Monitoring mechanical, color and anthocyanin changes during rehydration of raspberry-based products

2.1Fruits

Frozen raspberries (Rubus idaeus L.) of Autumn Bliss cultivar grown in Plottier (Neuquén province, Argentina) were used. After harvest, fruits were immediately individually quick frozen (IQF process) and stored at –22°C. The selection of the fruit to be used for this work, as well as the characterization of the frozen fruit were carried out as previously described by Sette et al. [13].

2.2Treatments

Fruits were subjected to sugar infusion pretreatments and further air- or freeze-drying. Infusions were performed at room temperature in glass vessels (8×16 cm). Different systems were prepared by immersing the frozen fruits into a mixture (dry or wet) of sucrose (osmotic agent) and preservatives. Potassium sorbate and sodium bisulphite were used as antimicrobial agents during infusion; sodium bisulphite also acts as inhibitor of enzymatic and non-enzymatic browning. Citric acid was also added in order to favor red hues of anthocyanins. The amount of sugar and chemical agents used were determined according to the weight of the fruit (100 g) and the final levels required after equilibration of the components between the food system and the generated syrup during infusion (a_w = 0.85). Three systems were prepared by using the methodology of Sette et al. [13]:

Samples with and without pretreatments were then subjected to different drying processes (air- and freeze-drying) to reach a final a_w value of 0.33 as described by Sette et al. [14]:

The concentration of additives, both in dried fruits as well as in rehydrated fruits, did not exceed the maximum values according to the “Codex General Standard for Food Additives” (Codex STAN 192–1995).

2.3Rehydration process

Rehydration process was carried out in a thermostated water bath at 25, 40, and 60°C, using 1 g of dehydrated fruit in 20 mL of water. Rehydration curves were obtained by weighing the samples at different intervals until reaching a plateau and/or a decrease in weight gain. For physicochemical studies, three specific values of time were chosen: 5 min, 20 min and the end of rehydration process. Before weighing, samples were taken out from the water bath and placed on filter paper in order to remove the unbound water.

2.4Modeling of rehydration kinetics

Rehydration curves were mathematically described by using two empirical models: Peleg’s model and the Weibull distribution function. Peleg’s model is represented by the following equation:

where t is the drying time (min), X is the water content (kg water/kg dry matter) at time t, X₀ is the initial water content (kg water/kg dry matter), k₁ parameter (kg dry matter/kg water.min) is defined as the inverse of the water transfer rate at the beginning of the rehydration process and k₂ (kg dry matter/kg water) is related to the maximum capacity of water [15], so that:

at   t = 0   dX/dt = 1/k₁

at   t⟶∞   X_e = X₀ + 1/k₂

Weibull’s model is described by the following equation:

Where t is the drying time (min), X_e is the equilibrium water content (kg water/kg dry matter), α and β are the kinetic parameters of the model. β parameter (1/min) is analogous to k₁ constant of Peleg’s model, since it measures the water absorption rate of the process when the superficial capillaries and cavities fill very fast. The shape parameter (α) (dimensionless) is an index of the product behavior during rehydration; it would be related to the different transport mechanisms that can take place such as diffusive, convective and relaxation mechanisms of the tissue matrix [15].

2.5Rehydration indices

In order to quantify the rehydration capacity of the dehydrated raspberries, two coefficients were calculated: the rehydration relationship (RR) and the rehydration ability (RA). The rehydration relationship (RR) was defined according to Wang and Chao [16] as:

The rehydration ability (RA) was defined according to Meda and Ratti [5] as:

Where m_rh is the weight of rehydrated sample (kg), m_dh is the weight of dehydrated sample (kg) and m_o is the weight of the fresh sample (kg).

2.6Sample analysis

2.7Statistical analysis

A completely randomized design was used. For all determinations, except for superficial color, volume recovery and mechanical properties, three replicates were measured. The results were expressed by mean and standard deviation of the mean (SD). An analysis of variance was performed to establish the presence or absence of significant differences in parameters according to the factors “rehydration temperature”, “type of infusion” and “drying method”. Multiple comparisons were carried out by using the Tukey test and significance level was set at p < 0.05. In the case of significant interactions between factors, the Tukey test was run for the interaction. For not significant interaction between factors, a Tukey test of main effects was performed. All statistical analyses were carried out using the data analysis software system STATISTICA version 8.0 (StatSoft, Inc., Tulsa, OK, USA).

The selected models were fitted to the rehydration curves, and the model parameters were determined by nonlinear least-squares regression analysis. The quality of fit was evaluated through the determination coefficient (r²), the root mean square error (RMSE) and the reduced chi square (χ²):

where: X_exp is the experimental water content and X_pre is the water content calculated with each model.

3.1Rehydration curves and fitting to the proposed models

Figure 1 shows the experimental rehydration curves obtained at different water temperatures (25°C, 40°C, and 60°C), and Table 1 shows the results of fitting the data with the Peleg’s model and the Weibull distribution function. For Weibull’s model, values of r² > 0.98 and RMSE<0.05 were obtained in most cases; therefore, this model was selected for the representation of the predicted data in the figures. Several authors obtained similar results when studying the rehydration kinetics in different plant products like air-dried and freeze-dried carrots [21], air-dried mushrooms [22] and air-dried aloe vera [12].

Fig.1

Comparison of experimental and predicted rehydration curves of raspberries: experimental data (symbols) and prediction given by Weibull’s model (lines): (a) 25°C, (b) 40°C, (c) 60°C. Air-drying (empty symbols and dotted lines) and freeze-drying (filled symbols and continuous lines). C (circles), WI-BAC (triangles), DI-BAC (diamonds).

The freeze-dried sample without pretreatment (CF) presented a higher rehydration rate than the air-dried control samples (CA) (Fig. 1) and the rate decreased at higher rehydration temperatures. The CF curve showed a steeper initial slope (>1/k₁ and 1/β) reaching its maximum rehydration capacity in a very short period of time (<1 min). It has been reported that during the first rehydration stage, water fills the air spaces present in the dehydrated food products [23]. The higher porosity of the freeze-dried sample may explain the higher rehydration rate compared to the CA sample. Previous studies on freeze-dried products have also obtained highly porous matrices with high rehydration capacity, due to the characteristics of the freeze-drying process [5, 24, 25]. At the early stage of the rehydration process, water rapidly fills the empty spaces of the freeze-dried matrices, producing an instantaneous increase in water absorption [24]. On the other hand, the high temperatures to which the tissue is subjected during air-drying lead to a structure of collapsed capillaries that reduce the rehydration rate [26]. This behavior occurred at 25°C and 40°C, but at 60°C both samples reached the maximum rehydration capacity at similar speed and time, reaching the same final water content after the rehydration process (≈k₂).

Temperature influences certain characteristics of the product such as the cell wall structure and cell membranes [27], normally producing the loss of nutrients and pigments which can be avoided in part with some pretreatments before drying. It has been shown that rehydration at temperatures below 40°C maintains the original structure of cell wall pectins improving the water absorption capacity of the tissue [28].

Pretreated raspberries (Fig. 1) had lower water incorporation capacity than control samples at all the rehydration times evaluated. This fact could be attributed to the changes in the chemical composition and the distribution of components in the tissue caused by the osmotic dehydration. When raspberries were immersed in the hypertonic solution, an increase in sugar content and a decrease in water content occurred. Also, this pretreatment caused the development of a concentrated solids surface layer, and the reduction of tissue porosity and shrinkage due to sugar infiltration [14]. A much lower water incorporation rate (>k₁) and a lower equilibrium value were evidenced, especially at 25°C, probably due to the presence of an additional resistance generated by the concentration of solutes in the fruit during the previous osmosis process. In the early stages of rehydration, this superficial sugar dissolves, making the absorption of water more difficult. This behavior was not observed in control dehydrated fruits (C), which exhibit the more abrupt initial slope in the rehydration rate with respect to the pretreated samples.

Regarding the characteristic parameters obtained by the Weibull’s model (Table 1), the temperature did not have a clear influence on the parameter α, since it decreased at 40°C, but at 60°C the behavior did not follow a particular trend. Similarly to the k₁ Peleg parameter, the β parameter evolved in an inversely proportional way to the applied rehydration temperature, except for the CF samples, where an increase of β and k₁ was observed with temperature. This behavior could be due to the damage on tissue porosity caused by the high temperatures on freeze-dried samples without pretreatment. On one hand during freeze-drying process a more porous structure is achieved, which usually allows a rapid intake of water. However, the effect of pre-freezing leads to a more fragile tissue that upon rehydration at high temperatures is disorganized and exhibits a porosity loss, leading to a lower capacity to recover the initial water of the fresh fruit.

3.2Rehydration indices, volume recovery and mechanical behavior

As rehydration behavior can be considered as a measure of injuries caused to the materials by treatments, some rehydration indices are often used as indicative of the loss of cell integrity occurred in the plant tissues not only during pre-drying treatments and subsequent drying, but also during rehydration per se. In this study, two indices were used to evaluate the rehydration performance. The RR index, named rehydration percentage, rehydratability or restoration ratio [9], that measures the relative mass increase of dried materials under rehydration in water, whereas RA index (rehydration ability) represents the matrix ability to recover the water lost during the dehydration processes [5]. Table 2 shows RR index calculated at three values of time selected as convenient for different applications of the raspberry products. The 5 min time corresponds to the consume period for the fruits included in a cereal mix; 20 min would be an appropriate time to recover the fresh fruit characteristics to be used in yogurts or desserts; and the equilibrium time allows an in-depth understanding of the rehydration process at the assayed conditions. At 25°C 5 min was enough time to fully rehydrate freeze-dried samples without pretreatments (CF), while air-dried raspberries (CA) at 20 min reached only 55% of their maximum rehydration capacity. At 40°C, CF samples also showed rapid water absorption, achieving 86% of the equilibrium value in 5 min. However, at 60°C CF samples presented a different behavior, as they exhibited lower and significantly different (p < 0.05) RR indices at each value of time, in contrast to the values observed for CA samples that increased with the rehydration temperature. However, Fig. 2-a shows that at 25°C, both, air- and freeze-drying led to the same recovery of fresh fruit water content (RA) at the end of the rehydration process, despite CF samples are more porous and presented ≈ 10% shrinkage according to Sette et al. [14]. Despite the instantaneous rehydration observed at short times, water-holding capacity diminished along time, probably due to the structural damage caused by the freezing step during the freeze-drying process.

Fig.2

Volume recovery, Vr_final (circles) and rehydration ability, RA _final (quadrates) at the end of the rehydration process raspberries obtained after the application of air-drying (empty symbols and dotted lines) or freeze-drying (filled symbols and continuous lines). Control samples (a), WI-BAC samples (b) and DI-BAC samples (c). Bars represent standard deviation of the means. Interaction between the three studied factors was significant for both variables.

As saturation moisture achieved by air-dried samples was much lower than that of fresh fruit (567±14 kg water/100 kg wet basis) and maintained at all rehydration temperatures (Fig. 1), it can be assumed that structural changes occurred in plant tissues during dehydration processes have been irreversible, restoring only 30% of fresh fruit water content and ≈ 50% of the initial volume (Fig. 2). These results demonstrate that, although freeze-drying protects the primary structure and the materials shape during water sublimation, with minimal collapse, a complete recovery of fresh samples in terms of moisture content was not attained. By analyzing the mechanical properties of these control fruits by back-extrusion tests (Fig. 3-a), a higher firmness in air-dried fruits was observed when compared to the freeze-dried ones after 5 and 20 min of rehydration at 25°C and 40°C. The mechanical behavior of air-dried control raspberries has been previously ascribed to the crust formation during air-drying, as well as to the greater shrinkage and the lower water content exhibited by the samples [14]. The freeze-dried fruits had a very low resistance to the extrusion force at all rehydration conditions. According to Prothon et al. [29], the fact that a tissue apparently has a better preserved structure, does not necessarily mean that its microstructure is not damaged and this appears to take place in freeze-dried raspberries, which have a much more fragile structure despite not showing structural collapse. Ice crystals formed during freezing step cause cell structure disruption, and, after sublimation, this generates, not only air-filled pores in the intercellular spaces of the fruit, but also new pores, resulting in an extremely porous structure. This will produce a texture loss during the subsequent rehydration. The above mentioned authors introduced the concept of “textural collapse” to describe this phenomenon.

Fig.3

Peak force (F_max, N) of raspberry products rehydrated for 5 min () and 20 min (), at 25°C (a), 40°C (b) and 60°C (c). Fresh sample and raspberries obtained after the application of air-drying or freeze-drying: control (C) and pretreated (WI-BAC, DI-BAC). Bars represent standard deviation of the means. Interaction between the three studied factors was significant.

The rehydratability of the pretreated samples was significantly reduced (p < 0.05) compared to their respective control samples due to the combination of different factors affecting the ability of the structure to absorb water (Table 2). Both infusion pretreatments (dry and wet) affected the ability of matrix to retain water to a greater extent in the case of freeze-dried fruits, especially in DI-BAC samples (Table 2). At 25°C these samples recovered only 3% of the total water removed in the successive dehydration processes and 50% of the initial fruit volume (Fig. 2-c), which is indicative of the significant structural changes these samples experimented not only during the infusion process but also during subsequent steps of freezing and freeze-drying. Previous studies showed a lower shrinkage of WI-BAC samples after freeze-drying process compared to DI-BAC [14]. This fact could explain the increased capacity of these samples to recover water and volume upon rehydration. By comparing the mechanical behavior of pretreated samples after rehydration (Fig. 3) it was verified that the use of DI-BAC pretreatments led to higher firmness at the different rehydration temperatures, which could be related to the lower water recovery ability and lower volume recovery (Fig. 2).

The RA values in pretreated samples increased at 40°C, but then decreased at 60°C (Fig. 2-b and c) probably because the ability to hold soluble solids diminished at high temperatures and also because the structural changes become irreversible. According to studies carried out by Maldonado et al. [30] when working with mango fruits, an increase in the rehydration temperature from 25 to 40°C improved the efficiency with minimum solids loss, but above 60°C the rehydration capacity decreased due to cell damage induced by heat. Studies with other fruits such as apple, banana and tomato have shown that an increase in the temperature from 25 to 40°C provoked not only an increase in water absorption but also a loss of tissue solids [31]. Studies carried out with broccoli showed a higher rehydration capacity at low temperatures, for a rehydration temperature range between 20–80°C [32]. These authors related water absorption capacity with the pectic polysaccharides content of tissue cell walls and with the effect of the rehydration temperature on the pectic enzymes activity. The chemical degradation of pectins induced by the heat generated during drying and/or rehydration influences water uptake and retention by the cellular tissue. The improvement observed in pretreated raspberries at 40°C could indicate that the permeability of the cell membranes was recovered and the surface barriers disappeared. However when rehydrating at 60°C the lesser ability of the tissue to recover the water lost during dehydration could indicate a greater degradation of cell walls and membranes by pectic polymers solubilization during sugar infusion and by thermal degradation during drying and/or rehydration with a probable significant solids loss to the rehydration medium (<RA). The results obtained provide evidence of structure damage incurred throughout the three stages of processing: pretreatment, dehydration (air or freeze-drying) and rehydration. According to Khalloufi and Ratti [33], structural and chemical alterations during the rehydration process itself, is one of the main factors that affect the ability of fruits to imbibe and retain water, but its influence is mainly ignored in the literature.

3.3Color and anthocyanin loss evaluation during rehydration

3.3.2Superficial color

Figure 4 shows the chromatic variables, chroma (Fig. 4-a), hue angle (Fig. 4-b), and luminosity (Fig. 4-c) for the dehydrated raspberry products upon rehydration at 25°C. Superficial color was evaluated at 25°C, 40°C and 60°C, however, no significant effect of temperature was detected, then, only results at 25°C are shown.

Fig.4

Chroma (C*_ab) (a), hue angle (h_ab) (b) and luminosity (L*) (c) of raspberry products rehydrated at 25°C for 5 min (), 20 min () and at the end of rehydration (). Raspberries obtained after the application of air drying or freeze-drying: control (C) and pretreated (WI-BAC, DI-BAC). Bars represent standard deviation of the means. Interaction between the three studied factors was significant for the variables C*_ab and L*. For h_ab variable significant interactions were obtained between “rehydration time” and “drying method” (superscript letters) and interaction between “rehydration time” and “type of infusion” (lowercase letters).

In general, freeze-dried samples showed higher chroma values than air-dried ones. Also, chroma values did not change along rehydration time for all the freeze-dried samples and control air-dried ones. Pretreated samples showed an important increase in hue angle at the final time of rehydration indicating a shift from red to orange hues. In the case of air-dried samples, the hue angle increase was accompanied by a C*_ab increase, reaching in all the pretreated samples to similar orange tones. This fact could be related to the damage on the tissue causing the complete loss of soluble contents of some drupelets exposing the yellowish walls, rendering in average an orange hue to the rehydrated fruit. Visual aspects of the rehydrated products can also be appreciated in the images shown in Figs. 5 and 6.

Fig.5

Images obtained after rehydration at 25°C of air-dried raspberries at different values of rehydration time. Control (C): 5 min (a), 20 min (b) and final (c). Wet infusion (WI-BAC): 5 min (d), 20 min (e) and final (f). Dry infusion (DI-BAC): 5 min (g), 20 min (h) and final (i).

Fig.6

Images obtained after rehydration at 25°C of freeze-dried raspberries at different values of rehydration time. Control (C): 5 min (a), 20 min (b) and final (c). Wet infusion (WI-BAC): 5 min (d), 20 min (e) and final (f). Dry infusion (DI-BAC): 5 min (g), 20 min (h) and final (i).

Regarding luminosity, all the rehydrated samples showed higher L* values than the fresh raspberry (L* = 24.4±2.2). Usually the drying processes promotes an increase in the luminosity values with respect to the original sample because during dehydration the water present in the intra- and intercellular spaces are replaced by air, providing dehydrated matrices with an opaque and pale appearance (>L*). This fact is related to diffusion of light in materials with interfaces of different refractive indices [35]. After the rehydration process, this phenomenon is expected to be reversed; however, due to pigments loss and structure changes occurring during drying and rehydration, the luminosity remains higher than the control fresh sample. Noteworthy, both freeze- and air-dried control samples preserved the red hues and did not show significant changes in L*during rehydration.