Rehydration of Apple Dried by Different Methods

Abstract

Apple var. Idared cut into cubes was dried by convection and freeze-dried. Dried cubes were equilibrated to variable water activities by storage over saturated salt solutions (range 0–0.33). Rehydration was done for one hour in water at 20°C. Increase in both mass and volume of cubes was followed during rehydration. Changes in height of a cube were recorded using a minimal displacement gauge. Electron transmission microscopy was used to measure cell wall thickness in raw and rehydrated material. Modes of drying affected the behavior of apple cubes upon rehydration. Convective dried apple imbibed water and swelled during rehydration, and a collapse of structure was observed at the beginning of the wetting process. Analysis of both mass and volume increase showed that at the beginning of rehydration capillaries and pores were filled with water and thereafter swelling of biopolymers occurred. Cell walls imbibed water, but their thickness after one hour of rehydration was half of that of raw apple. Freeze-dried apple increased in mass much faster than these dried by convection. Rehydration caused collapse of structure, which was not rebuilt during further wetting. On the other hand, cell walls swelled significantly during rehydration and reached thickness almost that of raw apple. It was inferred that freezing injured tissue, which after drying contained numerous discontinuities. Hence, swelling of domains, which did not form a continuous structure, was not propagated and expressed by the increase in volume on a macroscopic scale. Initial water activity of both investigated materials affected collapse of structure during rehydration. The higher the water activity, the faster the collapse.

Keywords:

• Convective drying
• Freeze-drying
• Structure collapse
• Cell wall thickness

INTRODUCTION

Drying causes chemical, physical, and biological changes in food. Evaporation of water desiccates the solid matrix of the material and increases concentration of solubles in remaining solution. Water removed from the material is, at least in part, replaced by air and contact with oxygen is substantially increased. These are the main features of convective drying of solid moist foods.[1]

Gradients of temperature and water concentration occur in the material undergoing drying. The gradients cause shrinkage stresses, which lead to changes in size and shape of the material.[2–6] Until the material contains sufficient amount of water and is viscoelastic the shrinkage stresses, deform the material uniformly. The decrease in volume corresponds to the volume of evaporated water. At the later stages of drying when the surface is dry, and the center is still wet, the shrinkage is slowed down or stopped; and, fissures, cracks, and crevices in the material are created by shrinkage stresses. Tearing forces incur mechanical damage to the material undergoing drying.[7]

Evaporation of water during the final stages of drying can lead to phase transitions. Dehydrated chains of biopolymers can approach each other and change from the random coil to the crystalline state. On the other hand, viscosity of the concentrated solution can be so high that crystallization of low molecular weight molecules will not be possible; a glassy state will form. Hence, dry product can be composed of crystalline and glassy domains, which add mechanical resistance to the material.[8–11] Occurrence of these domains and the relationship between them will depend on the kind of the material and its chemical composition, as well as variables such as the rate of drying and final water content.

The previously mentioned data shows that convective drying causes many changes in material subjected to dehydration. Hence, drying cannot be simply treated as a process of evaporation of water from the material. Evaporation of water accompanied by chemical, physical, and biological modifications creates a product with new properties and features. Because of this, it cannot be expected that addition of water to dry product will restore properties of the material before drying. Rehydration can, however, be used as a method of assessment of injures incurred to the material subjected to drying.[12–14]

Freeze-drying is a process of food dewatering in which most of the convective drying disadvantages are eliminated. Concentration of solubles occurs during freezing, but low temperatures reduce the extent of changes caused by that process. Presence of hydration water, which does not freeze out, partly protects biopolymers. Shrinkage stresses are not present, thus there is no volume change and shape deformation. Activity of native enzymes as well as oxidative processes are reduced. Hence, it can be expected that freeze-dried product should be of much better quality than the one obtained by convective drying. However, the mechanical injury to the material caused by growing ice crystals must be taken into account, and the extent of that injury can be followed by the behavior of the material during rehydration. The aim of this article was to investigate the influence of the drying method on the behavior of dried material during rehydration.

MATERIAL AND METHODS

Apple var. Idared was used in all experiments. It was cut into 1 cm cubes immersed for 5 minutes into 0.5% citric acid solution to reduce enzymatic browning. Cubes blotted with filter paper were spread in a single layer on a wire tray and dried by convection at hot air temperature 70°C and velocity 1.5 m/s. The load on a tray was 5 kg/m². Decrease in mass of the material undergoing drying was recorded continuously. Cubes blotted with filter paper were frozen at −21°C for 20 hours. Then the frozen material was freeze-dried at pressure 37 Pa and temperature of tray 25°C. Drying was done in freeze-dryer Alpha 1–4 (Christ, Switzerland) for 24 hours. Dried cubes were placed in a desiccator containing anhydrous calcium chloride for two weeks at 25°C in order to reach the same initial dry matter content. It was assumed that removal of small amount of water under these conditions does not change the structure of apple cubes attained during the drying process. Thereafter cubes were transferred to desiccators with saturated salt solutions and equilibrated for 2 weeks at 25°C.

Rehydration was done in distilled water at 20°C. Mass of cubes equivalent to 1 g d.m. was flooded with 100 g of water and left motionless for a prescribed time. Rehydrated material was separated from water on a sieve and blotted with filter paper before weighing. Dry matter content of raw, dried, and rehydrated apple was measured by drying at 98°C until a constant mass was reached. Volume of cubes was measured by volume displacement method[15] using toluene as the working fluid. Volume of water imbibed by dried cubes during rehydration was calculated from the following equation:

where: m—mass, g; s—dry matter content, fraction, V—volume; τ—at prescribed time. A change of height of a cube during rehydration was followed using a minimal displacement gauge (OBRN-Warszawa). A cube of dried material was placed in a vessel and immobilized with two pins (Fig. 1). A perforated plate of 1 cm² surface weighing 0.34 g was put on the upper cube surface. Vertically from the top a displacement gauge was put on the plate and zeroed. The moving part of the gauge was weighing 3.2 g. Hence a total pressure exerted on the cube was 35.4 Pa. Water at room temperature was added to the vessel and the change in cube height during rehydration was continuously registered in micrometers by the computer. Measurement was done in 10 replicates.

Figure 1 Experimental stand for measuring cubes height during rehydration. 1—rehydrated material; 2— vessel with water; 3—perforated plate; 4—minimal displacement gauge; 5—computer.

Changes in cell wall thickness caused by drying and rehydration were observed under electron microscope using JEM 1220 (Joel, Japan) and magnification 10⁴. Samples were prepared according to the procedure used to fix plant tissue for microscopic examination.[16, 17] Samples were fixed in a glutaraldehyde solution, and then in buffer with osmium oxide. Dehydration was done in successive increasing concentration of ethanol and then in acetone. Dehydrated material was embedded in epon, polymerazed, cut in microtom and dyed with uranyl acetate and lead citrate.

RESULTS

Material Characteristics

Both drying methods yielded material with similar final water content (Table 1). However, convective dried material had undergone substantial shrinkage, while freeze-drying preserved the size and the shape of apple cubes. There also was a difference in final water activity; freeze-dried material had a much lower water activity than that measured in convection dried apple. Storage of dried material over anhydrous calcium chloride for two weeks reduced water content but did not yield completely dry apple cubes. However, the difference in water content and water activity was within the experimental error, and both convective and freeze-dried materials could be treated as equivalent.

Table 1 Characteristics of dry materials.

Property	Convective drying	Freeze-drying
Initial dry matter, %	10.87	10.53
Initial water content, g/g d.m.	8.20	8.50
Final dry matter content, %	93.45	95.24
Final water content, g/g d.m.	0.0701	0.0500
Initial volume, cm^3	1	1
Final volume, cm^3	0.33	1
Water activity
raw material	0.98	0.98
dry cubes	0.14	0.07
After storage over CaCl2
water content, g/g d.m.	0.0150	0.0151
water activity	0.037	0.033

Convective and freeze-dried apple cubes stored over saturated salt solutions gained in mass and during two weeks practically reached the state of equilibrium. The difference between sample and surrounding air water activity for convective dried apple was +4.0% and for freeze-dried material +8.3%. In both cases, water activity of the stored material was higher than that of the surrounding air.

Rehydration

Rehydration in water at ambient temperature was followed for 60 minutes. Gain in mass of convective dried material was much slower and smaller than that observed with freeze-dried apple cubes. After one hour of rehydration, the mass of freeze-dried cubes increased more than twice the value reached by convective dried material (Figure 2). The mass of convective dried cubes increased three folds while mass of freeze-dried material increased almost 7 times. Convective dried cubes regain 35%, while freeze-dried material recovered 76% of its initial mass before dehydration. Increase of mass during rehydration was accompanied by leaching of solubles to the surrounding water (Figure 3). Leaching was fast and excessive for freeze-dried cubes and little slower for the convective dried material. After 10 minutes of rehydration freeze-dried cubes lost 80% of dry matter and convective dried apple some 60%. Further leaching was slow, but even after 60 minutes of rehydration, there was still a pronounced difference between convective and freeze-dried material.

Figure 2 Increase in mass of dried apple during rehydration.

Figure 3 Soluble losses during rehydration of dried apple.

Taking into account that imbibition of water was accompanied by leaching of solubles rehydration ability was calculated.[18] Calculated values presented in Table 2 show that dried apple water absorption capacity and dry matter holding capacity are both dependent on the mode of drying. Freeze-dried apple absorbed water much better than the convective one, while for dry matter holding capacity an opposite relation was observed. Finally freeze-dried apple showed a little better rehydration ability in comparison to that of these dried by convection. Volume of freeze-dried apple did not change during rehydration (Fig.4). In contract, volume of convection dried cubes increased slowly during rehydration and reached about 0.68 cm³ after one hour. The increase was two folds in respect to the volume of dry material. Taking into account that leached out solubles are replaced by rehydrating water, the increase in volume of convective dried cubes was theoretically calculated. At the beginning of the rehydration process, the increase in volume is smaller than the volume of imbibed water (Fig. 5). The increase in volume becomes equivalent to the amount of imbibed water after about 50 min. of rehydration.

Table 2 Rehydration indices for dried apple.

Index	Convective dried	Freeze-dried
WAC	0.3816	0.8268
DHC	0.1692	0.1121
RA	0.0646	0.0927
WAC = water absorption capacity; DHC = dry matter holding capacity; RA = rehydration ability.

Figure 4 Swelling of dried apple during rehydration.

Figure 5 Water in pores and capillaries in relation to total volume increase of convective dried apple cubes.

Changes in Cube Height during Rehydration

Cube height was measured continuously during rehydration. Data collected in this article showed that the material dried under the same conditions was very heterogeneous in its rehydrating properties. The spread of lines was large and proved that each cube behaved in individual way. Fig. 6 show changes of height of the convective dried cubes equilibrated to different water activities and subjected to rehydration. Regardless of the initial water activity, all cubes showed decrease in height during the initial stages of rehydration. The time the minimum in the cube height occurred was not strongly dependent on the cube water activity—it was from 3 to 8 minutes on average and became shorter the higher was the water activity of the cube. After the minimum was reached, the increase in height was observed, and the return to the initial value occurred after some time of rehydration. There were cubes, which recovered the height fast, and there were cubes that did not regain height even after 60 minutes of rehydration. The time reaching relative height equal 1 was the shorter the higher was the water activity of the material. Height of freeze-dried cubes quickly decreased upon water addition and remained constant during further rehydration (Fig. 7). The extent and time of structure collapse was dependent on water activity of the sample. The lower the water activity, the smaller the decrease in the cube height, and the longer the time the collapse occurred. At a_w = 0.019 the relative cube height at minimum was 0.979, and at a_w = 0.330 the height was 0.933 of the initial value. The time to reach the minimum height of the cube was 52 seconds at a_w = 0.019 and less than 2 seconds at a_w = 0.330. Further soaking in water did not affect the height of the cube and no swelling occurred. The velocity of structure collapse was calculated as the ratio between the maximum displacement of the gauge and the time of that displacement. In convective dried cubes, the velocity of collapse was in the range 8.06 × 10⁻⁷ to 1.11 × 10⁻⁶ m/s and in freeze-dried material it varied from 4.04 × 10⁻⁶ to 4.47 × 10⁻⁴ m/s. These data demonstrate that convective dried apple collapse velocity was much less dependent on water activity (difference 1.37 fold) than that of freeze-dried material (difference 110 fold).

Figure 6 Changes in height of convective dried apple cube during rehydration.

Figure 7 Changes in height of freeze-dried apple cube during rehydration.

Initial water activity of the sample influenced the variability of response of the freeze-dried cubes to rehydration. At low water activity, individual apple cubes differed substantially in respect to one another. Hence, standard deviation around the average value was large. At higher water activities, the samples were more uniform and standard deviation was smaller (Fig. 7).

Cell Wall Thickness

Observation of rehydrated material under electron microscope showed that cell walls of freeze-dried material swell indeed. Thickness of cell walls in raw apple was from 1875 to 2312 nm (average 2093 nm). Convective dried apple after rehydration have had cell walls 937–1375 nm thick, (average 1300 nm), hence some two times thinner than those of raw apple. In freeze dried and rehydrated material average cell wall thickness was 1890 nm, and was close to that of raw material. These data proved that freeze-dried material swollen during rehydration in expense of pores and capillaries. In consequence there was no macroscopic change in size and shape of the material.

DISCUSSION

Increase in mass of convective dried apple was not paralleled by the equivalent increase in volume; it showed that rehydrating water first filled the pores and capillaries and thereafter was absorbed by polymers and caused swelling of the material. Filling of capillaries prevailed over the swelling process during the first 20 minutes of rehydration. Hence, some wetness of the material must be reached in order to cause swelling of the polymers. Freeze-drying caused no shrinkage, therefore the swelling of rehydrating material could not be observed. But it did not mean that rehydration of freeze-dried material was a solely capillary filling process. Polymers present in cell walls also swelled, but this process was not pronounced on a macroscopic scale. Rehydration of cubes of dried apple subjected to small load causes collapse of the structure.

In convective dried apple, water adds some resistance to deformation, and the higher the water activity of cubes the smaller was the collapse. On the other hand, the higher the water activity of the freeze-dried material, the larger the collapse of structure. A very characteristic is the fact that collapsed structure of convective dried cubes was rebuilt during rehydration, while structure of freeze-dried material had no that ability. This suggested that structure of both materials differed substantially.

In convective dried apple, shrunken and compact structure imbibes water by capillary suction and wetted dry matrix begins to swell. The matrix forms an at least partly continuous network, which is able to transmit increase in its volume in one part to other parts. Hence, the swelling causes increase in volume, and the matrix continuity is responsible for its mechanical resistance. The data implies that cell wall network in convective dried apple is at least partly continuous. In freeze-dried material, swelling of some fragments of the network, although much larger than that observed in convective dried material, was not reflected in the macroscopic change of volume. Such a situation would occur if the network was fragmented and swelling of one domain was not transmitted to other domains. It resulted in inability to overcome the load and to restore collapse.

The previously presented apple structure alterations were caused by drying and inferred from rehydration behavior of the dried material conformed very well with published microscopic observations. On the basis of cross-section cavities, seen under the microscope, it was calculated that in a convective dried apple, 7 cavities were formed from 11 cells, while in freeze-dried material 2 cells were needed to form a void.[7] Thus, freeze-drying causes more extensive apple cell wall rupture than the convective process.

CONCLUSIONS

Convective dried apple rehydrated slowly, and at the beginning of the process, the filling of capillaries and pores prevailed over the swelling. Hence, the mass of the cube increased faster than its volume. Initial stages of rehydration were accompanied by the collapse of structure, which was followed by the swelling of the solid matrix. The swelling of substructures was superimposed and macroscopic increase in volume was observed. Freeze-dried apple rehydrated fast and extensively lost solubles to the surrounding water. The collapse of structure upon rehydration was observed, and the rate of that process was much faster than that occurring in convective dried apple. Moreover, collapsed structure was not rebuilt, although the cell walls of freeze-dried apple swelled and reached thickness close to that of raw material. It was suggested that freeze-dried material was strongly injured during freezing and its structure was discontinuous. Hence, the swelling of substructures was independent and did not cause increase in volume upon rehydration. Water activity, in the range 0–0.33, of both convective and freeze-dried material affected the collapse of structure. The higher the water activity, the faster was the collapse.