Xiao Hong Wei Pang Chang Le Wang Li Hong

biosystems engineering 105 (2010) 233–240

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/issn/15375110

Research Paper

Drying kinetics and quality of Monukka seedless grapes
dried in an air-impingement jet dryer
Hong-Wei Xiao a, Chang-Le Pang a, Li-Hong Wang a,b, Jun-Wen Bai a,
Wen-Xia Yang c,**, Zhen-Jiang Gao a,b,*
a

College of Engineering, China Agricultural University, P.O. Box 194,17 Qinghua Donglu, Beijing 100083, China
College of Machinery and Electricity Engineering, Shihezi University, Shihezi 832003, China
c
GanNan Normal University, Ganzhou, Jiangxi 341000, China
b

article info

Drying kinetics and quality of Monukka seedless grapes were investigated in an
Article history:

impingement dryer under different drying temperatures (50,55,60 and 65  C) and air

Received 13 September 2009

velocities (3,5,7 and 9 m s1). Results indicated that the effect of drying temperature on

Received in revised form

drying time was more distinct than air velocity. The moisture effective diffusivity ranged

13 October 2009

from 1.82  1010 to 5.84  1010 m s2 calculated using the Fick’s second law of diffusion.

Accepted 5 November 2009

The activation energy determined from Arrhenius equation was 67.29 kJ mol1. The

Published online 27 November 2009

hardness of dried Monukka seedless grapes changed from 9.53 to 17.16 N showing an
increasing trend as drying temperature increased. The retention ratio of vitamin C of the
samples varied from 10.26 to 39.73% compared to the fresh one. The results also illustrated
that the drying temperature was the major factor controlling the retention of vitamin C,
while there was no direct correlation between air velocity and vitamin C retention.
ª 2009 IAgrE. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

As one of the world’s most popular and largest fruit crops, grape
production all over the world was about 66,271,676 tons and
China produced about 6,250,000 t according to Food and Agriculture Organization (FAO) data for 2007 (FAO, 2008). The five
major grape producing countries are Italy (about 8.52  106 t),
France (about 6.50  106 t), China, the United States (about
6.11  106 t) and Spain (about 6.01  106 t) (FAO, 2008). In China,

the Xinjiang Uigur Autonomous Region is one of the main grape
production areas. In this region the Monukka seedless grape is
the mainly cultivated variety. This is an insect and diseaseresistant and high-yielding cultivar (Yang et al., 2009).
Grapes are a seasonal fruit being harvested during July–
September in China. Fresh grapes, having relatively high

moisture contents, are very sensitive to microbial spoilage
during storage, even under refrigerated conditions. Therefore,
within a few weeks following harvest they must either be
consumed or processed into various products. Drying is the
most common form for grape processing. It can process
grapes into raisins for longer shelf-life by reducing the moisture content to a low level. Raisins can be consumed either
directly as ready-to-eat food or as ingredients in biscuits,
breads and porridges.
The drying technology used is important for raisin quality.
Currently, almost all raisins are directly or indirectly produced
by natural drying in sunlight (Li et al., 2009). The most common
drying method is sun drying, which is traditionally practiced
in many countries (Pangavhane and Sawhney, 2002). This
technique has the advantages of simplicity and small capital

* Corresponding author. College of Engineering, China Agricultural University, P.O. Box 194,17 Qinghua Donglu, Beijing 100083, China.
** Corresponding author.
E-mail addresses: ywx0@sina.com (W.-X. Yang), cauzjgao@126.com (Z.-J. Gao).
1537-5110/$ – see front matter ª 2009 IAgrE. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.biosystemseng.2009.11.001

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biosystems engineering 105 (2010) 233–240

Nomenclature
D0
Deff
Ea
Me
R2
M0

2

1

Constant diffusivity basis (m s )
Effective diffusivity (m2 s1)
Activation energy (kJ mol1)
Equilibrium moisture content (kg [water] kg1 [dry
matter])
Correlation coefficient (dimensionless)
Initial moisture content (kg [water] kg1 [dry
matter])

investment. However, it requires large areas, high labour costs
and the drying time usually takes 15–20 days which is undesirable economically (Basunia and Abe, 2001; Doymaz, 2006).
Furthermore, the final product may be contaminated by dust
and insects and the exposure to solar radiation results in
colour deterioration (Andritsos et al., 2003). In order to improve
the quality, traditional sun drying techniques can be replaced
by industrial drying methods such as hot-air mechanical and
solar drying (Ertekin and Yaldiz, 2004). Much research has

been carried out to investigate the solar and hot-air drying of
grapes. Azzouz et al. (2002) experimentally evaluated the
drying kinetics and moisture diffusivity of convective drying
of grapes. Ramos et al. (2004) observed the microstructural
changes of grapes during hot-air drying. Fadhel et al. (2005)
studied the solar drying of grapes by three different processes
and found that the solar tunnel greenhouse drying was
satisfactory and competitive to a natural convection solar
drying process. Bennamoun and Belhamri (2006) investigated
the drying kinetics of grapes undergoing solar drying and
Esmaiili et al. (2007b) determined the thin-layer drying characteristics of seedless grapes when they were processed using
a tray dryer. Margaris and Ghiaus (2007) studied experimentally the hot-air drying characteristics of Sultana grapes.
Texture and nutrition are important quality attributes
evaluating the quality of dried grapes (Esmaiili et al., 2007a).
Apart from the variety and growing conditions, the texture
and nutritional value of grape raisins are mainly related to the
drying conditions (Mahmutoglu et al., 1996). Undesirable
changes in texture and nutrition of dried grapes may lead to
a decrease in its quality and marketing value. Usually, if the
texture of the dried grapes is softer the quality is better. In hotair drying, the nutritional quality of the product can be

adversely affected by high temperature and long drying time.
It was observed that if vitamin C is well retained during
drying, other nutritional components are also well preserved
(Lin et al., 1998). Therefore, vitamin C can be taken as an
indicator of the nutritional quality of Monukka seedless
grapes.
Air-impingement drying technology is an efficient drying
process and has been used successfully in both the paper and
textile industries. During air-impingement processing, the air
impinges on the product surface at high velocity, removes the
thermal boundary layers and increases the rate of heat
transfer (Anderson and Singh, 2006). The heat transfer coefficient is about 5 times higher than with cross-circulation
dryers (Seyedein et al., 1995) which greatly accelerates the
drying rate and reduces the drying time. However, to date few

Mt
n
r
R
t

MR
T

Moisture content at time t (kg water/kg dry matter)
Positive integer
Volume equivalent radius (m)
Universal gas constant (J mol1 K1)
Drying time (s)
Moisture ratio (dimensionless)
Drying temperature ( C)

research results concerning air-impingement drying characteristics and the subsequent quality of the dried grapes are
available (Yang et al., 2009). Therefore, the objectives of the
current work were to investigate the effect of air-impingement drying temperature and air velocity on the drying
characteristics of Monukka seedless grapes, to calculate
moisture effective diffusivity and activation energy, and to
evaluate the quality of dried grapes in terms of their texture
and vitamin C content.

2.

Materials and methods

2.1.

Raw material

Fresh Monukka seedless purple grapes were harvested from
Shihezi in the Xinjiang Uigur Autonomous Region of China.
The samples were checked carefully to discard spoiled fruit in
order to prevent contamination by bacteria or fungi. To ensure
the uniformity of the physical characteristics of experimental
materials, the grape samples with the same size (average
berry radius, length and weight is 7.6 mm, 34.1 mm and 4.34 g,
respectively) were selected. The average initial moisture
content of the grape samples was 4.24 kg kg1 in dry basis, as
determined by vacuum drying at 70  C for 24 h following the
Association of Official Analytical Chemists (AOAC) method
no.934.06 (AOAC, 1990). The samples were washed with tap
water to remove the dust then blown with air for about 5 min

to eliminate excess water from the surface. No chemical
pretreatments or preservatives were applied. The prepared
samples were wrapped with a plastic film and stored in
a refrigerator at 3  1  C and 90% relative humidity for several
hours in order to prevent moisture loss before the experiments were carried out.

2.2.

Experimental set-up and procedure

A schematic diagram of equipment used for air-impingement
drying is shown in Fig. 1.This apparatus basically consist of
series of round nozzles in lines, an electric heater to heat the
air, a centrifugal fan to supply the air flow and circulate the air
flow, and a Proportional-Integral-Derivative (PID) controller
(Omron, model E5CN, Tokyo, Japan) to control drying
temperature. The distance between the round impingement
nozzles and the up surface of grape samples is about 80 mm.
Air outlet velocity was measured with Founder Probe
Anemometer (Founder, China) having an accuracy of

biosystems engineering 105 (2010) 233–240

235

Fig. 1 – Schematic diagram of hot-air impingement dryer 1. PID controller 2. Frequency converter 3. Electric motor 4. Electric
heater 5. Centrifugal fan 6. Series of circular nozzles 7. Drying chamber 8. Temperature and air velocity sensor 9. Drying tray
10.grapes being dried.

0.1 m s1. During the drying experiments, temperature and
relative humidity of the surroundings is at the range of from
18.1  C to 33.2  C and from 22.3% to 42.3%, respectively. A
ACS100 Frequency Converter (ABB, Switzerland) was used to
vary the air velocity. After the dryer had reached steady state
conditions for the set points (at least 30 min), the grape
samples were spread into a single layer onto a stainless steel
wire mesh in the drying chamber. Each sample was located so
as not to touch the adjacent ones. The sample weight was kept
at 434.00  0.50 g for all runs. The experiments were performed according to Table 1. Weight loss of samples was
measured by means of removing the drying tray from the
drying chamber and weighing on an electronic balance
(0.01 g) at 1 h intervals during drying. It took less than 15 s to
weigh the sample. Drying was continued until the grapes
dried to raisins with 0.25 kg kg1 (d. b.) moisture content. The
product was cooled and packed in low density polyethylene
(LDPE) bags that were heat-sealed. The experiments were
replicated three times and the drying kinetics were calculated
as the average of the three replicates.

2.3.

Mathematical modelling of drying curves

Table 1 – Design for the experiments with run conditions
included.

1
2
3
4
5
6
7

Drying temperature ( C)

Air velocity (m s1)

50
55
60
65
60
60
60

5
5
5
5
3
7
9

Mt  Me
M0  Me

(1)

Where M0 is the initial moisture content, Me is the equilibrium
moisture content and Mt is the moisture content at time t. The
values of the equilibrium moisture content, Me are relatively
small compared to Mt or M0. Thus the Eq. (1) can be written in
a more simplified form as follows (Doymaz et al., 2004; Goyal
et al., 2007):
MR ¼

Mt
M0

(2)

The drying rate of grape samples during drying experiments
was computed using Eq. (3) and expressed as g [water] g1 [dry
solids] h1.
Drying Rate ¼

Mt1  Mt2
t2  t1

(3)

Where t1 and t2 are the drying times in hours at different times
during dying; Mt1 and Mt2 is the moisture content of grape
samples at time t1 and t2, respectively expressed on a dry basis.

2.4.

The moisture ratio (MR) of grape samples during drying
experiments was calculated using Eq. (1).

Run no

MR ¼

Calculation of moisture effective diffusivity

Generally, moisture effective diffusivity was used to describe
the drying characteristics, due to limited information on the
mechanism of moisture movement during drying and
complexity of the process, which may involve molecular
diffusion, capillary flow, Knudsen flow, hydrodynamic flow,
surface diffusion and all other factors (Souraki and Mowla,
2008). It was assumed that the whole drying process occurs
during the falling rate period and moisture diffusion controls
the process, the Fick’s second law of diffusion can be used to
describe the drying course of Monukka seedless grapes (Doymaz and Pala, 2002; Srikiatden and Roberts, 2006). The solution
of the Fick’s second law equation can be given as Eq. (4) with the
assumption that neglecting shrinkage, constant temperature
and diffusion coefficients and uniform initial moisture distribution (Matteo et al., 2000; Doymaz, 2006; Xiao et al., 2009).

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biosystems engineering 105 (2010) 233–240






Deff t
Deff t
M M  Me
6 1
1
þ 2 exp  22 p2 2
z
¼ 2 2 exp  12 p2 2
r
r
M0 M0  Me p 1
2




1
1
2 2 Deff t
2 2 Deff t
þ 2 exp  4 p 2
þ 2 exp  3 p 2
r
r
3
4


D
t
1
eff
ðn˛N/ þ NÞ
þ / þ 2 exp  n2 p2 2
r
n

a hollow planar base. The force was then applied to the
sample by a 2.0 mm spherical probe at a constant speed of
0.5 mm s1 with a penetration distance of 2.0 mm. The
maximum compression force of a rupture test of each sample
was used to describe the sample texture in terms of hardness.
All tests were duplicated and the average values were
reported.

(4)
2

1

Where, Deff is the moisture effective diffusivity (m s ); r is
volume equivalent radius of the Monukka grape samples, with
0.9  102 m as its value; t is the drying time expressed in
second (s); and n is positive integer. For long drying time, the
Eq. (4) can be simplified as Eq. (5) by taking the first term of
series solution (Crank, 1975; Matteo et al., 2000; Doymaz, 2006).


Deff t
M
6
¼ 2 exp  p2 2
r
M0 p

(5)

 

   2
M
6
p
ln
¼ ln 2  2 Deff t
r
M0
p

(6)

The natural logarithm form of Eq. (5) was given as Eq. (6).
The moisture effective diffusivity can be calculated using the
method of slopes. It is typically determined by plotting the
experimental drying data in terms of ln (MR) versus time. So
the slope can be given from the linear regression of ln (MR)
versus time curves, then the effective diffusion coefficients
(Deff) can be determined as shown in Eq. (7).

2.7.

Determination of vitamin C content

Vitamin C content of Monukka seedless grape samples was
determined by a titration method following the methodology
described by Marfil et al. (2008) with slight modification.
Vitamin C was measured as dehydroascorbic acid following
oxidation of reduced ascorbic acid with 2, 6-dichloro-indophenol (0.01 g/100 g solution) and expressed as mg vitamin C/
100 g raisins on wet basis. All determinations were performed
in duplicate. The vitamin C retention ratio of the dried
samples was then calculated using Eq. (10) as follows:
Retention ratio ¼

Vitamin C content of dried samples
Vitamin C content of fresh samples
 100%

(10)

3.

Results and discussion

3.1.

Drying curves

2

2.5.

(7)

Calculation of activation energy

Activation energy is the relative ease with which the water
molecules pass the energy hurdle while migrating within the
sample. The dependence of effective moisture diffusivity (Deff)
on drying temperature has been shown to follow an Arrhenius
relationship (Park et al., 2002; Srikiatden and Roberts, 2006;
Doymaz, 2007a,b; Singh and Gupta, 2007) presented as follows
(Eq. (8)):

Deff ¼ D0 exp 


Ea
RðT þ 273:15Þ

(8)

Where D0 is the constant diffusivity basis (m2 s1); Ea is the
activation energy (kJ mol1); R is the universal gas constant
with 8.31 J mol1 K1 as its value; T is the drying air temperature ( C). By taking the natural logarithm of both sides, the Eq.
(8) can be transformed into a linear-logarithmic form, as
shown in Eq. (9). Consequently, the activation energy (Ea) can
be calculated from the slope of ln (Deff) versus the reciprocal of
the temperature (1/(T þ 273.15)).


Ea
1
ln Deff ¼ lnðD0 Þ 
R T þ 273:15

2.6.

(9)

Textural properties

The textural properties of dried grape samples were evaluated
by a compressive test using a texture analyser (Instron 430,
Buckinghamshire, UK). A dried grape sample was placed on

To compare the effect of different drying temperature and air
velocity on the drying kinetics of Monukka seedless grapes,
the curves of MR versus drying time and curves of drying rate
versus moisture content under different processing parameters are shown in Figs. 2–4. From Figs. 2 and 3, it can be seen
that MR of grape samples decreased with the increase of
drying time. It can also be seen that the drying time taken to
reduce the moisture content of Monukka seedless grape
samples from the initial moisture 4.24 kg kg1 (d. b.) to a final
0.25 kg kg1 (d. b.) was 51, 45, 37 and 21 h with a constant hotair velocity of 5 m s1 at drying temperatures of 50, 55, 60, and
65  C, respectively. While it took 39, 37, 34 and 31 h at
a constant drying temperature of 60  C with air velocities of 3,

1.0
0.8

Moisture Ratio

r
Deff ¼  2 Slope
p

50 °C
55 °C
60 °C
65 °C

0.6
0.4
0.2
0.0
0

10

20

30

40

50

60

Drying time (h)
Fig. 2 – Drying kinetics of Monukka seedless grapes at
different drying temperatures with constant air velocity of
5 m sL1.

biosystems engineering 105 (2010) 233–240

drying the product could not provide a constant supply of
water for an appreciable period of time (Togrul and Pehlivan,
2003; Prakash et al., 2004; Singh and Gupta, 2007). It is clear
that diffusion is the dominant physical mechanism governing
moisture movement from interior to surface of Monukka
seedless grapes during drying process. Similar results have
been reported in literature by other researchers, such as Singh
et al. (2006) for sweet potato, Doymaz (2007a) for pumpkin
slices and Singh and Gupta (2007) for carrot.

1.0
0.9
0.8

Moisture Ratio

237

0.7
0.6
0.5
0.4
0.3

3.2.

0.2
0.1
0.0
0

5

10

15

20

25

30

35

40

45

Drying time (h)
Fig. 3 – Drying kinetics of Monukka seedless grapes at
different air velocities with constant drying temperature of
60 8C.

5, 7 and 9 m s1, respectively. This illustrates that increasing
the drying temperature and air velocity can enhance the
drying rate and decrease the drying time of Monukka seedless
grape samples. The effect of drying temperature on drying
rate of Monukka seedless grape samples was more distinct
than the effect of air velocity. This might be because moisture
diffusion from interior layer to grape surface controlled the
drying process and its rate mainly depended on drying
temperature. This is in agreement with the earlier research on
the drying of various vegetables and fruits such as Thompson
seedless grapes (Pangavhane et al., 2000), apricot (Bozkir, 2006)
and carrot (Xiao et al., 2009).
The drying rate versus moisture content curves of Monukka
seedless grape samples are illustrated in Fig. 4. The curves
show that the drying rate decreased continuously with
moisture content. The drying rate was rapid during the initial
period but it became very slow at the last stages during the
drying process. As shown in Fig. 4, there was no constant
drying rate period, and the entire drying process occurred
during a falling rate period. The absence of constant rate
period might be due to the reason that at initial stages of

Fig. 4 – Drying rate versus moisture content curves of
Monukka seedless grapes at different drying temperature
with constant air velocity of 5 m sL1.

Moisture effective diffusivity

The results of moisture effective diffusivity (Deff) of Monukka
seedless grape and other related products under different
drying temperatures are summarised in Table 2. It shows that
the Deff values of the Monukka seedless grape samples
changed from 1.82  1010 to 5.84  1010 m2 s1, which lie
within the general range from 1011 to 109 m2 s1 for food
materials (Madamba et al., 1996). It was noted that Deff
increased with the increase of drying temperature. This
phenomenon might be due to the fact that moisture effective
diffusivity depends on drying temperature, variety and
composition of the drying samples (Rizvi, 1986). When
samples were dried at higher temperature, increased heating
energy would increase the activity of the water molecules
leading to higher moisture diffusivity. This finding agrees with
that reported by Shi et al. (2008) for drying blueberries with
infrared radiation heating, Souraki and Mowla (2008) for
drying green beans using a fluidised bed dryer, Sharma et al.
(2009) for drying carrot slices by convective drying method,
Sharma et al. (2009) for drying garlic cloves undergoing
microwave-convective drying.

3.3.

Activation energy

The activation energy (Ea) of Monukka seedless grape samples
was 67.29 kJ mol1 calculated from the slope of the Arrhenius
plot as presented in Fig. 5. It is in the range of 12.7–
110.0 kJ mol1 for most food materials (Troncoso and Pedreschi, 2007). The activation energy is an indication of the
required energy to remove moisture from inside to the outside
of the drying product. For the sake of comparison activation
energy of grapes and other agricultural products are presented
in Table 3. From Table 3, it was found that the activation
energy of Monukka seedless grape is higher than the activation energy of Sultanin grapes, Chasselas grapes, potato slices,
broad beans, carrot slices, and chopped coconuts, but lower
than the activation energy of black tea, pumpkin slices and
nettle leaves.
This phenomenon might be due to the fact that the
components, tissue structures and specific surface area of the
product have a significant effect on its activation energy. In
general, the products which have high content of sugar or
pectin, compact tissue structures and small specific surface
area may have higher activation energy than the ones which
have low content of sugar or pectin, porous structures and
large specific surface area. Furthermore, pretreatments,
which can change the physical properties of the sample cell
structure or remove the waxy layer on the surface of the

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biosystems engineering 105 (2010) 233–240

Table 2 – Moisture effective diffusivity values of Monukka seedless grape and other products.
Products
Monukka seedless grape

Sultana seedless grape
Muscat black grape
Vinifera seedless grape
Mango slices
Mulberry
Plums
Apple slices

Drying temperature ( C)

Deff (1010 m2 s1)

50
55
60
65
50–70
60
50
55–65
60–80
65
55

1.82
2.92
3.65
5.84
7.91–35.00
3.82–12.80
3.34–8.46
2.62–4.39
2.32–2.76
2.17–2.40
5.00–16.00

products that impede moisture transport during drying, also
have an important effect on the activation energy of products.

3.4.

Textural properties

The texture of dried Monukka seedless grape subjected to
different process parameters is reported in terms of hardness
in Table 4. Its values ranged from 9.53 to 17.16 N, which was
the maximum breaking force of the dried Monukka seedless
grapes. It was also established that drying temperature (from
55 to 65  C) significantly effected the hardness of Monukka
seedless grapes during the drying process. In addition, its
hardness values showed an increasing trend as the drying
temperature increased. However, no significant difference
( p > 0.05) was found among the hardness values of the Monukka seedless grapes when they were dried at different air
velocities with a constant drying temperature. Hardness is an
important parameter used to investigate case hardening in
dried products, which is related to the strength of the structure under compression (Chong et al., 2008). The above results
are probably due to the fact that if Monukka seedless grapes
are dried at high temperature the water removal rate from the
surface is faster than the rate at which water migrates from
the interior and a hard layer, containing previously dissolved
solutes, is formed on the surface.

3.5.

References
Present work

Doymaz and Pala (2002)
Doymaz (2006)
Esmaiili et al. (2007b)
Goyal et al. (2006)
Maskan and Gogus (1998)
Doymaz (2004)
Karathanos et al. (1995)

Retention of vitamin C

The retention of vitamin C of Monukka seedless grape
samples under different drying conditions is presented in
Table 4. In all dried samples the retention ratio of vitamin C
was lower than 39.73%. The sample with the highest retention
of vitamin C was the that dried at the lowest drying temperature 50  C (2.25 mg vitamin C/100 g w. b.), whilst the most
degradation of vitamin C occurred at the highest drying
temperature 65  C (0.57 mg vitamin C/100 g w. b.). However,
drying air velocity did not significantly influence the vitamin
C retention ( p > 0.05). Hence it can be concluded that the
drying temperature was the major factor controlling the
retention of vitamin C, while there was no direct correlation
between drying air velocity and vitamin C retention. Similar
results were reported by Kaya et al. (2009b); Kuljarachanan
et al. (2009), and Miranda et al. (2009). The loss of vitamin C,
which is a thermo-sensitive compound, was probably due to
oxidation and thermal degradation (Hawlader et al., 2006;
Marfil et al., 2008).
In general, the quality of foodstuff is determined by its
nutrition. Vitamin C is very important for human health and
must be taken into the body. It has a protective effect against
lung, bladder and prostate cancers (Halliwell, 1994). It also has
antioxidant roles against oxidative damage and cardiovascular disease (Kritchevsky, 1992). An increase in drying
temperature has a negative effect on vitamin C preserving.
This finding suggests that in the grape drying process higher
drying temperatures should not be adopted for preserving
vitamin C and other nutritional components.

Table 3 – Activation energies of Monukka seedless grape
and other related products.
Products

Fig. 5 – Arrhenius-type relationship between moisture
effective diffusivity and drying temperature of Monukka
seedless grapes.

Ea (kJ mol1)

Monukka seedless grape 67.29
Sultanin grape
54.00
Chasselas grape
49.00
Potato slices
39.49–42.34
Broad beans
17.10–27.71
Carrot slices
23.00
Chopped coconut
25.93
Nettle leaves
79.87–109.00
Pumpkin slices
78.93
Black tea
406.03

References
Present work
Azzouz et al. (2002)
Azzouz et al. (2002)
Aghbashlo et al. (2009)
Hashemi et al. (2009)
Kaya et al. (2009a)
Madhiyanon et al. (2009)
Kaya & Aydin (2009)
Doymaz (2007a)
Panchariya et al. (2002)

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biosystems engineering 105 (2010) 233–240

Table 4 – The hardness and vitamin C retention of dried Monukka seedless grapes at different drying conditions
Run No.

Drying temperature
( C)

Fresh
1
2
3
4
5
6
7

50
55
60
65
60
60
60

Air velocity
(m s1)

Hardness
(N)

Retention of vitamin C
(mg/100 g)

Retention ratio
(%)

5
5
5
5
3
7
9

0.62  0.14d
9.53  0.68c
11.27  1.25c
14.52  0.74b
17.16  1.12a
15.21  0.79b
13.98  0.65b
14.83  0.84b

5.72  0.36a
2.25  0.21b
1.48  0.34c
0.94  0.29c
0.57  0.23d
0.86  0.26c
0.79  0.18c
1.03  0.27c

39.73  6.18a
25.42  6.67b
16.82  6.14b
10.26  7.06c
15.38  5.40b
14.06  4.03b
19.20  5.06b

Note: The values followed by different letters (a–d) in a same column are significantly different at p < 0.05.

4.

Conclusions

The effect of drying temperature and air velocity on the drying
kinetics and quality of Monukka seedless grapes, which subjected to thin-layer air impingement drying, were examined in
this investigation. In case of the drying kinetics the drying
temperature and air velocity were found to have an effect on
the drying rate of Monukka seedless grapes. Compared to air
velocity, the effect of drying temperature on the drying time
was more significant. Because the entire drying process
occurred during the falling period, Fick’s second law was used
to describe the drying characteristics of Monukka seedless
grapes. The moisture effective diffusivity (Deff) was determined
using the slope of a linear plot (ln (MR) versus time) and its value
changed from 1.82  1010 to 5.84  1010 m2 s1 for 5 m s1 and
temperature between 50 and 65  C. The drying temperature
was found to have a significant effect on Deff. The Ea of Monukka
seedless grapes was determined as 67.29 kJ mol1. In terms of
the quality of dried grapes, the texture and vitamin C retention
were studied. Regarding textural quality, the hardness values
showed an increasing trend as the drying temperature
increased but no significant relationship was found between air
velocity and sample hardness. The retention of vitamin C was
mainly controlled by the drying temperature. However, drying
air velocity did not show any significant influence on the
vitamin C retention ( p > 0.05).

Acknowledgements
This research is funded by the Science and Technology
Support Project for Xinjiang Production and Construction
Corps of China (No.2008ZJ28), the National High Technology
Research and Development Program of China (863 Program)
under Grant No.2007AA100406-04 and the Funding System for
Scientific Research Projects of Doctor Subject of Chinese
Advanced University (No.20060019011).

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