عنوان مقاله [English]
Background and objectives: Organic black grapes, which are produced in Sardasht, West Azerbaijan province of Iran, contain resveratrol and flavonoids and high amount of melatonin having health benefits. The relationship between equilibrium moisture content and water activity at specified temperature is expressed as sorption isotherms. Moisture sorption isotherms are useful in predicting shelf-life stability and in calculating the variations of moisture content during drying, storage and packaging. In physical, chemical and microbial analysis of food, data obtained from adsorption and desorption curves are very important factors. Isotherm curves are able to predict the maximum moisture retention in foods during processing. In this study, the moisture sorption isotherms of grape pulp (Siah-Sardasht cultivar) were determined using a static-gravimetric method at 30, 40, 50, 60 and 70°C with water activities over the range of 0.1-0.9.
Material and methods: Black grapes were obtained from a vineyard of Sardasht in the West Azerbaijan province of Iran. Nine saturated salt solutions with degree of purity over 98% for generating a range of relative humidity were used and in order to describe the sorption moisture isotherms, the BET, GAB, Henderson, Darcy-watt and Halsey equations were tested to fit the experimental data by using non-linear regression analysis method. The models were sorted based on the least mean absolute percentage error (P<10%), RMSE and χ2, and the highest R-square. The isosteric heat was determined using the Claussius–Clapeyron equation.
Results: At all of temperature levels, moisture sorption isotherm curves of Sardasht black grape pulp were of type III, according to BET. Moisture sorption isotherms obtained in this work were affected by temperature, and the equilibrium moisture content generally increased with increasing of temperature. According to the evaluation parameters of the models tested, P-value of Darcy-Watt model was consistently less than 10 % and its RMSE and χ2 were lowest, indicating its reliability in predicting moisture sorption behavior of the black grape pulp. The results also showed that, to predict moisture sorption in the grape pulp, BET and GAB models can be used only at high temperatures. The isosteric heat showed a marked increase with decreasing moisture content especially at less than 0.3% (d.b).
Conclusion: At the studied range of water activity, the equilibrium moisture content values of the black grape pulp increased with increasing temperature, and the reverse temperature effect wasn’t observed in the range studied of water activities. In spite of presence of sugar in the black grape pulp, no inversion phenomenon was observed in the relationship between equilibrium moisture content and water activity. Darcy-Watt model was found to be offering the best fits over the entire water activity and temperature range. The isosteric heat decreased with increasing of moisture content by reason of the approaching pulp water properties to the free water one.