Drying 2004 – Proceedings of the 14th International Drying Symposium (IDS 2004)
São Paulo, Brazil, 22-25 August 2004, vol. B, pp. 1435-1441
EFFECT OF DRYING METHOD ON MICROSTRUCTURAL
CHANGES OF APPLE SLICES
G. Reza Askari, Zahra Emam-Djomeh and S. Mohammad Ali Mousavi
Transfer Properties Lab. (TPL), Department of Food Science and
Engineering, Agricultural Engineering faculty, University of Tehran, Iran
Keywords: apple, air drying, microwave-assisted air drying, microstructure, SEM
Changes in structure and microstructure are very important
factors in the quality of dried fruits. In this study, thin slices
(d=22 mm, =4 mm) of apple (Golden Delicious) were dried
by hot air and combined method (hot air and microwave
drying) with or without osmotic pre-treatment (sucrose: 60%
w/w). To prevent undesirable enzymatic reaction and improve
porous structure, blanching pre-treating was carried in hot
water (80ºC, 1 min.). Microstructural changes have been
studied by Scanning Electron Microscopy (SEM). Physical
properties of dried products such as bulk density (?b), true
density (?t) and porosity (P) were also measured. The results
show that a combination of different drying methods can
prevent microstructural damages during drying and increase the
porosity of products.
Drying is one of the oldest and most cost-effective means of preservation of
grains, crops and foods in all varieties. From energy and environmental viewpoint as
well as in order to feed the growing population, it is important that drying technology
is improved to reduce spoilage and enhance quality of the products. Much has been
accomplished over the past decades as far as understanding and development of
drying technologies are concerned for food and agro-products (Mujumdar, 2000).
Dried vegetables and fruits are an important sector in the ingredient market.
Dehydrated vegetable products like mushrooms, broccoli, carrots, peas, onion, garlic,
corn and apple are suited to a broad range of food formulations including instant
soups, snacks, seasoning, stuffing, pasta salads, meat and rice dishes and casseroles
Most vegetables and fruits are generally dried convectively with heated air. Hot-air
dried vegetables are often difficult to rehydrate because of case-hardening and
shrinkage occurring during the drying process. The consumer demand has increased
for processed products that keep more of their original characteristics. Major
disadvantage of hot air drying of food is low energy efficiency and lengthy drying
time during the falling rate period. Prolonged exposure to elevated drying temperature
may result in substantial degradation in quality attributes, such as color, nutrients and
flavor, sever shrinkage also reduces bulk density and rehydration capacity (Feng et
al., 1998). The fact that fruits and vegetables collapse during dehydration has been
established before (Lozano et al., 1980). These researchers concluded that the slow
and difficult rehydration of dehydrated apples is explained by the development of
locked–in pores caused by cellular collapse during dehydration. Cellular shrinkage
during air drying was faster than the bulk shrinkage from full moisture to a moisture
content of 1.5% (W/W). After this point, the shrinkage coefficient was converged.
The result of the uneven shrinkage was an increased cellular porosity. This can be
explained by a three–dimensional rearrangement of tissue due to the cellular collapse
(Lozano et al., 1980).
The use of microwave energy has been of growing interest over the years. The interest
is dictated by the brief start times, volumetric heating due to microwave penetration
and reduced processing times making microwave an attractive source of thermal
energy. The shorter processing times can significantly reduce the production costs of
some products. So far microwaves have been used as the medium for energy input in
a wide range of applications including: heating, drying, sintering, vulcanizing and
sterilization (Lehne et al., 1999; Saltie and Datta, 1999; Prothon et al., 2001; Maskan,
2001). General advantages for the application of electromagnetic energy are:
• Rapid energy dissipation throughout the material and uniform heating,
• Avoidance of over drying or high surface temperatures,
• Possibility for puffing the material,
• Lower product temperatures in combination with a vacuum treatment,
• Minor migration of water soluble constituents,
• Possibilities for energy savings.
Some researchers reported that using microwave reduces drying time (25-90%) and
applying energy at lower level improves quality of final products, such as color,
rehydration capacity, density and porosity (Prabhanjan et al., 1994; Funebo et al.,
The objective of this investigation was to observe the effect of drying method on the
structural changes (density and porosity) of apple slices.
MATERIALS AND METHODS
Apples (Golden delicious) were purchased from a local supermarket and stored at
4ºC. A single batch of apples was used in these experiments, which were restricted to
a period of time. Because of the ripening of the apples it was always ensured that firm
apples were chosen for dehydration experiments. The firmness was also monitored by
always carrying out the puncture test on fresh apples each time the processed samples
were tested. Apples were washed and sliced (diameter: 22 mm, thickness: 4 mm). The
disks were immediately soaked in 20ºC tap water to prevent browning before all the
disks were cut. To prevent undesirable enzymatic reaction and to improve structural
properties, apple disks were blanched in hot water (80ºC, 1min)
The hot air drying experiments were performed in a lab dryer (tray dryer,
Armfield, England) controlled by a PC connected to the dryer. A programmable
domestic microwave oven (Butan, Iran) with maximum output of 1000W at 2450
MHz was used. The oven has the facility to adjust power supply (100-1000 W) and
the time of processing and was fitted with a turntable.
We used three different methods in our experiments:
Hot air drying: The dryer was operated at an air velocity of 1.5 m/s parallel to
drying surface at 75ºC. All experiments were carried out at constant temperature.
Moisture loss was recorded by digital balance at 10 min intervals during drying for
determination of drying curves. Apples were dried until equilibrium was reached.
Microwave–assisted air drying: Drying was carried out by a combination of
hot air–microwave techniques. Air drying was carried out until a certain moisture
content was reached (1.2, 0.8 and 0.4 kg/kg). Different microwave power intensities
(600, 400 and 200 W) were investigated in microwave drying at constant sample
thickness. It was observed that charring and simple boiling occurred at 600 and 400W.
So only the 200 W power level for 10 s was chosen. Our primary observation show
that using higher moisture content , yields a product with higher bulk density and
lower porosity thus samples with low moisture content (0.4 kg/kg) were used. One
glass containing the samples was placed at the center of oven turntable in microwave
cavity during treatment for uniform absorption of microwave energy. The turntable
was necessary to achieve the optimum oven performance and to reduce the levels of
reflected microwave onto the magnetron (Khraisheh et al., 1997).
Osmo-air drying: Apple slices were dehydrated in agitated (500 rpm) osmotic
solution (sucrose 60% W/W) at 40?C for 2 hours. The product/solution ratio was 1:10
(weight basis). Samples were removed from solutions and their surface was dried with
a filter paper for 5 min. Air drying of osmotically treated samples was carried out at
Water content: It was determined by an oven method. Apples were cut into pieces
with a knife and put into an oven (Heraeus RT 360) for 18h at 105ºC. The drying
curves were obtained by taking out two cubes from the oven and calculating the
moisture content every hour. It was assumed that the samples were representative.
Finally the water content was calculated.
Bulk density: The weight of samples (10 pieces) was taken with an analytical balance
(± 0.01g). The volume of the samples (Vb) was determined by the water displacement
method. Bulk density (?b) was calculated as:
? b =
True density: Samples were powdered and de-aerated to eliminate pores and air, then
volume (Vt) was measured. True density of samples was calculated as:
Porosity: Porosity (?) is obtained by following equation:
? = 1 ? ?
Scanning electron microscopy
A small section of tissue was cut from the inner part of the dried and rehydrated
sample and directly examined with a scanning electron microscope (XL30, Philips,
Data were subjected to analysis of variance (ANOVA) and means were compared
using Duncan’s multiple range test and MSTATC software. In addition, the
experimental data for physical, chemical and organoleptic evaluations were analyzed
at a significance level of 5%.
RESULTS AND DISCUSSION
True and bulk density values of dried and fresh apple slices are shown in table 1.
Calculated porosity values for dried samples are also presented in table 1. As it can be
seen, drying method has a significant effect on sample density and porosity. Air dried
samples (untreated) have lower porosity than others. During air drying a rapid
reduction of surface moisture and consequent shrinkage occurs. Prolonged exposure
to elevated drying temperature may result in severe shrinkage also reduces bulk
density and porosity.
Table 1: Density and porosity values for fresh and dried apple slices
1055 ± 1
554 ± 4.3
1187 ± 7.1
53.26 ± 0.1
668 ± 7.5
1114.3 ± 11
40 ± 0.4
1078 ± 4.7
61.7 ± 0.47
assisted air dried
Using an osmotic pre-treatment can prevent bulk density reduction because of less
shrinkage occurred during treatment due to solid penetration. Conversely, solid gain
can lead to the lack of porosity in products. Using a microwave stage as
complementary step after air drying produces more porous samples. However, other
researchers mentioned that bulk density was not affected by the process conditions or
increased by using a microwave stage (Funebo and Ohlsson, 1998; Funebo et al.,
2000). This divergence is due to the sequencing of microwave treatment. When the
microwave energy was used as complementary step after air drying, un-removed
water is heated suddenly then vapor exits from apple texture. As we observed during
heating, samples puffed and gained volume, but after drying, when product was
cooled, a small collapse was observed.
Four sample conditions were investigated by scanning electron microscopy
(SEM). Figure 2 shows the microstructure of samples.
a: Fresh apple tissue (25×)
b: blanched apple tissue (40×)
c: Air dried (75ºC) apple tissue (40×)
` d: Microwave–assisted air dried apple tissue (40×)
Figure 1- SEM images of fresh and treated apple tissue.
As it can be seen in fresh sample, apple tissue is composed of many well arranged
pores, after blanching some cell walls are disrupted and pores become larger than in
fresh ones. This fact helps drying process by accelerating the water removal (Fig. 2).
Figure 2- Drying curve of blanched and unblanched samples.
Microscopic image of air dried apple tissue shows that during drying cell volume is
reduced and tissue is collapsed. As mentioned before, these samples have low
porosity. Somewhat larger pores were found when a microwave treatment was added
to air drying (Fig1, d). We can deduce that the vapor induced during microwave
drying emanates from tissue and produces a more porous structure.
The present results show that the use of combined method of drying has in
comparison to conventional drying beneficial effects on structural properties of dried
products. Using microwave heating as final stage of drying may increase the porosity
of samples. However, our observations show that with this method we can not
produce a completely puffed product. So, it will be of advantage to add CaCl2 into the
osmotic solution. Its diffusion to the apple tissue can improve the firmness of dried
apple and prevent the collapse of tissue during cooling period after microwave
The authors wish to thank Tehran University research service for the financial
support (716/3/698) making it possible to carry out these experiments.
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