Enhancing the Nutritional Value and Preservation Quality of Strawberries through an Optimized Osmotic Dehydration Process

Abstract

This study aimed to optimize the osmotic dehydration process of strawberry slices by examining the effects of glycerol concentration, immersion time, and temperature on water loss and solid gain. Additionally, the study explored the use of chokeberry infusion to enhance the total phenolic content of the strawberries, thereby increasing their nutritional value. Using the Box–Behnken design and response surface methodology, the study identified optimal conditions to maximize water loss and solid gain. The findings revealed that high glycerol concentration (60% w/w) and temperature (50 °C), combined with optimal immersion times, significantly influenced water loss and solid gain. Experimental validations confirmed the model’s predictions, showing high prediction accuracy (87.37% to 87.69%) for water loss determination but moderate prediction accuracy (42.80% to 64.72%) for solid gain. The immersion of strawberry slices in a hypertonic osmotic chokeberry infusion for 170–220 min maximized the migration of natural antioxidants. Moreover, the osmotic dehydration process effectively reduced water activity and moisture content, enhancing the strawberries’ shelf stability. Furthermore, the addition of calcium chloride (CaCl₂) helped maintain the strawberries’ firmness and color during dehydration. Overall, the optimized osmotic dehydration process preserved the structural integrity and improved the nutritional profile and preservation quality of the strawberries, suitable for preparation of various fruit-based products.

1. Introduction

Strawberries are widely appreciated for their delicious flavor, taste, and nutritional benefits, but also for their low energy value of approximately 32 kcal/100 g. Rich in bioactive compounds, such as polyphenols, and essential micronutrients, including potassium, phosphorus, magnesium, iron, zinc, calcium, manganese, and dietary fiber, their health benefits are largely attributed to their polyphenol content, especially anthocyanins, and vitamin C, which contribute to their antioxidant potential [1]. Due to their seasonal nature and short postharvest life, effective preservation techniques are essential to maintain their availability and nutritional quality throughout the year. Several postharvest preservation methods have been explored to extend the shelf life of strawberries while retaining their sensory and nutritional attributes. Edible coatings are a promising method for extending strawberry shelf life. Studies have shown that coatings, such as carboxymethyl cellulose enriched with bacteriocins, can effectively reduce microbial spoilage while preserving fruit quality [2]. In addition, processing techniques, including freezing thermal treatment and high-pressure processing, have a significant impact on the retention of the above-mentioned bioactive compounds, affecting not only their phenolic properties but also their color and sensory characteristics [3,4]. Traditional drying methods, although effective, can have a negative impact on the quality of the fruit [4].

Osmotic dehydration (OD), a simple and energy-efficient preservation method, involves immersing fruit or vegetables in a hypertonic solution, resulting in water loss (WL) and solid gain (SG), which modifies their chemical composition and sensory characteristics [5]. OD is beneficial as it preserves natural attributes, reduces energy costs, prevents enzymatic browning, and retains fruit color and nutrients [6]. In addition, OD followed by drying reduces drying time and increases dryer efficiency, making it an attractive alternative for the production of nutritious fruit snacks [4,7,8]. OD has also been used to infuse fruits with minerals and bioactive compounds such as inulin, calcium, vitamins, and antioxidants, thereby improving their nutritional profile and sensory qualities [3,8,9,10,11]. Prior research has underscored the significance of several variables, including the type of the osmotic agent, the concentration of the osmotic solution, the duration of immersion, the temperature during the process, and the ratio of fruit parts to the osmotic solution, in influencing the efficacy of osmotic dehydration [12]. However, as the procedure is affected by a variety of factors, a comprehensive understanding of their combined effects, remains limited.

Studies have demonstrated that glycerol, as an osmotic agent, offers greater stability and is more effective in preventing microbial spoilage than conventional sucrose solutions, which tend to support the growth of fungi and yeasts [13,14,15]. As a hypotonic solution for the osmotic solution preparation, chokeberry Aronia melanocarpa infusion was selected for its rich antioxidant content. According to previous studies, chokeberry infusion is known to enhance the nutritional profile and color attributes of fruits and vegetables [3,4,16,17,18].

Temperature, immersion time, and glycerol concentration in the solution are considered to be the most important factors in the OD process as previously reported [19,20,21]. These variables could be chosen due to their critical roles in determining water loss, solid gain, and the overall quality of the dehydrated product. Despite its potential, the uptake of osmotic dehydration in the food industry has not been as extensive as expected, primarily due to a limited understanding of the counter-current flow phenomena associated with the process. Nevertheless, the demand for healthy, natural, nutritious, and tasty processed food products continues to rise, not only for finished products but also for ingredients used in complex foods such as ice cream, cereals, dairy, confectioneries, and bakery products [22].

Therefore, the aim of this study was to optimize the OD process of strawberry slices by investigating the combined effects of glycerol concentration, immersion time, and temperature on WL and SG. Additionally, this study explores the use of chokeberry infusion during OD to enrich the strawberries, enhancing their total phenolic content (TPC) and thus increasing their nutritional value. Using Response Surface Methodology (RSM), in particular the Box–Behnken design, this study sought to identify the optimal conditions that maximize both WL and SG while also improving the efficiency and effectiveness of the dehydration process and antioxidant transfer. The ultimate goal was to enhance the preservation quality and nutritional profile of strawberries, making them suitable for a variety of fruit-based products and extending their stability. Furthermore, this study evaluates the impact of these parameters on additional physicochemical characteristics such as, water activity, moisture content, firmness, and color, providing a comprehensive understanding of the osmotic dehydration process for strawberries. The optimization of this process not only contributes to improving the nutritional profile of strawberries but also offers practical applications for producing higher-quality fruit-based products with extended shelf life.

2. Materials and Methods

2.1. Sample Preparation

Fresh strawberries (Fragaria × ananassa) were sourced from a local market. The strawberries were uniform in size, color, and ripeness stage to ensure experimental consistency. They were washed thoroughly with tap water to remove any surface impurities and gently dried with paper towels. The initial humidity of the strawberries was 88.58 ± 1.71% (wet basis). The strawberries were cut into 10 ± 0.5 mm-thick slices, each weighing 7.5 ± 1.2 g.

For the osmotic solution preparation, an aqueous infusion of chokeberry and food-grade glycerol (Honeywell Specialty Chemicals Seelze GmbH, Seelze, Germany) were used. The food-grade glycerol was sourced from a local supplier, and the antioxidant-rich chokeberry infusion was prepared fresh for each experiment. Glycerol, also known as sugar alcohol (E 422, group I), is considered a non-toxic and safe additive according to Regulation (EC) No. 1333/2008 and was chosen as the osmotic agent due to its water binding capacity, which significantly reduces the water activity of the product and extends its shelf life. Regarding infusion preparation, dry chokeberry fruits were crushed and homogenized, then added to boiling water at a ratio 1:5 g/mL (w/v), and, after that, left for 1 day at room temperature (25 °C). The solution was afterward filtered under reduced pressure. To prevent textural deterioration during the dehydration process, 1.5% w/w calcium chloride (CaCl₂) (Chem-Lab NV, Zedelgem, Belgium) was added to the osmotic solution. CaCl₂ is known for maintaining firmness and structural integrity by forming chemical bonds with the plant tissue matrix, thereby preserving the texture of the dehydrated product [23]. It also aids in the formation of a compact surface layer, which facilitates higher rates of water loss and solid gain [22].

2.2. Osmotic Dehydration Procedure

Strawberry slices were initially immersed in the hypotonic chokeberry infusion for 15 min and, after that, the osmotic agents of glycerol and CaCl₂ were added in appropriate amounts, in order to obtain the desired glycerol concentrations of hypertonic osmotic solutions. The concentration of CaCl₂ in all osmotic solutions was maintained at 1.5% w/w. The samples were maintained at the specified temperature determined in the experimental design, using a temperature-controlled water bath. The immersion time varied according to the experimental design. After the designated immersion time, the strawberry slices were removed from each osmotic solution, blotted gently with paper towels to remove excess solution, and weighed immediately.

WL was calculated as the difference in weight of the strawberry slices before and after osmotic dehydration, expressed as a percentage of the initial weight. SG was determined by measuring the increase in weight of the strawberry slices due to the uptake of osmotic solutes, expressed as a percentage of the initial weight. WL (%) and SG (%) were calculated using the following equations [24]:

$$WL\left(\%\right)=\left({\displaystyle \frac{Wi-Wf}{Wi}}\right)\times 100$$

(1)

$$SG\left(\%\right)=\left({\displaystyle \frac{Wf-Wi-WL}{Wi}}\right)\times 100$$

(2)

where $Wi$ is the initial weight of the sample, and $Wf$ is the final weight after dehydration.

2.3. Experimental Design

A Box–Behnken design (BBD) was employed to determine the optimal conditions for osmotic dehydration process of strawberries. The design involved three factors at three levels (low (−1), medium (0), high (+1)), leading to a total of 15 experimental runs. Each experiment was conducted three times for accuracy. The independent variables investigated in the osmotic process were glycerol concentration in osmotic solution (40%, 50%, 60% w/w), immersion time (90 min, 180 min, 270 min), and temperature (30 °C, 40 °C, 50 °C). These variables were selected based on their documented influence on osmotic dehydration, with specific studies indicating that glycerol concentrations between 40 and 60% significantly impact water loss and solid uptake in fruits and vegetables [3,25]; immersion times ranging from 90 to 270 min have been shown to optimize mass transfer during osmotic dehydration; and temperature ranges of 30–50 °C play a critical role in accelerating the dehydration process while preserving the product’s quality [3,25,26,27,28,29,30,31,32]. The ratio of sample to osmotic solution was set to 1:8 (w/w), according to a series of preliminary experiments, where the ratio varied between 1:4 and 1:12, and was supported by literature findings to ensure sufficient interaction between the solution and the fruit [26,33,34]. The dependent variables, measured as responses, were WL and SG, which are critical indicators of dehydration efficiency and product quality in food processing [30]. All the experiments were performed in a randomized order to minimize the potential impact of uncontrolled factors and eliminate systematic errors. Three of the fifteen experiments were conducted with all factors at the medium (0) level to evaluate and confirm the model’s precision by calculating the average and the standard deviation of these experiments. The coded values in correspondence with the real values of the procedure, and the 15 experimental conditions determined by the Box–Behnken design are presented in

Table 1. Actual and coded values of the investigated factors.

Osmotic Dehydration Factors	Coded Values/Real Values
Independent Variables	−1	0	1
A: Glycerol Concentration (% w/w)	40	50	60
B: Temperature (°C)	30	40	50
C: Immersion Time (min)	90	180	270

and

Table 2. Experimental conditions for osmotic dehydration of strawberries.

Run Order	Glycerol Concentration (%)	Temperature (°C)	Immersion Time (Min)
1	50	50	90
2	60	30	180
3	50	40	180
4	60	50	180
5	60	40	90
6	50	50	270
7	40	40	90
8	40	30	180
9	50	40	180
10	50	30	270
11	40	50	180
12	40	40	270
13	50	30	90
14	60	40	270
15	50	40	180

, respectively. The OD factor ranges were selected based on values commonly reported in the literature, ensuring alignment with established research and maintaining relevance to practical applications [6,20,22]. While these ranges provide a robust framework for optimizing osmotic dehydration, it is acknowledged that exploring a wider range of variables might yield additional insights. However, the selected ranges were deemed appropriate for the objectives of this study, given the practical constraints of industrial applications, such as maintaining product quality and preventing texture or nutrient degradation at extreme conditions.

The experimental data were analyzed using RSM to develop mathematical models for WL and SG. The design of the experiments and the analysis of the experimental data were conducted using the software Minitab (trial version 20.4, Minitab LLC, State College, PA, USA, 2021). The linear regression method of the polynomial model, consisting of 10 coefficients, was used to describe the relationship between the response variable (Y) and the independent variables (A, B, and C) as shown in Equation (1). In this model, Y represents the response variable (WL or SG). The term β₀ denotes the estimated coefficient of the fitted response regression at the center point. The linear coefficients are represented by β₁, β₂, and β₃. The cross-product coefficients are β₁₂, β₁₃, and β₂₃, while the quadratic coefficients are β₁₁, β₂₂, and β₃₃.

Y = β₀ + β₁A + β₂B + β₃C + β₁₂AB + β₁₃AC + β₂₃BC + β₁₁A² + β₂₂B² + β₃₃C²

(3)

This comprehensive approach ensures a thorough understanding of how the independent variables influence the responses, thereby aiding in the optimization of processing conditions.

Analysis of variance (ANOVA) was performed to evaluate the significance of the model terms and interactions between the factors at a 95% confidence level (p ≤ 0.05). The software’s optimization module was then used to determine the optimal conditions for maximizing WL and SG. The optimization proposed three distinct osmotic conditions as the best options for this optimization. Based on this analysis, the software proposed three distinct osmotic conditions as the best options for achieving these objectives. These three recommended conditions were subsequently validated through experiments under the proposed optimized parameters to confirm the models’ robustness, predictability, and reproducibility. To verify the accuracy of the predictive model, the real experimental results were compared with the predicted values, and the prediction accuracy was calculated using the following formula:

$$\mathrm{P}\mathrm{r}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{A}\mathrm{c}\mathrm{c}\mathrm{u}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{y}\left(\mathrm{\%}\right)=100-\left|{\displaystyle \frac{PredictedValue-RealValue}{RealValue}}\right|\times 100$$

(4)

2.4. Determination of Quality Parameters

To investigate the migration of natural antioxidants from the chokeberry infusion to the strawberry slices during osmotic dehydration, as well as the strawberries’ quality parameters (firmness, color, moisture, and water activity), five additional experiments were conducted under optimal conditions for water loss and solid gain in terms of glycerol content and temperature. These experiments altered immersion times systematically to determine their effect on antioxidant transfer efficiency. All the individual experiments were performed in triplicate.

2.4.1. Total Phenolic Content Determination during Immersion in Chokeberry Infusion

The total phenolic content of the chokeberry infusion used for the osmotic solution preparation was measured as soon as it was prepared according to the modified micromethod of the Folin–Ciocalteu’s assay as described by Andreou et al. (2018) [35]. The TPC of the osmotically dried strawberry slices was determined, indirectly, by measuring the TPC in the osmotic solutions after the osmotic procedures. All measurements were performed in triplicate and the results were expressed as mg of gallic acid equivalents (GAE) per g of osmo-treated strawberry.

2.4.2. Quality Assessment

The texture of the osmotically dehydrated strawberry samples was assessed using the TA-XTplusC texture analyzer from Stable Micro Systems, Godalming, UK, following the methodology detailed by Giannakourou et al. (2023) [36]. The analysis was conducted on the strawberry slices using a 6.0 mm diameter cylindrical plunger in multiple regions to evaluate the firmness of the osmotically dehydrated samples. The color parameters of the osmo-treated strawberry slices were evaluated using a tristimulus chromatometer (model CR-400, Minolta, Tokyo, Japan). The instrument measured the color in terms of L*, a*, b*, and h values. The L* value represents the lightness of the sample, ranging from 0 (black) to 100 (white). The a* value indicates the red/green axis, with positive values signifying redness and negative values indicating greenness. The b* value represents the yellow/blue axis, where positive values denote yellowness and negative values indicate blueness. Additionally, the hue angle (h) was calculated to provide a comprehensive understanding of the color characteristics, representing the color tone of the sample. Water activity (a_w) measurements of the osmotically dehydrated strawberry slices were conducted using a water activity meter (4TE, METERGroup, Inc., Pullman, WA, USA). The moisture content of the fresh and osmo-treated strawberry’s samples was calculated at the end of vacuum drying (after 6 h) at 70 °C (Heraeus Instruments Vacutherm, ThermoScientific, Waltham, MA, USA) by measuring the weight loss resulting from heating, according to the official method AOAC 934.06 [37].

2.5. Statistical Analysis

The average values for TPC, color and texture parameters, water activity, and moisture content, along with their standard deviation, were reported after being calculated. These values were then analyzed using one-way ANOVA and post hoc test. Any probabilities below 0.05 were considered statistically significant (p < 0.05). The statistical analysis was conducted using the SPSS statistical software (IBM SPSS Statistics, version 29.0, Chicago, IL, USA).

3. Results and Discussion

3.1. Optimization of Osmotic Dehydration Process

Response surface methodology was utilized to optimize and maximize both dependent variables, i.e., WL and SG. In the context of food processing, increasing WL is paramount in osmotic dehydration processes, where the goal is to effectively remove moisture [38]. Conversely, enhancing solid gain is desirable when enriching products with bioactive compounds, thereby improving their nutritional profiles [14,27].

As mentioned in Section 2.3, three of the fifteen experiments were conducted with all factors set at the medium (0) level (Table 1) to verify the model’s precision by calculating the average and standard deviation of these experiments. The model proved to be quite precise, with the WL and SG mean values and standard deviations of these three similar experiments being 53.06 ± 5.52% and 12.36 ± 1.24%, respectively.

Initially, a comprehensive quadratic model, incorporating quadratic, linear, and linear two-way interaction terms, was selected to fit the data. The ANOVA results for the two key responses—WL and SG of the strawberry slices post-osmotic dehydration—are detailed in Tables S1 and S2 of the Supplementary Materials, respectively.

All response data were fitted to quadratic surfaces, and the corresponding Pareto charts, and regression equations for WL% and SG% are presented in Figure 1 and Figure 2, respectively, while their determination (R²), adjusted (R²adj), and predicted (R²pred) coefficients are presented in

Table 3. Determination (R²), adjusted (R²adj), and predicted (R²pred) coefficient for WL% and SG%.

	S	R-sq	R-sq (adj)	R-sq (pred)
WL%	4.17821	93.43%	81.61%	58.03%
SG%	1.20425	95.46%	87.27%	53.72%

. In all cases, the determination and adjusted coefficient were in close agreement, indicating a good fit of the Box–Behnken model to the data, with R² and R²adj values near 1 and a difference of less than 0.2. Moreover, the non-significant lack-of-fit values (p = 0.835 for WL and p = 0.562 for SG, Tables S1 and S2 of Supplementary Materials) further confirm the robustness of the models, suggesting they are suitable for predicting these responses. Moreover, the model significance was strong, with a p-value of 0.017 for water loss (WL%) and 0.007 for solid gain (SG%), indicating statistically significant models. Notably, the model could predict WL% and SG% values for new strawberry samples with over 58% and 53% accuracy, respectively. The significance of the model terms is presented in decreasing order in the Pareto chart (Figure 1 and Figure 2). The interaction plots for WL% and SG% (Figure 3 and Figure 4 respectively), were analyzed to assess the influence of the interaction terms of glycerol concentration (AB, AC), immersion time (AB, BC), and temperature (AC, BC).

Analysis of the Pareto charts and interaction plots provides valuable insight into the factors influencing WL and SG during osmotic dehydration. According to the Pareto charts, where factors exceeding the vertical red line are considered significant, it is evident that for WL (Figure 1), immersion time (C), as well as the concentration % of glycerol in the osmotic solution (A), were the most significant factors affecting the response. Temperature (B), along with its interactions with the other factors (AB, BC), did not show a statistically significant influence on WL. This findings align with Brochier et al. (2019) [39], who reported a strong influence of solute concentration on the dehydration process of kiwi fruit. Similarly, El-Aouar et al. (2006) concluded that the concentration of the osmotic solution was the primary factor affecting water loss during dehydration of papaya with sucrose, with immersion time being the next most important factor [40]. Although the interactions involving temperature and glycerol concentration (AB) were not statistically significant for WL, the interaction plots suggest that increasing both factors still contributes to higher water loss. The data show that increasing glycerol concentration correlated with higher water loss, while optimal water loss occurred at the medium level of immersion time (0), corresponding to 180 min. This observation reinforces the minor role of temperature in affecting WL, in agreement with the Pareto chart analysis.

For SG (Figure 2), the Pareto charts highlight immersion time (C) as the most influential factor, followed by temperature (B) and glycerol concentration (A). In contrast to WL, all three factors had a significant effect on SG values. Notably, the interactions between glycerol concentration and immersion time (AC) and between temperature and immersion time (BC) were significant for SG. The interaction plots for SG% (Figure 4) further demonstrated that extended immersion times combined with increased temperature (BC) or higher glycerol concentration (AC) resulted in higher solid gains. These findings are consistent with the study by Alam et al. (2019) [41], who reported that immersion time and temperature were the most significant factors affecting water loss and solid gains during osmotic dehydration of kinnow fruits in a sugar solution, followed by sugar concentration. Similarly, El-Aouar et al. (2006) [40] identified temperature as the primary factor influencing solid gain during the osmotic dehydration of papaya, with immersion time also being crucial. Additionally, the strong influence of temperature on SG during osmotic dehydration has been reported in studies on red algae and kiwiberries [42,43].

During the optimization of the OD process, Minitab software’s response optimizer was set to operate within the value range derived from literature data. The OD process optimization was constrained within literature-derived ranges to ensure model validity, as unconstrained optimization could lead to impractical conditions, such as excessive temperatures degrading phenolic compounds or high glycerol concentrations hindering the transfer of bioactive ingredients, thereby compromising the product’s quality, nutritional value, and industrial applicability. The optimization analysis, aimed at maximizing both WL and SG, identified and suggested three optimal experiments/runs, each involving glycerol concentration and temperature close to their high level (+1) but with varying immersion times. These experimental runs are summarized in

Table 4. Suggested experiments for WL% and SG% optimization.

Experiments/Runs	%Cglyc	T (°C)	t (Min)
1	60.0000	50.0000	270.000
2	60.0000	47.9199	205.187
3	59.9304	50.0000	193.414

, along with their predicted responses, real experimental results, and prediction accuracy. These solutions suggest that while high glycerol concentration and temperature are crucial for achieving high WL and SG, different immersion times can also produce optimal results. This flexibility in immersion time allows for tailored processing conditions depending on specific production requirements or constraints, providing valuable adaptability in industrial applications.

Table 5. Predicted and actual values of WL% and SG% under optimized conditions, and prediction accuracy.

Experiments/Runs	Predicted WL%	Predicted SG%	Composite Desirability	Experimental WL%	Experimental SG%	Prediction Accuracy (WL%)	Prediction Accuracy (SG%)
1	63.1979	20.2404	1.00000	55.2089	11.6984	87.37%	57.81%
2	62.5173	16.8397	0.99799	54.8344	10.8976	87.69%	64.72%
3	63.3045	16.5406	0.98121	42.9251	7.0793	67.81%	42.80%

presents the predicted and actual values of water loss and solid gain under the optimized conditions, as well as the prediction accuracy for each of these responses. The prediction accuracy for the optimized conditions varied, with water loss (WL%) showing high reliability in all runs (from 87.37% to 87.69%) except for the third experiment (67.81%). This indicates a strong ability of the model to predict water loss. In contrast, the accuracy for solid gain (SG%) was moderate to low, ranging from 42.80% to 64.72%, suggesting that the model was less reliable in predicting solid gain. The discrepancies between predicted and real values for solid gain highlight the need for further refinement in the model to better capture the influencing factors and interactions. These findings underscore the complexity of the osmotic dehydration process and the importance of continuous model improvement to achieve more precise predictions for both WL and SG.

3.2. Further Experimental Validation

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.

In Figure 5, the relationship between immersion time and the two key responses, WL% and SG%, is presented. It is clear that both water loss and solid gain increase consistently with increasing immersion time. However, at immersion times of 220 and 270 min, these parameters remained practically stable before increasing slightly again at 300 min. This plateau indicates a temporary equilibrium in the osmotic dehydration process, where the rate of water being drawn out and solids being infused balanced out. The subsequent increase at 300 min suggests that extending the immersion time beyond the equilibrium period allows for further mass transfer, possibly due to deeper penetration of the osmotic solution into the fruit tissue and more extensive cellular breakdown.

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.

3.2.1. Strawberries Enrichment with Natural Antioxidants from Chokeberry Infusion

The total phenolic content of the chokeberry infusion used for the osmotic solution preparation was found 1850.80 ± 144.04 mg GAE/L. Figure 6 presents the TPC that was transferred to the strawberry slices from the osmotic solution at the different immersion times. The results are expressed as mg of gallic acid equivalents (GAE) per g of osmo-treated strawberry.

The TPC values indicate the higher antioxidant migration from the hypertonic osmotic solution to the strawberries occurred between 170 and 220 min, with a significant (p < 0.05) reduction observed at longer immersion times. This pattern suggests an optimal immersion time to maximize the migration of the natural antioxidants from the chokeberry infusion to the strawberries, beyond which the efficiency of transfer decreases. The decrease in TPC of osmo-treated strawberries at 270 and 300 min could be attributed to the saturation of the strawberry tissues with antioxidants or the potential degradation of phenolic compounds. According to Giannakourou et al. (2021, 2023), after 120 min of osmotic dehydration on animal tissue, further enrichment with phenolic compounds was significantly decreased [36,46]. Additionally, it is claimed that increasing the duration of osmotic treatment has a negative effect on gallic acid migration in osmo-treated cucumbers [47]. Moreover, this reduction could possibly be attributed to the gradual diffusion of low-molecular-weight bioactive compounds towards the center of the sample, hindering further transfer of phenolic compounds. Additionally, Kowalska et al. (2019), using chokeberries in the osmotic solution, found that it effectively enriched the strawberries during the osmotic dehydration pretreatment [4].

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. Water activity (a_w), moisture content, and firmness at different Immersion Times.

Immersion Time (Min)	aw	Moisture Content (%)	Firmness (N)
0	0.9889 ± 0.0035 a 1	88.58 ± 1.71 a	2.41 ± 0.58 a
170	0.9669 ± 0.0031 b	82.25 ± 1.79 b	2.34 ± 0.72 ac
190	0.9330 ± 0.0182 c	74.49 ± 1.70 c	1.78 ± 0.10 ab
220	0.9002 ± 0.0131 d	61.36 ± 1.15 d	2.03 ± 0.18 ab
270	0.8880 ± 0.0143 d	59.07 ± 0.33 d	1.43 ± 0.20 bc
300	0.8754 ± 0.0154 d	53.09 ± 0.45 e	1.05 ± 0.16 b

¹ Different letters in the same column indicate a significant difference (p < 0.05).

.

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. Color parameters of strawberries during osmotic dehydration.

Immersion Time (Min)	L*	a*	b*	h
0	49.13 ± 3.17 a 1	19.97 ± 2.62 a	16.36 ± 2.68 a	39.19 ± 2.68 a
170	41.23 ± 3.20 b	21.54 ± 2.15 ab	11.00 ± 1.95 b	26.92 ± 2.78 b
190	43.44 ± 3.15 bc	21.25 ± 1.87 ab	12.96 ± 1.73 bc	31.31 ± 2.49 c
220	42.81 ± 3.71 bc	22.31 ± 3.92 b	13.11 ± 2.70 bc	30.34 ± 1.98 c
270	42.98 ± 3.12 bc	21.81 ± 4.52 ab	12.78 ± 3.13 bc	29.65 ± 2.17 c
300	44.85 ± 3.00 c	23.55 ± 2.3 b	14.19 ± 1.89 c	30.40 ± 1.86 c

¹ Different letters in the same column indicate a significant difference (p < 0.05).

. 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].

4. Conclusions

The optimization of the osmotic dehydration process for strawberry slices was successfully achieved by evaluating the effects of glycerol concentration, immersion time, and temperature on WL and SG. The study highlighted the significance of high glycerol concentration (around 60% w/w) and temperature (approximately 50 °C), along with immersion times at specific range (estimated 190–270 min), in maximizing water loss and solid gain. The prediction model demonstrated high reliability for water loss but showed a need for further refinement in predicting solid gain. Experimental validations confirmed the model’s robustness, with immersion times between 170 and 220 min being optimal for the migration of natural antioxidants into the strawberry tissue. At longer immersion times (270–300 min), the transfer of natural antioxidants decreased, possibly due to tissue saturation or phenolic compound degradation. The OD process was effective in reducing water activity and moisture content, enhancing the strawberries’ shelf stability, although additional processing steps may be required for long-term preservation. The study also confirmed that the firmness of strawberries decreased with prolonged immersion times, but significant changes were only observed at extended durations. The addition of calcium chloride played a crucial role in preserving the texture and color of the strawberries, contributing to their overall quality. These findings align with previous research indicating that OD processes generally preserve color and texture by minimizing air exposure and heat impact. In conclusion, the optimized OD process preserved the structural integrity and improved the nutritional profile of strawberries, offering valuable insights into the conditions necessary for effective dehydration. These results support the potential for OD to extend the shelf life and enhance the quality of strawberries, making them suitable for a wide range of fruit-based products. Further research should focus on refining the prediction models for solid gain and exploring additional variables that could influence the dehydration process. Finally, investigating the reuse or handling of the hypertonic osmotic solution, which changes in composition due to water transfer, would be important for future studies.