Osmotic Dehydration (OD), immersing food samples in osmotic solutions (such
as sucrose, glucose, corn syrup, maltose, sorbitol, etc.) is a viable process
for the partial removal of water from cellular material (such as fruits and
vegetables) without a phase change. The water from the food flows towards the
solution and, in an inverse sense, the solids from the solution to the product.
The type of osmotic agents always plays an important role in the osmotic dehydration
affecting the mass transfer of water and solids (Tregunno
and Goff, 1996; Panagiotou et al., 1999)
and Product characteristics (Forni et al., 1997;
Talens et al., 2003). The mass transfer is also
depended on the nature of the plant tissue and the process variables (Lenart
and Flink, 1984; Rahman and Lamb, 1990; Raoult-Wack
et al., 1991; Kaymak-Ertekin and Sultanoglu,
2000; Giraldo et al., 2003; Mujica-Paz
et al., 2003; Uddin et al., 2004).
The advantages obtained from osmotic dehydration include: providing the inhibition
of enzymatic browning and preventing volatile flavoring loss; providing the
required range of water and solute in food material to further processing; minimization
of thermal stress; a reduction of energy input compared to conventional drying
and freezing processes; enhancement of sensory quality; providing a fresh-like
state of raw material as Intermediate Moisture Food (IMF), etc. It is often
applied as a pretreatment to process certain food products, for example dried
products (Chottanom and Phoungchandang, 2005; Lombard
et al., 2008). Recently, many studies found a potential use of osmotic
dehydration for limiting biological compound loss during further processing
(Shi et al., 1999; Heredia
et al., 2009). For frozen products, it is an important pretreatment
for fruit freezing (Forni et al., 1997; Talens
et al., 2003). Low molecular weight sugars or sugar alcohol such
as sorbitol and xylitol are often added to food systems to give plasticizing
or cryostabilizing effects (Kim et al., 2004).
The osmotic dehydration presents clear advantages for preserving the quality
of thawed products, showing preferable measured-quality of color, drip loss
The aim of this study was to apply the osmotic dehydration to tomato freezing.
The influence of osmotic solution types on mass transfer and melting temperature
(Tm; Differential Scanning Calorimetry (DSC) method) of tomato was
analyzed. Other objectives were to monitor an alteration of lycopene content
during osmotic dehydration and frozen product quality after freezing.
MATERIALS AND METHODS
Tomato: Tomato (Lycopersicon esculentum Mill) samples located
in the north-eastern of Thailand were used. All tomatoes used in this study
were delayed for less than 48 h in a refrigerator at 4°C. For purposes of
analysis, two groups of tomatoes were established, a control group of fresh
tomatoes and an osmosed group. The tomatoes in each group were peeled, deseeded
and cut into 4 pieces and the Moisture Content (MC) (AOAC,
1990) of both groups was determined.
Dehydrofrozen procedure: The tomato pieces (180-200 g/container) in
the osmosed group were soaked in three osmotic solutions with the concentration
of 35 and 50% (w/w) for the maltose solution, 35 and 60% (w/w) for the sucrose
solution and 35 and 60% (w/w) for the sorbitol solutions. An OD time was 2 and
6 h. The mass ratio of tomato and solution was 1:6. The temperature of solution
and checking velocity was controlled at 35°C and 150 rpm, respectively.
Each sample was drained and then vacuum packed in a poly ethylene pouch. Batches
of the packaged samples were subjected to a conventional freezer at -18°C
for 4 months.
Freezing time estimation: The fresh and the osmosed tomatoes, soaked
for 6 h, were frozen (140-150 g for each group) to -18°C in an air blast
freezer at -35°C (iRiNox New HC series, Italy). The evolution
of the core temperature of samples was recorded by using a data logger to estimate
the freezing time required to reduce the temperature of sample from 21 to -18°C
(Karel et al., 1975).
Melting temperature (Tm) determination: The fresh and the
osmosed tomatoes with the solid mass fraction ranged from 0.05 to 0.50 were
used. The onset melting temperature of the samples was determined by using a
differential scanning calorimeter (Perkin-Elmer Pyris Diamond, USA). The instrument
was calibrated for temperature and heat flow using distilled water and indium.
Helium gas (99.99% purity) was used as the purge gas at a pressure of 20 lbs
in-2 (flow rate 40.0 mL min-1). The fresh control group
tomatoes and the osmosed group tomatoes were weighed in 40 μL aluminum
pan (PE NO 02190041). All samples were cooled with liquid nitrogen to -50°C,
held for 3 min and then heated at 10°C min-1 with a sample size
typically in the range of 6-8 mg.
Relationship between Tm and solid content values (mass fraction)
was evaluated. The polynomial equation has been used to describe the correlation
between Tm and water fraction of fresh tomato, r = 0.993 (Telis
and Sorbral, 2002) and between the initial freezing point and water fraction
of some fruits and vegetables (Dickerson, 1986). In this
study, the Tm values of each tomato sample were related to solid
content values (mass fraction) by using second-order polynomial equation as
where b0, b1 and b2 are the constant. Xs
is the solid fraction.
Mass transfer estimation: The tomatoes were determined the mass transfer
after osmotic dehydration process for 2 and 6 h. The parameters of mass transfer,
Weight Reduction (WR), Water Loss (WL) and Solid Gain (SG), during osmotic process
were estimated by using Eq. 2, 3 and 4,
respectively and expressed in g per 100 g initial sample as follows:
where m(t) and m(0) are the mass of tomato at time t
and the initial mass of tomato, respectively. W(0) and S(0)
are the initial water content (mass fraction) and solid content (mass fraction)
of the tomato, respectively. W(0) and S(t) are the water
content (mass fraction) and solid content (mass fraction) of the tomato at time
Color measurement: Color measurements of the 6 h osmosed tomatoes were
performed by using a Minolta color meter (CR-300). The coordinates of the color
CIE-L*a*b* of the tomato surface were obtained
by reflection. L*, a* and b* represent the
lightness, redness and yellowness values, respectively.
Lycopene measurement: The lycopene content (mg g-1 total
solids) of the 6 h osmosed tomatoes were spectrophotometrically determined on
extracts in petroleum ether in triplicate at 505 nm (Gould
and Gould, 1988) using a UV-Visible spectrophotometer (Milton Roy Spectronic
1201, USA). The lycopene content was quantified by using a standard curve of
95% purified lycopene (Sigma Chemical Co., St. Louis, USA) dissolved in petroleum
Drip loss measurement: After thawing at 20°C in a temperature-controlled
bath, each sample (6 h osmosed tomatoes) was removed from the pouch leaving
behind the drip. The pouch containing the drip was then weighted. Drip loss
was computed from the weight of drip and that of the sample and expressed as
a percentage loss based on the initial sample weight.
Sensory attributes: To quantity the sensory attributes, the thawed samples
were subjected to sensory analysers. The samples were subjectively rated by
16 sensory panels on the scale of 1-5 as follows: 1-very good; 2-good; 3-fair;
4-poor; 5-very poor.
Experimental design: A factorial in completely randomized design (23
factorial experiments) was used to evaluate the effect of osmotic solution type,
concentration and OD time on the mass transfer of tomatoes. A completely randomized
design was used for the other experiment. Each experiment was conducted with
three replications. The statistical analysis was processed by using software
of the package program, SPSS version 14.0 (SPSS Inc., Thailand). Analysis of
variance was performed by ANOVA procedures. Significant difference between experimental
means was determined by using the Duncans multiple range tests and the
Paired sample T-test. A significance of differences was defined at p<0.05.
RESULTS AND DISCUSSION
Table 1 shows the physical and chemical properties of the
sample tomatoes. The red and firm attribute (well developed) was used. The initial
moisture content of fresh tomato varied from 93 to 96%. The weight and hardness
values of the tomatoes were quite varied compared to other properties, especially
the moisture content and soluble solids (measured by a hand refractometer).
The moisture content, soluble solids and redness values were the major properties
for choosing the tomato samples in order to control the variation affecting
on mass transfer and product characteristics during osmotic dehydration.
Mass transfer parameters: Table 2 shows the effect
of osmotic solutions and osmotic dehydration time on the mass transfer of the
osmosed tomatoes. Each type of osmotic solutions had different potential to
decrease the initial moisture content and to increase solid content of the osmosed
|| Physical and chemical properties of fresh tomatoes
|*Measured by using an ATAGO hand-held refractometer
|| Mass transfer of water and solids of tomatoes during osmotic
|*g/ 100 g sample, Different letters in a column indicate significant
|| The b*/a* values of osmosed tomatoes in maltose, sucrose
and sorbitol solutions for 6 h osmotic dehydration
|*Paired samples T-test (p<0.05)
Sucrose and sorbitol solutions with 60% and 6 h soaking were the most effective
agents for water removal from tomato tissue but the sorbitol promoted two times
more solid gain into the tissue than that of sucrose, because of its low molecular
Sucrose and sorbitol solutions with a molecular weight of 342 and 182, respectively,
showed a potential for water removal during osmotic dehydration, compared with
a maltose solution with a molecular weight of 360.23. During osmotic dehydration,
low molecular weight solutions had higher corresponding osmotic pressure (Saurel
et al., 1994), which could flavor plant cell plasmolysis and enhance
water removal from tissue samples. In a similar result, Panagiotou
et al. (1999) found that, glucose seems more effective than sucrose
in the water loss and solid gain of apples, bananas and kiwi. The 40% glucose
concentration gave a higher water loss and solid gain than a sucrose concentration
at up to 50% of the same condition. Based on literature studied, it is known
that the mass transfer rate in osmotic dehydration is influenced by the two
major variables, process conditions (osmotic agent, concentration, temperature,
medium velocity, contact time, etc.) and the structure of biological material.
Generally, higher solid gains into food tissue are not required in frozen fruits
and vegetables because it adulterates the natural flavor of the products. Initially,
maltose solution and sucrose solution should be recommended for tomato freezing.
The samples with high WR and/or WL/SG values, soaked in 60% of sorbitol and
sucrose, should not be frozen because of tissue shrinkage. Generally, loss of
large amount of water and gain of small amount of solids into tissue lead to
the tissue shrinkage. Therefore, 6 h osmotic dehydration using 60% of sorbitol
and sucrose should be avoided for the tomato preparation before freezing.
Color parameters: Table 3 shows that the osmotic dehydration
did not clearly affect the color of tomato osmosed in maltose solution (L*,
a* and b* values are not shown). However, lower values
of the ratio of yellowness and redness (b*/a*) of osmosed
samples were found, particularly decreasing in the b*/a*
value of the osmosed samples in sucrose and sorbitol solutions. In this study,
the increase of coordinates a* was higher than the increase of coordinates
b*, contrasting to the color alternation of osmosed cherry tomato
reported by Heredia et al. (2009). Generally,
an increase of the chromatic coordinates a* and b* can
be promoted to the concentration of the liquid phase and the pigments in the
cellular tissue as a consequence of the osmotic dehydration.
Lycopene content: Figure 1 shows the effect of osmotic
solutions on the lycopene retention as osmotic progress. Maltose (50% concentration)
and sucrose (35% concentration) uptake in the tissue was the protective action
of lycopene in osmotic dehydration, compared to sorbitol and sucrose with 60%
concentration. The high concentration of sucrose and sorbitol solutions caused
the reduction of lycopene content compared to the low concentration. One possible
explanation may be that the effect of high osmotic pressure causes tissue damage,
inducing oxidation of lycopene.
||Lycopene content of (1) fresh tomato and (2) osmosed tomatoes
in 50% maltose, (3) 35% sucrose, (4) 60% sucrose and (5) 60% sorbitol solutions
for 6 h osmotic dehydration. Different letters indicate significant differences
|| Influence of osmotic solutions on freezing time of tomatoes
for 6 h osmotic dehydration
|Different letters indicate significant differences (p<0.05)
A similar result on the decrease of lycopene caused by a high concentration
of salt solution (20%) was reported by Heredia et al.
(2009). Maltose was reported to have the highest protective effect on chlorophyll
stability during storage at -10°C for kiwi fruits (Torreggiani
et al., 1993) and on color and ascorbic acid stability of apricots
(Forni et al., 1997).
Freezing time and melting temperature (Tm): Table
4 shows the freezing time of osmosed tomatoes. The effect of osmotic solution
on the freezing time could be observed. Even though small amount of time difference
was detected. This is possibly due to the high rate of shock freezing and low
volume of samples used in this study (Shock freezing mode can reduce the core
temperature of samples to -18°C within 4 h depending on type, thickness,
initial temperature, number and package of sample). The reduction of freezing
time by osmotic solutions could be explained that, the super cooling phenomenon
of water in tomatoes was influenced by the osmotic solids in the tissue which
promoted heterogenous nucleation, thereby accelerating the nucleation process.
Figure 2 shows the on set melting temperature (Tm)
of the tomato samples. The melting temperature of the tomatoes was shifted by
the osmotic solutions and there solid fraction values. As expected, the melting
temperature decreased with increasing solid content.
In the report of Baek et al. (2004), who were
analyzing the starch-sugar melting temperature by DSC method, it was observed
that the melting temperature decreased as proportional to molar concentration
of sugar and that the differences among the sugars (monosaccharides, hexoses
and pentoses) were minor.
||Melting temperature (Tm) of fresh ()
and osmosed (
sorbitol) tomatoes vs. solid fraction values
However, the melting temperature data of osmosed tomato were not colligatively
governed, showing a dependence on sugar structure.
Interestingly, the melting temperatures of the tomatoes osmosed in sucrose
and maltose solutions were lower than that of the fresh tomatoes and the tomatoes
osmosed in sorbitol solution, linking to unfrozen liquid level. Considering
that the empirical Eq. 5 to 8 fitted well
the experiment points of Tm of the fresh tomato (Eq.
5) and the tomato osmosed in maltose (Eq. 6), sucrose
(Eq. 7) and sorbitol (Eq. 8) solutions,
In a similar study, Cornillon (2000) found the influence
of solution concentration on the freezing point of an apple sample. They concluded
that a change of the freezing point of dehydrated fruits and the associated
enthalpy of crystallization was a function of the amount of sugar present in
the fruit. Telis and Sorbral (2002) found that the melting
point could be related with water fraction in tomato by using a second-order
polynomial model as well.
Sensory attribute and drip loss: The frozen tomatoes tested were color,
appearance, texture and overall acceptability. Figure 3 indicates
sensory scores and drip loss values of the product frozen 4 months frozen-storage
at -18°C. It is very clear from the results that the frozen tomato using
maltose was most acceptable (p<0.05), because of the good appearance and
low value of drip loss. The use of sucrose provided the highest drip loss that
was not different to the control (fresh-treatment).
||Sensory scores and Drip-loss values of (1) fresh tomato and
(2) osmosed tomatoes in 50% maltose, (3) 35% sucrose, (4) 60% sucrose and
(5) 60% sorbitol solutions for 6 h osmotic dehydration. Different letters
in each bar indicate significant differences (p<0.05)
Mass transfer during osmotic dehydration and characteristics of tomatoes was
influenced by the osmotic agent types and their concentration. The 50% maltose
was recommended for the frozen tomatoes, compared to sucrose and sorbitol, because
of their positive effect on WL/SG, color, lycopene retention, melting temperature
(linking to unfrozen liquid level) and good product quality.
Authors acknowledge the financial support received from Mahasarakham University,