Kinetics of dehydration and appreciation of the physicochemical properties of osmo‐dehydrated plum

Abstract The experiment was conducted to evaluate the dehydration kinetics and quantify its effect on the various physicochemical properties of the osmo‐dehydrated plum during storage at an ambient condition. The six treatments with a combination of three different sucrose–sodium chloride concentrations and two peeling conditions were selected in the experiment. Among the treatments, peeled plum dipped into 5% NaCl solution exhibited a faster drying rate. Concerning the rehydration properties of the osmo‐dehydrated plum, the whole plum immersed into 500B sucrose solution showed the highest reconstitution behavior and the lowest moisture content (wb). The highest values of water activity of 0.514 and the lowest values of texture 1.79 N‐mm2 were investigated in 500B sucrose treated whole plum. The peeled plum obtained the highest lightness (L), redness (a*), and yellowness (b*) compared to the unpeeled plum. Osmo‐dehydrated plum with high sugar solution contained more sugar and less total phenolic content nevertheless using only 5% NaCl resulted in less sugar and more total phenolic content after the treatment. The osmo‐dehydrated whole plums prepared in 500B sucrose scored the highest overall acceptability (8.0, e.g., like very much) followed by the 500B sucrose with peeled plum envisaged the sensory evaluation analysis. In conclusion, the osmo‐dehydrated plum treated in 500B sucrose and unpeeled condition performed better with a view to the overall plum quality, color, and acceptability judged by the expert panelists even after 12 months of storage at room temperature.


| INTRODUC TI ON
In Bangladesh, the demand of plum (Prunus domestica) usually meets up by importing from other countries like India, China, and Thailand (Mozumder et al., 2017). Spices Research Center of Bangladesh Agricultural Research Institute (BARI) released a plum variety namely "BARI Alu bukhara-1" which is high yielding and profit potential (Anonymous, 2014), but there is no available processing method to utilization of recently produced plum in Bangladesh. Hence, the suitable plum processing technique is needed. Various food processing techniques can be engaged to preserve fruits and vegetables; and dehydration is one of the most important operations that are widely practiced because of long time consumption (Chavan & Amarowicz, 2012). In recent years, there is growing demands by the customer for osmo-dehydrated plum with a comparatively long-life span, which preserve the attributes of fresh plum. In the case of fruit like plum, to obtain a fresh like plum implies certain operations such as whole or peeled and dip in sucrose-sodium chloride solution or often, partial dehydration of the plum. Osmotic dehydration has been the main effective method of dehydration with some advantages over other methods of drying. Therefore, osmotic dehydration has received remarkable attention in the use of moderate operating temperature, low energy process, reduced loss of volatile compounds, and better quality of the developed dehydrated plum (Lama, 2018).
Osmotic dehydration is a preservation process that is sometimes used as a pretreatment to enhance the quality of conventional dried plum (Monnerat et al., 2006). One of the most exoteric osmotic agents for fruits is sucrose because of its low cost, but other agents, such as glucose or concentrated fruit juices, are also used (Mandala et al., 2005;Rastogi et al., 2002). Osmotic dehydration is a counter flow process that results in solids gain, improving the textural and rheological properties of plum and other related fruits. It elevated the overall quality of plums as compared to conventional drying methods (Birwal et al., 2016). Consequently, the characteristics of the osmo-dehydrated plum can be varied by controlling temperature, sugar syrup concentration, the concentration of osmosis solution, time of osmosis, etc., which require osmotic concentration process faster. For fruits, the most commonly used osmotic agents were sucrose, glucose, and NaCl for vegetables (Chavan & Amarowicz, 2012). Bongirwar and Sreenivasan (1977) pointed out that the high temperature above 60°C modifies the tissue characteristics favoring impregnation phenomena and thus solid gain. Rahman and Lamb (1991) indicated the rate of sucrose diffusion is a function of solute concentration and temperature. As osmotic dewatering is a simultaneous counter-current mass transfer process, there are many changes in the chemical composition of food after osmotic treatment (Lewicki and Porzecka-Pawlak, 2005;Sablani and Rahman, 2003;Robert, 2008).
The process of reintroducing water to dried foods to reach similar water levels as in their initial state is called rehydration . The factor which affects rehydration of any osmodehydrated plum is the chemical composition of the dried fruits and vegetables, method and conditions of dehydration, solvent medium, and temperature (Taiwo & Adeyemi, 2009). In view of the physicochemical properties of fresh plum that could assist the dehydration and rehydrating properties of the osmo-dehydrated plum, this might be established in the present research.
The kinetics of dehydration, rehydration properties, and quality characteristics of dehydrated fruits such as mango, guava and reola (Kumar & Sagar, 2014), banana, apple, apple slices (Ghasemkhani et al., 2016), kiwifruit (Maskan, 2001), and longan (Chunthaworn et al., 2012). From the viewpoints of the above studies, the research on dehydration behavior of plum and physicochemical quality attributes of osmo-dehydrated plum is scare. Therefore, the effect of processing variables on the dehydration kinetics of plum along with the assessment of the physicochemical and rehydration properties of the osmo-dehydrated plum produced from fresh plum is the objectives set for the study.

| Collection and method of processing of plum
The plum fruits were collected from the Spices Research Centre, Bangladesh Agricultural Research Institute, Gazipur. The fruits were sorted, washed, and cleaned. Then, it was blanched in boiling water for 5 min and the plum was peeled by hand. The whole and peeled plum were dipped into 50 0 B sucrose, 45 0 B sucrose plus 5% sodium chloride solution, and only 5% sodium chloride solution for 1.5 hr.
Then, they were heated at 100°C for 2 min. For the preservation purpose, the KMS (1 g/L) and acetic acid (6 g/L) were added. The dehydration temperature was maintained at 60°C. After drying, the fruits were preserved in glass containers. Finally, the dehydrated fruits were analyzed at an interval of 3 months during storage for 1 year at room temperature.

| Mechanical drying
Cabinet dryer, Model OV-165 (Gallen Kamp Company) was used for the dehydration of the plum. The dryer consists of a chamber in which wetted plum could be placed. Air was blown by a fan pass through a heater and then across the trays of plums to be dried. The velocity of air was recorded (0.6 m/s) by an Anemometer. The dehydrated plum was taken for the determination of moisture content. Fresh plums (without peel and peel) at a constant loading density (0.5 kg/ft 2 ) were placed in trays in the drier, and drying was commenced in the drier at a constant air velocity (0.6 m/s) and a specific air-dry bulb temperature of 60°C. Weight loss was used as a measure of the extent of drying.
Fick's second law of diffusion (for plum dehydration) is applied for describing mass transfer during drying. The expression is as follows: where, M = Moisture content (dry basis); t = Time; D e = Effective diffusion coefficient.
The solution for an infinite slab, when dried from one major face (Booker et al., 1974;Crank, 1975;Islam, 1980) is: For low M e values and for moisture ratio, MR < 0.

| Determination of dehydration ratio
The dehydration ratio of the dried plum (without peel and peel) was calculated by the following formula: 2.3.2 | General procedure for rehydration (reconstitution) Rehydration means refreshing the dehydrated or dried plums in water. Six beakers of each 500 ml capacity were taken, and 100 ml of hot water (60°C) and 5 g of the dried samples were poured into each beaker. The wetted plum weight was taken in 5 min intervals up to 30 min. During the weighing process, the liquid portion was drained off and solid contents were transferred to a 4-inch diameter Buchner funnel separately fitted with filter paper to remove excess water from the plum by applying a gentle suction for a few seconds.
The rehydrated materials were removed from the funnel, and the weight is taken individually, and finally, the following relations were found:

| Water activity
Water activity of the dehydrated plum was determined by the chilled mirror technique using a Novasina water activity meter (Decagon devices Inc.).

| Measurement of osmo-dehydrated plum color
Dehydrated plum color was determined using a tristimulus colorimeter (CR-400, Minolta Corp., Japan) with 8-mm aperture and C light source at two equidistant points on the equator of each sample by using CIE color system on the L, a*, and b* color space where L, a*, and b* coordinates were recorded using D65 illuminants. A 10° standard observer was used as a reference system. L (lightness), a* (-greenness to + redness), and b* (-blueness to + yellowness) are the chromaticity coordinates.

| Measurement of texture
Osmo-dehydrated plum texture was analyzed using cross-sectional prove of Texture Analyzer TA.XT plus by back extrusion method.
The test mode compression was used to determine the working capacity of the instrument with a test speed of 1 mm/s and distance was 2.50 cm. The data analysis was performed by Texture Exponent Lite version 6.1.14.0 software (Stable Micro System) to find out the rupture force, and it was expressed as N.

| Measurement of sugar
Total sugar and reducing sugar were determined by Nelson (1944).
Reducing sugars were estimated as percent and calculated it as given below: The total sugar was estimated as percent and calculated as given under:

| Total phenol
Total phenolic content was extracted with 80% ethanol and was estimated based on their reaction with an oxidizing agent phosphomolybdate in Folin-Ciocalteau reagent under alkaline conditions (Bray & Thorpe, 1954). The developed blue color was measured at 650 nm in a UV-VS spectrophotometer (Shimadzu, Japan). The standard curve was prepared using different concentrations (8-32 μg/ml) of catechol, and the result was expressed as mg per 100 g on a fresh weight basis.

| Sensory evaluation
The sensory evaluation of the osmo-dehydrated plum was carried out at every 3 months interval during storage using a sensory taste questionnaire judged by expert sensory panelists. Each treatment was assigned a letter code to avoid biases among the panelists. The samples were presented to panelists in different orders to avoid order preference among the panelists. The osmo-dehydrated plum was rated by 10 experienced panelists who were asked to score samples based on the plum external color, off-flavor, firmness, sweetsour balance, and overall acceptance using a 9-point hedonic scale.

| Data analysis
The experiment was carried out completely randomized design (CRD), and all six treatments were replicated three times. The data were analyzed for ANOVA using computerized statistical software of R to compare the means and the level of significance of data.

| Effects of peeling and sucrose-sodium chloride concentrations on dehydration time
The fresh mature plum (whole and peeled) osmosed in different solutions was dried in the cabinet dryer at a constant temperature of 60°C using a single layer of material. The experimental data were analyzed by using Equation 3; and moisture ratio (MR) versus drying time (hr) were plotted on a semi-log coordinate, and regression lines were drawn in Figure 1. At constant loading density and constant temperature, the faster drying was observed for peeled plum than that of the whole plum. It was noted that the plum peel has a profound influence on dehydration rate, and it offers higher resistance in both heat and mass transfer with resultant higher drying time for peel less plum. For osmo-dehydrated plum, the drying rate constant and R-squared values were less in 50 0 B sucrose with whole plum and more in 50 0 B sucrose with peeled plum; the same trend was observed in another treated sample for whole plum and peeled plum, respectively, as shown in Table 1. It could be concluded that the rate constant of osmo-dehydrated peeled plum was decreased in all cases. This implies that at a specific moisture ratio, more amount of water is evaporated per unit area for a given time from the samples of peeled plum than that of the whole plum. This behavior is attributed due to broader mass transfer resistance given by the plum peel compared to the rest of the plum material (i.e., starchy endosperm, tube cell, epidermis, etc.). A similar result was reported by Pervin et al. (2007) for the effect of drying on bean seeds. It was observed that the NaCl concentration in plum gave a faster drying rate than that of the sucrose concentration.

| Rehydration characteristics of dehydrated plum
For dehydrated plum, the rehydration ratio for the peeled plum was higher than that of the whole plum for all the treated samples. For peeled plum, the highest rehydration ratio was 1.61 (T 6 ) followed by the whole plum it was 1.47 (T 5 ) and the same result was investigated in other treated samples. It was obvious that the plum peel has a significant effect on the rehydration of the plum. The peeled plum resulted in higher rate of drying that might have increased the rehydration rate of the plum as because of the cellular and structural disruption during drying. The reduced rate of shrinkage of the peeled plum has also influenced the attained of a higher rate of rehydration. The coefficient of reconstitution for whole and peeled plum; the highest values were 0.55 and 0.52 in the 50 0 B sucrose concentration, respectively, which was followed by the values of 0.44 and F I G U R E 1 Effect of peeling and various sucrose-sodium chloride concentrations on dehydration rate of plum at a constant temperature of 60°C. T1, 50 0 B sucrose in whole plum; T2, 50 0 B sucrose in peeled plum; T3, 45 0 B sucrose + 5% NaCl in whole plum; T4, 45 0 B sucrose + 5% NaCl in peeled plum; T5, 5% NaCl in whole plum; T6, 5% NaCl in peeled plum Abbreviations: T 1 , 50 0 B sucrose in whole plum; T 2 , 50 0 B sucrose in peeled plum; T 3 , 45 0 B sucrose + 5% NaCl in whole plum; T 4 , 45 0 B sucrose + 5% NaCl in peeled plum; T 5 , 5% NaCl in whole plum; T 6 , 5% NaCl in peeled plum. 0.43 in 45 0 B sucrose + 5% NaCl concentration, respectively, and the lowest values of 0.32 and 0.29 in only 5% NaCl concentration, respectively (Table 2), which indicated that the osmo-dehydrated plum possessed better reconstitution properties using different sucrose concentration than that of NaCl counterparts. This behavior may be attributed to the change in the rate of drying during osmotic treatments using various solutions (Kueneman et al., 1975).

| Physico-chemical properties of osmodehydrated plum
The osmo-dehydrated plum was stored in an ambient condition for one year. The changes in water activity (a w ) of stored osmodehydrated plum was seen in Table 3. There were significant differences observed due to variation in the solute concentrations as well TA B L E 2 Effect of peeling and various solutes concentrations on the rehydration characteristics of dehydrated plum Abbreviations: T 1 , 50 0 B sucrose in whole plum; T 2 , 50 0 B sucrose in peeled plum; T 3 , 45 0 B sucrose + 5% NaCl in whole plum; T 4 , 45 0 B sucrose + 5% NaCl in peeled plum; T 5 , 5% NaCl in whole plum; T 6 , 5% NaCl in peeled plum. Abbreviations: CV, Coefficient of variation; LSD, Least standard deviation; T 1 , 50 0 B sucrose in whole plum; T 2 , 50 0 B sucrose in peeled plum; T 3 , 45 0 B sucrose + 5% NaCl in whole plum; T 4 , 45 0 B sucrose + 5% NaCl in peeled plum; T 5 , 5% NaCl in whole plum; T 6 , 5% NaCl in peeled plum.

TA B L E 3
Effect of peeling and various sucrose-sodium chloride concentrations on the water activity (a w ) of osmodehydrated plum during storage as the peeling condition of the plum. In case of the peeling effect, initial a w (0.50) was found the highest in the whole plum and the lowest was 0.49 in the peeled plum. During the prolonged storage, Concerning the interaction between peeling conditions and solute concentrations, the a w for the whole plum was 0.514, 0.511, and 0.479 for the treatments of T 1 , T 3, and T 5 , respectively, and the percent increase was 21.79%, 21.14%, and 13.78% for the same treatments, respectively, which assumed due to the presence or absence of sucrose and NaCl in the plum. It might be happened due to temperature and humidity changes round the year during storage. The highest values of a w mean the increasing rate of water content for the treated sample of 50 0 B sucrose in the whole plum. In dehydrated plum, the higher water content may decrease the browning rate by diluting the reactive components of the plum and a similar investigation was observed by Labuza and Saltmarch (1981).  Abbreviations: CV, Coefficient of variation; LSD, Least standard deviation; T 1 , 50 0 B sucrose in whole plum; T 2 , 50 0 B sucrose in peeled plum; T 3 , 45 0 B sucrose + 5% NaCl in whole plum; T 4 , 45 0 B sucrose + 5% NaCl in peeled plum; T 5 , 5% NaCl in whole plum; T 6 , 5% NaCl in peeled plum.

TA B L E 4 (Continued)
gradually it was decreased up to 12 months of storage. Initially, the plum color was red and it decreased slowly up to the end of the storage period concerning the color coordinates a*. For the color coor- The influence of temperature on heat-sensitive compounds, such as carbohydrates, proteins, and vitamins, are responsible for the color degradation in fresh foods in addition to browning actions and pigment deterioration with drying processes (Hawlader et al., 2006;Maskan et al., 2002). Similar investigation has been pointed out by increased with the increasing of temperature (Adiletta et al., 2018).
The effect of peeling and solute concentrations on the texture of osmo-dehydrated plum during storage are given in Table 5, and the texture profile of osmo-dehydrated plum after 12 months of storage is shown in Figure 2. As shown in the Abbreviations: CV, Coefficient of variation; LSD, Least standard deviation; T 1 , 50 0 B sucrose in whole plum; T 2 , 50 0 B sucrose in peeled plum; T 3 , 45 0 B sucrose + 5% NaCl in whole plum; T 4 , 45 0 B sucrose + 5% NaCl in peeled plum; T 5 , 5% NaCl in whole plum; T 6 , 5% NaCl in peeled plum. for 45 0 B sucrose + 5% NaCl concentration. Concerning the interaction between peeling condition and sucrose-NaCl concentrations variation, the total sugar content for only sucrose treated plum was initially 68.12 and 64.51 for the T 1 and T 2 treatments, respectively, but after 12 months of storage it was decreased to 41.66 and 39.70, respectively. The total sugar was decreased by 38.84% and 38.46

TA B L E 5
percent for the treatments of T 1 and T 2 , respectively. Nevertheless, in the beginning, the total sugar content of the NaCl treated osmodehydrated plum was 5.74 and 5.32 in the treatments of T 5 and T 6 , respectively; subsequently, after 12 months of storage, it was decreased to 5.19 and 4.72, respectively. The reduction of total sugar content of NaCl treated plum was 9.58% and 11.28 percent for the treatments T 5 and T 6 , respectively. The observed variation was due to increase in moisture content and might also be due to conversion of sugar due to nonenzymatic browning reactions in the osmodehydrated plum (Nazaneen et al., 2015;Tomar et al., 1990). Sugar content in various treated plums varied significantly due to the variation of the sucrose-NaCl concentrations during osmotic treatments and peel conditions. As sucrose is used in plum, an increase in the content of sucrose makes the plum more caloric. For the reduction of the energy value of dried plums, sodium chloride can be used as an osmotic agent and a similar result was found by Robert (2008).
The plums treated with a higher percentage of sucrose along with peeling attributed to the higher values of reducing sugar and total sugar. This might be due to the effect of sugar syrups used for osmosis and the expose of the flesh of the plum after removal of the peel (Kumar & Sagar, 2014). The osmo-dehydrated plum gives a higher percentage of sucrose when sucrose is used as an osmotic agent as reported for the dehydrated mango slices and osmo-dried apple rings, respectively, during storage (Kumar, 2013).
The changes in total phenolic contents of stored osmo-dehydrated plum are presented in Table 7. For the effect of peeling, it was observed that the total phenolic content of 889.78 mg/100 g was found F I G U R E 2 Texture of osmo-dehydrated plum after 12 months of storage (Force vs Time). T 1 , 50 0 B sucrose in whole plum; T 2 , 50 0 B sucrose in peeled plum; T 3 , 45 0 B sucrose + 5% NaCl in whole plum; T 4 , 45 0 B sucrose + 5% NaCl in peeled plum; T 5 , 5% NaCl in whole plum; T 6 , 5% NaCl in peeled plum Abbreviations: CV, Coefficient of variation; LSD, Least standard deviation; T 1 , 50 0 B sucrose in whole plum; T 2 , 50 0 B sucrose in peeled plum; T 3 , 45 0 B sucrose + 5% NaCl in whole plum; T 4 , 45 0 B sucrose + 5% NaCl in peeled plum; T 5 , 5% NaCl in whole plum; T 6 , 5% NaCl in peeled plum.
as the highest in the peeled plum and 860.78 mg/100 g as the lowest in the whole plum during storage, and it was decreased slowly month by month. As to the effect of sucrose-sodium chloride concentrations, at beginning the highest total phenol of 937.61 mg/100 g was observed using only 5% NaCl concentrations which was followed by the value of 859.60 for the 45 0 B sucrose + 5% NaCl concentration.
The interaction between peeling condition and various solute concentrations, initially, the highest total phenol of 990.05 mg/100 g was seen in treatment T 6 and the lowest of 723.06 mg/100 g in treatment T 1 . Finally, the total phenolic content was slightly decreased after 12 months of storage at room temperature. It was happened because of the slower enzymatic reactions in dried plum at a lower temperature of storage as the temperature is a major factor in the initiation and feasibility of a chemical reaction. The phenolic contents occur to produce yellowish to brownish color (Clifford, 2000;Kumar, 2013) Table 8.
It was observed that the color, flavor, taste, sweet-sour balance, and bitterness had a significant effect on its overall acceptance.
According to the Table, it was observed that the overall acceptability of 7.17 was found as the highest for the whole plum and 6.67as the lowest for the peeled plum. As for the effect of concentrations, initially, the highest overall acceptability of 7.75 was observed in 50 0 B sucrose and followed by the value of 6.75 for 45 0 B sucrose + 5% NaCl treated plum. With regard to the interaction between peeling conditions and concentrations, initially, the highest overall acceptability of 8.50 was investigated in treatment T 1 and 8.0 was in treatment T 2 securing the second-highest score. Finally, the highest overall acceptability was continued in treatment T 1 up to the end of storage and it was 8.0 (i.e., like very much) that was judged by the Abbreviations: CV, Coefficient of variation; LSD, Least standard deviation; T 1 , 50 0 B sucrose in whole plum; T 2 , 50 0 B sucrose in peeled plum; T 3 , 45 0 B sucrose + 5% NaCl in whole plum; T 4 , 45 0 B sucrose + 5% NaCl in peeled plum; T 5 , 5% NaCl in whole plum; T 6 , 5% NaCl in peeled plum.

TA B L E 7
Effect of peeling and various solutes concentrations on the total phenol (mg/100g) of osmo-dehydrated plum during storage panelists. Panelists liked the osmo-dehydrated plums because of the balance of sodium chloride-sucrose percentage, less bitterness, attractive color, and overall taste as mentioned during judgment. The best color of the osmo-dried plum might be owing to the effect of KMS used in different treatments as well as the color retained due to the faster dehydration of the treated plum (Ahrne et al., 2003;Akpinar & Bicer, 2005).

| CON CLUS IONS
The research results were analyzed under the parameters of drying kinetics, rehydration properties, water activity, color, texture, sugar, total phenol, and overall acceptability of the osmo-dehydrated plum through sensory evaluation to assess the drying kinetics and the quality attributes of the osmo-dehydrated plum prepared from fresh plum during one-year storage in an ambient condition. The osmodehydrated plum prepared from whole plums osmosed in 50 0 B sucrose solution performed better considering the dehydration kinetics and analysis of the different quality attributes of the plums even after 12 months of storage at room temperature. Therefore, the developed technique would be helpful for the farmers/growers and traders for preparing osmo-dehydrated plum from fresh plum to prevent postharvest losses in addition to fulfill nation demand.

ACK N OWLED G M ENTS
The researchers would like to first express their profound gratitude and heartiest appreciation to the NATP Phase-II, BARC authority for providing an in-country scholarship to continue PhD study and research successfully. Also, we would like to extend our gratitude to PHTD and BARI authority for providing laboratory and manpower facilities to conduct this research work. Finally, we express thanks to Species Research Center, BARI for supplying fresh plum to conduct experiments.

CO N FLI C T O F I NTE R E S T
The author(s) declared no conflicts of interest with the research, authorship, and publication of this article.
TA B L E 8 Effect of peeling and various solutes concentrations on the overall acceptability of osmo-dehydrated plum during storage

E TH I C A L A PPROVA L
This research does not involve any human or animal testing.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.