Thermodynamic properties and sorption equilibrium of pestil (grape leather)

Moisture sorption isotherms (MSI) of pestil, a well-known grape leather in Turkey, were determined at 15, 25 and 35 °C over a water activity (aw) range of 0.06-0.98, using gravimetric static method. The Gugghenheim-Anderson-de Boer (GAB), Halsey, Oswin, and Peleg models were tested to fit the experimental data by using nonlinear regression analysis method. The MSI exhibited type III behaviour, and the inverse temperature effect was observed for the sample stored at 35 °C at higher aw values (>0.65). The Halsey and Peleg models adequately described the MSI. The thermodynamic properties of MSI (net isosteric heat, differential entropy, spreading pressure, net integral enthalpy and net integral entropy) were also evaluated for understanding of the properties of water and calculating the energy requirements for transport phenomena during processing. The net isosteric heat and differential entropy decreased with increasing moisture content and were characterized by two-parameter exponential decay and power-law models respectively. The spreading pressure of pestil increased with increase in water activity and decreased when the temperature increased to 35 °C. Net integral enthalpy increased to a maximum value of 20.25 kJ/mol and net integral entropy decreased to a minimum value at around monolayer moisture content. Thereafter, with increasing moisture content the enthalpy decreased and the entropy increased gradually.