Journal of Food Engineering 75 (2006) 267–274
www.elsevier.com/locate/jfoodeng
Influence of the osmotic agent on the osmotic dehydration
of papaya (Carica papaya L.)
Ânoar Abbas El-Aouar a,*, Patrı́cia Moreira Azoubel b,
José Lucena Barbosa Jr. a, Fernanda Elizabeth Xidieh Murr
a
a
Department of Food Engineering, College of Food Engineering, State University of Campinas, P.O. Box 6121,
ZIP 13083-970, Campinas, SP, Brazil
b
EMBRAPA Semi-Arid, BR 428, km 152, P.O. Box 23, 56302-970 Petrolina, PE, Brazil
Received 23 June 2004; accepted 18 April 2005
Available online 29 June 2005
Abstract
The objective of the present work was to study the influence of two different osmotic agents (sucrose and corn syrup) on the
osmotic dehydration of papaya slices (Carica papaya L.). The study was carried out using two factorial experimental designs, with
three independent variables whose levels varied from 44% to 56% w/w for concentration, from 34 to 46 C for temperature and from
120 to 210 min for immersion time. The responses of the experimental designs were the weight reduction (WR), water loss (WL),
solids gain (SG) and water activity (aw). The results showed that, considering the same osmotic pressure for both osmotic agents,
the values obtained for WR, WL and SG for dehydration in sucrose solutions were higher than those obtained in corn syrup solutions, due to their high viscosity and polysaccharide content. The opposite behavior was observed for aw. The models obtained for
the response variables followed a linear behavior except for SG.
2005 Elsevier Ltd. All rights reserved.
Keywords: Osmotic dehydration; Experimental design; Papaya
1. Introduction
Osmotic dehydration is widely used for the partial removal of water from plant tissues by immersion in a
hypertonic (osmotic) solution. The driving force for
the diffusion of water from the tissue into the solution
is provided by the higher osmotic pressure of the hypertonic solution. The diffusion of water is accompanied by
the simultaneous counter diffusion of solutes from the
osmotic solution into the tissue. Since the membrane
responsible for osmotic transport is not perfectly selective, other solutes present in the cells can also be leached
*
Corresponding author. Tel.: +55 19 3788 4057; fax: +55 19 3788
4027.
E-mail address: [email protected] (Â.A. El-Aouar).
0260-8774/$ - see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2005.04.016
into the osmotic solution (Rastogi & Raghavarao, 2004;
Sablani, Rahman, & Al-Sadeiri, 2002).
The rate of diffusion of water from any material made
up of such tissues depends upon factors such as: temperature and concentration of the osmotic solution, the size
and geometry of the material, the solution-to-material
mass ratio and the level of agitation of the solution. A
number of recent publications have described the influence of these variables on mass transfer rates during osmotic dehydration (Corzo & Gomez, 2004; Rastogi &
Niranjan, 1998; Rastogi, Raghavarao, & Niranjan,
1997).
The use of the osmotic dehydration process in the
food industry has several advantages: quality improvement in terms of color, flavor and texture, energy efficiency, packaging and distribution cost reduction, no
chemical pretreatment, providing required product
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Â.A. El-Aouar et al. / Journal of Food Engineering 75 (2006) 267–274
stability and retention of nutrients during storage (Rahman & Perera, 1999; Sablani et al., 2002).
The response surface methodology has been widely
and effectively used in industrial investigations and other
processes such as the development and/or improvement
of the nutritional products, due to its practical utility in
their optimization (Azoubel & Murr, 2003; Corzo &
Gomez, 2004). This methodology presupposes the use
of experimental design techniques to investigate and
learn about the functional form of the process or system
that involves one or more response variables that are
influenced by various factors or independent variables.
Therefore, our general objective was to study the
influence of two different osmotic agents (sucrose and
corn syrup) on the osmotic dehydration of papaya Formosa slices (Carica papaya L.). The specific objective
was to model the influence of the concentration and
temperature of the osmotic solution and the immersion
time on changes in weight reduction, water loss, solids
gain and water activity for both osmotic agents.
2. Material and methods
2.1. Raw material
Papayas (Formosa variety) with similar maturity (10–
12Brix) and weight (2.0–2.5 kg) were purchased in a
local market. The samples were hand-peeled and cut
into slices (30 · 50 · 5 mm) using a cutter designed for
this purpose.
2.2. Osmotic dehydration
Papaya slices, previously weighed and identified, were
immersed in the osmotic solutions (commercial sucrose
and solid corn syrup from corn syrup solid MORREX 1940 from Corn Products, Brazil) of given concentration (44–56% w/w) and temperature (34–46 C)
during a given immersion time (120–210 min). The studied range of immersion time was defined by a previous
kinetics in the central point of concentration and temperature (50% w/w and 40 C) for a period of 8 h, for
both osmotic agents. Therefore, identifying by the kinetic curves the region where there was no significant
variation in the drying rates for both agents, it was possible to find the more appropriate range of immersion
time to this study. A fruit:solution ratio of 1:10 was used.
The osmotic process was carried out in a shaker (TECNAL TE-421) at a constant agitation of 80 rpm. After
removing from the solution, the dehydrated slices of
each group were drained and blotted with absorbent
paper to remove the excess solution. The moisture content of the samples was gravimetrically measured using a
vacuum oven (635 mmHg) at 60 C for 24 h. The weight
and moisture content data of each sample were utilized
in order to calculate the response variables weight reduction (WR), water loss (WL) and solids gain (SG),
according to the following equations:
WR ð%Þ ¼
ðwi wf Þ
100
wi
ðwi X i wf X f Þ
100
wi
Xi
Xf
wi 1 100
wf 1 100
100
SG ð%Þ ¼
wi
WL ð%Þ ¼
ð1Þ
ð2Þ
ð3Þ
where wi and wf are the initial and final (time t) samples
weights, respectively, (g); Xi and Xf are the initial and
final (time t) samples moisture content, respectively, (g
water/100 g initial wet papaya).
Solutions and samples water activity (aw) were measured with a water activity meter (AquaLab Series 3
TE) at 25 C with an accuracy of ±0.003.
2.3. Experimental design and statistical analysis
According to Table 1, two factorial experimental designs were used with three independent variables whose
levels varied from 44% to 56% w/w for syrup concentration, from 34 to 46 C for temperature and from 120 to
210 min for immersion time, which required 11 experiments, including the center point. The responses of both
designs were weight reduction, water loss, solids gain
and water activity. Each experimental run was performed in triplicate.
It was assumed that a mathematical function u exists
for the response variable Y (WR, WL, SG and aw) in
terms of three independent process variables (Khuri &
Cornell, 1996), temperature, concentration and time:
Y ¼ uðT ; C; tÞ ¼ b0 þ b1 T þ b2 C þ b3 t þ b12 T C
þ b13 T t þ b23 C t
ð4Þ
where b is the equation coefficient and the subscripts 0,
1, 2, 3, 12, 13 and 23 correspond to the mean value of
the function u, temperature, concentration, immersion
time, interaction between temperature and concentration, interaction between temperature and immersion
time and interaction between concentration and immersion time, respectively.
In order to obtain the regression coefficients an analysis of variance (ANOVA) using the Statistica 5.0 (Statsoft, 1997) package was performed.
3. Results and discussion
3.1. Fitting models
Regression coefficients for the coded first-order polynomial equations are displayed in Table 2. The resulting
Â.A. El-Aouar et al. / Journal of Food Engineering 75 (2006) 267–274
269
Table 1
Experimental data for weight reduction, water loss, solids gain and water activity under different treatment conditions of solution temperature and
concentration and immersion time
Treatment
Sucrose
Corn syrup
T (C)
Ca(% w/w)
t (min)
WR (%)
WL (%)
SG (%)
aw
WR (%)
WL (%)
SG (%)
aw
34(1)
34(1)
34(1)
34(1)
46(+1)
46(+1)
46(+1)
46(+1)
40(0)
40(0)
40(0)
44(1)
56(+1)
44(1)
56(+1)
44(1)
56(+1)
44(1)
56(+1)
50(0)
50(0)
50(0)
120(1)
120(1)
210(+1)
210(+1)
120(1)
120(1)
210(+1)
210(+1)
165(0)
165(0)
165(0)
21.11
31.69
25.16
40.75
18.19
34.24
19.61
38.14
28.97
29.84
28.94
32.37
41.35
39.14
52.05
32.66
45.76
37.40
52.82
41.19
41.84
41.21
11.26
9.66
13.98
11.31
14.47
11.52
17.78
14.68
12.22
12.00
12.28
0.971
0.965
0.959
0.943
0.965
0.959
0.951
0.939
0.958
0.959
0.958
10.59
18.08
14.44
22.96
14.74
22.31
21.60
32.32
19.32
19.25
19.27
16.10
22.75
20.87
30.24
18.70
28.20
28.30
39.56
24.79
24.69
24.75
5.51
4.67
6.43
7.28
3.96
5.89
6.70
7.24
5.47
5.44
5.48
0.985
0.985
0.982
0.979
0.982
0.979
0.978
0.974
0.980
0.980
0.981
a
Water activity of sucrose solutions at 25 C: 0.954 (44% w/w); 0.939 (50% w/w); 0.920 (56% w/w). Water activity of corn syrup solutions at 25 C:
0.966 (44% w/w); 0.959 (50% w/w); 0.940 (56% w/w).
Table 2
Values of coded first-order polynomial regression coefficients
Coefficients
b0
b1
b2
b3
b12
b13
b23
Sucrose
Corn syrup
WR
WL
SG
aw
WR
WL
SG
aw
28.7854
1.0655
7.5942
2.3021
1.0512
0.9721
0.9349
41.6178
ns
6.3027
3.6582
0.8295
0.7094
0.7810
12.8324
1.5301
1.2916
1.3561
0.2217
0.2627
ns
0.9569
0.0028
0.0051
0.0084
ns
ns
0.0020
19.5330
3.1115
4.2899
3.1995
0.2846
1.0176
0.5226
25.3585
3.0994
4.5989
4.1536
0.5924
1.0885
0.5597
5.8255
ns
0.3091
0.9540
0.3078
0.0709
0.0372
0.9804
0.0020
0.0015
0.0023
0.0005
ns
ns
ns: non-significant (p > 0.05).
equations for both experimental designs were tested for
adequacy and fitness by analysis of variance (ANOVA)
according to Tables 3 and 4.
To convert the real values that include the studied
range of the independent variables in coded ones,
according to Eq. (4), the following equation can be used:
CV ¼
RV CP
DRVCP
It is important to point out that, for the analysis of
variance shown in Tables 3 and 4, the largest contribution in the residual value was due to the lack of fit, indicating that the experimental data presented good
reliability.
3.2. Influence of process variables
ð5Þ
where CV is the coded value; RV is the real value; CP is
the real central point value; DRVCP is the step change at
the central point.
Tables 3 and 4 summarize the results for linear and
cross-product terms for weight reduction, water loss,
solids gain and water activity for each solute; some
non-significant terms (p > 0.05) were eliminated, in
agreement with Table 2. According to Table 3, the fitted
models were significant (p 6 0.05), possessing low residual values and satisfactory values of multiple determination coefficients. By Table 4, only the model for the
solids gain was not predictive, presenting significant lack
of fit, therefore, the curves generated from this model
can only show the behavior of each independent variable on the responses.
Figs. 1 and 2 show the influence of concentration and
temperature of the osmotic solution and immersion time
on the weight loss, loss of water, solids gain and water
activity during the osmotic dehydration of papaya in sucrose and corn syrup, respectively.
According to Figs. 1(a) and (b), the osmotic solution
concentration was the most important effect on weight
reduction and water loss, followed by the immersion
time, and they positively affected these responses for
the sucrose agent. The effect of temperature of the osmotic solution practically did not influence the weight
reduction and it was negative. The weight reduction
exhibits a mass relationship among the whole flows involved in the osmotic process (mainly between water
loss and solids gain) and due to this which is more pronounced in the solids gain instead of the the water loss,
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Â.A. El-Aouar et al. / Journal of Food Engineering 75 (2006) 267–274
Table 3
Analysis of variance for weight reduction (WR), water loss (WL), solids gain (SG) and water activity (aw) in the osmotic dehydration of papaya in
sucrose solutions
Source
SS
DF
MS
Fc
Ft
Regression
Residual
Lack of fit
Pure error
Total
Weight reduction (WR)
536.2523
2.2065
1.6833
0.5232
538.4588
6
4
2
2
10
89.3754
0.5516
–
–
–
162.02
–
–
–
–
6.16
–
–
–
R2 = 0.9959
Regression
Residual
Lack of fit
Pure error
Total
Water loss (WL)
439.2592
2.5014
2.2267
0.2748
441.7606
5
5
3
2
10
87.8518
0.5003
–
–
–
175.60
–
–
–
–
5.05
–
–
–
R2 = 0.9943
Regression
Residual
Lack of fit
Pure error
Total
Solids gain (SG)
47.7299
2.1607
2.1192
0.0414
49.8905
5
5
3
2
10
9.5460
0.4321
–
–
–
22.09
–
–
–
–
5.05
–
–
–
R2 = 0.9567
Regression
Residual
Lack of fit
Pure error
Total
Water activity (aw)
8.6E-04
1.4E-05
1.3E-05
6.7E-07
8.8E-04
4
6
4
2
10
2.2E-04
2.4E-06
–
–
–
91.50
–
–
–
–
4.53
–
–
–
R2 = 0.9839
SS: sum square; DF: degree of freedom; MS: mean square; Fc: calculated F distribution value (p 6 0.05); Ft: tabulated F distribution value (p 6 0.05).
the negative effect of temperature found on weight
reduction could be explained. The temperature did not
significantly influence the water loss. Values of 40%
for WR and of 52% for WL can be obtained when the
concentration and the immersion time are in their highest levels, considering the whole temperature range,
practically.
Observing Fig. 1(c), for sucrose, the temperature was
the most important effect affecting solids gain, followed
by the immersion time. These effects positively influenced the solids gain for the osmotic process of papaya
in sucrose solutions. It is important to emphasize the
negative effect of the concentration in solids gain for sucrose. According to Heng, Guilbert, and Cuq (1990), it
should be mentioned that, in this case, the sugar gain
is less important. Generally speaking it appears that little sugar is obtained when the water outflow is fast and
significant. This occurs with a higher temperature, higher concentration and some agitation. Acceleration of
water loss without modification of solids gain, when
temperature or dehydration solution concentration is increased, is observed by many authors (Bongirwar &
Sreenivasan, 1977; Hawkes & Flink, 1978; Islam &
Flink, 1982). This effect is generally attributed to the
influence of natural tissue membranes as well as to the
diffuse properties of water and solutes as a function of
their respective molar mass. Low values of solids gain
(<10%) can be obtained using temperature of the osmotic solution and immersion time in their lowest levels,
with the concentration in its highest level.
Fig. 1(d) shows that the immersion time was the most
important effect on water activity, followed by concentration and temperature. All the effects were negative
on water activity for sucrose solutions. Values of aw
inferior to 0.945 can be obtained using the independent
variables in their highest levels.
In agreement with Fig. 2(a) and (b), the osmotic solution concentration was the most important effect on
weight reduction and water loss, followed by the immersion time and temperature of osmotic solution, and they
positively affected these responses for corn syrup. Values
of 27% for WR and of 35% for WL can be obtained
when the independent variables are in their highest
levels.
Considering the corn syrup agent, the temperature
did not influence the solids gain but it was positively
and strongly affected by the immersion time and with
less intensity by the solution concentration (Fig. 2(c)).
Fig. 2(d) shows that the immersion time and the temperature of the osmotic solution were the most important effects on water activity. All the effects were
negative on water activity for corn syrup solutions. Values of aw, inferior to 0.978 can be obtained using the
independent variables in their highest levels.
Â.A. El-Aouar et al. / Journal of Food Engineering 75 (2006) 267–274
271
Table 4
Analysis of variance for weight reduction (WR), water loss (WL), solids gain (SG) and water activity (aw) in the osmotic dehydration of papaya in
corn syrup solutions
Source
SS
DF
MS
Fc
Ft
Regression
Residual
Lack of fit
Pure error
Total
Weight reduction (WR)
317.6855
0.8274
0.8250
0.0024
318.5129
6
4
2
2
10
52.9476
0.2068
–
–
–
255.98
–
–
–
–
6.16
–
–
–
R2 = 0.9974
Regression
Residual
Lack of fit
Pure error
Total
Water loss (WL)
398.8631
1.6884
1.6833
0.0051
400.5515
6
4
2
2
10
66.4772
0.4221
–
–
–
157.49
–
–
–
–
6.16
–
–
–
R2 = 0.9958
Regression
Residual
Lack of fit
Pure error
Total
Solids gain (SG)
8.8553
1.7202
1.7193
0.0008
10.5754
5
5
3
2
10
1.7711
0.3440
–
–
–
5.15
–
–
–
–
5.05
–
–
–
R2 = 0.8373
Regression
Residual
Lack of fit
Pure error
Total
Water activity (aw)
9.3E-05
2.2E-06
1.9E-06
2.2E-07
9.5E-05
4
6
4
2
10
2.3E-05
3.6E-07
–
–
–
64.42
–
–
–
–
4.53
–
–
–
R2 = 0.9772
SS: sum square; DF: degree of freedom; MS: mean square; Fc: F distribution calculated (p 6 0.05); Ft: F distribution tabulated (p 6 0.05).
In agreement with Table 1, for the same solute concentration, the osmotic potential of sucrose solutions
is larger than corn syrup solutions. To compare between
both osmotic agents it is necessary to use solutions with
the same initial osmotic potential because water activity
is the driving force of the osmotic process.
According to Lewicki and Lenart (1995), excess pressure needed to reach the state of equilibrium between a
pure solvent and a solution is called osmotic pressure. In
the equilibrium state,
solution
Chemical potentialsolvent
solvent P ¼ Chemical potentialsolvent P
1
2
ð6Þ
Since in foods, water is the solvent, Eq. (7) can be
used:
P ¼ 4.6063 105 T lnðaw Þ
ð7Þ
where P is the osmotic pressure (Pa); aw is water activity
and T is the absolute temperature (K).
Fig. 3 illustrates the higher osmotic pressure of sucrose solutions compared to corn syrup ones, in terms
of the concentration of osmotic solution, using the water
activity data from Table 1.
Observing Fig. 3, it is possible to obtain a relationship between both curves considering the same osmotic
potential. It results in the following equation:
1.01
C Sucrose ¼ 0.83C Corn
syrup
ð8Þ
where C corresponds to the real value of osmotic solution concentration. Obeying the established relationship
in Eq. (8), it is possible to obtain the pairs of solution
concentrations for both osmotic agents corresponding
to the same osmotic pressure.
Taking into account Eq. (8) and according to Figs.
1(a) and 2(a) for an osmotic solution temperature of
34 C and an immersion time of 120 min, the difference
between the weight reduction values of both osmotic
agents, for the same osmotic pressure, indicates that
samples dehydrated in corn syrup solutions had approximately a weight reduction 30% less than samples processed in sucrose solutions. Considering an osmotic
solution temperature of 46 C and an immersion time
of 210 min, samples dehydrated in sucrose solutions
had approximately a weight reduction 20% less than
samples processed in corn syrup solutions due to the
small effect of temperature in this response variable
using sucrose solutions.
Figs. 1(b) and 2(b) show that the differences between
the water loss value of both osmotic agents, for the same
temperature, immersion time and osmotic pressure, indicate that samples dehydrated in corn syrup solutions
had approximately a water loss of 7–40% less than samples processed in sucrose solutions.
272
Â.A. El-Aouar et al. / Journal of Food Engineering 75 (2006) 267–274
Fig. 1. Influence of concentration and temperature of osmotic solution and immersion time on (a) WR, (b) WL, (c) SG and (d) aw for papaya
osmotic dehydration with sucrose solutions.
In agreement with Figs. 1(c) and 2(c), the differences
between the solids gain value of both osmotic agents, for
the same temperature, immersion time and osmotic
pressure, indicate that samples dehydrated in corn syrup
solutions had approximately a solids gain of 53–56% less
than samples processed in sucrose solutions.
According to Figs. 1(d) and 2(d), it was observed that
samples dehydrated in sucrose solutions reached a water
activity value of 1–3% less than samples processed in
corn syrup solutions.
On the whole, samples dehydrated in sucrose solutions had values of WR, WL and SG higher than those
Â.A. El-Aouar et al. / Journal of Food Engineering 75 (2006) 267–274
273
Fig. 2. Influence of concentration and temperature of osmotic solution and immersion time on (a) WR, (b) WL, (c) SG and (d) aw for papaya
osmotic dehydration with corn syrup solutions.
obtained for samples processed in corn syrup solutions,
except for aw, even when considering the same osmotic
pressure for both osmotic agents. The fact is that the
corn syrup solutions had visually a higher viscosity than
the sucrose ones.
The use of highly concentrated viscous sugar solutions creates major problems such as floating of food
pieces, hindering the contact between the food material
and the osmotic solution, causing a reduction in the
mass transfer rates. Mavroudis, Gekas, and Sjöholm
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Â.A. El-Aouar et al. / Journal of Food Engineering 75 (2006) 267–274
Osmotic pressure (Pa)
1.2x107
ever, the osmotic dehydration in corn syrup solutions
led to higher values of water activity.
Sucrose
Corn syrup
1.1x107
1.0x107
Acknowledgements
9.0x106
The authors acknowledge CAPES (Coordination of
Perfectioning Superior Level Staff) and Corn Products,
Brazil.
8.0x106
7.0x106
6.0x106
5.0x106
References
6
4.0x10
44
46
48
50
52
54
56
Concentration (% w/w)
Fig. 3. Relationship between concentration and osmotic pressure of
sucrose and corn syrup osmotic solutions.
(1998) affirm that the increase of agitation level could be
a good alternative for this case. In early works (Bongirwar & Sreenivasan, 1977; Ponting, Watters, Forrey,
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Chemical and physical properties of corn syrup solids
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Moisture content max. 5.0%; dextrose equivalent 38–
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Katsanidis, and Nickolaidis (1995), overall mass
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molecular size of the osmotic solute. The larger the solute size, the lower the sugar uptake under fixed process
conditions.
4. Conclusions
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on weight reduction, water loss, solids gain and water
activity during the osmotic dehydration of papaya slices
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