FREEZE-DRYING CHARACTERISTICS OF STRAWBERRIES

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Drying kinetic, as well as color and volume variation, of whole and sliced strawberries were investigated after freeze- drying under various temperatures (30, 40, ...
DRYING TECHNOLOGY, 20(1), 131–145 (2002)

FREEZE-DRYING CHARACTERISTICS OF STRAWBERRIES F. Shishehgarha,1 J. Makhlouf,2 and C. Ratti1 1

Department of Soil and Agri-food Engineering, and 2 Department of Food Science and Nutrition, Laval University Ste-Foy, (QC) G1K 7P4, Canada

ABSTRACT Drying kinetic, as well as color and volume variation, of whole and sliced strawberries were investigated after freezedrying under various temperatures (30, 40, 50, 60 and 70 C). Dehydration time increased proportionally to the thickness of the product and heating plate temperature markedly reduced it. Freeze-drying caused a pronunciation in red color of strawberries. A decrease in hue angle by 22.5% (skin) and by 42.4% (pulp) was noted, with no significant effect of freeze-drying temperature up to 70 C. The strawberries had a volume reduction of 8% (whole) and 2% (sliced) due to freezedrying although the level of shrinkage was also independent of freeze-drying temperature. However, the percentage of collapsed strawberries increased with process temperature. At heating temperatures higher than 50 C, the strawberry dry layer temperature was higher than the estimated glass transition temperature of dried fruit, increasing the risk of collapse. Key Words: Collapse; Color; Glass transition temperature; Shrinkage; Small fruits 131 Copyright & 2002 by Marcel Dekker, Inc.

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INTRODUCTION Removal of water from food products in order to increase their shelf life can be achieved by different dehydration techniques. However, shrinkage phenomena which depends on the interstitial mobility (1) affect markedly the quality of final product in classical dehydration methods (2). Berries for example, with delicate structure and high levels of water content, are very difficult to dehydrate by classical methods (3, 4), specially during air-drying when collapse causes considerable damage in their physical structure (2). According to Sullivan et al. (5), blueberries are very susceptible to skin rupture during air-drying which causes severe bleeding. In addition, dried berries produced by conventional air-drying have poor rehydration characteristics (6). The effect of osmotic dehydration of strawberries (prior to conventional drying) was also investigated by some researchers (4, 6). Disruption of cell membranes and cell wall alteration were generally present. In addition to the negative impact of drying on final product quality, the important concentration of anthocyanins contained in most types of berries (7), which are a good source of antioxidants (8), can be deteriorated during air-drying process (9). Anthocyanins are very unstable (9) so high temperatures and oxygen during drying and storage greatly destabilize these compounds (10). The change in anthocyanin concentration due to processing affects product color markedly, since there is a good correlation between both variables (9, 7). Vacuum freeze-drying of biological products is the best method of water removal with end products of highest quality, compared with other dehydration techniques (11, 12). This dehydration method has been widely used to obtain high quality instant coffee and dairy bacterial cultures (13, 14). However, its application to other foods has a potential for development, particularly nowadays when the consumers are demanding for higher food quality. Freeze-dried strawberries were found to be of excellent color and flavor (4, 7), with high rehydration capacity (4). Freeze-dried blueberries had the highest retention of important components, soluble solids and color, as compared to berries dehydrated using other methods (10). Despite of its capability of providing a very high quality dehydrated product, freeze-drying is an expensive method and the high costs of process limit its application to industrial scale. Therefore, the quality of dehydrated products, as well as cost and time for processing, should all be taken into consideration in order to carry out the operation under optimal conditions. In recent years, the interest to find the influence of processing conditions (chamber pressure, heating plate temperature, quality of the raw material and freezing rate) on freeze-drying time of strawberries, production rate and quality of final product has increased. Although numerous, these studies did

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not always lead to the same conclusions. Hammami and Rene´ (15) studied the influence of pressure and temperature, on both product quality and freeze-drying time. They found that color of strawberries depends deeply on process temperature, but no significant effect of heating plate temperature on shrinkage was observed. However, Roos (16) showed that strawberries are susceptible to collapse at relatively high temperature conditions. In this study, strawberries freeze-dried at 20 C were of higher quality than those at 60 C, which were slightly different in appearance and taste. Pa¨a¨kko¨nen and Mattila (17) have found that, during freeze-drying, the structure, color and aroma of strawberries remained unchanged, but low processing temperatures improved sensory quality of dried fruits. According to Iwanin and Mittal (18), for the maintenance of texture quality in freezedried strawberries, high temperatures can be applied along with very low heating rates, to prevent meltdown and cell collapse. Very few studies involved product thickness and its effect on the quality of freeze-dried materials (19). However, product thickness is an important optimization factor in this process since its reduction certainly decrease freeze-drying time, preventing collapse which is a temperature–time phenomena (20). On the other hand, thin materials may be easily overheated in a long process. Consequently, the effects of product thickness and heating plate temperature on the color and volume of freeze-dried strawberries as well as on drying time were investigated in this work in order to determine optimum freeze-drying conditions.

PROCEDURE Samples Preparation Strawberries, (var. Seascape, Quebec) were obtained the day of their harvest. After elimination of unripe and rotten fruits, part of the strawberries was cut into 5 and 10 mm thick slices, and the rest was used as whole. Both whole and sliced strawberries were frozen at 40 C in a monolayer bed (Sanyo medical freezer, MDF 235). Color and volume of strawberries (both whole and slices) were determined before freezing and after freeze-drying.

Freeze Drying Experiments Experiments on the effect of heating plate temperature on final product quality were conducted in a vacuum freeze-dryer (Virtis Freezemobile, 25 EL) having a condenser temperature of 92 C. The vacuum level was less than

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5 ml and the heating rate, 1 C/min. Five plate temperatures (30, 40, 50, 60 and 70 C) were used over different freeze drying times (12 h in the case of slices and 24 and 48 h in the case of whole fruits). Four thermocouple probes were used to monitor and control shelf and product temperature during the process. Internal temperature of whole strawberries was recorded by placing probes at the center of each sample. Kinetics studies were performed in 1000 ml standard Pyrex flasks, attached to the external vacuum gages on the manifold of the freeze-drier, and exposed at room temperature (26  1 C). Drying curves were obtained by weighing strawberries at different times during freeze-drying, up to final moisture content of less than 1 g/100 g dry mater. Dry mass of fruits was obtained gravimetrically after drying in a vacuum oven (55 C) for 72 h in the presence of P2O5. Both experiments (quality and kinetics) were done in triplicate with three sampling at each time.

Color Whole strawberries (freeze-dried during 48 h) and 5 and 10 mm slices (freeze-dried during 12 h) were used for color and volume determination. Color measurements were performed using a colorimeter (Minolta CM-300 f 8 mm measuring area and diffuse illumination/0 viewing geometry), directly on the skin and pulp of fruits. The colorimeter was calibrated with a white standard tile. Color was recorded using the CIE-L*a*b* uniform color space, according to the CIELAB 76 convention (21), where L* indicates lightness, from completely opaque (0) to completely transparent (100), a* is a measure of redness and b* of yellowness. The three CIE-L*a*b* values were further used to calculate color saturation index (C ¼ [a*2 þ b*2]1/2) and hue angle (h* ¼ Arctan b*/a* ). Collapsed samples were not included as part of the color results.

Volume Volume measurements were performed using a balance (Mettler Toledo AT) with an accuracy of 104 g, equipped with a density determination kit (Mettler Toledo), which measures density of solids based on Archimedes principle. The samples were weighed in air and in water. Before soaking into water, dried fruits were wrapped in a thin layer wax film to prevent diffusion of water into the fruits. The volume of the wax film was considered negligible. Collapsed samples were not included as part of the volume reduction results.

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Collapse Collapsed samples were detected after each treatment by visual observation (22) and also measuring the specific volume of the samples (23). A volume reduction higher than 15% was considered collapse. The number of collapsed samples was counted in order to determine their percentage per treatment.

Statistical Analysis All results are the means of nine data points (three measurements done in three replicas of each drying experiment). Data were analysed by analysis of variance within a completely randomised design. Duncan’s multiple tests were used for mean discrimination (5% level of probability), using Statistical Analysis System (24).

RESULTS Dehydration Curves Freeze-drying curves (at room temperature) of whole and sliced strawberries are presented in Figure 1. Drying rates are fast during the first phase of drying (two, three and eight first hours for 5 mm, 10 mm and whole strawberries respectively) slowing down at the end of the process as expected from the increase of the dry layer resistance to heat transfer throughout the process. As can be also observed from Figure 1, the rate of water loss is also significantly dependent on slice thickness. In Table 1, experimental freezedrying times for 5 and 10 mm thickness strawberries are presented at several water contents. In this table, prediction values of freeze-drying time for 10 mm slices, calculated as if it varies proportionally to the smallest thickness of the product (in this case 5 mm), as well as the percentage difference between actual and predicted values, are also presented. It can be observed that, for most water contents, predicted and actual drying times for 10 mm are very close, indicating that freeze-drying time of strawberries can be considered as proportional to thickness. Other authors found previously a linear relationship between freeze-drying time and thickness, as Sharma and Arora (19) for yoghurt under different heating transfer modes and Saravacos (25) in the case of apple and potato. In the literature however, freeze-drying time is often presented as proportional to the square of the piece size (26, 27) since the process is usually explained from the

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Figure 1. Drying curves of whole (m), 5 mm (f) and 10 mm (g) slices of strawberries, freeze-dried at room temperature (26  1 C) (Xo ¼ initial moisture content in dry basis, X ¼ moisture content dry basis).

Table 1. Freeze-Drying Times for 5 and 10 mm Slice Thickness Strawberries (E%: Percentage Difference Between Actual and Predicted 10 mm Freeze-Drying Time Values) Freeze-Drying Time (h) X/Xo

5 mm

10 mm

10 mm*

0.05 0.1 0.2 0.4 0.5 0.6 0.7 0.8

5 4.23 3.40 2.3 1.78 1.25 0.89 0.54

10.7 8.9 6.4 4.8 3.4 2.5 1.78 1.07

10 8.46 6.8 4.6 3.56 2.5 1.78 1.08

*predicted.

diffusion theory. King (28) explained the anomaly respect to pure diffusion theory based on an externally controlled (boundary layer) freeze-drying process, which could clearly explain the linear relationship between freezedrying time and product thickness.

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Figure 2. Residual moisture content of strawberries after freeze-drying at varying heating plate temperatures (g whole freeze-dried for 24 h, œ whole freeze-dried for 48 h, e 5 mm and f 10 mm slices freeze-dried for 12 h. (X/Xo)res ¼ moisture content after freeze-drying/initial moisture content, dry basis.

The residual moisture content of whole strawberries after 24 and 48 h freeze-drying, and of 10 mm slices after 12 h freeze-drying, is presented in Figure 2 as a function of heating plate temperature. An important effect of temperature on residual moisture content was observed. This positive effect was found to be more pronounced as the heating plate temperature increases from 30 to 40 C (see Figure 2). At higher temperatures the reduction of residual moisture content due to temperature is less important. This phenomenon could be explained by the decrease in matrix viscosity and the closing of micro-pores due to imminent collapse in the strawberry tissues near 50 C (16).

Color The sole analysis of parameters a* and b* is not enough to interpret changes in color during processing. According to Abers and Wrolstad (29),

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the hue angle (h* ¼ tan1 b*/a*) has the most significant correlation with visual scores and the saturation index (C ¼ a*2 þ b*2)1/2 gives a good indication of the amount of color. Thus, changes in hue angle and saturation index of pulp and skin of strawberry during freeze-drying are presented as a function of plate temperature in Figures 3 and 4, respectively. The pronunciation in red color of strawberry observed during freeze-drying leads to a decrease in hue angle of about 22.5% in the case of skin and 42.4% in the case of pulp compared to the fresh fruit. The important decrease of hue angle of pulp with freeze-drying is a result of a significant decrease in b* (45.4%). Saturation index of skin did not change with freeze-drying, but it showed a slight decrease in pulp measurements. The color reinforcement observed during freeze-drying may be interpreted from different points of view. It has been previously attributed to pre-freezing step (30) and pigment concentration due to water reduction (15). But this phenomenon can also be explained by the effect of pH. According to Flink (31), the concentration of Hþ increase during the freezing process and it can be changed by up to 3 pH units. Bell and Labuza (32) found that pH of reduced-moisture solid systems appeared to be lower than that in the hydrated state, because dilution changes the chemical environment of the solutes and potentially the pH. On the other hand, anthocyanin color varies according to pH (33). In acid aqueous solutions anthocyanins exist as a mixture of four structures in equilibrium: flavylium cation (red), quinoidal base (blue), carbinol pseudobase (colourless) and chalcone (colourless or light yellow). Therefore, a decrease in Hþ concentration during freeze-drying can change the anthocyanin equilibrium in the production of flavylium cation, which may increase the red color intensity. Despite the marked pronunciation of red color during freeze-drying, no significant effect of drying temperature on color was observed, as determined statistically using SAS (5%). Also, the statistical analyses of pulp color showed that color changes during freeze-drying were independent of the product thickness.

Volume The results of volumetric measurements indicated that shrinkage due to freeze-drying were 8% for whole strawberries and less than 2% for slices. These results were comparable to those found by Jancovie´ (3) for raspberries and blackberries. No significant effect of processing temperature on volume reduction of fruits was observed. However, the number of collapsed samples during freeze-drying increased considerably with processing temperature.

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ho

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hue angle(h*)

25 20 15 10 5 0 30

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Figure 3. Variation in hue angle of skin (a), and pulp of 5 mm (b) and 10 mm (c) sliced strawberries during freeze-drying at different plate temperatures (ho ¼ initial hue angle).

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Figure 4. Variation in color saturation of skin (a), and pulp of 5 mm (b) and 10 mm (c) sliced strawberries during freeze-drying at different plate temperatures (Co ¼ initial saturation index).

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Figure 5. Percentage of collapsed strawberries during freeze-drying at different plate temperatures (œ detected by volume reduction, e detected by visual observation).

Collapse of macroscopic structure causes the most detrimental changes during dehydration, decreasing markedly the quality of dehydrated foods (1). Figure 5 shows the percentage of collapsed samples as a function of plate temperature as assessed by both methods previously described. At temperatures higher than 50 C this percentage exceeds 20%, which can be considered a high loss in the uniformity of final products quality. Collapsed products showed a large decrease in volume as well as color and appearance changes. Glass transition temperature, Tg, can be defined as the temperature at which an amorphous system changes from the glassy to the rubbery state (34). It has been reported that when the temperature of a product is higher than Tg at the correspondent water content, the quality of foodstuffs can be seriously altered (35). Some authors explain this by the effect of viscosity. Indeed, above Tg, the viscosity drops considerably and reactants become more mobile to take part in deterioration reactions. If viscosity decreases to a level that facilitates deformation, the matrix can flow eventually closing pores, and excessive volumetric shrinkage and loss of structure may occur (23). The glass transition temperature of freezedried strawberries was found to be 38 C (36). The internal temperature of strawberries during freeze-drying at different heating plate temperatures is shown in Figure 6 together with the glass transition temperature of freeze-dried strawberry. If the heating plate temperature is higher

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Figure 6. Internal temperature of strawberries during freeze-drying at different OOOOOOOO plate temperatures (m 30, e 40, f 50, t 60, œ 70 C). The dotted line ( ) represents the glass transition temperature of dried product.

than 50 C, the temperature of the product at the end of the process (dry layer) is higher than the Tg of dry solids, which could explain the high percentage of structural collapse observed above when this temperature is exceeded.

CONCLUSIONS An increase in heating plate temperature up to 70 C during freezedrying process of strawberries allows a considerable increase in drying rate reducing processing time. No significant effect of heating plate temperature on colour or volume results was found. However, the strawberries freezedried at the temperatures lower than 50 C were found to be of better visual quality, while those freeze-dried at higher temperatures, seemed to have slightly suffered from an excessive heating. In addition, a high percentage of collapsed strawberries were obtained at temperatures higher than 50 C. The last result, combined to product internal temperature measurements, indicated that this phenomenon can be interpreted from glass transition theory. Therefore, the determination of Tg value of dry products could be used as a simple method to determine the appropriate heating plate temperature for freeze-drying in order to preserve the quality of final product.

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ACKNOWLEDGMENTS The authors would like to thank the NSERC (Natural Sciences and Engineering Research Council of Canada) and FCAR (Formation de Chercheurs et l’Aide a` la Recherche de Que´bec) for their financial support.

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