Rehydration modeling and characterization of dehydrated sweet corn

Abstract The increasing demand of rehydrated foods is due to its better storage stability at ambient conditions and not requiring refrigeration. Prior to drying at 55, 60, 65, and 70°C in a hot air tray dryer, hot water blanching (HB), steam blanching (SB), and microwave blanching (MB) were employed as pretreatments. Rehydration of dried pretreated sweet corn kernel was performed in boiling water. The pretreatments and drying temperatures were independent factors that affected the dependent factors such as rehydration ratio, total sugar, ascorbic acid, geometric mean diameter, color, sensory evaluation, water absorption, mass, and geometric mean diameter. Peleg, Weibull, and newly proposed models were considered to describe the change in moisture content during rehydration. The proposed model performed better than other models and indicated the rise in equilibrium moisture content of rehydrated sweet corn with an increase in dehydration temperature of sweet corn due to higher R 2 (0.994), and lower chi‐square (0.005) and RMSE (0.064). The rehydrated sweet corn obtained from samples processed with microwave blanching and dehydration at 70°C showed higher retention of total sugar, ascorbic acid, geometric mean diameter, and color.

blanching and thus nutritional quality is also improved (Pravitha et al., 2022).
Generally, dehydrated food products need to be rehydrated before their edible use (Gan et al., 2016). The nutritional and sensory characteristics are affected by the process of dehydration followed by its rehydration (Zhu et al., 2022). The gain of moisture lost during drying is also dependent on the temperature of drying (Singh & Talukdar, 2020). A faster and shorter dehydration period enhances the absorption capacity of the dehydrated food material (Dehghannya et al., 2018).
A rehydration study is required for identifying the quantum of damage in the structure of food during drying, which overall affects the rehydration characteristics of foods (Liu et al., 2021). The swelling of the food material and imbibing of water into dehydrated food takes place during rehydration along with the leaching of soluble solutes from the food (Sagar et al., 2018;Singh et al., 2022). The quality always remains a major concern along with energy conservation for optimizing any dehydration process . The structural variation in absorption behavior and structural matrix due to temperature change was reported by Mounir (2015), although the change in absorbed water remained nonsignificant.
Mass transfer kinetics modeling throughout the rehydration process can be described using empirical models. Fick's second law is used for preparing any diffusion models (İlter et al., 2018). The model proposed by Peleg is popular for various foods with porous structures due to its simplicity to other equations (Zielinska & Markowski, 2016). The Weibull model is also popular in food engineering applications due to its simplicity, which uses the probabilistic approach and also enhances the notable performance (Serment-Moreno, 2021). Because of its versatility and simplicity, the standardized Weibull distribution model has recently been effectively employed to assess the drying kinetics of agricultural goods (Dai et al., 2019). The dehydrated sweet corn may remain a promising stable product, which can be used in several food applications like salads, pizza, sweet dishes, etc. after rehydration. However, no research has been carried out on the pretreatment such as hot water blanching (H B ), steam blanching (S B ), and microwave blanching (M B ) for the determination of rehydration characteristics for rehydrated sweet corn. Therefore, the study was undertaken to observe the rehydration kinetics of dried sweet corn kernels with different pretreatments and drying temperatures to examine the probability of modeling of rehydration processing.

| Raw material
The raw material was procured from Godhra, Gujarat, India. Before drying experiments, the sweet corn kernels (Variety: Madhuri) were separated manually with a hand knife. The moisture content of the procured sweet corn kernel on a wet basis was 75 ± 3.3%.

| Pretreatments
The sweet corn kernels were blanched using hot distilled water at 100°C for 120 s, blanched using steam for the 90s, and blanched using a microwave at 900 W power level for 60 s. The durations of different blanching methods were selected in the preliminary blanching study according to the appropriateness of blanching (Kachhadiya et al., 2018).

| Drying experiment
The blanched sweet corn kernels were dried in a tray dryer (M/s Nova Instruments Pvt. Ltd., India). The kernels were spread over the trays in 5 mm thickness for drying in thin layers. The dryer consists of a heater for heating the air, which is blown using forced air convection.
The tray was loaded with the sample amounting to 4.67 ± 0.1 kg/m 2 as tray load. The mass of the samples before and after the drying was noted using a digital weighing balance (Shimadzu Corporation, Japan, Precision 0.01 g). The kernels were dried at 55, 60, 65, and 70°C drying temperatures. The drying process endpoint was observed till no reduction in mass was observed.

| Rehydration experiment
Rehydration experiments were performed in boiling water in a beaker kept on a hot plate. At every five-minute interval, the sweet corn kernels were taken out from the beaker. The samples were placed on tissue paper for removing surface moisture before weight measurement of the samples (Ranganna, 2000).

| Rehydration kinetics modeling
The equilibrium moisture content cannot be estimated independently during rehydration due to many changes with long steeping times, therefore, becoming difficult to measure. The additional parameters for equilibrium moisture content in the rehydration kinetics models are used as suggested by Peleg's model (Patero & Augusto, 2015).
Two already existing models namely Weibull's model and Peleg's models were used. Furthermore, a newly proposed model was also used, which was conceptualized based on the logarithmic variation considering the opposite trend of moisture content during rehydration in comparison to dehydration in thin-layer drying models. All three models are represented in Table 1.

| Rehydration ratio
The rehydration of the dehydrated sweet corn kernel was studied in terms of the rehydration ratio. It is defined as the ratio of the mass of

| Rehydration modeling
Rehydration needs a long steeping time and models used for rehydration modeling incorporate additional parameters for equilibrium moisture content (Lopez-Quiroga et al., 2020). In this study, the experimental rehydration data of sweet corn kernel were converted to the moisture content at different treatments. The Weibull's, Peleg, and proposed new models were used to fit the moisture content with respect to the time for rehydration.
Weibull model can be expressed as: where, X wt , X wo , and X eq represent average instantaneous moisture content at any instantaneous time t, initial, and equilibrium moisture con- Peleg's model can be expressed as: The Peleg rate constant is represented by the k 1 parameter, Peleg capacity constant is represented by k 2 related to equilibrium moisture content. At the time t = ∞, the equilibrium moisture content can be represented by: The mechanism of rehydration indicates the absorption of water in the kernels, which is opposite to the mechanism of dehydration by the removal of moisture. Therefore, a new model is proposed based on opposite trends of the thin-layer drying model, that is, logarithmic in place of exponential models, and is represented as: Where k is a parameter indicating the rehydration rate.
The experimental data were analyzed statistically using

| Total sugar and ascorbic acid
The method was used for the sweetcorn samples after rehydration to determine the total sugar and the 2,6-dichlorophenol-indophenol method was used for the determination of ascorbic acid ).

| Geometric mean diameter
The where, L, W, and T are mutually normal dimensions and can be used to represent the length as the maximum dimension, width as normal to length, and thickness as normal to the length and width. All the dimensions were measured in millimeters (mm).

| Total color difference
The color change was estimated using the following Equation (9).
The subscript '₀' refers to the color value of the fresh kernel. The total color difference should be as minimum as possible to maintain the color of the kernels and can be represented as: where, L*, a*, and b* represent the degree of lightness to darkness with an initial value of Lₒ*, redness to greenness with an initial value of aₒ*, and yellowness to blueness with an initial value of bₒ*, respectively (Jeevarathinam et al., 2021;Pravitha et al., 2022).

| Sensory evaluation
Sensory evaluation was conducted for rehydrated samples, which were kept in boiling water for 60 minutes (Xu et al., 2018). A semitrained panel of 15 panelists comprising faculty and postgraduate students were asked to access the rehydrated samples based on a hedonic rating test ranging from 1 to 9 indicating 1 as 'dislike extremely' and 9 as 'like extremely' according to their opinion for color, odor, taste, and overall acceptability (Ranganna, 2000). The samples were kept on a (1) Rehydration ratio (RR) = Mass of rehydrated sample, g Mass of dried sample, g Petri dish and tests were conducted in uniform light, and the background of the coded samples was not informed to the panelists.

| Statistical analysis
Rehydration characteristics and physicochemical parameters of rehydrated sweet corn were determined in triplicates. The statistical analysis was performed using Origin-Pro9.2 (Origin Lab Corporation, Northampton, MA, USA) and Microsoft Excel (MS office 2016). For the mean comparison, 5% level of significance was set.

| Water absorption
The rehydration ratio of sweet corn samples dried before pretreatment and different temperatures is shown in Figure 1 (I  The results in Figure 1 (II) indicate that rehydration rates were higher for all samples when the drying air temperature was increased. This is likely due to the fact that higher temperatures result in shorter drying times, leading to less structural and cellular damage. It was reported that minor disruption of cell wall polymers for orange by-products during dehydration was about 50-60°C (Garau et al., 2007). Similar findings were observed in the study for purplefleshed sweet potatoes, where the compact tissue of the dried product was formed through continuous drying which resulting in a comfortable and steady destruction of the cells and structure (Liu et al., 2017).

| Rehydration kinetics
The Peleg parameters k 1 , k 2 , and Xeq are shown in Table 1 Another parameter β represents the rate of rehydration and the time required to achieve a 63% level of rehydration, which varied from A similar variation was also observed in control and hot water-

| Mass
The fresh kernel mass was about 0.44 g in fresh sweet corn. The mass of pretreated sweet corn kernels reduced from 0.11 to 0.14 g at drying temperatures from 55 to 70°C ( Figure 3I). The mass of fresh  in the mass of samples. In the previous study, it was observed that due to gaining moisture the dimensions were augmented and it was also reported that deflation in the dimensions of sweet corn was due to losing the moisture during the microwave heating process (Kachhadiya et al., 2018). Similar results were also obtained by Popaliya and Kumar (2022) for sweet corn kernels by using microwave blanching and a reduction in the geometric mean diameter and mass of the sweet corn kernels sample was reported.

| Total color difference
The total color difference after rehydration of sweet corn kernel of control, H B , S B, and M B samples with fresh samples was 31. 23-26.34, 27.63-21.23, 25.61-19.36, and 15.23-11.12 at drying temperature varying between 55 and 70°C, indicating more retention color of microwave-blanched samples ( Figure 4I). The minimum change in color of microwave-blanched samples during drying and rehydration is due to effective blanching and less destruction of color pigments.  (Liu et al., 2015). Delfiya et al. (2018) studied the microwave blanching effect and brine solution on the carrots and reported that the complete inactivation of enzymes causes color changes and also caused a decrease in total color. It was also reported that microwave blanching is more effective for the inactivation of enzymes and retention of total color values. The increase in total color changes due to the lower microwave power level and the reduction in total color changes just because of the longer time of microwave exposure (Srinivas et al., 2020).

| Total sugar and ascorbic acid
Total sugar after dehydration of sweet corn kernel of control, H B , S B , and M B samples varied from 5.21 to 5.53 g/100 g, 6.13 to 6.63 g/100 g, 6.61 to 7.26 g/100 g, and 7.21 to 7.41 g/100 g at drying temperature from 55 to 70°C indicating retention of higher sugar in

| Sensory evaluation
An increasing trend of overall acceptability with an increase in drying temperature was observed during the sensory evaluation of the rehydrated samples. The maximum sensory score of rehydrated samples was 8.62, indicating the highest preference for samples blanched with microwave and dehydrated at 70°C among the treatments in the study (Figure 4(IV)). The lower score at lower temperature drying may be attributed to the longer drying periods, while the lower score in sensory analysis in control, H B , and S B samples may be attributed to the partial rupturing of kernels and color degradation of the kernels.

| Compound correlation
This correlation describes the results of the study investigating the effect of different blanching methods and drying temperatures on the physical, chemical, and sensory properties of sweet corn. Compound correlation was used to analyze the data, which is a statistical F I G U R E 4 Variation of (I) total color difference, (II) ascorbic acid, (III) total sugar, and (IV) overall acceptability of sample after rehydration at different pretreatments and drying temperatures.  technique used to identify the similarities and differences between multiple process variables and characteristics of the product.
In Figure 5a, a dendrogram was used to represent the compound correlation among all the treatments based on physical properties.
The dendrogram shows that samples dried at 55 and 60°C are similar and can be kept in one cluster, while samples dried at 65 and 70°C form another cluster at about 50% of the total distance on the vertical axis, indicating the classification of both clusters based on temperatures. The subclusters of samples within each cluster based on drying temperature also suggest that the physical properties of samples dried at a specific temperature are more similar to each other than to samples dried at a different temperature. This indicates that the drying temperature has a more prominent effect on physical properties compared to the type of blanching methods used. The vertical distance of clusters reduced with an increase in temperatures, indicating that samples treated at higher drying temperatures had more similar physical properties. In Figure 5b, the overall compound correlation of physical, chemical, and sensory properties was analyzed. The results showed that microwave-blanching (M b ) samples had overall similar properties. The microwave-blanching samples of 60 s dried at 70°C showed the highest overall acceptability and had higher retention of ascorbic acid and total sugar. The results also indicate that the type of blanching method and drying temperature significantly affect the overall properties of sweet corn.

F I G U R E 5
Compound correlation for (a) physical properties and (b) overall properties of rehydrated sweet corn. Overall, the compound correlation analysis helped to identify the similarities and differences between multiple variables and allowed the researchers to draw conclusions about the effect of blanching method and drying temperature on the physical, chemical, and sensory properties of sweet corn.

| CON CLUS ION
The rehydration ratio varied from 2.56 to 3.54for all the samples to recent studies many researchers have reported that soaking in hot water at 50°C for 20 min was the most effective rehydration method for producing high-quality, rehydrated sweet corn with good color, texture, and sensory properties. These findings can contribute to the development of effective strategies for preserving and utilizing surplus sweet corn, thereby reducing food waste and increasing food availability, particularly in areas with limited access to fresh produce.