applied  
sciences
Article
Performance Evaluation of an Inflatable Solar Dryer for Maize
and the Effect on Product Quality Compared with Direct
Sun Drying
Janvier Ntwali 1,* , Steffen Schock 1, Sebastian Romuli 1 , Christine G. Kiria Chege 2 , Noble Banadda 3,†,
Gloria Aseru 3 and Joachim Müller 1
1 Tropics and Subtropics Group, Institute of Agricultural Engineering, University of Hohenheim,
70599 Stuttgart, Germany; schock.steffen@uni-hohenheim.de (S.S.);
Sebastian_Romuli@uni-hohenheim.de (S.R.); joachim.mueller@uni-hohenheim.de (J.M.)
2 Alliance of Bioversity International and the International Center for Tropical Agriculture (CIAT), Africa Hub,
Nairobi P.O. Box 823-00621, Kenya; C.Chege@CGIAR.ORG
3 Department of Agricultural and Biosystems Engineering, Makerere University, Kampala P.O. Box 7062, Uganda;
gloriaaseru50@gmail.com
* Correspondence: janvier.ntwali@uni-hohenheim.de; Tel.: +49-711-459-23119
† Dedicated to the memory of author Noble Banadda, who passed away during the preparation of this manuscript.
Featured Application: Postharvest handling, drying, food safety, renewable energy.



 Abstract: Maize is an important staple in Africa, which necessitates immediate drying to preserve the


postharvest quality. The traditional drying of maize in the open sun is prone to adverse weather and
Citation: Ntwali, J.; Schock, S.; extraneous contamination. In this study, the drying performance of an inflatable solar dryer (ISD)
Romuli, S.; Chege, C.G.K.; Banadda,
was compared to direct sun drying (DSD) in Gombe Town, Wakiso District (Uganda) by analysing
N.; Aseru, G.; Müller, J. Performance
the moisture content, yeasts, moulds, aflatoxin, and colour. The maximum temperature inside the
Evaluation of an Inflatable Solar ◦ ◦
Dryer for Maize and the Effect on ISD reached 63.7 C and averaged 7 C higher than the ambient temperature. Maize was dried using
Product Quality Compared with both methods to a moisture content below 14% after two days. In one of the received maize lots that
Direct Sun Drying. Appl. Sci. 2021, 11, was already heavily contaminated after harvest, drying with DSD and ISD reduced the aflatoxin
−1
7074. https://doi.org/10.3390/ content from 569.6 µg kg to 345.5 µg kg−1 and 299.2 µg kg−1, respectively. Although the drying
app11157074 performance in terms of drying time and product quality regarding colour, yeast, and mould was
similar for both drying methods, the advantage of ISD in reducing the risk of spoilage due to sudden
Academic Editor: Rafael rain is obvious. A strategy for the early detection of aflatoxins in maize is recommended to avoid
López Núñez contaminated maize in the food chain.
Received: 28 June 2021 Keywords: drying characteristics; food safety; innovative solar drying; maize quality; mycotoxin;
Accepted: 28 July 2021 solar bubble dryer; Africa
Published: 30 July 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in 1. Introduction
published maps and institutional affil-
iations. Maize (Zea mais) is the most cultivated cereal crop in Uganda, with an annual pro-
duction of 2.9 million tons cultivated on 1.1 million hectares [1]. Improving the quality
of the produce and reducing the postharvest losses is a major objective of agricultural
research [2]. Furthermore, better food quality and quantity contributes to the food security,
nutrition, and economy of the country. The district of Wakiso, in the Lake Victoria Cres-
Copyright: © 2021 by the authors.
cent agro-ecological zone (AEZ), is one of the main maize-producing districts in Uganda.
Licensee MDPI, Basel, Switzerland.
This article is an open access article Farmers in this district and across the country face several postharvest challenges leading
distributed under the terms and to losses of maize. Most of the postharvest losses are mainly a result of poor storage
conditions of the Creative Commons management that causes the infestation of yeasts and moulds, as well as with insects and
Attribution (CC BY) license (https:// rodents [3]. In humid tropical regions like Uganda, the infestation with moulds results in
creativecommons.org/licenses/by/ the production of mycotoxins that have detrimental consequences on human health and
4.0/). affect the economy [4]. An estimated 25–40% of cereal grains worldwide are contaminated
Appl. Sci. 2021, 11, 7074. https://doi.org/10.3390/app11157074 https://www.mdpi.com/journal/applsci
Appl. Sci. 2021, 11, 7074 2 of 14
by mycotoxins [5]. The tropical regions are classified as the most conducive regions for
mycotoxin contaminations [6]. Similar issues were revealed in research on the effect of stor-
age time on the contamination of maize in different AEZs of Uganda [7]. It was found that
most farmers could hardly dry maize to the required storage moisture content of <14%. At
the same time, it was determined that the contamination levels with aflatoxins were higher
with longer storage times, with an average contamination level of 30.2 µg kg−1, especially
in humid zones. In East Africa, the maximum limit for an aflatoxin contamination of maize
was fixed at 10 µg kg−1 [8]. The European Union Commission set the maximum limit
of total aflatoxins to 10 µg kg−1 for maize intended for sorting and processing, but this
limit is 4 µg kg−1 when maize is intended for direct consumption [9]. The USA Food and
Drugs Administration set, on the other hand, a limit of 20 µg kg−1 for the total aflatoxins
in maize [10]. A study on the level of exposure of the Ugandan population to aflatoxin
showed that the exposure was ubiquitous among the rural population and that this ex-
posure was suspected to be related to the consumption of contaminated food products
like posho (also called ugali, a stiff porridge made of maize flour) and groundnuts [11,12].
Aflatoxins are a group of mould metabolites produced by strains of Aspergillus flavus and
Aspergillus parasiticus [13]. There are four major aflatoxins: B1, B2, G1, and G2, and B1 is
the major aflatoxin produced by toxigenic A. flavus strains [14,15]. A. flavus contamination
can occur in the field, as well as during storage, but happens most commonly in storage
under favourable conditions. These are the moisture content of the grains, the storage
temperature, and the relative humidity in the storage facility all related to the water activity
(aw) [6]. Another essential quality parameter for maize, which is affected by drying, is
the grain colour. The colour of maize is evaluated using the CIE Lab colour space. It was
shown that slight changes in maize colour could occur during an extended drying time for
maize [16].
Maize producers in the tropical and subtropical regions face difficulty in drying the
harvested maize to the desired moisture content for storage [17]. High humidity and
frequent rainfall during the harvesting season delay the drying process and can result in
a loss of quality by enhancing mould infestation [7,18]. A moisture content of <14% is
necessary to keep the aw of maize grain below the safe storage level of 0.6 [19]. In humid
and sub-humid tropical regions, maize is harvested with a moisture content around 30%
w.b., necessitating drying before safe storage. In the district of Wakiso, conventional sun
drying is a common practice for maize drying, which is done by spreading grains on a mat
to expose them to sunlight [20]. The shortcomings of this method are an uneven moisture
removal; exposure to insects, rodents, birds and dust; and failure to reach a safe moisture
content due to a high dependency on the weather conditions [21]. An alternative and
affordable method for smallholder farmers is the use of solar dryers. There are different
types of solar driers used to dry cereals in Africa, but due to economic and technological
constraints, their application has been obscured [22]. The solar tunnel dryer is a forced–
convection direct- type solar dryer that was developed at the University of Hohenheim [23].
It was first applied to dry fruits and showed advantages by significantly reducing the
drying time and mass loss while protecting the crops from dust, rain, animals, and in-
sects [24]. Its application in cereal drying, initially with paddy rice, was promising, but
improvements were needed, such as the ability to mix the grains to achieve a homogeneous
drying process [25]. The inflatable solar dryer was developed in collaboration with the
International Rice Research Institute (IRRI) as an improved version, which is adapted to
cereal crops in tropical regions [26,27]. Asemu et al. [28] conducted experiments on the use
of an inflatable solar dryer to dry maize. Assuming a thin layer drying model, they proved
that the diffusion approach fitted their results better.
The objective of this study was to assess the performance of an inflatable solar dryer
(ISD) compared to open sun drying, referred to as direct sun drying (DSD), for maize drying
with regard to the product quality. The quality parameters assessed were the moisture
content, mycotoxins (total aflatoxins), yeasts, and moulds, as well as maize colour. The
results of this research allowed us to draw conclusions on the use of an inflatable solar
Appl. Sci. 2021, 11, 7074 3 of 14 
 
Appl. Sci. 2021, 11, 7074 3 of 14
moisture content, mycotoxins (total aflatoxins), yeasts, and moulds, as well as maize 
colour. The results of this research allowed us to draw conclusions on the use of an 
inflatable solar dryer in the postharvest handling of maize and recommendations on the 
dryer in the pofsuthtuarrev eascttihoannsd floinr gmoafizme ahizaendanlidngr etcoo amvmoiedn cdoantitoanmsionnattihone  fiunt tuhree vacatliuoen schfoarin. 
maize handling to avoid contamination in the value chain.
2. Materials and Methods 
2. Materials and Methods
2.1. Inflatable S2o.l1ar. IDnrflyaetra(bISleD S)olar Dryer (ISD) 
The ISD (SBD T25h,eG ISraDin (PSBroDI n2c5.,, GZraaminbParloes I,ntch.e, ZPahmilibpapleinse, sth) ew Pahs iulispepdinfoers)t hweads ruysiendg for the drying 
experiments, wexitphertihmeetnwtso, mwaitihn tchoem tpwoon menatisnu csoemd ptoonmenantsu ufascetdu rteo tmheanISuDfacbteuirneg thaeU IVSD-  being a UV-
stabilized transsptaabreilniztepdo tlyraenthsyplaerneent( PpEo)lyfielmthy(1le5n0eµ (mPEt)h fiiclmk), (w15h0i cµhmis thuiscekd),f worhtihceh tios puspeadr tfor the top part 
of the dryer; aonfd thaer edirnyfeorr;c eadndb laa crkeinpfoolrycveidn ybllacchklo priodlyev(iPnVyCl c)hfillomridaes (tPhVe Cbo) tftiolmm aosf tthhee bottom of the 
dryer, both condnreycetre,d bboytha czoinpnpeecrt[e2d6 ]b.yT ha ezdiprpyeerr h[2a6d].a Tlhene gdtrhyoefr 2h6adm aa nlednagtwh iodft h26o fm2 amn.d a width of 2 
Figure 1 showsmp. iFctiguurerse o1f sthhoewdsr ypeirctduurerisn ogf tthhee edxrpyeerri mduernitnsg.  the experiments. 
 
(a)  (b) 
Figure 1. InFfliagtuarbele1 s. oInlaflra dtarybeler s(IoSlaDr):d (ray)e vri(eIwSD fr):o(ma) ovuietswidfer ofmulloyu ltosaideedfu wlliythlo 1a0d0e0d kwg iotfh m10a0iz0ek agnodf (mb)a iinzesiadned view during the 
drying expe(bri)minensitd sehvoiwewingd uthrein lgoctahteiodnr yoifn dgifefexrpeenrti mseennstosrsh oawndin mgetshhe bloacgast (ioMnBo).f different sensors and mesh
bags (MB).
The air flow was produced by one 12-V DC axial flow fan (RDF2589B12N18S, Runda 
The air floEwlewctraosnpircos dCuOce.,d LbTyDo, nSeh1e2n-zVheDnC, Caxhiianlafl) opwowfaenre(Rd DbFy2 o5n89eB 11020N-W18 Sp,eRauk nsdoalar panel and a 
Electronics CO7.5,-ALThD s,olSahre bnazthteerny,. TChein rae)coprodwede raevderbaygeo naier v10e0lo-Wcityp deuakrinsogl tahrep darnyeinl ga nedxperiments was 
a 75-Ah solar b4a.t1t3e rmy. Ts−h1.e Mreaciozred wedasa vsperaegaed aoirn vae tlaorcpitayudliunr inegxth teo dthryei InSgDe xfopre rdiimreecntt suwna dsrying (DSD). 
4.13 m s−1. Maize was spread on a tarpaulin next to the ISD for direct sun drying (DSD).
2.2. Drying Experiments 
2.2. Drying Exp2e.r2i.m1.e Enxtsperimental Site and Raw Material 
2.2.1. Experimental Site and Raw Material
Three experimental batches of maize were dried using both an ISD and under direct 
Three expseurnim (DenStDal) bnaetacrh tehseo cfitmy aoifz Ge womerbeed inri ethdeu Wsinakgisboo tDhiastnriIcStD ofa Undgaunnddae. rTdhier eecxtperimental site 
sun (DSD) neaisr ltohceacteitdy aot fcoGoormdibneatiens t0h°e32W′0a1k.2is″ oS Danisdt r3i1ct°3o7f′3U9g.0a″n Ed. aT.hTeh eexpeexrpiemriemntesn ttoaolk place during 
site is located at coord ◦ ′ ′′ ◦ ′ ′′the montinhas toefs J0ul3y2 an01d. 2AuSgaunstd, t3h1e d37rie3s9t. 0moEn.tThhs eine txhpee Lriamkeen Vtsicttooorika pClraecsecent AEZ, with 
during the moannt hasveorfaJguel y67a nmdmA uofg urasitn, ftahlel. dTrhiee satvmeroangteh ssoinlart hreadLiaaktieonV iocft o4r3i8a.4C rWes cme−n2 twas measured 
AEZ, with an athverorauggeh6o7utm thme oefxrpaeirnifmalel.nTtsh uesainvger aa gseolsaorl adrartaad loiagtgioenr o(Df 4L3-183.4 W m
−2
1 LUX, Vwolatcsraft, Hirschau, 
measured throGuegrhmoauntyth).e Mexapizeer iwmaesn stuspupsliinegd abyso lloacradl afatarmloegrsg einr  (bDaLgs-1 a3n1dL wUaXs, lVooaldtcerda ifnt,to the ISD and 
Hirschau, Germany). Maize was supplied by local farmers in bags and was loaded into
the ISD and spread on the bottom surface in a 2-to-3-cm-thick layer. For DSD, maize was
spread on a tarpaulin with the same grain layer thickness next to the ISD. The experiments
Appl. Sci. 2021, 11, 7074 4 of 14
were conducted during the daytime and ended when the moisture content of the grains
reached the recommended level (14% w.b.). The moisture content was measured at the
beginning of the drying experiment (to account for the difference in the moisture content
as they were harvested at different periods of time) and at constant intervals until the end
of the experiment.
2.2.2. Performance Evaluation
Miniature USB data loggers for the temperature and relative humidity (DL-181THP,
DL-210 TH, and DL-111K; Voltcraft, Hirschau, Germany) were placed at different locations
inside the ISD, as well as on the tarpaulin for DSD to monitor the drying process (see
Figure 1). The sensors were collecting temperature and relative humidity data at constant
intervals during each experimental period for each batch. For monitoring of the moisture
content of the grains, mesh bags were placed inside the ISD and weighed every hour. Three
samples per batch were collected at the beginning of the experiments, as well as at the end
for the quality analysis. At the beginning of each experiment, the samples were collected
randomly in different bags supplied by farmers. At the end of drying, the samples were
also collected randomly from different positions in the dryer. The samples were then
thoroughly mixed to make an aggregate sample. They were finally transported to the
laboratory and stored at −18 ◦C in sealed bags before analysis.
2.2.3. Absolute Humidity Calculation
Temperature and relative humidity data were used to calculate the absolute humidity
x, expressed in g kg−1, using Equation (1):
ϕ/100·p
x = 0.622· s
p− · (1)ϕ/100 ps
where ϕ is the relative humidity in %, p is the ambient pressure in hPa, and ps is the
saturation pressure in hPa, which was calculated using the formula given by Tetens, shown
in Equation (2) [29]:
7.5 · ϑ
p = 1.33 × 10( ϑ+237.3 + 0.66)s (2)
where ϑ is the temperature of the air in ◦C.
2.2.4. Moisture Content Analysis
The moisture content of maize samples was measured using the oven method. The
sample was ground and homogenized. From the sample, 10 g was placed in weighed
crucibles and dried in an oven (Memmert UM 700, Memmert GmbH & Co. KG, Schwabach,
Germany) for 24 h at 105 °C, and the final weight was taken. The moisture content MCwb
(%) was calculated using Equation (3):
m − m
MC i fwb = ·100 (3)mi
where mi (g) is the initial mass before drying, and mf (g) is the final mass after drying.
To compare the drying experiments with different initial MC, the moisture ratio MRt
(-) was calculated using Equation (4):
MCt − MCMR eqt = − (4)MCin MCeq
where MCt (d.b.) is the moisture content at time t, MCin (d.b.) is the initial moisture content,
and MCeq. (d.b.) is the equilibrium moisture content.
Appl. Sci. 2021, 11, 7074 5 of 14
2.3. Quality Analysis
2.3.1. Yeasts and Moulds Enumeration
For yeasts and moulds enumeration, the pour plate method was used as suggested in
the standard method [30]. The test sample was extracted in a stomacher (BagMixer 400 SW,
Interscience, Saint Nom la Brétèche, France) with peptone water. The extract (1/10) was
further diluted to a dilution of 10−6. A 1-mL droplet of each dilution was transferred to
a Petri dish, and approximately 20 mL of agar (Dichloran Rose-bengal Chloramphenicol)
were added and left to solidify. The Petri dishes were incubated at 25 ◦C for 1 week with
intermediary colony counting after four days. Petri dishes with countable colonies were
considered for the log yeasts and moulds colony count N divided by the sample dry weight,
expressed as CFU g−1 dry weight, using(Equation (5) [31]:)
N log ∑
C
= 10 w.(n1+0.1· ·
(5)
n2) d
where ∑C is the sum of the colonies counted on the considered plates, n1 is the number
of plates counted in the first dilution, n2—is the number of plates counted in the second
dilution, w—is the dry weight, and d—the dilution from which the first count was obtained.
2.3.2. Total Aflatoxins Analysis
A competitive enzyme immunoassay method by RIDASCREEN® was used for the
quantitative determination of the total aflatoxins. A ground and homogenized sample of 2 g
was extracted with 100 mL of 70% methanol, and the extract was filtered with No. 1 filter
paper. The filtrate was diluted to the ratio of 1:7 and 50 µL of the extract, and the standards
were added to their respective wells for total aflatoxins determination. A conjugate, as well
as the antibodies, were added to each well and left to react for 30 min at room temperature
(20–25 ◦C). The liquid was poured out of the wells, and they were washed with buffer
solution three times. Chromogen was added to each well and left to incubate for 15 min at
room temperature. A stop solution was then added. The measurements were performed at
450 nm with a UV photometric microwell reader (Epoch, BioTek, Winooski, VT, USA). The
percentage absorbances of the different standards were calculated from the zero standard
and drawn on a semi-logarithmic graph against the aflatoxin concentration to obtain
a calibration curve. The concentrations contained in the samples were read on the curve
and further multiplied by the corresponding dilution factor to obtain the concentration
that was corrected to be reported on a dry basis.
2.3.3. Colour Measurement
Colour measurements were taken using a reflectance colorimeter (CR–400 Minolta
Co, Ltd., Chiyoda, Japan) with a 2◦ observer that closely matched the CIE 1931 standards
observer [32]. A standard white plate with D65 illumination (Υ = 93.7, x = 0.3158, and
y = 0.3324) was used to calibrate the instrument prior to the measurements. The measure-
ments were taken in triplicate and randomly on different maize kernels spread on a plate
by placing the instrument on top of the kernel. The colour parameters were expressed
in terms of L* related to the lightness (L* = 0 for black and L* = 100 for white), a*, which
represents the intensity in green–red (a* < 0 for green and a* > 0 for red), and b*, which
describes the intensity in blue–yellow (b* < 0 for blue and b* > 0 for yellow). From the
a* and b* values, the chroma and hue were computed. Chroma (C*) indicates the colour
saturation, which is proportional to its inte√nsity and is calculated using Equation (6):
C∗ = a∗2+b∗2 (6)
Appl. Sci. 2021, 11, 7074 6 of 14
For the hue, an angle of 0◦ indicates a red hue, while the angles of 90◦, 180◦, and 270◦
indicate yellow, green, and blue hues, respectively(. To)obtain the hue, Equation (7) was used.
b∗
ho= arctan ∗ (7)a
Colour alterations were estimated and expressed as the total colour difference ∆E* of
the samples before and after drying u√sing Equation (8):
∆E∗ = ∆L∗2 + ∆a∗2 + ∆b∗2 (8)
2.4. Statistical Data Analysis
Significant differences were determined by an analysis of variance (ANOVA) using
the Generalized Linear Model (GLM) procedure of SAS® 9.4 (SAS Institute Inc., Cary,
NC, USA). The least square difference (LSD) option was used to evaluate the differences
among the means. Graphs were constructed with the software Origin Pro version 2019a
(OriginLab® Corporation, Northampton, MA, USA).
3. Results
3.1. Performance Evaluation
3.1.1. Temperature, Relative Humidity, and Water Content in Air
The results of the temperature and relative humidity collected by the sensors installed
inside the ISD and on the tarpaulin for DSD are presented in Figure 2, together with the
calculated absolute humidity (water content in the air) for batch 1.
A significant higher temperature was recorded in the ISD compared to the conditions
under DSD (pα≤0.05 = 0.0001). An average temperature of 47 ◦C was recorded in the ISD,
7 ◦C higher than the temperature under DSD. The maximum temperature in the ISD was
reached at 13:30 on the second day of the drying experiment with a temperature of 63.7 ◦C,
while, for DSD, the maximum temperature was only 54 ◦C. An average relative humidity
of 28% was recorded in the ISD, and for DSD, the average relative humidity was 34%. The
minimum relative humidity of 14% was recorded inside the ISD at the same time when
the temperature was highest, while, for the DSD, the minimum relative humidity was
19%. The change of the relative humidity in the ISD and for DSD is a typical characteristic
of a psychometric heating process where the increase in air temperature at a constant
humidity results in a reduction of the relative humidity [29].
The absolute humidity, which is the measurement of the amount of water vapour in
the air; inside the ISD, it was higher than the one observed for DSD. This is explained by
the uptake of vapour from the evaporation of water in the maize grains. For the same
reason, the absolute humidity was higher at the outlet of the dryer compared to the outlet
of the solar collector. The maximum absolute humidity of 27.37 g kg−1 was recorded at
13:00 at the outlet of the ISD, the time when the temperature was highest. The saturation
deficit was 10% lower than the one recorded for sun drying, an indicator of an efficient
drying process. A similar pattern in absolute humidity was observed in maize drying by
Sanghi et al. [33], which was attributed to a higher drying rate, as the temperature was
higher inside the ISD.
Appl. Sci. 2021, 11, 7074 6 of 14 
 
ho=arctan ba**  (7)
Colour alterations were estimated and expressed as the total colour difference ∆E* of 
the samples before and after drying∆ Eu*s=ing∆ ELq*2u+a∆tiao*n2+ (8∆)b: *2 (8)
2.4. Statistical Data Analysis 
Significant differences were determined by an analysis of variance (ANOVA) using 
the Generalized Linear Model (GLM) procedure of SAS® 9.4 (SAS Institute Inc., Cary, NC, 
USA). The least square difference (LSD) option was used to evaluate the differences 
among the means. Graphs were constructed with the software Origin Pro version 2019a 
(OriginLab® Corporation, Northampton, MA, USA). 
3. Results 
3.1. Performance Evaluation 
3.1.1. Temperature, Relative Humidity, and Water Content in Air 
Appl. Sci. 2021, 11, 7074 The results of the temperature and relative humidity collected by the sensor7s oifn1-4
stalled inside the ISD and on the tarpaulin for DSD are presented in Figure 2, together 
with the calculated absolute humidity (water content in the air) for batch 1. 
 
FFigiguurree 22. .TTeemmppeerraatuturree aanndd rreelalattivivee hhuummididitiyty ((aa) )aanndd aabbssoolulutete hhuummididitiyty ((bb)) inin tthhee iinnffllaattaabbllee ssoollaarr 
ddrryyeerr ((IISSD)) and for diirreecctt ssuunnd dryryiningg( D(DSDSD),)s, hsohwown nex exmepmlaprlialyrilfyo rfobra tbcahtc1ho f1t hofe tdhrey idnrgyeinxgp eerximpeerni-t.
ment. 
3.1.2. Drying Curves
The variation of the moisture content during the drying process of maize in the three
batches with use of ISD and direct sun drying is shown in Figure 3.
The total drying time for the maize in the ISD was 19.9 h and 10.3 h distributed over
3 days for batches 1 and 2, respectively. For batch 3, it required only 9.5 h to dry the maize
to the required moisture content. The starting moisture content for the ISD was 26.6% for
batch 1 and 23.6% for batch 2, and the final moisture content was 11.9% and 12.3% for batch
1 and 2, respectively.
For batch 3, the starting moisture content was 17%, and a final moisture content of
11% was reached in 2 days. A small re-humidification of the maize in batch 1 was observed
on the third day of the experiment, as a result of the humid conditions prevailing during
the night on that day. Due to the difference in the initial moisture content, the moisture
ratio was used for a better comparison of the different treatments.
Table 1 shows the t-test comparison of the moisture content of the samples dried by
both methods among the three batches.
The starting moisture content for maize dried in the ISD was 23.5% for batch 1 and
21.3% for batch 2, and the final moisture content was 11.9% and 13.2% for batches 1 and 2,
respectively. For the maize in batch 3, the starting moisture content was 15.1%, and a final
moisture content of 12.5% was reached in 2 days. The significance t-test did not show
a difference in the moisture content among the different drying methods (t-test, α = 0.05),
except for batch 3. The drying temperature was sufficient to reduce the moisture content to
the recommended level within a reasonably short period [33].
Appl. Sci. 2021, 11, 7074 7 of 14 
 
A significant higher temperature was recorded in the ISD compared to the conditions 
under DSD (pα≤0.05 = 0.0001). An average temperature of 47 °C was recorded in the ISD, 7 
°C higher than the temperature under DSD. The maximum temperature in the ISD was 
reached at 13:30 on the second day of the drying experiment with a temperature of 63.7 
°C, while, for DSD, the maximum temperature was only 54 °C. An average relative hu-
midity of 28% was recorded in the ISD, and for DSD, the average relative humidity was 
34%. The minimum relative humidity of 14% was recorded inside the ISD at the same time 
when the temperature was highest, while, for the DSD, the minimum relative humidity 
was 19%. The change of the relative humidity in the ISD and for DSD is a typical charac-
teristic of a psychometric heating process where the increase in air temperature at a con-
stant humidity results in a reduction of the relative humidity [29].  
The absolute humidity, which is the measurement of the amount of water vapour in 
the air; inside the ISD, it was higher than the one observed for DSD. This is explained by 
the uptake of vapour from the evaporation of water in the maize grains. For the same 
reason, the absolute humidity was higher at the outlet of the dryer compared to the outlet 
of the solar collector. The maximum absolute humidity of 27.37 g kg−1 was recorded at 
13:00 at the outlet of the ISD, the time when the temperature was highest. The saturation 
deficit was 10% lower than the one recorded for sun drying, an indicator of an efficient 
drying process. A similar pattern in absolute humidity was observed in maize drying by 
Sanghi et al. [33], which was attributed to a higher drying rate, as the temperature was 
higher inside the ISD. 
3.1.2. Drying Curves 
The variation of the moisture content during the drying process of maize in the three 
Appl. Sci. 2021, 11, 7074 8 of 14
batches with use of ISD and direct sun drying is shown in Figure 3. 
Figure 3. Moisture content (MC) (a) and moisture rat io (MR) (b) vs. time for drying maize in the
Fiingfluartea b3l.e Msooliasrtudrrey ecron(ItSeDnt) (aMndCf)o (rad) iarnecdt msuonisdturyrein rga(tDioS (DM)Ro)f t(hbr)e vesb. atitmchee sf:oBr 1d,rBy2in, gan mdaBiz3e.  in the 
inflatable solar dryer (ISD) and for direct sun drying (DSD) of three batches: B1, B2, and B3. 
Table 1. Moisture content (MC) before drying (fresh) and after drying in the inflatable solar dryer
(ISD) and direct sun drying (DSD).
Batch 1 Batch 2 Batch 3
MCw.b% ± SD MCw.b% ± SD MCw.b% ± SD
ISD 11.9 ± 0.4 a 13.2 ± 0.3 a 12.5 ± 0.5 a
DSD 11.9 ± 0.1 a 12.3 ± 0.3 a 11.0 ± 0.0 b
Means followed by the same letter are not significantly different (p ≤ 0.05).
3.2. Quality of the Dried Maize
3.2.1. Yeasts and Moulds
The results of the yeast and moulds analysis for different drying treatments of the
three batches are summarised in Figure 4.
The yeast log10 colony count ranged from 5.0 to 6.0 CFU g−1, and the moulds log10
colony count ranged from 5.2 to 5.8 CFU g−1, with no significant differences among
treatments. The average yeast log10 count was 5.5 ± 0.4 CFU g−1, 5.7 ± 0.4 CFU g−1,
and 5.7 ± 0.4 CFU g−1 for fresh maize and maize dried with ISD and DSD, respectively.
Similar results were observed for moulds where the log10 colony count ranged from 4.4 to
6.3 CFU g−1. The mean log10 colony count in the three treatments was 5.4 ± 0.2 CFU g−1,
5.7 ± 0.6 CFU g−1, and 5.6 ± 0.6 CFU g−1 for fresh maize and maize dried with ISD
and DSD, respectively. The short time of drying was not sufficient for the moulds to
develop significantly.
Appl. Sci. 2021, 11, 7074 9 of 14
Appl. Sci. 2021, 11, 7074 9 of 14 
 
 
FFigiguurere 44. C. Coloolnonyy cocuounntst sppere rgg ddryry mmatattetre rfofro ryeyaesats t(a()a )anandd mmououldlds s(b(b) )inin mmaiaziez ebebfeofroer eddryryiningg (f(rfersehsh) )
ddrireiedd inin ththe eininflflaatatabblele sosolalar rddryryere r(I(SISDD) )anandd ddrireided uunnddere rddiriercetc tsusunn (D(DSDSD);) t;hthrere ebabtacthchese s(T(Trereaatmtmeenntsts 
wwitihth sasammee lelettetersrs aarere nnoot tssigignnifiificcaanntltyly ddififfeerreenntt, ,pp ≤≤ 0.0.50)5. ).
33.2.2.2.2. .TTootatal lAAflflaatotoxxinin CCoonntetennt t
TThhee reressuultlsts oof fththee aannaalylyssisis oof fththee tototatal laaflflaatotoxxinin inin frferesshh mmaaizizee aanndd mmaaizizee ddrrieiedd uunnddeerr 
ddirierecct tssuunn aanndd wwitihth ISISDD aarree pprreesseenntteedd inin FFigiguurree 55. .
The total aflatoxins analysis showed a high variation in contamination among the
batches of fresh maize that were delivered by different farmers. Whereas batches 1 and
2 were well-below the threshold of 10 µg kg−1 fixed by the East African Standard maize
specification, and batch 3 was severely contaminated prior to drying, reaching a value
of 569.6 µg kg−1. The lower initial MC compared to the other batches suggests that
an uncontrolled drying occurred before delivery to the experimental site, resulting in
a high production of aflatoxins. During drying, the aflatoxin content in batch 3 could be
significantly reduced to 345.5 µg kg−1 for DSD and to 299.2 µg kg−1 for ISD. However,
these values were still far above the level for safe consumption. In batch 2, the aflatoxin
content of fresh maize of 3.3 µg kg−1 was not affected by drying, whereas, in batch 1, the
initial aflatoxin content of 3.1 µg kg−1 significantly increased to 12.1 µg kg−1 for DSD
and to 11.1 µg kg−1 for ISD. The reason might be the higher MC during the first night
as compared to batch 2 that favours the production of aflatoxins in a humid atmosphere.
Appl. Sci. 2021, 11, 7074 10 of 14
Appl. Sci. 2021, 11, 7074 10 of 14 
 As a consequence, drying should start in the early morning to reduce the MC sufficiently
before sunset.
 
FFiigguurree 55.. TToottaall aaffllaattooxxiinn ccoonntteenntt iinn ffrreesshh mmaaiizzee,, mmaaiizzee ddrriieedd iinn tthhee iinnffllaattaabbllee ssoollaarr ddrryyeerr ((IISSDD)),, aanndd 
maaizizee ddrireiedd uunnddere rdidriercetc stusnu n(D(SDDS)D fo) rf othret htherteher beeatbchatecsh (eTsre(Tatrmeaetnmtse nwtsithw sitahmsea lmetetelrest taerres naorte snigo-t
nsiifgicnaifinctlayn dtliyffderifefnetr,e pn t≤, p0.≤05)0.. 05).
3.2.3T. hCeo ltooutarlo affDlartioexdinMs aainzaelysis showed a high variation in contamination among the 
batchTesh eofm freeasnh vmalauizees ftohratt hweeLre*, dae*,libv*e,rcehdr obmy ad,ifhfueree,natn fdartmotearlsc.o Wlohuerrdeiafsfe braentccheefso 1r manadiz 2e 
wbeefroer ewderlly-bineglo(wfr etshhe) ;tmhraeiszheodldri eodf i1n0a µngi nkflga−t1a fbilxeedso lbayr dthrey eEra(sISt DA)f;raicnadn dSitraenctdsaurnd dmryaiizneg 
s(pDeScDifi)caarteiopnr,e asnendt bedaticnh T3a wblaes2 s.everely contaminated prior to drying, reaching a value of 
569.6 µg kg−1. The lower initial MC compared to the other batches suggests that an uncon-
tTraobllleed2 .dCryolionugr opcacruamrreetder sbeoffomrea izdeeldirvieedryb ytoI StDhec oemxppearriemd etontDaSl Dsi(tme, eraenssuclotimngp airnis oan hti-gtehs tp).ro-
duction of aflatoxinLs*. During dray*ing, the aflabto*xin contenCt i*n batch 3 cohu◦ld be signi∆fiEca*ntly 
reduced to 345.5 µg kg−1a for DSD and to 299.2 µg kg−1 a a for ISD. Ha owever, thease values were 
still FfarershISD above 
6
6t
3h.e4 ±1.6 ±lev
0.5 1.9 ±
2.e4la for 2s.a2f±e 
0c.30.o1nsum
1p5.t3io±n1. .I3n ba15a 15.5 ± 0.6 a 1tc5h
.4 ±
.7 ±2, 
1.3
0t.h6ea afl
1a.t5o±xin0.01.4 ± 0. 0coa nte2n.8t ±of
- 1f.7re3sah 
maizDe SoDf 3.3 µg6 −16k.0g± 5w.3aas no2t. 0af±fe0c.t2eda  by1 6d.1ry±in2g.3, awhe1r6e.1as±, i2n.3 baatch1. 41,± th0e.0 ianitia5l. 1af±la3to.0xain 
conteLnStD of 3.1 µg k6.g5−91 significa0n.4tl8y increase2d.5 t4o 12.1 µg 2k.5g5−1 for DSD0. 0a2n3d to 11.11 µ1.g26 kg−1 
fNoro tIeSs:DM. eTahnsef orellaowsoend  bmy itghehsta bmee tlhetete rhaigrehneort MsigCni fidcuanrtilnygd itfhfeere fnitr(spt ≤ni0g.0h5t). as compared to batch 2 
that favours the production of aflatoxins in a humid atmosphere. As a consequence, dry-
ing shTohueldre sstualrtts isnh tohwe etahraltyt mheorrenwinags tnoo rseidguncifie ctahnet MdiCff esurefnficceieinntlcyo lboeuforrdeu seutnosetht.e drying
method. The L* a* b* values for the grains show that the maize was white in colour, as
3L.2*.3w. aCsohloiguhr ,owf Ditrhieads Mligahiztet endency towards pale clean. The b* value, which indicates
the vTahreia mtieoannf vroamlueyse flolor wtheto L*b,l au*e,, bw*, acshrpoomsiati, vheu,ea, nanidnd toictatli coonlooufrt dhieffyereellnocwe ftoern mdeanizcey 
boeffomrea idzreyginrgai (nfsre[s3h4)];. mTahizeer delraietidv einly ahni ignhfelarta∆bEle* svoallaure doryf egrr (aIiSnDs )t;r aenadte dirbeyctD suSDn dmryeianngs 
(tDhSaDt t)h aerree pwreassenatesdli ginh tTeafbfleec t2.o f drying on the colour. However, this change was not
statistically significant. Experiments in another study showed that temperatures of 120 ◦C
Taanbdleh 2i.g Choelrouhra pdaarasmigetneirfis coafn mtaiinzfle uderinecde boyn ISmDa ciozme pliagrhedtn teos Ds,SrDe d(mneasns,s aconmdpyaerlilsown tn-teessst)[. 19].
Our results are consistent with those of Prachayawarakorn, Soponronnarit, Wetchacama
and ChinnabuLn*[ 16], who aan*a lysed the imb*p act of differCen*t drying mehth° ods on the∆cEol*o ur of
F  a  a  a  a  amareizseh and63fo.4u ±n d0.5tha t th1e.9i m± 0p.a3ct wa1s5n.3o ±t  s1i.g3ni fica1n5t..4S ±e v1e.3ra l oth1e.5r ±sc 0ie.0nt ific publ-ic ations
haIvSeDa lso 6in1d.6i c±a 2te.4d a the2m.2i n±o 0r.1im a pac1t5o.5f  s±o 0la.6r ad ryin15g.7o n± a0.p6 ra odu1c.t4’s ±c 0o.l0o ua r [325.]8.  ± 1.73 a 
DSD 66.0 ± 5.3 a 2.0 ± 0.2 a 16.1 ± 2.3 a 16.1 ± 2.3 a 1.4 ± 0.0 a 5.1 ± 3.0 a 
LSD 6.59 0.48 2.54 2.55 0.023 11.26 
Notes: Means followed by the same letter are not significantly different (p ≤ 0.05). 
The results show that there was no significant difference in colour due to the drying 
method. The L* a* b* values for the grains show that the maize was white in colour, as L* 
Appl. Sci. 2021, 11, 7074 11 of 14
4. Discussion
Inflatable solar dryers have recently become popular in tropical regions, but there is
a lack of research on their performance and their effect on the quality of dried products. In
this research, the performance of an inflatable solar dryer was compared with direct sun
drying (DSD), and the impact on maize quality was evaluated. Five maize quality attributes
were evaluated: moisture content, mycotoxins (total aflatoxins), yeasts, and moulds, as
well as colour. The temperature in the solar collector was, on average, significantly higher
compared to the ambient temperature (Figure 2). The observed drying temperatures
were suitable for drying maize with a batch drier. For maize grain intended for flour
milling, a maximum temperature of 65 ◦C is recommended in order to avoid that the
high temperature affects the chemical structure and deteriorates the quality [36]. A higher
temperature was associated with a lower relative humidity and resulted in higher drying
rates. Salvatierra-Rojas, Nagle, Gummert, de Bruin and Müller [26] developed an ISD
for drying paddy rice, and the experiments showed that rice was dried to a moisture
content of 8% and 12% in dry and rainy seasons, respectively. Asemu, Habtu, Delele,
Subramanyam and Alavi [28] evaluated the drying characteristics of maize grain dryed in
a solar bubble dryer, where the moisture content reached a safe storage moisture content
after 40 h. Comparable performances were achieved for our experiments in terms of the
moisture content and drying time. The cascades observed in these curves are characteristic
for solar drying systems. They are caused by the diurnal cycle of solar radiation, which, in
turn, affects the air temperature [37]. In terms of moisture content reduction, there was no
significant difference in the moisture content reduction in maize dried in the ISD compared
to the DSD method, because maize reaches the recommended storage moisture content
at a comparable time with both methods. Numerous experiments on maize grain storage
have shown that maize with an initial moisture content below 14% w.b. could keep the
quality attributes like germination capacity, low ethanol and acetic acid contents, and low
microbial count under long-term hermetic storage [38]. To stay below this threshold in our
experiment, <14%, was considered as a safe moisture content for storage, and drying was
stopped when the moisture content reached this level (Figure 3).
The quality parameters analysed showed a slight effect on the maize quality, mostly
on the yeast and moulds and the total aflatoxins (Figures 4 and 5). The total aflatoxins was,
on average, higher compared to the standard level of 10 µg kg−1 [8]. The results of the
yeast and moulds in Figure 4 showed a nonsignificant (p ≤ 0.05) effect of the treatment on
the contamination level. Schemminger et al. [39] modelled the impact of ambient air drying
on mould growth using a batch dryer. The results showed a mould colony count of up to
107 CFU g−1 in the uppermost layer after 400 h of drying [40]. This is a high level of colony
count in comparison to the maximum 1.5× 106 CFU g−1 observed in maize dried under ISD
in 3 days. The mould count could not be reduced, but the risk of an increase in the colony
count is the minimum, because the maize was dried to a low moisture content, which
does not allow further mould growth. Kaaya and Kyamuhangire [7] reported that mould
infection rate varied with the length of storage and dryness of the region, with a highest rate
of 50% of the total analysed samples found in the moist mid-altitude regions of Uganda. In
Figure 5, a high variation in the total aflatoxin contamination levels of the different samples
analysed was observed. The high prevalence of the total aflatoxins in maize in Uganda was
previously reported [7,9,10]. In their cross-sectional survey conducted in five major markets
in Kampala on different staple crops, Osuret et al. [41] found that 40% of the analysed
samples were contaminated by aflatoxins. They also proved that maize samples that were
within the permissible safe storage levels of moisture content were less prone to aflatoxin
contamination. Raters and Matissek [40] investigated the thermal stability of aflatoxins
and showed that Aflatoxin B1 was completely degraded at 150 ◦C when in the protein
matrix. The experimental results in our research showed a slight reduction in the aflatoxin
contents in drying treatments compared to fresh maize for batch 3, which might be because
of the exposure to high temperatures inside the dryer or under sun drying. Evidences
exist of up to a 30% aflatoxin reduction in contaminated maize exposed to the sun for
Appl. Sci. 2021, 11, 7074 12 of 14
a prolonged period [42]. The contamination level in batch 1, on the other hand, increased
from 3.1 µg kg−1 to 12.1 µg kg−1 and 11.1 µg kg−1 for DSD and ISD, respectively. Battilani
et al. [43] showed that aflatoxin production is the maximum at temperature between 25 and
30 ◦C and, mostly, aw above 0.9. Drying with ISD should be planned as best as possible
to maximise the solar energy and prevent contamination by mycotoxins. Experiments on
rice in Burkina Faso also showed that the ISD dried rice had less aflatoxins and impurities
compared to conventional sun-dried rice [27]. This slight effect observed for maize in our
research was not significant enough to reduce the contamination level of aflatoxins.
5. Conclusions
In this research, three batches of maize at different moisture contents were successfully
dried both by ISD and DSD. The temperature, relative humidity, and moisture content
were monitored throughout the experiments. The temperature reached a level sufficient to
dry maize with insignificant damage to the quality and was enough to dry maize to the
recommended moisture content. The maximum temperature inside the ISD was 63.7 ◦C,
with an overall average of 47 ◦C. A moisture content below the recommended level of <14%
was reached. The original aflatoxin contamination was very high in one of the received
maize lots, and a minor reduction by drying was observed in this case. The colour was not
significantly affected by the drying process. Although the drying performance in terms of
drying time and product quality was similar for both drying methods, the advantage of ISD
in reducing the risk of spoilage due to sudden rain was obvious. To maximise the drying
performance and minimise the risk of contamination during drying, it is recommended start
drying early in the morning to reduce the moisture content sufficiently before sunset. The
effect on the aflatoxins was not enough to reach the standard recommendations when the
original contamination was high; thus, a rapid and non-destructive method to determine
the mycotoxin content of maize ought to be developed to detect contamination at an early
stage of the value chain before drying.
Author Contributions: Conceptualisation, J.N., S.S. and S.R.; methodology, J.N. and S.S.; validation,
J.M.; formal analysis, J.N.; investigation, J.N., G.A. and S.S.; writing—original draft preparation, J.N.;
writing—review and editing, S.S., S.R., C.G.K.C., N.B., G.A. and J.M.; visualisation, J.N.; supervision,
J.M.; project administration, S.S. and funding acquisition, S.S., C.G.K.C. and J.M. All authors have
read and agreed to the published version of the manuscript.
Funding: This research was financially supported by the German Federal Ministry for Economic Co-
operation and Development (BMZ) through the project 15.7860.8-001.00 “Making Value Chains Work
for Food and Nutrition Security of Vulnerable Populations in East Africa”, led by the International
Centre for Tropical Agriculture (CIAT). The main author was supported by scholarships awarded
by DAAD.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: The authors wish to acknowledge the Community Enterprises Development Or-
ganization (CEDO) in Uganda for their enormous support during the experimental period in Uganda.
The authors appreciate the contribution of Sabine Nugent for language editing and the laboratory
team of the Agriculture Engineering in the Tropic and Subtropics group for their contribution in the
laboratory analysis. Special gratitude is also addressed to Luka Chupona for his assistance.
Conflicts of Interest: The authors declare no conflict of interest.
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