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Negar Hafezi et al./ Elixir Agriculture 77 (2014) 29135-29139
Available online at www.elixirpublishers.com (Elixir International Journal)
Agriculture
Elixir Agriculture 77 (2014) 29135-29139
Evaluation of drying rate and quality characteristics of potato slices during
drying by infrared radiation heating method under vacuum
Negar Hafezi∗, Mohammad Javad Sheikhdavoodi, Seyed Majid Sajadiye and Mohammad Esmail Khorasani Ferdavani
Department of Mechanics of Agricultural Machinery and Mechanization, Faculty of Agriculture, ShahidChamran University, Ahvaz,
Iran.
ARTICLE INFO
Ar t i cl e h i st o ry :
Received: 16 July 2014;
Received in revised form:
25 November 2014;
Accepted: 11 December 2014;
K e y w or d s
Potato, Drying rate,
Vacuum, Infrared radiation,
Shrinkage, Rehydration.
ABSTRACT
The effect of drying behavior on drying rate and quality characteristics of potato slices
in a vacuum- infrared drying system was studied. In this work, the effect of the infrared
radiation powers (100, 150 and 200 W) and vacuum levels (20, 80, 140 mm [Hg] and
atmosphere pressure) at different thickness (1, 2 and 3 mm) on drying rate, shrinkage
percentage and rehydration capacity were investigated. From the study, it was
concluded that IR power level has significant effects to processing time and drying
rate.The processing time reduced,while drying rates were higher with increased in IR
power. Drying rate curve of potato slices at initially time of drying because of surface
moisture evaporation in the ascending phase and afterward due to the start of influence
of water from within of material to surface descending phase occurs. Also shrinkage
percentage increased with increase of sample thickness. In other words, shrinkage was
decreased at different thickness with increase of infrared radiation power and vacuum
level. It was found that the long period of drying and increase of sample thickness may
have contributed to a decrease in rehydration capacity. However, rehydration capacity at
temperature 100°C for 3 min was more than temperature 25°C in duration 12 min.
© 2014 Elixir All rights reserved.
Introduction
Processed foods are normally changed in numerous ways
from their fresh counterparts with regard to appearance, flavor,
texture, nutrient content and microbiota. Consumers demand
high quality and convenient products with natural flavor and
taste and greatly appreciate the fresh appearance of minimally
processed food (Anjum et al., 2006).Fruits and vegetables are
agricultural products that are known for their rich vitamins, high
concentration of moisture and low fats. They are highly
perishable due to excess moisture present in them especially at
harvest. Fruits and vegetables are seasonal crops and are mostly
available during the production season. Potatoes are members of
the Solanaceae family. Of the many tuber- forming Solarium
species, the onethat is most widely cultivated is Solarium
tuberosum. In 2007 total world production of potatoes wasmore
than 320 million tonnes, and about 66% were consumed by
people as food. The other 34%production is used as animal feed,
and as potato starch in pharmaceuticals, textiles, adhesives
industries.In industry, the drying technologies have been widely
used for processing food products. Potato is rich in
carbohydrates, proteins, phosphorus, calcium, vitamin C, βcarotene and has high protein calorie ratio. Amongst the world’s
important food crops, Potato is the fourth important food crop
after wheat, rice and maize because of its’ great yield potential
and high nutritive value. The ratio of protein to carbohydrate is
higher in potato than in many cereals and other tuber crops
(Marwaha et al., 1999). It constitutes nearly half of the world’s
annual output of all root and tuber crops and has always
remained in the top ten since last twenty years. India ranks
fourth in area with 14 lakh hectares and the third largest country
in the world in production of potato after China and Russian
federation with a production of 294.94 million tonnes and
productivity of 17.86 tonnes per hectare (Faisal et al., 2013).
Tele:
E-mail addresses: nhafezi@yahoo.com
© 2014 Elixir All rights reserved
Drying is defined as a process of moisture removal due to
simultaneous heat and mass transfer. The moisture can be either
transported to the surface of the product and then evaporated, or
evaporated internally at a liquid vapour interface and then
transported as vapour to the surface (Gogus, 1994). The
technique of dehydration is probably the oldest method of food
preservation practiced by mankind. The use of artificial drying
to preserve agricultural products has expanded widely, creating
a need for more rapid drying techniques and methods that
reduce the large amount of energy required in drying processes.
New and/or innovative techniques that increase drying rates and
enhance product quality have achieved considerable attention.
Drying is the most common form of food preservation. This
process improves the food stability, since it reduces
considerably the water and microbiological activity of the
material and minimizes physical and chemical changes during
its storage (Hatamipour et al., 2007). Drying of food products
has been a very important industrial sector for many years.
Drying is the most energy intensive process in food industry.
Therefore, new drying techniques and dryers must be designed
and studied to minimize the energy cost in drying process
(Kocabiyik and Tezer, 2009).
Microbial activities are not active when the moisture
content of a product is below 10 %. Hence, harvested vegetables
must be stored dry (5% moisture content wet basis) (FAO, 1981)
to prevent attack and deterioration by activities of
microorganisms and fungi. Considering the fact that the highest
energy consumption in agriculture is associated with drying
operations, different drying methods can be evaluated to
determine and compare the energy requirements for drying a
particular product. Dried fruits and vegetables can be produced
by a variety of processes. These processes differ primarily by
the type of drying method used, which depends on the type of
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Negar Hafezi et al./ Elixir Agriculture 77 (2014) 29135-29139
food and the type of characteristics of the final product
(Mujundar, 1987).
The quality evaluation of the dried product was carried out
on the basis of response variables viz. Rehydration Ratio,
Shrinkage Percentage, Color and the Overall Acceptability.
Besides the nutrient contents, most of the previous studies on
drying of fruits and vegetables considered the rehydration
characteristics of dried samples. The rehydration rate and ratio
were obtained by soaking dried samples in water at determined
temperature and period of time. Rehydration is a complex
process aimed at the restoration of previously dried materials in
contact with water. It is generally accepted that the degree of
rehydration is dependent on the degree of cellular and structural
disruption. During dehydration, irreversible rupture and
dislocation occur resulting in loss of integrity and hence a dense
structure of collapsed, greatly shrunken capillaries with reduced
hydrophilic properties as reflected by the inability to rehydrate
fully (Lewicki, 1998). Pre-drying treatments and drying induce
changes in structure and composition of plant tissues (Lewicki,
1998), which results in impaired rehydration properties. One of
the most important physical changes that the food suffers
during drying is the reduction of its external volume. Loss
of water and heating cause stresses in the cellular structure
of the food leading to change in shape and decrease in
dimension. Shrinkage of food materials has a negative
consequence on the quality of the dehydrated product.
Changes in shape, loss of volume and increased hardness
cause in most cases a negative impression in the consumer.
There are, on the other hand, some dried products that have had
traditionally a shrunken aspect, a requirement for the consumer
of raisins, dried plums, peaches or dates Several authors have
tried to relate the effect of collapse and porosity with the
kinetics and extension of some chemical reactions in foods
undergoing drying and further storage (Mayor and Sereno,
2004).
Infrared radiation has significant advantages over
conventional drying. Among these advantages are higher drying
rates giving significant energy savings, and uniform temperature
distribution giving a better quality product. Therefore, it can be
used as an energy saving drying method (Mongpraneet et al.,
2002). In IR drying, special infrared lamps are used to extract
moisture from the material being dried. In this method, the air
surrounding wet matter flows using a suction device (vacuum
pump) to remove humidity released by the matter from its
vicinity in order for it to face less resistance while avoiding
material surface saturation with dump. Infrared radiation drying
has the unique characteristics of energy transfer mechanism. In
vacuum drying method due to lack of oxygen in dryer ambiance
and unwanted reduction of reactions in food, the quality of dried
food in this method is higher than the others (Motevali et al.,
2011 a, b). Also applying vacuum in food drying causes
expansion of air and vapor and creates puff state in the matter.
Due to the high energy consumption in this method, vacuum
drying can be used for highly sensitive and high value-added
products (Motevali et al., 2011a, b). For drying under vacuum
(or vacuum drying), moisture within the product being dried
evaporates at lower temperatures (lower than 100°C) giving
better product quality, especially in the cases of foods or
agricultural products, which are heat-sensitive in nature. When
the advantages of the two dying methods are combined, energy
efficiency of the drying process is enhanced and degradation of
dried product quality is also reduced. Earlier attempts to apply
infrared to drying of agricultural materials have been reported in
the literatureDoymaz, 2011; Sharma et al., 2005;Abe & Afzal,
1998. Combined infrared radiation and convection or vacuum
drying has also been reported as promising (Kouchakzadeh, and
Haghighi, 2011; Nimmol, 2010; Swasdisevi et al., 2009). Far
infrared drying of potato achieved high drying rates with
infrared heaters of high emissive power (Masamura et al., 1988).
The drying rate was also reported to be increased when the
electric power supplied to the far infrared heater was increased
and consequently the temperature of the sample was also
observed to be high. Akpinar et al. (2005) found that the potato
slices are sufficiently dried in the ranges between 60 and 80℃
and 20%–10% relative humidity at 1and 1.5 m/s. Reyes et al.
(2007) found that the type of dryers and the drying temperature
had a strong effect on drying rate and on the color and the
porosity of the dried potato slices, while the rehydration
capacity and the maximum penetration force were not affected.
Gowen et al. (2006) studied the rehydration properties of
microwave -hot air combination drying of chickpeas and
performed an optimization for the process. The report suggested
that high levels of microwave power could adversely affect
rehydrated product quality. It was found that high power level
resulted in low retention of product quality. Pimpaporn et al.
(2007) studied the influence of various pretreatments and drying
temperature on the low-pressure superheated steam drying
kinetics and quality parameters of dried potato chips. Khraisheh
et al. (2004) studied the quality and structural changes of
potatoes during microwave and convective drying. They
reported that air drying led to higher structural changes than in
the case of microwave drying. Mongpraneet et al. (2002)
examined the drying behavior of the leaf parts of welsh onion
undergoing combined far infrared and vacuum drying. The
results showed that the radiation intensity levels dramatically
influenced the drying rate and the dried product qualities. Singh
et al. (2006) found that rehydration rates of sweet potato slices
were dependent of drying condition and rehydration
temperature. Hernandez et al. (2000) proposed a linear relation
for shrinkage of foods as a function of moisture content.
Hatamipour and Mowla (2002) reported a linear correlation for
volume change and empirical relation for axial contraction of
carrots during drying in a fluidized bed dryer with inert
particles.
Figure 1. A schematic diagram of a vacuum- infrared drying
system: 1) humidity sensor; 2) thermocouples; 3) infrared
lamp power controller; 4) voltmeter; 5) infrared lamp; 6)
vacuum gauge; 7) vacuum break-up valve; 8) vacuum
pump; 9) webcam; 10) electronic balance; 11) sample tray;
12) drying chamber; 13) laptop; 14) air outlet duct.
The objective of this study is to examine the drying
behavior of potato slices by drying rate and quality
characteristics using combination of infrared radiation heating
method with the vacuum operating condition.
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Negar Hafezi et al./ Elixir Agriculture 77 (2014) 29135-29139
Experimental set-up, materials and methods
Experimental set-up
A laboratory scale vacuum- infrared dryer, developed at the
Agricultural Machinery and Mechanization Engineering
Laboratory, ShahidChamran University (Iran).A schematic
diagram of the apparatus for combined vacuum and infrared
radiation drying system is shown in Fig. 1. The dryer consists of
a stainless steel drying chamber, which is designed to withstand
lower level of pressure; a laboratory type piston vacuum pump,
which is used to maintain vacuum in the drying chamber; an
infrared lamp with power of 250 W (OSRAM, Slovakia), which
is used to supply thermal radiation to a drying product; and a
control system for the infrared radiator.
Materials and Methods
Sample Preparation
Fresh potatoes were purchased from a local market in
Hamadan province (Iran). The samples were stored in
refrigerator to prevent undesirable effect at about 5-6oC and
relative humidity of about 85%. Potatoes were peeled, washed,
and cut into sliced with thickness of 1, 2 and 3 mm by a manual
cutter. The samples were blanched in solution containing 2%
NaCl for 3 min. The initial moisture content of the fresh samples
was 77% (wet basis, wb), which was determined in triplicate by
using a convection oven at 70°C for 24 h (AOAC, 1990). Drying
experiment potato slices was dried in a vacuum chamber with
vacuum levels i.e., 20, 80 and 140 mm [Hg]; IR power of 100,
150 and 200 W. The distance between the infrared lamp and the
sample tray was set at 15 cm. The change of the mass of the
sample during drying was detected continuously using an
electronic balance (Lutron, GM- 1500P, Taiwan) with an
accuracy of ±0.05 g. The temperatures of the drying chamber
and of the drying sample were measured continuously using
thermocouples (SAMWON ENG, SU-105KRR). In start of
experiments relative humidity and temperatures of the drying
chamber were measured (respectively 35% and 50°C). The
drying experiments were performed until the sample moisture
content of 6-7% (w.b.) was obtained
Theoretical principle
Modelling of drying kinetics
The moisture content of the samples was found during the
drying process at different length of the dryer using equation (1).
wet material); Mo is the initial moisture content in wet basis (kg
water/kg wet material).
Determination ofdrying rate
The drying rate is expressed as the amount of the
evaporated moisture over time. The drying rate of potato slices
was calculated using the following equation:
W − Wd
Mw = w
× 100
Ww
RC =
(1)
Where: Mw is the moisture content wet basis (%); Ww is the
initial weight of potato samples (gr); Wd is the dry weight of
potato samples (gr)
Because of the variation in initial moisture content of fresh
potato, moisture ratio was used to describe the drying behavior
of potato in this study. To calculate the drying rate, an
appropriate empirical equation was fitted to the experimental
moisture removal data (drying curve) and was then
differentiated with respect to time. To find a suitable
mathematical model, the moisture content data at different
thickness, vacuum levels and infrared power were converted to
the moisture ratio (MR, dimension less) expression by using
following equation.
MR =
Mt −Me
Mo −Me
(2)
Where: MR is the moisture content ratio; Mt is the moisture
content at any drying time wet basis (kg water/kg wet material);
Me is the equilibrium moisture content wet basis (kg water/kg
DR =
MCt + dt − MCt
dt
(3)
Where, DR is the drying rate, (kgH2O/kg wet matter. min);
MCt+dt is the moisture content at time t+dt, (kgH2O/kg wet
material); MCt is the moisture content at time t (kgH2O/kg wet
material) and dt is the time between two sample weighing (min).
Measurements of volume and shrinkage of fresh and dried
potato slices
The shrinkage percentage is a drying quality assessing
parameter and it must be least for better drying as it directly
affects the rehydration quality of the dried product. The
shrinkage percentage was calculated after determining the size
of the potato slices before and after drying using toluene
displacement method (Mohsenin, 1986). For each measurement,
three slices were randomly selected. Shrinkage of potato slices
at the end of drying process was calculated using the following
equation (Koc et al., 2008).
V
% SKG = 1 − × 100
V0
(4)
Where V0 and V denote the initial and dried volume of the same
potato slice, respectively.
Measurements of rehydrationcapacity of dried potato slices
The measurement of the water rehydration rate was based
on the following procedure. The rehydration tests measured the
gain in weight of dehydrated samples (~1 g), dehydrated
samples were rehydrated in 200 g of distilled water at 25°C and
100°C for 12 min and 3 min, respectively. 25°C represents the
condition of room temperature and 100°C as the temperature at
boiling point of water. Samples were withdrawn, drained,
wrapped in absorbent tissue to remove surface water and
weighed on an analytical balance. Rehydration capacity (RC)
was calculated by Eq (5).
(M t − M r )
(M i − M r )
(5)
The amount of initial Mi and residual moisture content Mr of the
samples (w.b.) was determined from the moisture content of
fresh and dried potatoes, respectively. The moisture of the
rehydrated product Mt was calculated from the sample weight
before and after rehydration. The rehydration measurements
were made three times for all the samples and average values
were reported (Reyes et al., 2007).
Results and discussion
Calculation of drying rate
Figure 2-4. Shows the variations of drying rate with drying
time at vacuum level 140 mm [Hg] based on the thickness of
potato slices at the various IR power level of 100, 150 and 200
W. Drying rate curve at vacuum levels of 20 and 80 mm Hg and
atmosphere pressure was observed such as vacuum level of 140
mm Hg. Drying rate curve of potato slices at initially time of
drying because of surface moisture evaporation in the ascending
phase and afterward due to the start of influence of water from
within of material to surface descending phase occurs.
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Negar Hafezi et al./ Elixir Agriculture 77 (2014) 29135-29139
direct proportion have to the volume changes. This table display
that factors vacuum and IR power have indirect proportion to
the volume changes. In other words, shrinkage was decreased at
different thickness with increase of infrared radiation power and
vacuum level.
Table 1. Correlation coefficients of the ratio of the volume
measured by toluene displacement
Figure 2. Effect of thickness on drying rate of potato slices at
vacuum level of 140 mm Hg and at IR power of 100 W
Calculation of rehydration capacity
The long period of drying and increase of sample thickness
may have contributed to a decrease in rehydration capacity.
Figure 6. Shows that at temperature 25°C for 12 min at the
power level of 150 W, vacuum 20 mm [Hg] and thickness of 1
mm, however, fresh-like properties were obtained.
Figure 3. Effect of thickness on drying rate of potato slices at
vacuum level of 140 mm Hg and at IR power of 150 W
Figure 4. Effect of thickness on drying rate of potato slices at
vacuum level of 140 mm Hg and at IR power of 200 W
Calculation of shrinkage percentage
Figure 5. Shows that maximum of shrinkage percentage
(85.31%) and minimum of shrinkage percentage (61.79%) was
computed at thickness of 3mm and 1 mm, respectively.
Figure 5. Shrinkage percentage of potato slices in vacuuminfrared drying system
According to Table 1 the factor of thickness has most
relationship with volume changes. This coefficient suggests that
Figure 6. Rehydration capacity of potato slices at
temperature 25°C for 12 min in vacuum- infrared drying
system
Figure 7. Shows that fresh-like properties were obtained at
temperature 100°C for 3 min at the power level of 200 W,
vacuum 80 mm [Hg] and thickness of 1 mm. According of
results rehydration capacity at temperature 100°C was more than
temperature 25°C. In both temperature, value minimum of
rehydration was observed at IR power of 100 W and vacuum
level of 20 mm [Hg] at thickness of 3 mm.
Figure 7. Rehydration capacity of potato slices at
temperature 100°C for 3 min in vacuum- infrared drying
system
Conclusions
From the study, it was concluded that IR power level has
significant effects to processing time and drying rate. The
processing time reduced, while drying rates were higher with
increased in IR power. Data analysis showed that shrinkage
percentage increased with increase of sample thickness. This
means that Maximum of shrinkage percentage (85.31%) and
minimum of shrinkage percentage (61.79%) was computed at
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Negar Hafezi et al./ Elixir Agriculture 77 (2014) 29135-29139
thickness of 3mm and 1 mm, respectively. The rehydration
process at 100ºC yielded the highest rehydration capacity at
thepower level of 200 Wvacuum 80 mm [Hg] and thickness of 1
mm.
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