. This may be because, once the target maximum temperature was achieved, the MW power was applied in pulses rather than continuously. The authors noted that MW energy disperses unevenly within the drying chamber, caus- ing certain regions of the sample to receive more energy than others and resulting in hot and cold spots. To reduce this uneven heating, one effective approach is to maintain a narrow temperature range, which promotes more uniform heat distribution and decreases the likelihood of these spots. Increasing speed or modifying the rotation dynamics of the sample turntable could enhance exposure to a more consis- tent microwave field for all areas. Additionally, implement- ing sample movement during the drying process along with adaptive control systems can address the challenges of inho- mogeneity in MWAFD, ultimately producing higher-quality and more evenly dried products. In recent years, pulse-spouted microwave-assisted freeze drying (PSMWAFD) has been proposed as a solution to address the uneven heating challenge of MWAFD. This technique changes the position of the food materials within the drying chamber, thereby enhancing the consistency of MW heating. Figure  2 shows a schematic diagram of the PSMWAFD, MWAFD, and FD systems. To change the posi- tion of materials in the MW field, an airflow spout system is placed at the bottom of a cylindrical MW freeze drying chamber. A solenoid valve controls the spout time and inter- val [61]. Wang et al. [62] explored the product quality of stem lettuce slices by applying MW heating and pneumatic pulse agitation in a laboratory system. The PSMWAFD achieved superior quality (uniform and compact microstruc- ture, greater hardness) of dried stem lettuce slices with little discolouration (ΔE 37.68), a greater rehydration capacity (RC) (0.36 g water/ g dehydrated sample), and a consistent and more compact microstructure compared to MWAFD dried samples (ΔE 38.79, RC 0.35  g water/ g dehydrated sample). PSMWAFD heated the food more uniformly and consistently, improving food quality and overall efficiency. Li et al. [63] studied the drying efficiency of yam slices dur- ing PSMWAFD. The results revealed that PSMWAFD can reduce the drying time by 41.38% compared with FD and save total energy consumption by up to 34.4% compared with the conventional FD. Infrared Radiation-Assisted Freeze Drying (IRAFD) IR are electromagnetic waves with a wavelength between 0.75 μm and 1 mm, corresponding to frequencies between 0.3 THz to 400 THz. They are categorised into three by assessing drip loss and tissue firmness. The results showed that DF increased product firmness (N) from 35 to 52%; convective DF decreased drip loss in carrots, sweet red bell peppers, and cucumbers. Advanced Freeze Drying Technologies (AFDTs) Microwave-Assisted Freeze Drying (MWAFD) This technique utilises microwave (MW) power as a heat source for sublimation. Heat is generated via dipole rotation and ionic conductance in an electromagnetic field within the drying medium [57]. Conventional freeze dryers integrate microwave energy into the drying process in two main con- figurations [58]. In the first configuration, MW fields were applied at the beginning of the FD to provide sublimation heat. This approach significantly increases the rate of FD by facilitating deeper penetration of MWs into food material, resulting in more uniform volumetric heating. The second configuration involved two stages: FD during the primary drying stage, followed by microwave-assisted vacuum dry- ing during the secondary drying stage. The commercial use of MWAFD is gaining popularity owing to its reduced energy demand, shortened processing time, and improved uniformity in temperature distribution across the material through volumetric heating, which leads to improved dried food quality. Cao et al. [59] analysed var- ious MW power levels (1, 1.5, and 2 W g− 1) to assess their effects on energy supply and product quality. The findings indicated that 1.5 W g− 1 was the most effective for achieving a balance between energy efficiency and quality retention, reducing total energy consumption by up to 42% compared to FD, while maintaining high levels of flavonoids and chlo- rophyll (11.7 and 12.8 g kg–1). The author also found that a maximum reduction of 48% in energy consumption for MWAFD can be achieved compared to FD, and the dry- ing time can be shortened by over 43%. Chen et al. [23] compared the energy consumption and drying time of FD and MWAFD, finding that MWAFD resulted in a 34.5% reduction in energy consumption and a 33.3% reduction in drying time compared to FD. Notably, MWAFD may pres- ent disadvantages related to the uneven distribution of the electromagnetic field. This unevenness can lead to localised overheating and ice melting, potentially compromising the quality of dried products. Kalinke & Kulozik [60] examined how temperature (40 °C, 50 °C, 60 °C, 70 °C) and micro- wave (MW) power (180 W, 200 W, 220 W, which correlates to 1.50 W/g, 1.67 W/g, and 1.83 W/g of the initial sample weight) affected both energy consumption and inhomogene- ity, particularly focusing on the degree of uneven process- ing in MWAFD. Their results indicated that setting higher maximum temperatures for the drying process significantly 1 3 Food Engineering Reviews In the IRAFD process (Fig. 3), IR provides the energy required for water sublimation, minimising energy con- sumption compared to conventional FD, which uses heat- ing plates [65]. The major advantages of IR drying include reduced energy loss and drying time, as well as the pres- ervation of the quality of the dried product [66]. Several studies have been conducted on the application of IRAFD to enhance the drying efficacy and product quality of fresh foods, including bananas (24), mushrooms (67), sweet pota- toes (68), and pumpkins [67]. For instance, Khampakool et al. [24] investigated the efficacy of IRAFD in produc- ing dried banana snacks. The study revealed that IRAFD separate bands: near-infrared (0.75–2.0  μm), mid-infrared (2.0–4.0 μm), and far-infrared (4.0–1000 μm). IR radiation is transmitted from the source to the food sample without heating the surrounding air. When IF reaches a food sam- ple, its energy is absorbed by the molecules in the material, increasing its vibrational states. Infrared radiation exhibits a shallow penetration depth into food compared to MWs and RFs. Therefore, energy absorption occurs mainly near the surface, resulting in elevated temperatures that surpass those of the surrounding air. Consequently, heat transfer from the product surface to its interior is facilitated through conduction [64]. Fig. 3  Schematic diagram of IRAFD [65] Fig. 2  Schematic representation of freeze drying system for PSMWAFD, MWAFD, and FD: 1 feeding ball valve, 2 plate valve with 3-mm diameter hole, 3 and 22 microwave heating cavity, 4 mag- netron, 5 and 16 circulating water unit, 6 drying chamber for MFD and PSMFD, 7 and 11 pressure gauge, 8 solid–gas separator, 9 gas flow electromagnetic valve, 10 gas flow adjustable valve, 12 nitrogen gas source (7–12 only for PSMWAFD),, 13 and 29 fiber optic temperature sensor, 14 and 24 sample, 15 drying chamber with a jacket for FD, 17 control panel, 18 vapor condenser, 19 vacuum pressure transducer, 20 refrigerator unit, 21 vacuum pump unit, 23 water load pipe, 25 Teflon tube, 26 gas distributer, 27 fixed unit for drying chamber holder, 28 silicon rubber stopper 4 (×4) [62] 1 3 Food Engineering Reviews industry is classified into two categories: (1) low-frequency (20–100 kHz), high-energy (≥ 1 Wcm− 2), and (2) high-fre- quency (above 100  kHz), low-energy (≤ 1  Wcm− 2) ultra- sound [70]. Combining US with other food processing methods, such as drying or frying, enhances their efficiency. US induces cavitation and the sponge effect, generating sequential compressions and expansions within the mate- rial, which lead to mechanical stresses [71]. Mechanical stress accelerates the removal of water vapour through the food matrix and increases the diffusivity of water vapour within the food matrix, leading to an increase in the effec- tive mass transfer rate [72]. There are two distinct methods of transferring ultrasound for drying: airborne ultrasound and direct contact ultrasound. Airborne ultrasound involves transmitting ultrasound waves through air to the material being processed. On the other hand, direct contact ultra- sound involves the direct transmission of ultrasound waves through a medium, such as a liquid or solid, in direct contact with the material being processed. However, airborne ultra- sound cannot be applied in a vacuum. Hence, ultrasound is transmitted by direct contact with shelves supporting foods for FD. Dai et al. [73] incorporated an ultrasonic vibration plate into an FD device to examine its effect on the size of ice crystals and the rate of sublimation on carrot slices. According to the results, US increased nucleation at lower supercooling levels, and the sublimation time of the sono- crystallized samples was reduced by 21.80% compared to that of the control. Similarly, research by Schössler et al. [74] revealed that the application of US shortened the sub- limation time by 11.5%, without affecting bulk density, colour, ascorbic acid content, and rehydration properties of the dried product. Furthermore, in addition to increasing the drying rate and shortening the processing time, USAFD has been shown to positively influence the nutritional qual- ity of the product, rendering it an attractive alternative to FD. Gong et al. [75] investigated the effects of UAFD on the drying kinetics, colour parameters, phytochemical com- position, antioxidant capacity, and microstructure of flos sophorae immaturus. The drying time using UAFD was reduced by 40% compared to FD. In addition, the total phe- nolic and rutin contents increased by 29% and 27%, respec- tively. UAFD resulted in more porous structures, along with improved colour retention and antioxidant capacity. Instant Controlled Pressure Drop-Assisted Freeze Drying (FD-DIC) The concept of controlled pressure drop (DIC), also referred to as explosion puffing drying (EPD), is based on the ther- momechanical effects associated with sudden pressure drops to vacuum (approximately 5 kPa) in samples pretreated with saturated steam (0.1–0.6 MPa) [76]. This process is divided significantly reduced the drying time by up to 213 min com- pared to 696 min for FD, resulting in more than 70% time savings. Hnin et al. [68] compared infrared drying (IRD), FD, and IRAFD in terms of quality and energy consumption for two types of potato chips. They found that IRAFD can reduce drying time by up to 25–29% and energy consump- tion by 43–46% compared to FD while maintaining higher product quality (colour, odour, texture, water activity, expansion ratio, and bulk density). Numerous studies indi- cate that combining IRAFD with other heating sources or implementing pretreatments before drying can yield higher drying rates, shorter processing times, and improved pres- ervation of nutritional value. Lao et al. [25] investigated the combined effects of IRAFD and MVD on the drying time, energy consumption, and quality parameters of kale yogurt melts. They showed that IRAFD shortened the drying time by 9.54% compared to FD, whereas IRAFD-MVD short- ened the drying time by 31.08% compared to IRAFD. Fur- thermore, IRFD-MVD and IRFD consumed approximately 30.56–46.11% and 15.54–34.46% less energy than FD, respectively. Wu et al. [69] reported that ultrasound pretreat- ment of sweet potatoes before IRAFD significantly acceler- ated drying rates and shortened processing times, resulting in lower energy consumption. In addition, the pretreated sample had a 4–42% higher β-carotene content than the untreated sample. Khampakool et al. [24] employed a semi- customised pilot-scale dryer to evaluate the effectiveness of IRAFD. A variety of IRAFD trials were conducted, includ- ing continuous IRAFD at 2.7 kW/m², IRAFD at 2.7 kW/m² with a 20% weight reduction (WR), and IRAFD at 2.7 kW/ m² at 20% WR to 4.0  kW/m² at 0  °C. The findings indi- cated that the minimum drying time for continuous IRAFD at 2.7 kW/m² was 213 min. This was followed by a drying time of 294 min for IRAFD at 2.7 kW/m² at 20% weight reduction (WR) to 4.0 kW/m². Additionally, the drying time was recorded as 330 min for IRAFD at 2.7 kW/m² with a 20% WR, while conventional FD required 696  min. The authors also assessed electrical energy consumption using an electric energy meter and discovered that FD exhibited the highest total energy consumption, recording 27.0 × 103 kJ, along with the lowest drying rate of 3.3 g/h. In contrast, IRAFD-2.7 kW/m² at 20% WR demonstrated a consump- tion of 13 × 10³ kJ while achieving a drying rate of 7.0 g/h. Meanwhile, IRAFD-2.7 kW/m² at 20% WR to 4.0 kW/m² at 0 °C with a drying rate of 8.0 g/h, consuming a total energy of 11.6 × 103 kJ, whereas continuous IRAFD-2.7  kW/m² consumed 8.4 × 103 kJ with a drying rate of 10.9 g/h. Ultrasound-Assisted Freeze Drying (USAFD) Ultrasound (US) increases the speed and efficiency of various processes in the food industry. Its use in the food 1 3 Food Engineering Reviews nutrient loss, and elevated energy consumption, especially during extended drying periods [82]. Therefore, alternative pretreatment techniques have been introduced to address the shortcomings of conventional methods. Alternative pretreatments, such as MW blanching (MWB) and RF blanching (RFB), ultrasound (US), high-pressure process- ing (HPP), pulsed electric fields (PEF), cold plasma (CP), and electrohydrodynamic (EHD) treatment, have recently been developed to minimise nutrient loss and enhance FD efficiency (Table 3). Microwave Blanching (MWB) and Radio Frequency Blanching (RFB) MWB and RFB are based on the production of ions in water by the application of electromagnetic fields [83]. In the MWB process, products are placed in a metal chamber, and a device transfers MWs to the chamber. The electromag- netic field directly interacts with food, heating the samples effectively. However, in the RFB process, products are placed between electrodes, and RF energy is generated by a triode valve. Ferreira et al. utilised MWB on broccoli by-products and assessed its impact on hot air drying and FD. Their results indicated that the MWB-treated samples required less pro- cessing time and energy compared to the untreated ones. For example, untreated samples required 64  h and consumed 69,120  kJ of energy for air drying, as well as 164  h and 130,382 kJ for FD. Conversely, MWB samples also required 64 h and used 69,120 kJ of energy for air drying, and 164 h with 130,382 kJ for FD. This study also reported that the untreated sample contained a total carotenoid of 1609 mg/g in FD and 911 mg/g in air-dried samples, whereas the MWB- treated sample contained a total carotenoid of 2688  mg/g in FD and 963 mg/g in air-dried samples. Additionally, the untreated sample had a total pigment content of 5712 mg/g in FD and 2817 mg/g in air-drying, while the MWB-treated sample had a total pigment content of 8258 mg/g in FD and 2556 mg/g in air-drying [84]. RF heating has advantages of a larger penetration depth compared to MW and higher heating rates compared to thermal blanching, resulting in shorter processing times and better-quality retention. Tsubaki & Matsuzawa [85] tested different frequencies of electromagnetic waves in the range of RFs (27 to 200 MHz) to enhance the FD pro- cess of model aqueous solutions, including pure water and NaCl. For water, the application of RFB and MWB signifi- cantly reduced the duration of FD by 67% (27 MHz). In the case of NaCl, the RF application led to a 71% reduction in FD duration (27 MHz). This reduction is attributed to the high dielectric loss tangent of the aqueous NaCl solution in the RF range, which heats ice water through RFs due to into five parts: (1) pre-drying the sample to a desired mois- ture content, (2) establishing water equilibrium, (3) apply- ing high-temperature steam, (4) rapidly reducing pressure to create a vacuum, and (5) completing drying under vacuum [8]. This technique minimises energy consumption owing to its efficient usage of vapour and protects heat-sensitive components through short heating times [77]. DIC-assisted freeze drying (FD-DIC) has been used to dry various substances, including food, pharmaceuticals, and biological materials. During the FD stage, the mate- rial is first frozen; then, a vacuum is applied for partial dry- ing. This vacuum condition rapidly increases the number of cracks and fractures within the material, decreasing the moisture content by approximately 50%. The instant con- trolled pressure-drop technique is then applied to the sec- ondary drying stage [78]. Steam is administered for a brief duration, typically approximately 10  min, followed by a sudden and controlled pressure reduction to approximately 3  kPa via a vacuum pump. Lyu et al. [30] compared the drying properties of FD and FD-DIC on papaya chips; FD reached the desired moisture content in 1800  min, while FD-DIC samples took 962 min. The authors also reported no significant colour differences, with FD-DIC samples exhibiting a crispier texture than FD. Yi et al. [29] evaluated the effects of FD-DIC, DIC-assisted hot air drying (HAD- DIC), and FD on the physicochemical properties, colour, antioxidant activity, expansion ratio, texture, rehydration, hygroscopicity, and microstructure of jackfruit chips. FD- DIC samples exhibited the highest expansion ratio (119%), superior colour (ΔE, 6.5), and improved texture (hardness 42  N, crispness 19 n (n = numbers of peak)) compared to HAD-DIC and FD. Moreover, FD-DIC dried jackfruit chips demonstrated stronger antioxidant capacity and higher reten- tion of phenolics and carotenoids compared to the other two methods. In a similar study, Chen et al. [79] reported that FD-DIC dried black mulberry had the best texture charac- teristics (hardness 29.46 N, crispness 30 n), a higher rehy- dration ratio of 1.65 g water/g dehydrated sample), better colour (hue angle 0.03), and an excellent overall sensory assessment score compared to FD, HAD-DIC, and HAD. Alternative Pretreatment before Freezing and FD The pretreatment of food prior to the drying process serves two distinct purposes: it reduces the drying time and enhances the quality of the dried material [80]. Con- ventional pretreatment methods typically encompass hot water blanching (HWB), steam blanching, hyperosmotic solutions, alkaline solutions, sulfation, and acid liquor [81]. However, these conventional pretreatments may also pose potential challenges, including chemical absorption, qual- ity degradation, inadequate rehydration, structural failure, 1 3 Food Engineering Reviews Sample Freezing or freeze drying method Pretreatment Pretreatment condition Findings Refer- ence Broccoli by-products Hot air drying (HAD) Freeze drying (FD) Microwave blanching (MWB). MW power: 800 W, Time: 2 min. The untreated sample had a total carotenoid of 1609 mg/g in FD and 911 mg/g. The MWB-treated sample had a total carotenoid of 2688 mg/g in FD and 963 mg/g. The untreated sample had a total pigment of 5712 mg/g in FD and 2817 mg/g. The MWB-treated sample had a total pigment of 8258 mg/g in FD and 2556 mg/g. [84] NaCl FD Radiofre- quency balancing (RFW). RFB: 27 and 200 MHz. RFB treatment reduced the duration of water by 67% RFB treatment reduced the duration of NaCl by 71%. [85] Strawberries FD Pulse-spouted microwave- assisted freeze drying (PSMWAFD) Ultrasound- assisted osmotic dehydration (USOD). USOD: sucrose: 100 g solution con- tains 30 g sucrose and 10 g maltodextrin. Fruit to solution: 1:4 (w/ w), US treatment: 15 min at 25 ± 5 ºC and 45 Hz; ultrasonic power was 180, 240, 360 W. PSMWAFD reduced drying time by 45% compared to conven- tional freeze drying. USOD treatment provided an additional 10% reduction in drying time. The untreated sample’s total phenolic compound (TPC) and total flavonoid content (TFC) were 6.89 mg/g and 8.84 mg/g, respectively. The TPC and TFC of the USOD (180 W) treated samples were 5.11 mg/g and 8.00 mg/g, respectively. The TPC and TFC of the USOD (240 W) treated samples were 9.56 mg/g and 6.99 mg/g, respectively. The TPC and TFC of the USOD (360 W) treated samples were 8.38 mg/g and 6.57 mg/g, respectively. [91] Pineapple Slices Microwave- assisted Vacuum freeze drying (MWAVFD) Osmotic dehydration (OD), Ultrasound (US). OD: 20°Bx, 30°Bx, and40°Bx, time: 20, 30 and 40 min. US: 60, 90, and 120 W, time: 20, 30 and 40 min. The untreated sample consumed energy 50.26 kWh. The sample pretreated with OD at 40ºBx for 40 min used a minimum of 42.09 kWh of energy. The sample pretreated with US at 120 W for 40 min used a minimum of 35.18 kWh of energy. [140] Carambolas FD High hydro- static pressure (HHP). HHP: 50, 100, 150, 200 and 250 MPa. Time 15 min, Temperature: 25 ºC. The untreated sample needed 18 h to reduce the moisture content below 8%. HPP treatment at 50, 100, 150, 200 and 250 MPa required 12, 12, 12, 10 and 10 h, respectively. The TPC and TFC of the untreated sample were 11.34 mg/g and 10.77 mg/g, respectively. The TPC and TFC of the HPP-50 MPa sample were 11.83 mg/g and 11.46 mg/g, respectively. The TPC and TFC of the HPP-100 MPa sample were 12.50 mg/g and 11.81 mg/g, respectively. The TPC and TFC of the HPP-150 MPa sample were 12.77 mg/g and 12.38 mg/g, respectively. The TPC and TFC of the HPP-200 MPa sample were 13.32 mg/g and 12.75 mg/g, respectively. The TPC and TFC of the HPP-250 MPa sample were 13.36 mg/g and 12.73 mg/g, respectively. [95] Strawberry slices FD HPP. HPP: 50, 100, 150, 200 and 250 MPa, respectively, for 5 min at 25 °C. The untreated sample took 36 h to dry. The sample treated with HPP-50 required 32.76 h to dry. The sample treated with HPP-100 required 32.06 h to dry. The sample treated with HPP-150 required 30.6 h to dry. The sample treated with HPP-200 required 27.72 h to dry. The sample treated with HPP-250 required 27.36 h to dry. [95] Apple slices FD Pulsed electric field (PEF). PEF: Specific ener- gies of 0.5 and 1 kJkg− 1 and a field strength of 1.07 kVcm− 1. The distance between the elec- trodes (made of stain- less steel): 280 mm. PEF treatment reduced processing time by 57% compared to untreated apple slices. The effective water diffusion coefficient increased by 44% due to PEF application. [100] Table 3  Application of novel pretreatment before freeze drying for various food products 1 3 Food Engineering Reviews and heat transfer. As a pretreatment, this approach accel- erates drying rates, reduces processing time, conserves energy, and enhances product quality. Fan et al. [88] inves- tigated the impact of the US on carrot slices as a pretreat- ment at different power levels (150, 240 and 300 W) using IRAFD. The results showed that the drying rate increased with US power; the drying time was shortened by 20.7%, 23.7%, and 22.6% at 150 W, 240 W, and 300 W, respec- tively, compared to untreated samples. The study also found that US pretreatment notably affected carotene retention, with a 33% increase. Li et al. applied US (200 W,40 kHz, 25 min), freeze-thawing (− 20 ± 0.5 ºC, 24 h), and a com- bination of both (Freeze-thawing-US) as pretreatments in the VAFD process of apricot slices, analysing the effects of these pretreatments on drying conditions and product qual- ity. The study reported that the total energy consumption for the control, US, freeze-thawing, and freeze-thawing-US groups was 64.33, 52.96, 44.06, and 41.39 kWh/kg, respec- tively. Compared to the control group, the total energy consumption of the US, freeze-thawing, and freeze-thaw- ing-US treatment groups decreased by 17.67%, 31.51%, the interaction of ions in ice with electromagnetic waves, as demonstrated by molecular dynamics simulation. Zhang et al. [86] reported that RF blanching of apple slices had a faster heating rate, reaching the target temperature in half the time compared to HWB. They also found that HWB had higher weight loss and discolouration than RFB. In another study, Gong et al. [87] investigated RF heating as a novel dry-blanching method for carrot cubes. Their findings revealed that RF blanching reduced peroxidase activity by 90–95%, while preserving hardness, redness, and vitamin C content better than HWB. Ultrasound (US) In recent years, there has been considerable interest in using ultrasound pre-treatment for food products prior to drying, effectively reducing drying time while maintaining the qual- ity of food products. When US pretreatment is applied to foods immersed in water, the cavitation of US waves pro- duces micromechanical shock waves that disrupt cellular structure, create pores on the surface, and increase mass Sample Freezing or freeze drying method Pretreatment Pretreatment condition Findings Refer- ence Red bell pepper FD Hot water blanching (HWB). PEF. Ultrasound (US), PEF-US. HWB: 98 ºC, 3 min. PEF: Specific energy: 1–3 kJkg− 1, electric field strength: 1.07 kVcm− 1. US: Frequency: 21 kHz, Power: 300 W, Time: 30 min. PEF-US: PEF + US, US-PEF. The untreated sample required a drying time of 800 min. HWB treatment required a drying time of 245 min. PEF treatment required a drying time of 305 min. US treatment required a drying time of 245 min. PEF-US treatment required a drying time of 230 min. US-PEF treatment required a drying time of 455 min. [104] Sea Cucumber FD Electrohy- drodynamic (EHD). Voltage 45 kV, Tem- perature 18 ºC, Rela- tive humidity 45%. The untreated sample required a total drying time of 20 h and consumed 168,000 kJ of energy to remove 1 kg of water. The EHD-treated sample required 17.5 h and consumed 13,200 kJ of energy to remove 1 kg of water. The final shrinkage values for untreated and treated samples were 4.10% and 10.75%, respectively. [118] Chilean sea cucumber FD EHD. Voltage 20, 30, and 45 kV, Time 30, 45, and 60 min, Tempera- ture 25 ºC, Relative humidity 54.8%. The highest initial drying rate was observed with the 30 kV-30 min pretreatment. [119] Haskap FD Cold plasma (CP). CP: Frequency 40 Hz, power of 200 W, vacuum of 85 Pa, time: 15, 30, 45, 60, 75 s. The sample without CP treatment needed 21.67 h. The sample with CP treatment for 15 s needed 21.67 h. The sample with CP treatment for 30 s needed 19.67 h. The sample with CP treatment for 45 s needed 18.67 h. The sample with CP treatment for 60 s needed 17.67 h. The sample with CP treatment for 70 s needed 15.67 h. [112] Apples and potatoes FD CP. CP: Frequency 2.45 GHz, power 1.2 kW and a gas flow of 20 L min− 1, time 2.5, 5, 7–10 min. CP treatment for 10 min reduced polyphenol oxidase by 42% for apples and 10% for potatoes. CP treatment for 10 min reduced peroxidase by 65% for apples and 89% for potatoes. [113] Table 3  (continued) 1 3 Food Engineering Reviews Zhang et al. [95] investigated the effects of HPP pretreat- ment on drying efficiency and the physical and chemical properties of carambola chips. The study revealed that the time required for the untreated sample to reduce the residual moisture below 8% was 18 h. After different pressure treat- ments (50, 100, 150, 200, and 250 MPa), the required times were 12, 12, 12, 10, and 10 h, respectively. The drying time was shortened by 33.3%, 33.3%, 33.3%, 44.4%, and 44.4% in each case. This reduction may be due to HPP treatment increasing cell permeability. The microstructure analysis also indicated that as pressure increased, the number of irregular pores grew. The increment of these microscopic pore areas provides more ample space for water evaporation and mass transfer during the drying process. Consequently, it reduces the adhesion between water and surrounding struc- tures, thereby improving the efficiency of the drying process. Additionally, HHP treatment altered the cellular membrane, resulting in improved extraction and increased phenolic compound content. Consequently, Zhang et al. [95] found that at 250 MPa, the total phenolic content (TPC) rose from 11.34 to 13.36 mg/g, while the total flavonoid content (TFC) of the control sample increased from 10.77 to 12.73 mg/g. Zhang et al. investigated the effects of HHP pretreatment on the drying efficiency and physicochemical properties of strawberry slices during VAFD [96]. The authors concluded that the drying duration for the untreated sample was 36 h (moisture content below 10%), which decreased to 32.76, 32.04, 30.6, 27.72, and 27.36  h for samples pretreated at 50, 100, 150, 200, and 250 MPa, respectively. Additionally, the HPP pretreatment significantly enhanced the total antho- cyanin content by 34.65%, 47.28%, 135.41%, 272.7%, and 263.83% for the samples pretreated at 50, 100, 150, 200, and 250 MPa, respectively. While HPP pretreatment provides many advantages, it may also result in some negative effects if not carefully optimised. For instance, excessive pressure can lead to dis- colouration or toughening in certain foods, as seen in some high-pressure frozen or thawed items. The rehydration capacity of HPP-treated products can differ. In particular cases, like beetroot cubes, the rehydration capacity reduces when pressure exceeds 100 MPa, whereas samples treated at 100 MPa demonstrate a higher dry matter holding capacity and improved rehydrability compared to others [97]. Thus, it is essential to optimise both pressure levels and treatment duration to enhance the benefits of HPP while reducing any adverse effects on product quality. Pulsed Electric Field (PEF) Pulsed electric field (PEF) applies micro to millisec- ond repeated high-voltage electrical pulses (20–400  kW, give voltage too, 20–50 kV) to foods within a processing and 35.66%, respectively. This is likely due to the US and freeze-thaw pretreatments damaging the cells of the apricot slices, creating porous structures that enhance the diffusion of free water within the cells, facilitating water migration during the drying process, and thereby accelerating the drying rate. The above findings also agree with the results reported by Xu et al. [90], who found that the application of US and different freeze-thawing pretreatments (freeze-air thawing, freeze-water thawing) reduced the drying time by 25.0–62.50% and the total energy consumption was 24.28– 62.35% less [89]. Applying the US directly to fruits and vegetables, such as mango and avocado, poses challenges. To overcome this limitation, the US can be combined with osmotic dehy- dration (OD) as a pretreatment, referred to as ultrasound- assisted osmotic dehydration (USOD). This technique combines the benefits of osmotic dehydration with the acoustic effects of ultrasound, resulting in improved mass transfer, reduced drying time, and enhanced retention of nutritional and textural properties. Jiang et al. investigated the effect of USOD pretreatments (100 g of solution con- tains 30 g of sucrose and 10 g of maltodextrin, US power: 180, 240, 360  W) on the drying and quality character- istics of pulse-spouted microwave-assisted freeze-dried (PSMWAFD) strawberries. Their study concluded that, compared with conventional FD, PSMWAFD could reduce the drying time by approximately 45%. USOD could further decrease the drying time by about 10% due to the improved dielectric properties of the treated strawberries. The study also reported that the USOD-treated sample groups retain more total phenolic content than the untreated samples. This may be because the USOD treatment significantly reduces the drying time, resulting in a higher retention rate of total polyphenols. Additionally, the total flavonoid content of the test group is lower than that of the control group, which may be due to a greater loss of flavonoids during the USOD pro- cess [91]. High-Pressure Processing (HPP) HPP pretreatments can reduce processing time and improve the drying efficiency of food products [92]. HPP treatment causes modification of the cellular structure, reducing the turgor pressure and affecting water permeability and mass transfer. According to Witrowa-Rajchert et al., HPP leads to cell decomposition, facilitating diffusion and positively influencing mass transport during subsequent processes [93]. Yucel et al. reported similar findings, indicating that the HPP enhances cellular infiltration, increases moisture diffusion, and improves mass transfer, drying rate, and rehy- dration time [94]. 1 3 Food Engineering Reviews tests, and found that PEF treatment led to better shape retention, prevented shrinkage, and resulted in tissue pore enlargement. A case study investigated by Lammerskitten et al. [103] investigated the influence of PEF pretreatment on the microstructure of strawberry dice using scanning electron microscopy (SEM) and µ-CT image visualisation. The SEM images revealed that untreated samples exhibited a higher density and more compact structures, while PEF- pretreated samples displayed large pores (Fig.  4a). It was also observed that untreated samples exhibited significant deformation and shrinkage. In contrast, PEF pretreated sam- ples showed a more uniform shape, reduced shrinkage, and resulted in optically better quality (Fig. 4b). Images of µCT revealed that PEF-treated samples exhibited greater volume retention and preserved the original shape more closely than untreated samples [103]. However, a notable disadvantage of PEF pretreatment is its influence on product colour. Studies have consistently shown that samples treated with PEF often exhibit reduced colour stability compared to those that are untreated. For example, Rybak et al. [104] assessed the effects of PEF, ultrasound (US), and hybrid treatment (PEF-US) on freeze- dried Red Bell Peppers. The results revealed that the PEF- US treatment yielded the highest colour difference value (ΔE) of 23.9, followed closely by PEF at 25.4, HWB at 23.1, and US at 9.4. Cold Plasma (CP) Plasma, the fourth state of matter alongside solid, liquid, and gas, is an ionised gas composed of a wide variety of spe- cies, including electrons, positively and negatively charged chamber. This process results in improved permeability and rupture of biological cell membranes. Recently, various studies have demonstrated that PEF enhances mass trans- fer and reduces drying time without causing unpleasant changes in food products [98]. Foods are placed between two electrodes in a chamber with an aqueous medium, usu- ally at room temperature. The polarisation of the cell mem- brane leads to permeabilisation and destruction of the cell tissues, thus increasing the mass and heat transfer during subsequent drying [99]. Lammerskitten et al. conducted an analysis of the impact of PEF (specific energies of 0.5 and 1  kJ/kg, and a field strength of 1.07  kV/cm) on the ther- mophysical properties and water adsorption characteristics of dried apples subjected to VAFD. The findings revealed that a similar moisture ratio (0.004) required 840 min for the untreated sample, whereas the FEP-treated sample necessitated only 368 min, which is half the duration of the untreated sample [100]. The improvement in drying effi- ciency is attributed to the decrease in internal resistance to mass and heat transfer resulting from electroporation induced by the application of PEF prior to the drying pro- cess. Additionally, PEF treatment causes permeabilisation and cellular tissue destruction, improving mass and heat transfer mechanisms. Jalte et al. [101] studied the effects of PEF pretreatment on potato tissue before air-blast freezing. PEF treatment induced significant tissue damage, result- ing in an accelerated freezing rate, more uniform shapes, reduced shrinkage, and an improved overall appearance of the dried food. Parniakov et al. [102] evaluated the effect of PEF pretreatment of apple tissue before vacuum cool- ing and FD processes. The authors performed microscopic and macroscopic analysis, including capillary impregnation Fig. 4  Micro-(a) and macro (b) images of untreated and PEF- treated freeze dried strawberry dices [103] 1 3 Food Engineering Reviews increased the collision between the CP and the haskap sur- face, which resulted in surface cracks, reduced structural integrity, and the formation of surface pores. This increase in roughness facilitated rapid moisture discharge, reducing the drying time. The analysed microstructure indicated that the untreated haskap surface had a distinct reticular struc- ture with consistent porosity. Extended processing time resulted in damage to the mesh structure, leading to defor- mation, increased roughness, irregularity, and an increase in pore size, ultimately causing the mesh structure to deterio- rate and eventually disappear. Bußler et al. [113] examined the activity of the main enzymes, polyphenol oxidase (PPO) and peroxidase (POD), in apple and potato tissues treated with CP prior to FD. The scientists indicated that in apple tissue, PPO activity decreased to approximately 42% of its initial value following 10 min of CP treatment, whereas in potato tissue, PPO activity diminished more significantly to roughly 10% under identical conditions. POD activ- ity decreased by approximately 65% in apples and 89% in potatoes following a 10-minute CP treatment, indicating a noticeable and enzyme-specific inactivation effect. However, most existing research on the application of CP to food drying has primarily focused on hot air drying, with little attention paid to FD. Only a few studies have exam- ined the effects of CP on FD foods, with a primary focus on nutritional changes and drying time. As a result, the poten- tial of CP as a pretreatment for FD requires a comprehensive investigation, particularly into its effects on drying kinet- ics, microstructure, and quality, as well as mechanisms of action. ions, free radicals, molecules in ground or excited states, and photons [105]. CP is generated by subjecting gases, whether present or flowing between two electrodes, to high- intensity electric fields [106]. These fields ionise the gas molecules, releasing free electrons, which then collide with other gas molecules, leading to the formation of additional ionised species in cascading reactions [107]. Their tempera- ture differs because of the thermodynamic non-equilibrium between electrons and heavy species. This is because elec- trons are far lighter than ions and neutral molecules; hence, only a small amount of the total energy is transferred [108]. In the food processing industry, CP can be implemented via various techniques, such as dielectric barrier discharge (DBD), plasma jet (PJ), corona discharge (CD), RF, and MW, as shown in Fig. 5 [107]. Cold Plasma (CP) is used to extend the shelf life of fresh food by deactivating harmful microorganisms and enzymes on the surface of foods [109, 110]. Compared to the ther- mal process, CP uses less water, lower temperatures, and reduces operation costs. The combination of CP pretreat- ment with drying techniques, such as FD or hot air drying, shortens the drying time and enhances the nutritional value of the dried products [111]. CP combined with FD not only increases the drying efficiency but also improves the qual- ity characteristics of dried food. Li et al. [112] evaluated the feasibility of using CP pretreatment to improve the FD process for haskap. The pretreatment was conducted at a frequency of 40 Hz, with a power of 200 W and a vacuum of 85 Pa, for varying durations (0, 15, 30, 45, 60, and 75 s). Their results suggested that the overall drying process was not influenced by a 15  s CP treatment. The drying time decreased by 9.22%, 13.84%, 18.46%, and 27.68% for CP treatments of 30, 45, 60, and 75 s, respectively. This may be attributed to the fact that the duration of the CP treatment Fig. 5  Basic configuration of CP system: (a) Dielectric barrier discharge (DBD), (b) Plasma jet (PJ), (c) Corona discharge (CD), (d) Radiofrequency (RF), and (e) Microwave (MW) [107] 1 3 Food Engineering Reviews of the EHD-pretreated samples. Tamarit-Pino et al. [119] applied EHD pretreatment on Chilean sea cucumbers prior to FD and investigated the effect of various EHD voltages (20, 30, and 40 kV) and exposure times (30, 34, and 60 min). The authors noted that EHD significantly increased the moisture evaporation rate while reducing processing time compared to control samples. In particular, the highest dry- ing rate was reported at 30 kV for 30 min of pretreatment, generating more accessible porous spaces and facilitating faster moisture removal due to accelerated heat and mass transfer rates. Challenges and Limitations of AFTs and AFDTs Recent advancements in freezing and FD have significantly enhanced food preservation by boosting efficiency, qual- ity, and flexibility. These advancements involve integrating innovative technology and refining conventional processes to tackle common challenges such as high energy consump- tion and prolonged drying times. These innovative technol- ogies also enhance production efficiency and better preserve nutrients and flavours through low-temperature processing, which inhibits enzymatic activity and microbial growth [120, 121]. Studies have shown that combining novel tech- nologies with CF and FD systems can minimise drying time by 30 to 40%, depending on the type of technology, such as MW, RF, US, HPP, IR, etc [63]. Challenges in Implementing Advanced Technologies However, AFTs and AFDTs systems, particularly those that use ultrasound, microwaves, or other sophisticated equip- ment, are typically more expensive than conventional FD processes and may pose challenges for smaller operations. Furthermore, this specialised equipment might require fre- quent maintenance, increasing the total production costs. One of the main drawbacks of AFTs and AFDTs is the com- plexity of process control. Specific control parameters, such Electrohydrodynamic (EHD) EHD is a non-thermal and environmentally friendly method to prevent quality deterioration in heat-sensitive food prod- ucts. This technique facilitates rapid drying at lower tem- peratures, thereby reducing the risk of adverse impacts on product quality. Other advantages of EHD technol- ogy include its low energy consumption, non-mechanical design, simplicity, and rapid control. EHD drying consists of exposing wet products to power- ful electric fields, causing the elimination of water through the aerodynamic effects of “ionic,” “electric,” or “corona” winds [114]. This process relies on aerodynamic action and operates by applying high voltage differences between two electrodes, namely the emitter and collector [115]. The elec- trical parameters governing EHD drying include voltage, current type (direct or alternating), and polarity. The dry- ing chamber employs a vertically adjustable electrode with a pointed needle toward a stationary horizontal metal plate (Fig. 6) [116]. The metal plate is grounded and serves as the surface on which the products to be dried are positioned. When an electric field is applied to products, the charges within those products experience electrical forces, resulting in the movement of charges. This movement induces the repulsion or attraction of charged particles, facilitating the removal of water from the surface and enhancing heat and mass transfer [114]. In recent years, numerous studies have employed elec- trohydrodynamic (EHD) as a pretreatment before drying, owing to its high drying rate, low energy consumption, and protection of heat-labile components of dried foods [117]. Bai et al. [118] investigated the impact of EHD pretreat- ment followed by FD on the quality of sea cucumbers. The EHD pretreated samples showed significant reductions in drying time and a 67% decrease in energy consumption compared to the untreated sample. Additionally, shrinkage was reduced, and the rehydration rate and protein content increased, resulting in improved sensory characteristics Fig. 6  EHD pretreatment [116] 1 3 Food Engineering Reviews uniform heat distribution than low-fat foods [126]. How- ever, achieving uniform freezing or drying across large batches of materials in an industrial setting is often challeng- ing. As these techniques may work differently depending on the material or product characteristics, achieving consistent quality across large volumes is a major hurdle. However, standards and guidelines for some of these innovative tech- niques are limited. This poses challenges for adopting this technology in industries requiring standardised food safety, quality, and shelf-life processes. Process Optimisation Using Real-Time Monitoring and Machine Learning To address these challenges, process optimisation serves as an effective strategy, implemented through the introduc- tion of a dynamic FD cycle that utilises real-time sensors to accurately regulate temperature and pressure. For example, Liu et al. [127] developed a practical solution for the food processing industry, demonstrating that combining infrared spectroscopy (FTIR) with machine learning (ML) may be an efficient, quick, and non-destructive way to predict com- plicated FD process parameters. The authors indicated that the utilisation of FTIR spectral data, following appropri- ate preprocessing and the selection of characteristic wave- lengths through techniques such as principal component analysis, offers valuable insights into sample properties that can be directly employed to forecast FD process parameters, including pre-freezing temperature and time, sublimation temperature and time, as well as desorption temperature and time. Furthermore, the prediction models were rigorously tested by comparing model outputs to experimental data. In one case, the FD curves generated from the predictions showed over 96% agreement with experimental data, indi- cating the model’s reliability and precision. Innovations in Equipment and Smart Control Systems Moreover, mathematical modeling and simulation can enhance drying parameters and shorten cycle duration. Equipment innovation presents a key strategy for manag- ing scale-up and investment costs. This approach can intro- duce improved tray and shelf designs that ensure uniform heat transfer, incorporate heat recovery systems, and utilise AI-driven smart control systems for precise monitoring and informed decision-making, thereby enhancing both effi- ciency and user-friendliness of the innovative setup. Role of AI in Monitoring and Optimising FD Processes AI-powered optical sensing devices have become valuable tools for accurately monitoring the FD process. Throughout as processing time, voltage, frequency, temperature, pres- sure, and freezing rate, can lead to technical difficulties. A slight deviation in these factors may result in overprocess- ing, structural damage, improper drying, or a reduced nutri- tional value. For example, runaway heating is a common issue in RF heating, leading to uneven heating [122]. In US- assisted freezing, the duration of US radiation is a factor that enhances the nucleation process during freezing. Kiani et al. [70] analysed the ultrasound radiation time on agar gels and found that an ultrasound irradiation time of 1 s was inad- equate to initiate nucleation, 3 s was optimal, while times exceeding 5 s hindered the nucleation process. Scale-Up Limitations and Material-Specific Constraints Consequently, scaling AFTs and AFDTs from pilot- or labo- ratory-scale setups to large-scale manufacturing is a signifi- cant challenge. The scalability of these novel technologies for industrial applications presents numerous technical challenges, particularly in terms of equipment design. For example, most HPAF systems are currently at the pilot scale, and scaling up to industrial levels requires larger pressure vessels and more complex process control systems. HPAF requires robust pressure vessels capable of withstanding extreme conditions, including high pressures of up to 200 MPa [123]. Therefore, it needs a high-pressure generator along with various auxiliary systems such as vacuum pumps and a refrigeration unit. Similarly, USAFD faces challenges in expanding its operations in the industry due to the lack of ultrasonic reactors that can maintain a steady cavitation intensity on an industrial scale. The complexity of scaling up these reactors arises from considerations like equip- ment geometry, power intensity, and frequency selection. Researchers have suggested the development of multi-trans- ducer systems and tubular resonators as potential solutions to this issue; however, this approach also raises the total cost involved [124]. Return on Investment (ROI) represents a critical determinant impeding the scaling of innovative technologies within the industry. Various pivotal factors, such as process optimisation, energy efficiency, and product quality, significantly influence the ROI associated with the adoption of AFTs and AFDTs. A recent study has indicated that minimising energy costs can effectively reduce overall production expenses. Furthermore, it has been demonstrated that IRAFD systems result in a reduction in energy expen- ditures of nearly 25% compared to CF, which can accelerate the ROI timeline [125]. Some materials, such as high-fat foods, oily substances, or biological materials with complex protein structures, impact the efficiency of AFTs or AFDTs because they do not freeze as effectively or are adversely affected by novel freezing techniques. For example, high-fat foods have less 1 3 Food Engineering Reviews to greater consumer acceptance compared to conventional meals. Many consumers expressed doubts about the con- cept of longer shelf life for chilled ready meals with fewer additives. However, consumer acceptance of RF technology is hindered by a lack of uniform heating [133], and MW is influenced by concerns about the potential health effects of microwave heating [134]. Consumer acceptance of US and DIC technology is mostly favourable, with US considered a natural and environmentally sustainable processing method [135], while consumers appreciate DIC-processed products for their enhanced texture and flavour [136]. However, most of these studies have focused on tech- nical parameters such as drying rate, drying time, energy consumption, and product quality, while few have explored the effects of novel technologies on freeze-dried foods from a sensory and consumer perspective, revealing a gap in cur- rent research. Conclusions Freezing and FD are essential food processing preservation methods; however, they are hindered by lengthy process- ing times and high energy requirements. These challenges can be mitigated by integrating innovative technologies or appropriate pretreatments into these systems. The review paper summarises the advances in combining CF and FD with environmentally friendly technologies and high- lights the positive impact of innovative pretreatments on efficiency and nutritional properties. AFTs such as RFAF, HPAF, MFAF, HVEFAF, and DF lead to the formation of small and uniform ice crystals, which improve product quality, minimise nutritional loss, and reduce energy con- sumption. AFDTs, such as MWAFD, IRAFD, USAFD, and FD-DIC, have shown potential to accelerate drying rates, shorten processing times, reduce energy consumption, and preserve food quality. Alternative pretreatment techniques before FD have shown promise for reducing nutrient loss, shortening processing time, saving energy, and producing high-quality dried foods. However, most of these advanced technologies have pri- marily been showcased at the research level or pilot scale. Scaling them for commercial use will entail cost consider- ations; however, these can be offset by potential decreases in processing time, improved energy efficiency, and, in cer- tain cases, enhanced product quality. Future research should focus on the techno-economic and life cycle assessment of these AFTs and AFDTs, as well as on introducing combina- tions of advanced technologies, including the adoption of AI and machine learning, to optimise drying cycles in real- time. In future, the integration of two or more innovative technologies with CF will be regarded as a more advanced the process, these sensors, which include hyperspectral imaging and near-infrared spectroscopy, offer real-time information on the amount of moisture present [128]. They allow operators to continuously monitor moisture lev- els, ensuring the successful completion of the sublimation phase. Real-time data enables quick adjustments to process parameters, leading to optimal drying and preventing over- processing. FD involves multiple steps, including primary and secondary drying. AI sensors can identify slight varia- tions in moisture content and predict when the product will reach its ideal drying state [129]. This technique saves time and energy by reducing the possibility of over-drying and under-drying. AI optical sensing also significantly enhances energy management, potentially reducing overall process- ing time and energy consumption. Consumer Acceptance of Novel Technologies Although many new technologies are constantly being developed to enhance production efficiency and improve product quality, their adoption and implementation in indus- try are greatly influenced by consumer acceptance. Con- sumers’ tendency to be wary of new technology, especially in the food sector, can be impacted by several factors, such as concerns about ingesting unknown components in food, a perceived loss of naturalness, and the potential effects of new technology on the environment, workforce, and society at large [130]. Introducing new food processing technolo- gies often presents challenges, and the food industry con- siders the acceptance of consumers and other stakeholders during the decision-making process. Although consumers usually express concerns about new processing technolo- gies, perceptions differ depending on consumer behaviours and the technology involved. Generally, familiarity with the technology or positive associations with its name enhance acceptance. For instance, responses to high-pressure meth- ods (like pressure cooking, which can be done at home) have been far more moderate than reactions to irradiation technology, which has raised major objections among con- sumers. (Issues surrounding consumer trust and acceptance of existing and emerging food processing technologies). Khouryieh et al. [131] conducted a survey among food experts, including managers, scientists, and technologists from 421 food processing companies. The study revealed that HPP was the most commonly used non-thermal tech- nology, with 35.6% of respondents indicating its use, fol- lowed by PEF at 20%, and CP and oscillating magnetic fields, both of which were reported by 14.1% of respon- dents. Sorenson and Henchion [132] explored consumers’ perceptions and potential to purchase chilled ready meals produced using HPP. Their findings revealed that most par- ticipants regarded HPP as a safe and natural method, leading 1 3 Food Engineering Reviews 2. Afolabi IS (2014) Moisture migration and bulk nutrients interac- tion in a drying food systems: a review. Food Nutr Sci 58:692–714 3. James C, Purnell G, James SJ (2015) A review of novel and innovative food freezing technologies. 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