Optimization of sonication time, edible coating concentration, and osmotic solution °Brix for the dehydration process of quince slices using response surface methodology

Abstract The goal of this work was to examine the effects of sonication time, edible coating concentration (with guar gum), and °Brix (sucrose solution) on the osmotic dehydration (OD) parameters (mass reduction, water loss, soluble solids gain, and rehydration ratio) and the appearance properties (color indices and surface area) of quince slices using a response surface methodology (RSM) approach based on the central composite design (CCD), for the optimization of the process. The process parameters, sonication treatment time (5–10 min; 40 kHz and 150 W), edible coating concentration using guar gum (0.05%–0.15%, w/w), and osmotic concentration using sucrose solution (20%–50%, w/w), were investigated and optimized for OD of quince slices. After each OD process, the quince slices were dehydrated in an oven at 70°C for 240 min. Results demonstrated a good correlation between empirical data with the linear model. Using the optimization method, optimum input operating conditions were determined to be a sonication time of 5 min, guar gum concentration of 0.05%, and sucrose concentration of 37.19°Brix. At this optimum point, the OD process of quince slices reached the optimal mass reduction (17.74%), water loss (25.77%), soluble solids gain (8.03%), rehydration ratio (206.19%), lightness (77.6), redness (0.60), yellowness (34.84), total color change (ΔE) (8.92), and area changes (7.59%).


| INTRODUC TI ON
Quince (Cydonia oblonga) from Rosaceae family is a tree cultivated as a medicinal and nutritional plant. This fruit is a rich source of vitamins A, B, and C, fibers, and tannin. Dehydration is one of the main preservation methods employed for the storage of quince fruit. Fresh and dehydrated quince fruit are used to make jam, marmalade, jelly, and quince pudding (Dehghannya et al., 2018;Salehi & Kashaninejad, 2018). Osmotic dehydration (OD) has been used as a pretreatment for many fruits and vegetables. During OD, water is removed from tissue by putting the fruit particles in a sugar solution. OD technique decreases the thermal damage to either color or taste, prevents enzymatic browning, and results in better protection of the nutritional component during dehydration process (Dehghannya et al., 2018;Salehi, 2023). Recently, some researchers used the OD technique for improving the quality properties of dehydrated quince fruit and found that this technique is a suitable method for the pretreatment and dehydration of quince samples (Dehghannya et al., 2018;Kutlu, 2021;Turkiewicz et al., 2020). Turkiewicz et al. (2020) used OD technique as a pretreatment before drying Japanese quince fruit using a convective and vacuum microwave dryer. Their results showed that the OD process significantly shortened the combined drying time compared with non-OD samples.
Using pretreatment such as ultrasound has been shown to reduce OD time and increase water loss during OD (Salehi, 2023).
Ultrasound pretreatment accelerates the mass transfer in dehydration and drying of fruit and vegetable slices mostly due to the breakdown of cells and the creation of microchannels (Salehi, 2020).
A sonication frequency range of 20 kHz to 1 MHz is usually used in fruits and vegetables processing due to its physicochemical effect (Allahdad et al., 2019). The influence of pretreatment osmoticultrasonic dehydration on the rehydration kinetic of quince at three temperatures was investigated by Noshad et al. (2012). The results of this study showed that the osmotic-ultrasonic dehydration pretreatment caused samples with lower water absorption ability in comparison with the untreated samples because of cell permeabilization due to the process stress.
Coatings with edible gums have been widely analyzed, aiming to improve the quality and shelf-life of fruit and vegetable products and decrease the soluble solids gain during OD. In addition, they used food slices for dehydration, a technique that can increase the nutritional and sensorial attributes of dehydrated products (Salehi & Satorabi, 2021). Guar gum is the powdered endosperm of the seeds of Cyamopsis tetragonolobus, which is a leguminous crop. It is a natural polysaccharide with non-toxicity, safety, quick solubility in cold water, biodegradability, biocompatibility, lower prices, and easy availability characteristics. Due to its various functional characteristics, it is a promising biopolymer for the development of packaging or edible coating films (Thombare et al., 2016).
Response surface methodology (RSM), comprising sophisticated mathematical and statistical techniques, is used for the improvement and optimization of different processes (Roshanpour et al., 2023;Yang et al., 2023). In a study, Eren and Kaymak-Ertekin (2007) proposed optimal conditions using a RSM test setup for OD of potato as 22°C for temperature, 54.5% for sucrose concentration, 14% for salt concentration, and 329 min for treatment time. At this optimal point they could achieve 52.9% mass reduction (MR), 59.1% water loss, 6.0% soluble solids gain, and 0.785 water activity. In another study, Bchir et al. (2020) proposed optimal conditions using a RSM test setup for ultrasound-assisted OD of pomegranate seeds as 41°C for temperature, 60°Brix for sucrose concentration, and 240 min for treatment time. At this optimal point they could achieve 27.39% MR, 31.7% water loss, 4.25% soluble solids gain, and 7.39 × 10 −9 m 2 /s effective moisture diffusivity (D eff ).
This study aims to examine the effect of ultrasound pretreatment on the OD kinetics of coated quince slices searching for the optimal operating conditions (immersion time in the ultrasound bath, concentration of guar gum, and concentration of osmotic solution) that maximize MR, water loss, rehydration ratio, lightness (L*), and yellowness (b*), and minimize solid gain, redness (a*), total color change (ΔE), and area changes (ΔA) using RSM. In this work, after each OD process, the quince slices were dehydrated in an oven at 70°C for 240 min.

| Fresh materials
Fresh quince fruits (C. oblonga L.) were purchased from the market at Hamedan, Iran. The fresh and uniform-sized quinces with no external damage were selected, and with the aid of an industrial slicer (food slicer, model AF-23; Girmi) and a stainless steel ring mold cutter were cut into 5-mm thick and 3.28 cm diameter slices. The initial quince slices moisture content was 81.42% w.b. (moisture determination was performed in a Shimaz oven, at 70°C for 4 h).

| Ultrasound pretreatment
To apply the sonication treatments on the quince slices, a Backer vCLEAN1-L6 ultrasonic bath (Iran) was employed with a frequency of 40 kHz and a power of 150 watts. The tank of the device was filled with 5 L of distilled water and, then, after the temperature of the water reached to 20°C, the quince slices were placed directly in the bath. Sonication pretreatments were applied to the quince slices in two replicates.

| Edible coating and osmotic dehydration procedure
In this study, the treated samples with ultrasound were placed in a 200-mL glass beaker containing sucrose and guar gum (in the determined concentrations), and then, beakers were placed in a water bath (R.J42; Pars Azma Co.) at 50°C for 60 min (Figure 1). After the termination of each experiment, quince slices were removed from the bath with forceps and wrapped immediately with a clean kitchen towel (microfiber towel) to remove excess moisture.

| Air drying
The fresh and treated samples (before and after treatments) were dried (in single layer) at 70°C for 240 min using a laboratory scale dryer (Shimaz) equipped with temperature controllers and digital indicating controllers, an electrical heater, and a fan.

| Rehydration ratio (RR) of quinces samples
Dried quince slices were weighed and immersed for 20 min in water (100 mL) at 50°C. The rehydration ratio was determined as the ratio of the final mass of samples (rehydrated quince slices) over the initially dried quince slices mass (after oven) (Salehi et al., 2022).

| Mass reduction, water loss, and soluble solids gain
Before ultrasound treatment and after the osmotic, drying, and rehydration processes, the mass of the samples was measured using a digital balance (with an accuracy of ±0.01 g, model SL1000, Kia Laboratory, Iran). The dehydrated quince slices were analyzed for MR, water loss (WL), and soluble solids gain (SG) (Salehi et al., 2022).

| Surface color and area measurement
Color measurement of food products has been used as an indirect measure of other quality attributes because it is simpler, faster, and correlates well with other physicochemical characteristics (Pathare et al., 2013). The color analysis of untreated (control) and treated quince slices were assessed from all the various pretreatment methods using the Image J software (V.1.42e), which gave L*, a*, and b* values (where L* represents lightness, a* represents redness, and b* represents yellowness). A scanner (HP Scanjet-300) was used to photograph the quince slices. The fresh quince slices exhibited a yellow color, with L*, a*, and b* being 84.95, −1.02, and 33.14, respectively. The color variation was made with respect to the fresh sample. Equation 1 was used to compute the ΔE of the quince slice after OD (Salehi, 2019): In addition, the surface area of the quince slice after OD was calculated using the threshold color plugin in the image analysis software (Image J, V.1.42e). The average surface area of fresh quince slices was 8.45 cm 2 . The area changes (ΔA) of the quince slice after the OD process was calculated using Equation 2: where ΔA is the area changes (%) and A 0 and A t (cm 2 ) are the areas of fresh and dehydrated quince slices, respectively.

| Experimental design and optimization by response surface methodology
In this study, we used Design Expert Version 13 software as a design and analysis tool to conduct experiments. Regression coefficients (RCs), the significance of the process variables, conformity of the experimental data to models, and optimal response variables can be Schematic of the ultrasound pretreatment system and osmotic dehydration process of quince slices.
The central composite design (CCD) is the most commonly used fractional factorial design used in the RSM model (Bhattacharya, 2021). In this article, we employed the CCD technique for designing the experimental collection. The design included 20 experimental points and it is adopted by adding 6 central points and 6 axial points to 8 full factorial designs (Table 2).
This study aims to examine the influences of ultrasound pretreatment on the OD of coated quince slices to search for the optimum point (immersion time in the ultrasound bath, concentration of guar gum, and concentration of osmotic solution) that maximize MR, water loss, rehydration ratio, lightness, and yellowness, and minimize solid gain, redness, ΔE, and area changes using RSM (CCD). The experimental data were fitted to a second-order polynomial model (Bchir et al., 2020;Yoo et al., 2023). The significant terms in the models were found by analysis of variance (ANOVA) for each dependent variable. The fitness of the model was probed via the comparison of the F test, lack of fit, RC, predicted RC, adjusted RC, and p value. Significance was judged by calculating the probability level that the F statistic estimated from the data is <5% (Roshanpour et al., 2023). The lack-of-fit values were checked corresponding to the variation of the data around the fitted model designed.

| Mass reduction
The linear model (suggested model) reveals the role of all independent variables in the dependent variables. The results of ANOVA revealed that the linear model was significant (.0001 < p < .04) for all dependent variables (Table 3). Also, the lack-of-fit values for all dependent variables are not significant (p > .05) relative to the pure error, showing good response to the linear model. Table 3 shows that the linear term of sonication time had the most significant (p < .05) effect on the MR of quince slices during the OD pro-

| Water loss
Edible coatings are thin layers of edible material added on the surface of fruits to improve the appearance, maintain the quality, decrease water loss, and extend shelf-life (Odetayo et al., 2022).
Results of various runs of OD of quince slices are shown in Table 3.
An ANOVA was performed to characterize the significant impact of process variables on each dependent variable. The data from this table show that the sonication time has the major influences on the water loss of quince slices during the OD process. Also, the linear term of °Brix has a significant (p < .05) effect on the water loss of quince slices subjected to OD. Çağlayan and Barutçu Mazı (2018) reported similar results showing that the pretreatment time (for 40, 80, and 120 min) and the osmotic solution concentration (containing 40% and 60% sucrose) are significant factors affecting MR, water loss, and soluble solids gain of pumpkin slices.
The linear term of guar gum concentration did not has a significant (p > .05) impact on the water loss of quince slices. The RCs (Table 4)

| Soluble solids gain
The results of Table 3

| Rehydration ratio
Rehydration of food particulates is one of the qualitative parameters in the dehydrated product and aimed at the restoration of raw material properties when dehydrated material comes in contact with water. Rehydration is a complex phenomenon affected by numerous factors that typically include pretreatment methods, dehydration technique, food structure and composition, and medium viscosity (Marabi & Saguy, 2004;Noshad et al., 2012). The results of Table 3 demonstrate that the sonication time, guar gum concentration, and °Brix did not have significant (p > .05) effects on the rehydration ratio

| Lightness
Color is a main quality parameter in the fresh and processed food products, and it influences consumer's choice and preferences (Pathare et al., 2013). The results of Table 3  Impacts of OD parameters on the changes in the lightness of quince slices during the OD process were seen in Figure 6. As can be seen in Figure 6 In addition, Figure 6 shows the influence of guar gum concentration and sucrose solution concentration on the lightness of quince  Table 3 shows that the linear term of guar gum concentration had the most significant (p < .05) effect on the redness of quince slices during the OD process. However, the linear terms of sonication time and °Brix did not have a significant (p > .05) effect on the redness of quince slices subjected to OD. The RCs show the maximum positive portion of guar gum concentration (β 2 = 38.89) followed by sonication time (β 1 = 0.548). In addition, the appropriate model as fitted corresponds to Equation (8):

| Redness
The redness parameter of pretreated quince slices is shown in Figure 7. As can be seen in Figure 7, the redness index increased

| Total color change
One of the most significant color parameter used to measure the color variation between fresh and processed food is the ΔE (Pathare  Impacts of the OD parameters on the changes in the ΔE of quince slices during the OD process were seen in Figure 9. It was considered that the ΔE increased with the increase in sonication time and guar gum concentration. Also, it was observed that the ΔE decreased with the increase in sucrose concentration. The effects of ultrasound, OD, and osmosonication pretreatments on color of dried ginger slices were investigated by Osae et al. (2019). The results of this study showed that the osmosonication pretreatment increased the ΔE of the dried ginger samples compared to ultrasound and OD. In addition, Oladejo et al. (2017) reported that the highest ΔE values were obtained in osmosonication pretreated sweet potato compared to (10)

| Area changes
The values in Table 3 confirm  to 10 min. While it was observed that area changes (%) of quince slices during OD decreased with the increase in guar gum concentration and sucrose solution concentration from 0.05% to 0.15% and from 20 to 50°Brix, respectively.

| The optimum condition
According to the design expert software optimization step, the desired target for each operational condition (sonication time, guar gum concentration, and °Brix) were chosen as "minimize and within the parameter range," while the responses MR, water loss, rehydration ratio, lightness, and yellowness were defined as "maximize" and the responses soluble solids gain, redness, ΔE, and area changes were defined as "minimize." It was observed that the optimum condition used for OD of quince slices was found to be sonication time = 5 min, guar gum concentration = 0.05%, and sucrose concentration = 37.19°Brix. In this condition, the OD of quince slices reached the optimal MR (17.74%), water loss

ACK N O WLE D G E M ENTS
The present study was financially supported by Bu-Ali Sina University, Hamedan, Iran.

FU N D I N G I N FO R M ATI O N
None.

CO N FLI C T O F I NTE R E S T S TATE M E NT
None.

DATA AVA I L A B I L I T Y S TAT E M E N T
All data generated or analyzed during this study are included in this published article.

E TH I C S S TATEM ENT
This study does not involve any human or animal testing.