Journal of Agricultural Chemistry and Environment, 2014, 3, 13-19
Published Online October 2014 in SciRes. http://www.scirp.org/journal/jacen
http://dx.doi.org/10.4236/jacen.2014.34B003
How to cite this paper: Olalusi, A. (2014) Hot Air Drying and Quality of Red and White Varieties of Onion (Allium cepa).
Journal of Agricultural Chemistry and Environment, 3, 13-19. h ttp://dx.doi.org/10.4236/jacen.2014.34B003
Hot Air Drying and Quality of Red and White
Varieties of Onion (Allium cepa)
Ayoola Olalusi
Department of Agricultural and Environmental Engineering, Federal University of Technology, Akure, Nigeria
Received 20 August 2014
Abstract
This study was undertaken to investigate the drying characteristics of red and white varieties of
onion (Allium cepa) in a convective dryer. The drying characteristics of onion slices were ex-
amined at air temperatures of 50˚C, 60˚C and 70˚C and sample thicknesses of 2 mm. During the
drying experiments, air velocity was held stable at 0.9 m/s. The effects of air temperature on the
drying characteristics and quality parameters such as vitamins C and D, macro minerals (Mg, Na, P,
Ca and K), non-enzymatic browning and thiolsulphinate of the dried onion slices were determined.
For each of the plots of the drying curves from the mechanical drying tests, there was an absence
of or just a very brief appearance of a constant rate period because onions are hygroscopic and
hygroscopic foods tending to quickly enter the falling rate period; drying time decreased consi-
derably with increased temperature. The time required to reduce the moisture content to any
given value was dependent on the drying conditions, being the longest at an air temperature of
50˚C, 0.9 m/s and the shortest at an air temperature of 70˚C and air velocity of 0.9 m/s. The loss of
vitamin C, which is a thermo-sensitive compound, was the least at an air temperature of 50˚C.
Keywords
Air Temperature, Onion, Drying Characteristics, Quality, Moisture Content
1. Introduction
Onion (Allium cepa) has been widely used even in ancient times as seasonings, for medical uses and as foods [1].
The importance of onion s lies in the flavour that it imparts to several other dishes. Onion is characterized by its
distinctive flavour and pungency which are due to sulphur containing compounds available in the scales of bulbs.
Onion is a basic flavouring in the kitchen. It is used as a vegetable, or as a spice to bring out the flavour of other
dishes without overpowering them. Onions are the indispensable vegetable, the strong (yet sweet) cornerstone of
modern cooking, not just in our culture but around the world. For centuries, onions have been thought to have
certain medicinal and disease prevention powers; modern science has begun to show that there may be some fact
in the ancient lore. At the same time, onions are low in calories and are a source of dietary fiber. Bulb onions
also provide vitamin C, with one medium onion providing 15 to 20 percent of the daily requirement. In the
United States, products that contain onion extract are used in the treatment of topical scars [21]; some studies

A. Olalusi
14
have found their action to be ineffective, while others found that they may act as an anti-inflammatory or bacte-
riostatic [3]. 3-Mercapto-2-methylpentan-1-ol in onion was found to have an antioxidant potent that inhibits pe-
roxynitrite-induced diseases.
In Nigeria, onion (Allium cepa) is majorly used as a flavouring agent. There is a general increase in demand
for processed onions because of the high perishability of fresh onion. To satisfy this demand several types of
natural onion products are currently available to food processors. These include dehydrated onion in powder or
pieces: onion oil obtained by the distillation of fresh onions and offered as is diluted in vegetable oil or in form
of an emulsion; onion juice concentrated to a viscous syrup or thick paste and also offered in dry form as disper-
sion on salt or dextrose [4]. Most of these processed onions are imported into the country.
Onions stored for long periods are subject to storage rot, sprouting, rooting, and loss of water making dehy-
dration necessary as a method of preservation [5]. Prices of onion tend to oxalate during the dry season before
harvest begins, due largely to the inability of the farmers to store the highly perishable crop. Preparing onions
are often accompanied by watery eyes, caused by the breakdown of onion cells when cut; these cells release
sulphurous substances that are in contact with the surrounding air, and create a new molecule (allyl sulphate)
that irritates the eyes; the stronger the onion, the more it stings the eyes. Also the appearance of pieces of onions
is not generally acceptable by people despite the love for onion flavour in most of their foods.
The rate of browning reaction is influenced strongly by the temperature and the moisture content of the food
during dryin g; drying temper atures above 65˚C have a significant influence on the colour of dried onion [6] [7].
This has led to the demand for increase in the efficiency of onion dehydration to obtain a high-quality finished
product. The objective of this work is to determine the effects of air temperature on the drying characteristics
and quality of red and white onion var ie ties .
2. Materials and Method
2.1. Materi als
2.1.1. Onions
Two fresh well graded onion varieties, namely Red and White were purchased from a local market (Iloro) in
Akure, Ondo State, Nigeria. The onions were peeled manually by removing the skin and the first layer. After
peeling, it was washed with tap water and left on an aluminum trays for 1 hour to allow the residual wash water
on the surface of the onions to evaporate, the onions were sliced using a vegetable slicer in the direction perpen-
dicular to the vertical axis. The onion s lices were 2 mm thick.
2.1.2. Experimental Dryer
The drying experiments were conducted in a laboratory using the mechanical convective dryer [8] which con-
sists of the following: centrifugal blower, a heating chamber (with 2-no 1.5 kW heater), a plenum chamber and a
drying chamber. The air velocity could be kept constant during drying experiments at any desired level ranging
from 0.3 up to 1.8 m/s.
The air passes into the central section of the duct where four trays of the material to be dried are suspended in
the air stream. The entire dryer was insulated to avoid heat losses and substantial temperature differences across
the test sections. The temperature of the air stream was measured by means of k-type thermocouples and con-
trolled by a power regulator. Drying air velocity was measured by means of a hot-wire anemometer. The onion
slices (2 mm thick) were spread in a net placed on the baskets which were made from stainless steel wire net.
2.2. Experimental Procedures
2.2.1. Drying Experiment
The dryer was run idle for about 1 hour to achieve a steady state in respect of pre-set experimental drying co ndi-
tions before each drying run. About 100 g of onion slices after weighing were uniformly spread on the net and
placed in the basket inside the drying chamber. The weight of the onion samples (white and red onions) was
measured at intervals of 30 minutes. Drying was stopped when there was no noticeable change in the weight of
the samples. The mechanical drying experiments were conducted at 0.9 m/s air velocity and 50˚C, 60˚C and
70˚C air temperatures.

A. Olalusi
15
2.2.2. Analytical Methods
The moisture content of fresh onion and the moisture content of the dried products were determined using the
oven method [9]. The extent of brown pigment formation in the onion samples was determined by the measure-
ment of the optical index. Phosphorus was determined routinely by the Vanado-Molybdate colorimetric or
Spectrophotometric method. Thiolsulphinate, vitamins D, ascorbic acid (vitamin C), calcium, pota ssium and so-
dium content were determ i ne d using the of ficial m e t hod of A.O.A.C. [10].
Sensory evaluation was performed on the dried-milled onion samples using 20 panelists [11]. The product
was evaluated on quality characteristics of texture, appearance, aroma, taste, feel and general acceptability on a
9-point Hedonic scale ra ting with 9 as like ex tremely and 1 as dislike extremely. The scores were then subjected
to analysis of variance (ANOVA). Panelists were from the university community and cut across age and sex.
3. Results and Discussion
3.1. Effect of Drying Air Temperature on Drying Characteristics of Onions
The graphs of moisture content versus drying time and drying rate versus drying time at air velocity of 0.9 m/s
and air temperature of 50˚C, 60˚C, 70˚C are shown in Figure 1 and Fig ure 2 for the red and white onion varie-
ties respectively. The drying curves are typical to ones for food stuffs. The moistur e content of onion slices de-
creased exponentially with elapsed drying time. As the air temperature increase d, other drying conditions being
same, the drying curves became steeper indicating higher moisture removal rates thus resulted into substantial
decrease in drying time (t). A t air velocity of 0 .9 m/s drying time for the on ion slices at air te mperature of 50˚C,
60˚C and 70˚C were about 13 h, 12.5 h and 11 h, respectively. The moisture content reduced rapidly at the be-
ginning and then decreased slowly with increase in drying time. Both the external factors and the internal mois-
ture transport mechanism controllin g the drying processes in the two main drying time regimes are important in
determining the overall drying rate of the product [12]. The drying rates were higher in the beginning of the
drying process and gradually reduced as the drying process progressed. This was because of more heat energy is
absorbed by the water at the product surface initially, resulting into faster drying, and with the product surface
drying out subsequently, heat penetration though the dried layer decreased thus retarding the drying rates. The
drying rates increased with increase in air temperature, other conditions remaining the same and thus reducing
the drying time. The dehydration rate of onion slices was found to be greatly dependent on the air temperature.
Figure 1. Drying curves for mechanical dried red onion slices at 50˚C, 60˚C and 70˚C air temper-
atures.
0
10
20
30
40
50
60
70
80
90
100
0246810 12 14
% MC (wb)
Time (Hours)
50°C air temperature
60°C air temperature
70°C air temperature

A. Olalusi
16
Figure 2. Drying rate against time for mechanical dried red onion slices at 50˚C, 60˚C and 70˚C air temperatures.
In other words, the dehydration rate increased with an increase in the air temperature. The initial moisture con-
tent of onion slices were observed to be 90.89% and 88.55% wet basis red and white variety respectively, the
final moisture contents was 10.87% and 10.89% at 50˚C, 9.58% and 9.28% at 60˚C, 7.69% and 6.39% at 70%
for the red and white onions respectively. This shows that the higher the air temperature, the lower the final
moisture content. The drying curv es also mostly exhibited a falling rate period. Similar results were obtained by
Krokida et al. (2003) [13] using potato, carrot, pepper, garlic, mushroom, onion, leek, pea, corn, celery, pump-
kin, and tomato; Babalis and Belessiotis (2004) [14] using figs; Sima l et al. (2005) [15] using kiwi fruit; Akpi-
nar (2006) [16] using par sley, mint, and basil leaves; Miranda et al. (2009) [17] using Aloe Vera and Doymaz et
al. (2006) [18] using dill and parsley leaves.
3.2. Effect of Drying Air Temperature on the Quality of Dried Onions
Tables 1-4 show the effect drying air temperature on vitamins C and D, some mineral content and the thiosul-
phinate content and optical index in dried onion samples. ANOVA carried out to see the effect of drying air
temperatures on onion slices (red and white) revealed th at at air temperatures of 60˚C and 70˚C red v ariety an d
the white variety at 50 ˚C, there was a significant difference of 5% level with the raw onions (red and white). For
60˚C, 70˚C white variety a nd 50˚C red variety, there was no significant difference with the raw onions (red and
water). An increase in drying air temperature had a negative effect on quality for both vitamin C and vitamin D.
This is due to the break down at high temperatures and the sensitivity of vitamin C to heat to oxidation degrada-
tion [19] [20]. There was a loss in the vitamin C content as seen in Table 1 by the significant difference in the
dried samples at the three temperatures for the red and white varieties from the raw onions. However, at 50˚C
for the loss in the vitamin C content is not significant. This is because vitamin C is sensitive to heat and oxid a-
tion. An increase in drying air temperature has a negative effect on quality of both vitamins C. The destruction
rate is higher at high moisture contents than at others stages of drying. There was also a significant difference in
vitamin D for the red and white varieties at 50˚C, 60˚C, 70 ˚C from the raw red and white onions respectively but
the loss in vitamin D was lesser at 50˚C for both varieties. Th is is also similar to the drying of milk were the vi-
tamin D content is greatly decreased by drying. There was a significant difference between the onions dried at
50˚C, 60˚C and 70˚C for (r ed and white), as shown in the ANO VA Table 3, in the non-enzymatic browning as
measured by the optical index increased with an increase in time and temperature during drying. Similar obser-
vation was reported by Ertekin and Gedik (2004) [21]. The best colour in relation to the raw samples for the red
and white variety was at 50˚C air temperatures for the red and white dried onions. The quantity of thiolsulphi-
0
5
10
15
20
25
02 4 68 10 12 14
Drying Rate (g/h)
Time (Hours)
50°C air temperature
60°C air temperature
70°C air temperature

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17
Table 1. Analysis of the vitamins C and D content in dried onion samples.
Samples Vitamin C Vitamin D
Raw red 6.43b ± 0.01 0.12a ± 0.001
Raw white 6.51a 0.11b ± 0.01
Red 50˚C 6.25def ± 0.10 0.024hi ± 0.003
Red 60˚C 6.18fg ± 0.004 0.021i ± 0.001
Red 70˚C 6.12g ± 0.02 0.016j ± 0.001
White 50˚C 6.36bc ± 0.010 0.04e ± 0.001
White 60˚C 6.18fg ± 0.13 0.03f ± 0.001
White 70˚C 6.24ef ± 0.01 0.029fg ± 0.001
Similar letters in the exponential in the same row show there are no significant differences (p-value < 0.05).
Table 2. Analysis of the mineral content in dried onion samples.
Samples Na Ca K P
Raw red 3.43ab ± 0.01 21.72a ± 0.02 1.59E2a ± 0.01 33.70a
Raw white 3.50a ± 0.01 20.85b ± 0.01 1.52E2b ± 0.01 32.95a ± 0.01
Red 50˚C 2.75fg ± 0.20 17.18ef ± 0.22 1.45E2de ± 1.40 27.75de ± 0.15
Red 60˚C 2.44hi ± 0.17 15.05g ± 0.85 1.37E2f ± 2.75 26.00f ± 0.20
Red 70˚C 2.27i ± 0.09 13.85h ± 0.1 1.30E2g ± 4.15 23.05g ± 0.35
White 50˚C 3.25bc ± 0.01 18.42d ± 0.02 1.43E2e ± 0.01 30.50b
White 60˚C 2.88ef ± 0.22 16.70f ± 0.20 1.39E2f ± 0.20 27.35e ± 0.45
White 70˚C 2.61gh ± 0.13 13.37h ± 0.47 1.30E2g ± 1.43 26.70ef ± 2.00
Similar letters in the exponenti al in the same row show t here are no significant differences (p-value < 0.05).
Table 3. Analysis of the thiosulphinate content and optical index in dried onion samples.
Samples Thiosulphur Optical
Raw red 0.103b ± 0.01 0.25a ± 0.01
Raw white 0.073c ± 0.001 0.17def ± 0.001
Red 50˚C 0.024d ± 0.001 0.25a ± 0.003
Red 60˚C 0.02e ± 0.001 0.24ab ± 0.02
Red 70˚C 0.018f ± 0.001 0.22bc ± 0.002
White 50˚C 0.013g ± 0.001 0.16efg ± 0.001
White 60˚C 0.012g ± 0.001 0.18de ± 0.023
White 70˚C 0.009h ± 0.001 0.19de ± 0.04
Similar letters in the exponential in the same row show there are no significant differences (p-value < 0.05).
nate decreased with an increase in drying time and temperature. Thiolsulphinate loss increased with a decrease
in moisture content to a certain level.
There was a significant difference in the mineral content at the three air temperatures for the red and white
varieties but the red and white variety at 50˚C had the highest mineral content for K, Na, Ca and P however, all

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Table 4. Analysis of the organoleptic properties of dried onions.
Samples Taste Aro ma Appearance Texture Overall acceptability
Red 50˚C 7.70a ± 1.13 7.55a ± 1.00 8.15a ± 1.23 8.00a ± 1.17 8.10a ± 1.02
Red 60˚C 7.50a ± 1.15 7.25a ± 1.52 7.70ab ± 1.17 7.25abc ± 1.45 7.50a ± 1.28
Red 70˚C 7.25a ± 1.16 7.05a ± 1.10 7.30b ± 0.92 6.90bc ± 1.17 7.65a ± 1.09
White 50˚C 7.45a ± 1.10 7.35a ± 0.93 7.50ab ± 1.05 6.65c ± 1.31 7.55a ± 1.00
White 60˚C
White 70˚C
7.40a ± 1.57
7.40a ± 1.05
7.55a ± 1.05
7.50a ± 1.05
7.65ab ± 1.31
7.60ab ± 1.31
7.60ab ± 1.27
7.40abc ± 1.27
7.60a ± 1.14
7.45a ± 1.14
Similar letters in the exponential in the same row show there are no significant differences (p-value < 0.05).
dehydrated samples showed a considerable decrease in mineral content with respect to the fresh samples as
shown in Table 2. Nevertheless, the mineral content may vary widely among vegetables, depending on several
factors such as ripeness, variety, s oil type, the use of fertilizers, intensity and exposur e time to sunlight, temper-
ature, rain, and cultiva tion area [22] [23]. Analysis of variance (ANOVA) was carried out on the white and red
varieties and the result shown in Tabl e 4 reveals that there was no significant difference in the aroma, taste and
overall acceptability of th e red and white onion varie ty at 50˚C, 60˚C and 70˚C air temperatures but there was a
significant difference appearance and texture in at p < 0.05 at 50˚C, 60˚C and 70˚C for the red and white onion
variety. The red onion variety dried at 50˚C had the be s t organoleptic pro pe rty.
4. Conclusions
In this study of mechanical convective drying of red and white onion slices is presented. Based on the experi-
mental results reported herein, the following conclusions can be made :
1) Drying time decreased considerably with increased temperature.
2) The time required to reduce the moisture content to any given value was dependent on the drying condi-
tions, being the highest at an air temperature of 50˚C, 0.9 m/s and the lowest at an air temperature of 70˚C and
air velocity of 0.9 m/s.
3) For each of the plots of the drying curves from the mechanical drying tests, there was an absence of or just
a very brief appearance of a constant rate period because onions are hygroscopic and hygroscopic foods tending
to quickly enter the falling rate period.
4) An increase in drying air temperature had a negative effect on quality for both vitamin C and vitamin D
which is a thermo -sensitive compound; this was likely due to the elevated processing temperature and period of
exposure. Dried red and white onion at an air temperature of 50˚C had the highest vitamin C retention after dry-
ing.
Acknowledgements
Special thanks to Deborah Okeke for her assistance in data collection.
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