Mass transfer in osmotically dehydrated apple stored at temperatures above zero

Available online at www.sciencedirect.com

Journal of Food Engineering 86 (2008) 140–149 www.elsevier.com/locate/jfoodeng

Mass transfer in osmotically dehydrated apple stored at temperatures above zero Anna Kamin´ska, Piotr P. Lewicki *, Paweł Malczyk Department of Food Engineering and Process Management, Faculty of Food Technology, Warsaw Agricultural University (SGGW), Nowoursynowska 159c, 02-776 Warsaw, Poland Received 11 June 2007; received in revised form 31 August 2007; accepted 18 September 2007 Available online 25 September 2007

Abstract Material dehydrated in sucrose solution by osmosis for 3 h at 30 °C and for 1 h at 70 °C was stored for 24, 72 and 144 h at 5, 9, 15 and 20 °C. Internal gradients of dry matter and sucrose concentration caused mass transfer in the material. Dry matter content in the surface layers was close to 40%, while sucrose concentration was between 20% and 23%. At the distance of 10 mm from the mass transfer surface dry matter content was still higher than that in raw apple, but sucrose concentration was that of raw apple. Concentration profiles of sucrose changed with time and storage temperature. The higher was the temperature the faster was the diffusion of sucrose. In material osmosed at 30 °C for 3 h and stored for 72 h at 5, 9 and 15 °C sucrose concentration gradients were still present. However, after 144 h at 9 °C the concentration of sucrose became independent on the distance from the mass exchange surface. Diffusion of sucrose in material osmosed at 70 °C for 1 h was much faster than that observed in samples osmosed at lower temperature. The effective diffusion coefficient of sucrose was dependent on temperature and time of storage, and distance from the mass transfer surface as well. Since distance was related to concentration a relationship between sucrose concentration and effective diffusion coefficient was found. The effective diffusion coefficient was of the order of 109–1012 m2/s. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Apple; Osmotic dehydration; Mass transfer; Diffusion coefficient; Sucrose concentration profile

1. Introduction The research and development on food preservation methods concentrates on processes during which the structure of the product does not deteriorate and its nutritional value is maintained. Osmotic dehydration is one of such mild processes. It is based on the removal of water from the material of interest without a change in phase (Fito et al., 2001; Lenart, 1990). This water removal process lead into concentrated solutions of soluble solids having higher osmotic pressure and lower water activity (Kaymak-Ertekin & Sultanoglu, 2000). The permeability of plant tissue is low to sugars and high molecular weight compounds, hence the material *

Corresponding author. Tel.: +48 22 5937560; fax: +48 22 5937576. E-mail address: [email protected] (P.P. Lewicki).

0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.09.020

is impregnated with the osmoactive substance in the surface layers only. Penetration of an osmoactive substance, except sodium chloride, is limited to 2–3 mm whereas changes in water content are observed up to depth of 5 mm (Bolin, Huxsoll, & Jackson, 1983; Lenart, 1986; Lenart & Lewicki, 1981). However, osmotic dehydration is only a type of pre-treatment, since it does not ensure that the product will possess sufficient durability (shelf life) and microbiological stability (Lewicki & Lenart, 2007). Therefore, combination of osmotic pre-treatment with the cooling and freezing processes may bring about positive results. The method of ‘‘dehydrofreezing” (D-F) is based on the initial removal of water from the product (loss of mass is close to 50%), followed by freezing (LaBelle & Moyer, 1966). Reduction in mass can be accomplished through drying or osmotic dehydration. Osmotic dehydration is more economical method than

A. Kamin´ska et al. / Journal of Food Engineering 86 (2008) 140–149

141

Nomenclature A a b c D d d.m. f g l m

constant constant constant sucrose concentration (%) effective diffusion coefficient (m2/s) constant dry matter content constant constant thickness of the sample (m) mass (g)

drying and the quality of partly dewatered product is better. The loss of heat labile components and the unappealing changes in appearance and texture of the end product are prevented, as well. The final effect is a frozen product with a decreased mass and volume, characterized by pleasant appearance after thawing. Storage at cooling conditions after osmotic dehydration is a part of research on ‘‘dehydrofreezing” method and can extend shelf-life of the product as well. Osmotic dehydration is a complex process of countercurrent mass transfer between the plant tissue and hypertonic solution. The flux of osmoactive substance penetrating the osmosed tissue changes its chemical composition. There is also a flux of native substances leaving the tissue (Lewicki & Lenart, 2007). The osmotic pressure gradient is the driving force for osmotic mass transfer. This driving force depends on the concentration and temperature of the osmotic solution. An increase in temperature or concentration, under otherwise similar conditions, results in an increase in osmotic pressure gradient, resulting in an increased mass transfer and thereby higher values of effective diffusion coefficients (Mastrantonio, Pereira, & Hubinger, 2006; Mayor, Moreira, Chenlo, & Sereno, 2006; Rastogi, Raghavarao, & Niranjan, 1997; Sereno, Moreira, & Martinez, 2001). Influence of the main process variables, such as concentration and composition of the osmotic solution, temperature, immersion time, pre-treatment procedures, agitation, nature of food and its geometry, solution/sample ratio on the mass transfer mechanism and the product quality have been studied extensively (Aktas, Fujii, Kawano, & Yamamoto, 2006; Beristain, Azuara, Cortes, & Garcia, 1990; Contreras & Smyrl, 1981; Islam & Flink, 1982; Mastra´ngelo, Rojas, Castro, Gerschenson, & Alzamora, 2000; Mavroudis, Gekas, & Sjo¨holm, 1998; Pereira et al., 2004; Rastogi & Raghavarao, 1994). The models based on the Fick’s second law do not necessarily simulate the osmotic dehydration process. Countercurrent fluxes of osmoactive substance, water and soluble components of the cell sap can interact and effective diffusivities of individual components will not represent events occurring during osmotic

x s T

distance from the dehydrated surface (m) time (s) temperature (K)

Subscripts f final n number of element in series o initial v weighing dish x distance s time

dewatering of plant tissue. Moreover, assumption that the whole resistance to mass transfer is in the solid is also a kind of unrealistic simplification. Reaction of living tissue to osmotic stress is also not taken into account in description and interpretation of osmotic dehydration (Lewicki & Lenart, 2007). Research on ‘‘dehydrofreezing” method conducted so far has concentrated mainly on the quality aspects of the final product. The mass transfer process inside osmotically dehydrated material stored at different temperature practically has not been studied. Mass transfer during storage time is important because of the following reasons:  After osmotic dehydration concentration of solutes in the surface layers is high and measured water activity is sufficiently low to assure limited storage stability. During storage concentration equilibration occurs and concentration of solutes in the surface layers decreases. Hence, water activity of the surface layers increases and the time of storage stability becomes shorter than it would be expected on the basis of the initial value of water activity.  After osmotic treatment structure of the material is altered mainly in the surface layers. Diffusion of solutes into the tissue causes further plasmolisis and changes in structure and texture of the osmosed material. Hence, quality of the osmosed material becomes function of storage time and temperature. Therefore, a time span between osmotic treatment and further processing becomes an important variable. Because of those expected changes undergoing in osmosed material during storage it is necessary to know how fast is the migration of solutes in the material. Moreover, tools to predict the time of solute concentration equilibration are needed. The aim of this study was to determine the profile of dry matter and sucrose concentration in apples osmotically dehydrated, and the effect of storage time and temperature above zero on the concentration equilibration process.

A. Kamin´ska et al. / Journal of Food Engineering 86 (2008) 140–149

142

2. Materials and methods 2.1. Materials Apples v. Idared were used. Slices (40  40 mm and 20 mm thick) were cut out from the flesh part of fruits.

was determined by a drying method according to Polish Standard, that is at 98 °C until constant weight was reached. Dry matter content was calculated based on the following equation: d:m: ¼

2.2. Osmotic dehydration Osmotic dehydration was conducted in 61.5% sucrose solution (Kaymak-Ertekin & Sultanoglu, 2000; Panagiotou, Karathanos, & Maroulis, 1999), in two variants: variant I – 3 h at 30 °C and variant II – 1 h at 70 °C. Osmotic solution concentration and time/temperature variables were chosen so as to obtain sucrose concentration profiles close each to other in osmosed apple. Apple slices were immersed in the preheated to prescribed temperature solution to a depth of 18 mm, leaving the upper surface in contact with air. Mass ratio of the osmotic solution to the dehydrated material was 4:1 w/w. The solution was slightly circulated during the process. After the specified dehydration time, the samples were separated from the osmotic solution, rinsed with spray of cold water and dried on a filter paper. Osmotic dehydration was performed separately for each storage temperature. 2.3. Storage of samples After osmotic dehydration samples were single packed into aluminum foil and stored in a refrigerator set to a prescribed temperature. The storage temperature/time relations are given in Table 1. 2.4. Determination of the spatial distribution of the dry matter content A cylinder 20 mm in diameter was cut out from the apple slice osmotically dehydrated. Then, using a device equipped with a micrometer screw, slices 0.5 mm thick were cut from the cylinders, beginning from the surface which was in direct contact with osmotic solution. The dry matter content was determined in slices cut at the following distance from the surface: 0–0.5; 0.5–1.0; 2.5–3.0; 4.5–5.0; 6.5–7.0; 9.5–10.0 mm. Dry matter content in those slices

Temperature (°C) 5

9

15

20

24 72 144 – – –

24 72 – 24 72 –

24 – – 24 – –

Time (h) 30 °C/3 h

70 °C/1 h

24 72 144 24 72 144

ð1Þ

2.5. Determination of the spatial distribution of sugar concentration The concentration of sugar was measured at the same time as the concentration of dry matter. Slices were cut out from apple cylinder the way described above. Sugar content was measured by colorimetric method using 3, 5-dinitrosalicylic acid (DNS reagent) (Toczko & Grzelin´ska, 1997). Simple sugars reduce the nitro group of 3,5-dinitrosalicylic acid to the amine group. Intensity of orange color given by amine groups depends on the amount of sugar in the sample. Each slice of apple (0.5 mm thick) was mixed separately with 20 ml of water. This mixture was heated at 90 °C for 30 min and then filtered. Then 2 ml of DNS reagent was added to 2 ml of filtered liquid and heated at 90 °C for 10 min. After cooling to room temperature in a cold water bath the absorbance was recorded with a spectrophotometer at 550 nm. To determine the amount of non-reducing sugars hydrolysis was done. To 5 ml of filtered solution, 1 ml of 6-M hydrochloric acid was added. The hydrolysis was done at the temperature of 90 °C for 30 min. After cooling to room temperature 1 ml of 6-M sodium hydroxide was added and then mixture was filled up with water to 10 ml. Then 2 ml of the DNS reagent was added to 2 ml of hydrolyzed mixture and heated at 90 °C for 10 min. Absorbance was recorded the way described above. The amount of sucrose was calculated from the difference between total and reducing sugars. Measurement was done in triplicates. 2.6. Modeling mass transfer For all samples the sucrose concentration profile at a given time of storage was described by the following equation: cx;s ¼ a þ b  edx þ f  egx ¼ uðxÞ

Table 1 Temperature and time of storage of osmodehydrated samples Process

ðmf  mv Þ 100 ðmo  mv Þ

ð2Þ

When x = 0, then cx,s = a + b + f, that is the sugar content at the surface of osmosed sample. This equation for each variant of storage was selected using Table Curve 2D (Systat Software Inc., San Jose, CA, USA). When maximum distance was assumed to be l = 20 mm, then the average sugar concentration in the sample could be calculated by the following equation: Z 10 1 c ¼ ða þ b  edx þ f  egx Þ dx ð3Þ 20 0

A. Kamin´ska et al. / Journal of Food Engineering 86 (2008) 140–149

Diffusion coefficient of sucrose in osmosed apple tissue was calculated solving the Fick’s second law at appropriate initial and boundary conditions (Appendix A). The following solution was obtained: 1 p  n  x 2 2 Ao X p n Ds ð4Þ þ An  e l 2 cos cx;s ¼ l 2 n¼1 where An ¼

2 l

Z

l

uðxÞ  cos 0

pnx dx for n ¼ 0; 1; 2; . . . l

ð5Þ

where Ao ¼ c

ð6Þ

143

storage time of fruits. Sucrose content in raw apple was between 1.5% and 1.8%. The average sucrose concentration was 1.69 ± 0.07%. In comparison, Pijanowski, Mro_zewski, Jarczyk, and Drzazga (1976) reported that in Poland raw apple dry matter content was on the average of 15.0% and sucrose contributed 2.5%. Kunachowicz, Nadolna, Przygoda, and Iwanow (1998) found the sucrose content in apple was at the level of 1.5–2.0% while dry matter content was at the level of 14.0%. Sucrose content in raw apple did not depend on the distance form the dehydrated surface and varied within the level of standard deviation (±0.05%). 3.2. Dry matter spatial distribution in osmosed apple

and An ¼

2  b  d  l  ½ðg  lÞ2 þ ðn  pÞ2 ð1  edl cosðn  pÞÞ 4

2

2

ðn  pÞ þ ðn  p  lÞ ðd 2 þ g2 Þ þ ðd  gÞ  l4 2

þ

2

2  f  g  l  ½ðd  lÞ þ ðn  pÞ ð1  egl cosðn  pÞÞ ðn  pÞ4 þ ðn  p  lÞ2 ðd 2 þ g2 Þ þ ðd  gÞ2  l4

ð7Þ

It has been found that diffusion coefficient was related to the distance from the interface. The relationship was approximated by the following equation using TableCurve2D software: D¼

Do  1 þ 0:02 x

ð8Þ

Eq. (4) was solved using Mathcad 2001 Professional System.

Osmotic dewatering resulted in increased concentration and spatial distribution of dry matter in apple tissue. Dry matter concentration in the surface layers was close to 40% (Fig. 1) and depended on the osmotic process parameters. Longer osmosis at lower temperature resulted in higher dry matter content concentration (43.69%) in comparison to the process done at higher temperature and a shorter time (39.00%). However, the differences occurred to the depth of about 2 mm from the interface. In further layers there was no difference in dry matter concentration between two osmotic processes studied. Moreover, dry matter concentration at the distance from 4 to 10 mm from the surface was constant and higher than the initial dry matter concentration. 3.3. Sucrose concentration profiles in osmosed apple

2.7. Statistical analysis of data

3. Results and discussion 3.1. Sucrose concentration profile – raw apple

After 3 h of osmotic dehydration the average sucrose content in the first slice was 22.79 ± 1.36%, and changes in the sugar concentration occurred until the depth of 5.0 mm from the dehydrated surface (Fig. 2). At the depth of 5.0 mm sucrose concentration reached the level of raw apple (1.75 ± 0.10%). It confirmed the earlier research done by Pałacha and Kamin´ska (2001) and Lenart and Lewicki (1981).

45

dry matter content, %

In the presented work, two types of inaccuracies were encountered. One was arising from the analytical error of sucrose concentration assay. The second one was due to natural variability of apple tissue structure and composition. Analytical method used to measure sucrose concentration was found to be very precise and reproducible. Standard deviation was on the level of ±0.01% and the relative error of the measured value was less than ±1%. Inaccuracies arising from natural variability of apple tissue structure were much larger. The relative error varied from ±2.23% to ±18.23%. The average value of that error was ±8.13%. In further statistical analysis variability between samples was taken into account as that which affected reported data much more than the variability within the sample. The following statistical measures were calculated using software Excell (Microsoft): standard deviation, analysis of variance and root mean square (RMS).

40

osmosed at 30°C for 3h

35

osmosed at 70°C for 1h native dry matter concentration

30 25 20 15 10 0.000

0.002

0.004

0.006

0.008

0.010

distance, m

Dry matter content in raw apple was found to be in the range 10.4–16.2% and depended on the maturity and

Fig. 1. Relationship between dry matter concentration and distance from the mass transfer surface.

A. Kamin´ska et al. / Journal of Food Engineering 86 (2008) 140–149

osmosed at 30°C for 3h

30

osmosed at 70°C for 1h 25 native sucrose concentration 20 15 10 5 0 0.000

0.002

0.004

0.006

0.008

0.010

distance, m Fig. 2. Relationship between sucrose concentration and distance from the mass transfer surface.

After 1 h of osmotic dehydration at 70 °C the sucrose content in the first slice was lower than that observed in samples osmosed at 30 °C for 3 h (20.18 ± 4.00%). Higher temperature of osmotic dehydration caused more significant changes in apple tissue. At the depth of 5.0 mm from the surface, sucrose content was nearly two times higher than that in raw apple (Fig. 2) and it was 3.10 ± 0.31%. However, at the distance of 10.0 mm from the osmosed surface the concentration of sucrose reached the level characteristic for raw apple. Comparison of dry matter and sucrose concentration profiles shows that surface layers are simultaneously dewatered and impregnated with sugar, while tissue situated further from the surface is only dewatered. In raw apple sucrose constituted about 11% of dry matter. After osmotic dewatering the share of sucrose in dry matter increased in surface layers to some 60%, while at the distance of 10 mm it dropped to less than 10%. It means that dewatering concerns apple tissue situated further from the surface than that impregnated with sucrose. These observations are consistent with compartmental model of mass transfer during osmotic dehydration. In that model it is assumed that at the beginning of the process a superficial layer of solute in the surface of the material is formed. It creates resistance to further penetration of solute into the tissue but it is favorable to water removal due to large concentration gradients (Raoult-Wack et al., 1991). It is also in agreement with the advancing disturbance model developed by Salvatori, Andres, Chiralt, and Fito (1999). 3.4. Sucrose concentration profile – apple osmosed at 30 °C for 3 h and stored under different conditions After osmotic dehydration samples were stored under different conditions: 24 h at temperature of 5, 9, 15 and 20 °C; 72 h at 5, 9 and 15 °C, and 144 h at 5 and 9 °C. The difference in sucrose concentration caused mass transfer inside the material during storage. Mass transportation was dependent on tissue temperature and storage time.

The higher was the storage temperature, the faster was the diffusion process. In general, sucrose moved toward the interior of the sample. After 24 h of storage at 5 °C sucrose content near the surface (0.5 mm) decreased to 19.54 ± 3.45%. That corresponds to 85.6% of the initial sucrose concentration (22.79 ± 1.36% in samples after osmotic dehydration). At the distance of 5.0 mm sugar content approached the level of raw apple. At higher temperature (9 °C) at the distance of 5.0 mm sucrose content was almost two times higher than that in raw apple but at the distance of 10.0 mm it was still as low as in raw apple. After 24 h at 20 °C at the distance of 0.5 mm sugar content decreased to 16.01 ± 0.93%. At the distance of 5.0 mm it was higher then that in samples stored for the same time at the temperature of 5 °C, and reached the level of 4.86 ± 0.13%. In those samples increase in sucrose content was also noticed at the distance of 10.0 mm (Fig. 3). Longer time of storage (72 h) caused further diffusion of sucrose in osmosed apple tissue. At the temperature of 5 °C the concentration of sucrose near the surface (0.5 mm) was 17.06 ± 3.11% and it corresponded to 75% of the initial value (Fig. 4). Concentration of sucrose after 72 h of

sucrose concentration, %

sucrose concentration,%

35

stored at 5°C stored at 9°C stored at 15°C stored at 20°C

20

15

10

5

0 0.000

0.002

0.004

0.006

0.008

0.010

distance, m Fig. 3. Influence of storage temperature on sucrose profiles in apple osmosed at 30 °C for 3 h and stored for 24 h.

sucrose concentration, %

144

stored for 24h stored for 72h stored for 144h

20

15

10

5

0 0.000

0.002

0.004

0.006

0.008

0.010

distance, m Fig. 4. Influence of storage time on sucrose profiles in apple osmosed at 30 °C for 3 h and stored at 5 °C.

A. Kamin´ska et al. / Journal of Food Engineering 86 (2008) 140–149

3.5. Sucrose concentration profile – apple osmosed at 70 °C for 1 h and stored under different conditions Apple osmosed for 1 h at 70 °C was stored for 24 h at the temperature of 5, 15 and 20 °C; 72 h at 5 and 15 °C;

Table 2 Sucrose concentration in selected slices of osmodehydrated apple stored under variable conditions Storage parameters

Sucrose concentration (%)

Time (h)

0–0.5

4.5–5.0

9.5–10.0

19.54 ± 3.45 17.26 ± 0.81 16.65 ± 0.67 16.01 ± 0.93

1.80 ± 0.32 3.48 ± 0.16 4.19 ± 0.17 4.86 ± 0.28

1.81 ± 0.32 1.78 ± 0.08 1.93 ± 0.08 2.26 ± 0.13

5 9 15

17.06 ± 3.11 16.36 ± 2.35 13.42 ± 1.64

2.71 ± 0.49 4.33 ± 0.62 5.70 ± 0.70

1.56 ± 0.28 2.17 ± 0.31 2.05 ± 0.25

5 9

11.19 ± 0.54 8.35 ± 0.31

6.79 ± 0.33 7.62 ± 0.29

5.47 ± 0.27 7.56 ± 0.28

Apple osmosed at 70 °C for 1 h 24 5 16.93 ± 2.48 15 14.44 ± 0.83 20 11.77 ± 0.27

3.96 ± 0.58 5.90 ± 0.33 6.54 ± 0.15

3.99 ± 0.58 4.54 ± 0.26 5.42 ± 0.13

Distance from the osmosed surface (mm) Temperature (°C)

Apple osmosed at 30 °C for 3 h 24 5 9 15 20 72

144

72

5 15

9.07 ± 0.62 7.98 ± 0.36

7.70 ± 0.53 7.10 ± 0.33

5.32 ± 0.36 7.56 ± 0.35

144

5

8.05 ± 0.18

7.29 ± 0.16

6.79 ± 0.15

and 144 h at 5 °C. Mass transfer in samples was observed during storage and its rate was higher than that in samples osmosed for 3 h at 30 °C. After 24 h at 5 °C sucrose content near the surface was 84% of concentration reached after osmotic dehydration. At the distance of 5.0 and 10.0 mm increase in sugar content was statistically significant and it was more than 2 times higher than that in raw apple (Table 2). Storage of osmosed apple at higher temperature causes increase in mass transfer rate. After 24 h of storage at 20 °C, sugar content at the distance of 0.5 mm from the surface was 11.77 ± 0.27% and at 10.0 mm it was 3 times higher than that in raw apple. Storage of samples for 72 h at 5 °C caused decrease of sucrose content at the distance 0.5 mm to 9.07 ± 0.62% and that corresponds to 45% of the initial sucrose concentration. At the distance of 10.0 mm from the dehydrated surface sugar content was 3 times higher than that in raw apple. The increase was the same as that in samples stored for 24 h at 20 °C. At 15 °C a state of equilibrium was reached after 72 h. A state close to equilibrium was reached at 5 °C when samples were stored for 144 h (Table 2). In general, mass transfer process was faster in samples osmosed for 1 h at 70 °C than that in samples dewatered for 3 h at 30 °C and stored under the same conditions. After storage for 24 h at 5 °C, in samples dewatered for 1 h at 70 °C concentration of sucrose at the distance of 5.0 mm was 3.96 ± 0.28% while samples osmosed at 30 °C for 3 h showed sucrose concentration equivalent to that of raw apple (Fig. 5). Osmotic dehydration at higher temperature causes serious changes in apple tissue structure (Spiess & Behsnilian, 1998) and because of that sugar transport in samples dewatered for 1 h at 70 °C was faster than that observed in samples osmosed at 30 °C for 1 h. 3.6. Diffusion coefficients Calculated effective diffusion coefficients are dependent on temperature and time of storage and distance from

sucrose concentration, %

storage at 5 °C approached the level of raw apple at the distance of 10.0 mm from the dehydrated surface. Increasing temperature to 9 °C caused increased concentration of sucrose at the distance of 10.0 mm. Sugar content reached almost the same level as that noticed in samples stored 24 h at 20 °C. After 72 h of storage at 15 °C sugar content near the surface of samples decreased significantly from 22.79 ± 1.36% to 13.42 ± 1.64%. At the distance of 5.0 mm from the dehydrated surface sucrose content was 3.5 times higher than that in raw apple and at the distance of 10.0 mm sucrose content was almost the same as that observed in apple stored 24 h at 20 °C. Prolonging the storage time caused further diffusion of sucrose. Storage for 144 h at 5 °C decreased sucrose content at the distance of 0.5 mm to 11.19 ± 0.54%. It means that sucrose concentration in surface layers dropped by half. At the distance of 5.0 mm from the surface sucrose concentration was almost 4 times higher than that in raw apple. At the distance of 10.0 mm sugar content was 5.50 ± 0.27%. During the same time of storage at the temperature of 9 °C almost equilibrium state was reached. Concentration of sucrose in surface layers was 8.35 ± 0.31% and at the distance of 10.0 mm it was 7.56 ± 0.28% (Table 2).

145

osmosed at 30°C for 3h

20

osmosed at 70°C for 1h 15

10

5

0 0.000

0.002

0.004

0.006

0.008

0.010

distance, m Fig. 5. Influence of osmotic dewatering parameters on sucrose profiles in apple stored for 24 h at 5 °C.

A. Kamin´ska et al. / Journal of Food Engineering 86 (2008) 140–149

146

tration was related to distance from the mass exchange surface by Eq. (2), a relationship between sucrose concentration and effective diffusion coefficient was expected (Fig. 7). The lower was the sucrose concentration, the higher was the value of effective diffusion coefficient. It is worth to notice that the influence of temperature of

diffusion coefficient, D*1011 m2/s

16 stored at 5°C

14 12

stored at 9°C stored at 15°C

10

stored at 20°C

8 6 4

Table 4 Influence of time and storage temperature of apple osmosed for 3 h at 30 °C on sucrose effective diffusion coefficient

2 0 0.000

0.002

0.004

0.006

0.008

0.010

distance, m Fig. 6. Influence of storage temperature and distance from the mass transfer surface on effective diffusion coefficient in apple osmosed at 30 °C for 3 h and stored for 24 h.

the interfacial surface (Fig. 6). Relationship between effective diffusion coefficient and the distance was approximated by Eq. (8) and Do was introduced as the coefficient independent on distance. Values of Do are collected in Table 3. The effective diffusion coefficient of sucrose (Deff) in osmosed apple was found at the level of 1012 m2/s for the distance of 0.5 mm from the dehydrated surface. In comparison, Kaymak-Ertekin and Sultanoglu (2000) found that effective diffusion coefficients of water and sucrose for apple slices dehydrated 8 h at 30 °C (66% of osmotic solution) were in the range of 1011 m2/s. Lazarides, Gekas, and Mavroudis (1997) found diffisivities of water in osmosed apple of the order of 1010 m2/s at temperatures from 20 to 50 °C and sucrose concentration from 45% to 65%. However, the diffusivity values reported by Azuara, Cortes, Garcia, and Beristain (1992) Conway, Castaigne, Picard, and Voxan (1983) were higher and they were in the range of 108–109 m2/s. It is worth to mention that most of reported diffusion coefficients are calculated on the basis of an average solute concentration and for the undergoing osmotic dewatering, that is for the process when external and internal resistances to mass transfer play a role. For both processes of dehydration, effective diffusion coefficients were dependent on the distance from the dehydrated surface (Tables 4 and 5). Since sucrose concen-

Table 3 Influence of dewatering parameters and time and temperature of storage of osmosed apple on diffusion coefficient D0 Process

Temperature of storage (°C)

Time of storage (h) 24

72

144

30 °C/3 h

5 9 15 20

9.65E11 2.75E10 3.35E10 4.15E10

1.35E10 1.66E10 3.88E10

8.85E10 1.03E09

5 15 20

1.65E10 9.68E10 2.91E09

2.65E09 1.25E08

1.25E08

70 °C/1 h

Time of storage (h)

Distance (m)

Diffusion coefficient (m2/s) Temperature of storage (°C) 5

9

15

20

24

0.00025 0.00075 0.00275 0.00475 0.00675 0.00975

1.19E12 3.49E12 1.17E11 1.85E11 2.44E11 3.16E11

3.40E12 9.94E12 3.32E11 5.28E11 6.94E11 9.01E11

4.14E12 1.21E11 4.05E11 6.43E11 8.45E11 1.10E10

5.12E12 1.50E11 5.02E11 7.96E11 1.05E10 1.36E10

72

0.00025 0.00075 0.00275 0.00475 0.00675 0.00975

1.67E12 4.88E12 1.63E11 2.59E11 3.41E11 4.42E11

2.05E12 6.00E12 2.01E11 3.19E11 4.19E11 5.44E11

4.79E12 1.40E11 4.69E11 7.45E11 9.79E11 1.27E10

144

0.00025 0.00075 0.00275 0.00475 0.00675 0.00975

1.09E11 3.20E11 1.07E10 1.70E10 2.23E10 2.90E10

1.27E11 3.70E11 1.24E10 1.97E10 2.59E10 3.36E10

Table 5 Influence of time and storage temperature of apple osmosed for 1 h at 70 °C on sucrose effective diffusion coefficient Time of storage (h)

Distance (m)

Diffusion coefficient (m2/s) Temperature of storage (°C) 5

15

20 3.59E11 1.05E10 3.52E10 5.58E10 7.34E10 9.54E10

24

0.00025 0.00075 0.00275 0.00475 0.00675 0.00975

2.04E12 5.96E12 1.99E11 3.17E11 4.16E11 5.41E11

1.20E11 3.50E11 1.17E10 1.86E10 2.44E10 3.17E10

72

0.00025 0.00075 0.00275 0.00475 0.00675 0.00975

3.27E11 9.58E11 3.20E10 5.09E10 6.69E10 8.68E10

1.54E10 4.52E10 1.51E09 2.40E09 3.15E09 4.10E09

144

0.00025 0.00075 0.00275 0.00475 0.00675 0.00975

1.54E10 4.52E10 1.51E09 2.40E09 3.15E09 4.10E09

A. Kamin´ska et al. / Journal of Food Engineering 86 (2008) 140–149

1.2e-10 5°C 1.0e-10

9°C

8.0e-11

15°C

6.0e-11 4.0e-11 2.0e-11 0.0 0

2

4

6

8

10

12

14

16

18

20

sucrose concentration, % Fig. 7. Relationship between sucrose concentration and sucrose effective diffusion coefficient in apple osmosed at 30 °C for 3 h and stored at variable temperature.

storage on effective diffusion coefficient was dependent on sucrose concentration. At high sucrose concentrations the effect of temperature was negligible. At lower sucrose concentrations the higher was the storage temperature the stronger was the concentration influence on effective diffusion coefficient. Parameters of osmotic process also affected effective diffusion coefficient. Effective diffusion coefficients were higher for samples dehydrated at 70 °C for 1 h than those calculated for samples osmosed at 30 °C for 3 h. The diffusion coefficient Do was dependent on sample storage time and temperature. It increases with temperature, but the relationship is rather not of the Arrhenius type (Fig. 8). This observation can be due to either metabolic activity of the tissue or structural alterations caused by sucrose concentration gradients changing with time of storage. Osmotic dehydration affects structure of cells (Lewicki & Porzecka-Pawlak, 2005) and cell membranes and changes metabolic activity of the tissue. However, these changes are in the surface layers and the tissue as a whole is metabolically active. Lewicki, Gondek, Witrowa-Rajchert, and Nowak (2001) have shown that osmosed apple

30°C/3h, stored for 24h 30°C/3h, stored for 72h 70°C/1h, stored for 24h

-20

15

10

5

0 0

0.00355

0.00360

20

F 30 °C, 3 h

70 °C, 1 h Calculated vs. experimental sucrose concentration

Influence of time of storage

Calculated vs. experimental sucrose concentration

24

5 9 15 20

268.30 339.98 780.79 137.48

0.0442 0.0752 0.1387 0.1449

45.07 – 46.02 66.85

0.0007 – 4.5425 0.5198

72

5 9 15

35.32 277.89 88.11

0.6701 1.5253 0.5049

6.78 – 4.02

7.8941 – 8.1029

144

5 9

11.54 2.78

0.1234 0.0362

4.31 –

3.7013 –

0.00365

reciprocal of temperature, 1/K Fig. 8. Arrhenius plot for apple osmosed and stored under different conditions.

15

Table 6 Statistical F values for comparison of experimental and calculated sucrose concentrations in apple subjected to osmotic dewatering

-23

0.00350

10

Fig. 9. Relationship between experimental and calculated sucrose concentrations in osmosed apple stored at temperatures above zero.

-22

0.00345

5

sucrose concentration, experimental

Time Temperature Influence (h) (°C) of time of storage

-21

-24 0.00340

20

Parameters of storage

-19

Ln (D0)

tissue is respiring, hence it is metabolically active. In this experiment it is evident that the longer the time of storage the higher the effective diffusion coefficient. This suggests that during storage some changes of tissue structure occur, which facilitate mass transfer. Effective diffusion coefficients reported in Tables 4 and 5 were calculated on the basis of average sucrose concentration profiles. They are loaded with the average uncertainties arising from the natural variability of apple tissue structure and composition. In order to estimate that average uncertainty average effective diffusion coefficients were substituted into Eq. (4) and sucrose concentration at a given distance from the surface of the sample was calculated. Calculated and experimental values were compared (Fig. 9). All points are symmetrically scattered around diagonal, and show that the error of sucrose concentration estimation is the larger the lower is the concentration. At sucrose concentrations higher than 10% the RMS is close to ±5%, while at concentrations close to 2% it is as large

sucrose concentration, calculated

diffusion coefficient, m2/s

1.4e-10

147

148

A. Kamin´ska et al. / Journal of Food Engineering 86 (2008) 140–149

as ±25%. Taking into account that natural sucrose concentration in apple varies between 1.5% and 1.8% that is by some ±10%, it is evident that the calculated values are biased by a larger error than it would be expected due to natural variability. Analysis of variance was done for calculated and experimental sucrose concentrations. The F values are collected in Table 6. Statistically significant influence of the distance from the sample surface on sucrose concentration is evident for more than 80% of samples. The difference between calculated and experimental values was statistically unimportant. The data presented in Table 6 shows that the closer is the sucrose concentration to the equilibrium state the less evident is the influence of distance from the mass exchange surface on its value.

Appendix A Osmosed product packed hermetically and stored at constant temperature constitutes a closed system in which migration of components is caused by concentration gradients. There is no mass exchange with the surroundings, and an initial component distribution occurs. Geometry of infinite plate is assumed and solution of the Fick’s second law is sought for the appropriate boundary conditions.

c0,τ

4. Conclusions During the osmotic dehydration process sucrose molecules penetrate the sample, that leads to the significant concentration gradients inside the dehydrated material. These concentration gradients depend on dehydration parameters and material structure. Osmotic dehydration at 30 °C for 3 h resulted in gradients of dry matter and sucrose concentration in the surface layers. At the distance of 5.0 mm from the surface, dry matter and sugar content approached the level characteristic for raw apples. Higher temperature of osmotic dehydration lead to the deeper migration of osmotic substance. Samples dehydrated at 70 °C for 1 h showed concentration of sugar equivalent to that of raw apple at the distance of 10.0 mm from the dehydrated surface. The internal gradient of sucrose concentration causes mass transfer inside the samples during storage. The rate of this process depends on tissue temperature and storage time. Diffusion of sucrose was faster during storage in samples dehydrated at 70 °C for 1 h than that in samples dehydrated at 30 °C for 3 h. Samples dehydrated for 1 h at 70 °C reached state of equilibrium after 144 h of storage at 5 °C, while samples dehydrated for 3 h at 30 °C were far from the equilibrium state under these storage conditions. Effective diffusion coefficients were found to be in the range of 109–1012 m2/s, dependent on osmotic process and storage parameters. The coefficients are dependent on sucrose concentration, and the dependence is affected by temperature of storage. At high sucrose concentrations the influence of temperature on the relationship between effective diffusion coefficient and sugar content is negligible. At low sucrose concentrations the effect is the stronger the higher is the storage temperature. Influence of storage temperature on the diffusion coefficient Do is not of the Arrhenius type. This can be due to metabolic activity of osmosed tissue and structural changes occurring during storage and caused by sucrose concentration gradients.

cx,0 = ϕ (x)

x x=0

D

x=l

o2 c oc  ¼0 ox2 os

Initial boundary conditions: s¼0 06x6l cx;0 ¼ uðxÞ Boundary conditions: s>0

x¼0

s>0

x¼l

oc ¼0 ox oc ¼0 ox

The following solution was obtained using Fourier’s method: 1 p  n  x 2 2 Ao X p n Ds cx;s ¼ þ A n  e l2 cos l 2 n¼1 where 2 An ¼ l

Z

l

uðxÞ  cos 0

pnx dx; l

A0 ¼ c

References Aktas, T., Fujii, S., Kawano, Y., & Yamamoto, S. (2006). Effect of some osmotic pre-drying treatments on drying kinetics, desorption isotherms and quality of vegetables. In I. Farkas (Ed.), Proceedings of the 15th

A. Kamin´ska et al. / Journal of Food Engineering 86 (2008) 140–149 international drying symposium (pp. 877–883). Go¨do¨llo¨, Hungary: Szent Istva´n University Publisher. Azuara, E., Cortes, R., Garcia, H. S., & Beristain, C. I. (1992). Kinetic model for osmotic dehydration and its relationship with Fick’s second law. International Journal of Food Science and Technology, 27, 239–242. Beristain, C. I., Azuara, E., Cortes, R., & Garcia, H. S. (1990). Mass transfer during osmotic dehydration of pineapple rings. International Journal of Food Science and Technology, 25, 576–582. Bolin, H. R., Huxsoll, C. C., & Jackson, R. (1983). Effect of osmotic agents and concentration on fruit quality. Journal of Food Science, 48, 202–204. Contreras, J. M., & Smyrl, T. G. (1981). An evaluation of osmotic concentration of apple rings using corn syrup solids solutions. Canadian Institute of Food Science and Technology, 14, 301–314. Conway, J. M., Castaigne, F., Picard, G., & Voxan, X. (1983). Mass transfer considerations in the osmotic dehydration of apples. Canadian Institute of Food Science and Technology, 16, 25–29. Fito, P., Chiralt, A., Betoret, N., Gras, M., Cha´fer, M., & MartinezMonzo´, J., et al. (2001). Vacuum impregnation and osmotic dehydration in matrix engineering. Application in functional fresh food development. Journal of Food Engineering, 49, 175–183. Islam, M. N., & Flink, J. N. (1982). Dehydration of potato: II. Osmotic concentration and its effect on air drying behaviour. Journal of Food Technology, 17, 387–403. Kaymak-Ertekin, F., & Sultanoglu, M. (2000). Modeling of mass transfer during osmotic dehydration of apples. Journal of Food Engineering, 46, 243–250. Kunachowicz, H., Nadolna, I., Przygoda, B., & Iwanow, K. (1998). Food composition tables (p. 512). Warszawa: Polish Institute of Food and Nutrition (in Polish). LaBelle, R. L., & Moyer, J. C. (1966). Dehydrofreezing of red tart cherries. Food Technology, 20(10), 105–108. Lazarides, H. N., Gekas, V., & Mavroudis, N. (1997). Apparent mass diffusivities in fruit and vegetable tissues undergoing osmotic process. Journal of Food Engineering, 31, 315–324. Lenart, A. (1986). Influence of plant tissue properties on diffusion of osmoactive substances. Zeszyty Problemowe Postepow Nauk Rolniczych, 297, 351–355 (in Polish). Lenart, A. (1990). Osmotic dehydration of apples as a pre-treatment to convection drying of fruits and vegetables. Przemysł Spo_zywczy, 44(12), 307–309 (in Polish). Lenart, A., & Lewicki, P. P. (1981). Water diffusion in apple tissue subjected to osmotic dehydration. Zeszyty Naukowe SGGW-AR. Technologia Rolno-Spozywcza, 14, 33–47 (in Polish). Lewicki, P. P., Gondek, E., Witrowa-Rajchert, D., & Nowak, D. (2001). Effect of drying on respiration of apple slices. Journal of Food Engineering, 49, 333–337. Lewicki, P. P., & Lenart, A. (2007). Osmotic dehydration of fruits and vegetables. In Handbook of industrial drying (3rd ed., pp. 665–687). Boca Raton, FL: Taylor & Francis. Lewicki, P. P., & Porzecka-Pawlak, R. (2005). Effect of osmotic dewatering on apple tissue structure. Journal of Food Engineering, 66, 43–50.

149

Mastra´ngelo, M. M., Rojas, A. M., Castro, M. A., Gerschenson, L. N., & Alzamora, S. M. (2000). Texture and structure of glucose-infused melon. Journal of the Science of Food and Agriculture, 80, 769–776. Mastrantonio, S. D. S., Pereira, L. M., & Hubinger, M. D. (2006). Mass transfer and diffusion coefficient determination in osmotically dehydrated Guavas. In I. Farkas (Ed.), Proceedings of the 15th international drying symposium (pp. 860–864). Go¨do¨llo¨, Hungary: Szent Istva´n University Publisher. Mavroudis, N. E., Gekas, V., & Sjo¨holm, I. (1998). Osmotic dehydration of apples – Effects of agitation and raw material characteristics. Journal of Food Engineering, 35, 191–209. Mayor, L., Moreira, L., Chenlo, F., & Sereno, A. M. (2006). Effective diffusion coefficients during osmotic dehydration of pumpkin with ternary solutions of NaCl and sucrose. In I. Farkas (Ed.), Proceedings of the 15th international drying symposium (pp. 892–899). Go¨do¨llo¨, Hungary: Szent Istva´n University Publisher. Pałacha, Z., & Kamin´ska, A. (2001). The impact of osmotic pre-treatment on the freezing process of apples. Chłodnictwo, 36(3), 44–47 (in Polish). Panagiotou, N. M., Karathanos, V. T., & Maroulis, Z. B. (1999). Effect of osmotic agent on osmotic dehydration of fruits. Drying Technology, 17, 175–189. Pereira, L. M., Rodrigues, A. C. C., Saranto´poulos, C. I. G. L., Junqueira, V. C. A., Cunha, R. L., & Hubinger, M. D. (2004). Influence of modified atmosphere packaging and osmotic dehydration on the quality maintenance of minimally processed guavas. Journal of Food Science, 69, 172–177. Pijanowski, E., Mro_zewski, S., Jarczyk, A., & Drzazga, B. (1976). Technology of fruit and vegetable products (in Polish). Warsaw: PWRiL. Raoult-Wack, A.-L., Petitdemange, F., Giroux, F., Guilbert, S., Rios, G., & Lebert, A. (1991). Simultaneous water and solute transport in shrinking media. Part 2. A compartmental model for the control of dewatering and impregnation soaking process. Drying Technology, 9, 631–648. Rastogi, N. K., & Raghavarao, K. S. M. S. (1994). Effect of temperature and concentration on osmotic dehydration of coconut. Lebensmittel Wissenschaft und Technologie, 27, 564–567. Rastogi, N. K., Raghavarao, K. S. M. S., & Niranjan, K. (1997). Mass transfer during osmotic dehydration of banana: Fickian diffusion in cylindrical configuration. Journal of Food Engineering, 31, 423–432. Salvatori, D., Andres, A., Chiralt, A., & Fito, P. (1999). Osmotic dehydration progression in apple tissue. II. Generalized equations for concentration prediction. Journal of Food Engineering, 42, 133–138. Sereno, A. M., Moreira, R., & Martinez, E. (2001). Mass transfer coefficients during osmotic dehydration of apple in single and combined aqueous solutions of sugar and salt. Journal of Food Engineering, 47, 43–49. Spiess, W. E. L., & Behsnilian, D. (1998). Osmotic treatments in food processing, current state and future needs. In C. B. Akritidis, D. Marinos-Kouris, & G. D. Saravakos (Eds.). Proceedings of the international drying symposium (Vol. A, pp. 47–56). Thessaloniki, Greece: Ziti Editions. Toczko, M., & Grzelin´ska, A. (1997). Biochemistry exercises. Assay of sugar content in the plant material. Warszawa: Wydawnictwo SGGW (in Polish).