To further explore the mechanisms of WL and SG in the OD process, and then to investigate the transfer of antioxidants into strawberries, additional experiments were conducted at maximum glycerol concentration and temperature, with varying immersion times of 170, 190, 220, 270, and 300 min. These specific immersion times were selected based on the optimization results, which indicated that this range was critical for maximizing water loss and solid gain. Previous studies have consistently shown that immersion time plays a key role in determining dehydration efficiency [
25,
28,
34]. By focusing on this time range, we aimed to gain deeper insights into how immersion time influences not only WL% and SG% but also the migration of natural antioxidants into the strawberries from the hypertonic osmotic chokeberry infusion, as well as the physicochemical parameters of the strawberries such as water activity (a
w), moisture content, firmness, and color.
It has been reported that in the early stages of the osmotic process, the driving force between the product and the hypertonic solution is the strongest, resulting in more pronounced solid uptake rates [
30]. As the process progresses, the osmotic pressure reduces due to mass transfer between the phases. Over time, as water migrates from the sample to the medium and solute moves from the solution to the sample, the concentration gradient decreases, leading to lower dehydration rates [
44,
45]. This is also consistent with Kowalska et al. (2023) [
3], who observed that water loss and solid gain were most pronounced in the early stages of osmotic dehydration. Moreover, this rapid initial loss can be attributed to the significant pressure difference between strawberry cells and the surrounding hypertonic solution, which promotes quick water molecule diffusion. As dehydration continues, this pressure difference diminishes, leading to structural changes in the strawberry tissue and approaching a dynamic equilibrium in mass transfer [
34]. Finally, Brochier et al. (2019) [
39] observed that solid gain reached equilibrium after 250 min in a 65 °Brix solution at 25 °C for kiwi fruit.
These findings align with the optimization model, which predicted that immersion time, along with glycerol concentration and temperature, plays a crucial role in maximizing water loss and solid gain. The model identified specific conditions that optimize these responses, and the observed experimental results validate these predictions. The close agreement between the predicted and real values of WL% and SG% underlines the model’s robustness and reliability.
Moreover, the model suggested that maximum glycerol concentration and temperature, combined with optimal immersion times, would yield the best results for both water loss and solid gain. Experimental validation confirms this by showing that longer immersion times were beneficial, albeit with a plateau at certain durations, which the model also hinted at through the prediction of diminishing WL and SG values beyond a certain time point.
3.2.2. Quality Assessment
The effectiveness of the osmotic dehydration process for strawberries was assessed by analyzing several quality parameters. These parameters included water activity (a
w), moisture content, texture, and color, alongside the previously presented results for water loss (WL), solid gain (SG), and natural antioxidants transfer. The values of water activity, % moisture, and firmness of the strawberry samples in the different immersion times are presented in
Table 6.
The results showed a clear trend of decreasing a
w and moisture content with increasing immersion time, which aligns with results from other research studies [
44,
48,
49]. The fresh strawberries’ samples had an a
w of 0.9889 and a moisture content of 88.58%. These values significantly (
p < 0.05) decreased as the immersion time increased. At 170 min, the a
w was reduced to 0.9669, and the moisture content was reduced to 82.25%. A further reduction in a
w and moisture content was observed with increasing immersion times, reaching the lowest (
p < 0.05) values at 300 min with an a
w of 0.8754 and a moisture content of 53.09%. The significant reduction in a
w and moisture content over time indicates the effectiveness of the osmotic dehydration process in removing moisture from the strawberries. The decrease in a
w to below 0.9, especially at 220 min and beyond, suggests that the strawberries had reached a level where microbial growth was significantly inhibited, enhancing their shelf life and stability. The moisture content also was reduced significantly, reaching just over 53% at 300 min, which is within the optimal range for dried product storage as recommended by Omolola et al. (2017) [
50]. Generally, the minimum a
w at which microorganisms can grow is 0.60, but these numbers are variable; for example, halophilic bacteria can grow at 0.75, and most bacteria require a
w levels of about 0.87 for growth. Dried fruits and vegetables, typically have a low a
w of 0.70 [
51]. The osmotic dehydration process was effective in significantly reducing both the water activity (a
w) and moisture content of the strawberries, with the lowest values observed at the longest immersion times. The reduction in a
w to below 0.9 and moisture content to just over 53% demonstrates the efficiency of the process in inhibiting microbial growth and enhancing the shelf stability of the product. However, while these reductions contribute to the overall quality and shelf life of the dehydrated strawberries, the levels achieved suggest that additional processing steps, such as further drying or appropriate packaging, may be necessary to ensure the product’s long-term safety and stability.
Firmness is a critical quality attribute in dehydrated strawberries, influencing both consumer acceptance and the textural integrity of the final product. Initially, the fresh strawberries had a firmness of 2.41 ± 0.58 N. With increasing immersion time, there was a general trend of decreasing firmness, although this decrease was not significant in most cases. Specifically, the firmness values at 170 min (2.34 ± 0.72 N) and 190 min (1.78 ± 0.10 N) showed a reduction compared to the initial firmness, but these changes were not statistically significant (
p ≥ 0.05). At 220 min, firmness slightly increased to 2.03 ± 0.18 N, indicating a temporary stabilization of the fruit’s structural integrity during this phase of dehydration. Beyond 220 min, the firmness of the strawberries continued to decline, reaching 1.43 ± 0.20 N at 270 min and further decreasing to 1.05 ± 0.16 N at 300 min. The changes in firmness at these longer immersion times were statistically significant to the initial firmness of the slices. This suggests that the structural integrity of the strawberries was relatively well maintained throughout most of the osmotic dehydration process, with significant changes only observed at extended immersion times. The observed decrease in firmness found during osmotic dehydration aligns with findings from previous studies. Torreggiani and Bertolo (2019) explained that the deformation of the middle lamella, which leads to turgor loss and displacement of intercellular substances, causes textural changes [
52]. The primary reasons for the decrease in firmness include the dissolution of pectin, turgor loss, and tissue shrinkage [
53,
54,
55]. Although some water loss and solid gain can temporarily stabilize firmness, osmotic dehydration generally results in a softer texture in fruits and vegetables [
56]. The type and concentration of the osmotic solution has a significant impact on the rate of osmotic dehydration and the food material’s texture [
30]. For example, increasing the osmotic solution concentration with added sugar can increase the hardness of fruits like mangoes due to sugar’s structural strengthening effect [
57]. However, excessively high concentrations can lead to structural deterioration and decreased firmness, as seen in studies on potatoes and cranberries [
58,
59]. The use of firming agents, such as calcium lactate and calcium chloride, in the osmotic solution has been shown to enhance the firmness of fruits and vegetables by interacting with the carboxyl groups of pectin in the cell wall [
57,
60]. Prosapio and Norton (2018) successfully used calcium lactate to achieve a better texture during osmotic dehydration [
61].
In summary, while the firmness of strawberries generally decreased during osmotic dehydration, the changes were not significant until longer immersion times were reached. This indicates that the strawberries’ structural integrity was mostly preserved throughout the osmotic dehydration process. The addition of calcium chloride (CaCl2) in the osmotic solution significantly helped maintain texture by forming chemical bonds with the plant tissue matrix, preventing textural deterioration.
To assess the impact of the dehydration process on the visual appeal of the samples, the color parameters L*, a*, b*, and h were measured, and their values at the different immersion times are presented in
Table 7. These color parameters were crucial in assessing the visual quality and any changes in the appearance of the strawberry slices due to the osmotic dehydration process. The values of L* indicates that the strawberries darkened initially, as the value dropped significantly at 170 min, remained stable until 270 min, and further increased at 300 min. The overall trend indicates that the strawberries darkened initially but regain some lightness with extended immersion times. The a* value increased, reaching its peak at 300 min with a value of 23.55. This increase suggests a more intense red color, which is desirable as it can be associated with ripeness. The b* value decreased significantly from 16.36 to 11.00 during the 170 min of the procedure, showing a reduction in yellowness. Then, it was followed by a gradual increase until the end of the procedure. The hue angle was initially 39.19, representing a yellowish red color. Its value decreased significantly to 26.92 at 170 min, indicating a shift towards a redder hue, followed by an increase to 30.40 by the end of the osmotic dehydration. The overall decrease in hue angle suggests that the strawberries became redder during the dehydration process, which was consistent with the increase in the a* value. The appearance of the osmotically treated strawberry slices at different immersion times is shown in
Figure 7.
In conclusion, osmotic dehydration seems to effectively preserve the color of strawberries, with initial darkening followed by partial lightness recovery and an overall increase in redness. The results align with previous research indicating that OD generally preserves color due to reduced air exposure and minimal heat impact [
62]. The variations in color parameters depend on factors such as the concentration of osmotic solutions, temperature, and pretreatment methods [
54,
63]. Finally, the addition of calcium chloride as a firming agent may also contributed to the color retention by interacting with cell wall components [
60,
64].