Moisture desorption isotherms of grape skin (Siah Sardasht cultivar)

Introduction: Sorption isotherms are important in modeling, designing, optimizing the drying process and equipment, predicting the storage time, calculating changes in humidity during storage, and selecting the appropriate packaging materials. Due to the diverse and complex composition of foods, theoretical estimation of these curves is not possible and requires experimental measurements. Desorption isotherms permit the estimation of water activities corresponding to the prevailing ambient relative humidity during the drying process.
Material and methods: Black grapes were obtained from vines of Sardasht in the Western Azerbaijan province of Iran. After weighing about one gram of fresh skin in the Petri dish, they were put in the air-tight glass jars containing the saturated salt solutions. The static gravimetric method was used according to the instructions COST 90 (Wolf, Spiess, et al. 1985) at 20, 30, 40, 50, and 60°C for determining moisture desorption isotherm of fresh black grape skins (Siah Sardasht cultivar). Nine saturated salt solutions were selected to give different relative humidity in the range of 0.1-0.9. A small quantity of toluene was used in jars to prevent fungal activity in samples at relative humidity above 50%. Three replications of the same experiment were done. The samples were placed in jars and allowed to equilibrate with the surrounding air at the selected temperatures in an incubator until there was not more than 0.001g difference between two consecutive scaling. The temperatures were checked and controlled within ±1 °C. The equilibrium moisture content of the skins was determined by drying in the vacuum oven at 50˚C for 48h. The GAB, BET, D’Arcy- Watt, Henderson, and modified Halsey equations were fitted to experimental data using regression analysis. The efficiency of fit was evaluated with P<10%), least RMSE and χ2, maximum R2 (Ayranci 1990, kaymak ertekin 2004). The net isosteric heat is an important thermodynamic parameter derived from moisture sorption isotherms. Clausius-Clapeyron equation is often used in determining the net isosteric heat by plotting lnaw against 1/T at a specific moisture content of the material and determining the slope (Majd 2013).
Results and discussion: According to the results, the moisture desorption behavior of the grape skin was dependent on temperature, and at a constant water activity, the equilibrium moisture content of the skins decreased with an increase in temperature. This behavior is generally ascribed to a reduction in the number of active sites due to physical and chemical changes induced by temperature; the extent of decrease, therefore, depends on the nature or constitution of the food. The equilibrium moisture of grape skin was highest at 20 °C and lowest at 60 °C. This finding is consistent with the results of other researchers who have reported the opposite effect of temperature on given water activity. In Italian grape skin, a downward trend in equilibrium moisture has been reported in a specific water activity with an increase in temperature from 35 to 75 °C (Gabas et al. 1999). Studying persimmon skin desorption isotherm showed that equilibrium moisture content increases with decreasing temperature in constant water activity (Talis et al. 2000). In plum peel, equilibrium moisture content was highest at 20 °C and lowest at 70 °C (Gabas et al. 2000). In lemon peel, a decrease in equilibrium moisture has been reported with increasing temperature from 20 to 50 °C (Garcia et al. 2008), in mango peel with decreasing temperature from 20 to 44 °C, a decreasing trend in equilibrium moisture content from 0.622 to 0.203 Observed (Souza et al. 2015).
Conclusion: By considering the lower values of χ2, RMSE and P< 10% and highest adj -R2, R2, the D’Arcy -Watt model had the best fit with the experimental data at all examined temperatures and relative humidity so it can be used to predict and estimate the equilibrium humidity of the samples. The GAB model also had an acceptable fit at 30 °C. According to the results of the net isosteric heat curve, with increasing moisture of grape skin, the amount of net isosteric heat decreased. This fact may be explained assuming that sorption occurs at low moisture contents on the most active sites, hydrophilic groups, while, as moisture content increases water molecules bind with less active sites resulting in lower isosteric heats (Gabas et al., 1999). The net isosteric heat for black grape skins increased with decreasing equilibrium moisture from 0.6 to 0.2 (based on the dry matter) with a gentle slope from 195 to 480 kJ / kg. By reducing the moisture content from 0.2 to 0.1 (based on the dry matter), this value increased significantly from 480 to 1093 kJ / kg, which is due to the strong bond between water molecules and the constituents of grape skin at low humidity. In this way, at least in moisture contents less than 0.2, the increase in net isosteric heat of desorption should be taken into account to simulate the energy requirement for the drying process of the grape skin. Similar results have been shown to increase the amount of net isosteric heat by decreasing the equilibrium moisture content for plum peel (Gabas et al. 2000), Italian grape peel (Gabas et al. 1999), lemon peel (Garcia et al. 2008), and persimmon peel (Talis 2000).