Thermodynamic properties and sorption equilibrium of pestil (grape leather)
Introduction
Production of fruit leather is one of the oldest methods used to increase the shelf life of fruits. Grape, apricot, and plum are the some examples used to prepare pestil (fruit leathers), however pestil mainly refers to grape pestil. Pestil, which mainly consist of juice and starch, is stored for a long-term period. Water activity has long been considered as one of the most important quality factors especially for long-term storage. All chemical and microbial deterioration reactions are directly affected with changing water activity. Phase transitions (dissolution, sugar crystallization) of high sugar containing foods are also related to water activity changes (Labuza, 1984). Therefore, the determination of water activity and moisture content relation is significant and this relation is described by moisture sorption isotherms.
The production steps of pestil show some differences according to the geographical regions. Also different ingredients like sugar, starch, or flour could be used in the production of pestil. Not only composition but also the applied processes have an affect on moisture sorption characteristics since heating or other process methods may cause changes in sorption properties of food constituents (Kaymak-Ertekin & Gedik, 2004).
Several empirical and semi-emprical equations have been proposed to correlate equilibrium moisture content, temperature and aw (Chirife & Iglesias, 1978). Mostly applied equations for high sugar containing foods have been GAB, Halsey, BET, Henderson, and Oswin (Tsami et al., 1990, Maskan and Gögˇüş, 1997, Kaymak-Ertekin and Gedik, 2004).
The application of thermodynamic principles to sorption isotherm data has been used to obtain more information about the properties of water, food microstructure, and physical phenomena on the food surfaces, and sorption kinetic parameters (Rizvi and Benado, 1984, Aviara and Ajibola, 2002, McMinn and Magee, 2003). The net isosteric heat of sorption (qst), differential entropy (Sd), spreading pressure (Φ), net integral enthalpy (Qin) and net integral entropy (Sin) are used as thermodynamic functions for analysis of sorption isotherms. The net isosteric heat of sorption or differential enthalpy shows the energy requirement for removing moisture from food material (water–solid binding strength) and has a practical use in complete drying calculations and modeling of energy. The net integral enthalpy or net equilibrium heat of sorption indicates the binding strength of water molecules to food particles and could be a measure of the food–water affinity (Aviara, Ajibola, & Oni, 2004). The changes in integral enthalpy may provide a measure of the energy changes occurring upon mixing of water molecules with sorbent during sorption processes (Telis, Gabas, Menegalli, & Telis-Romero, 2000). The differential entropy (Sd) of a material may be related to the number of available sorption sites at a specific energy level (Madamba, Driscoll, & Buckle, 1996). The changes in entropy could be used in exergy balance giving valuable information about energy utilization in food processing (Rotstein, 1983). Also, the order/disorder concept, useful for the interpretation of processes that take place during moisture sorption such as dissolution, crystallization and swelling, is related with entropy variation (Aviara, Ajibola, & Dairo, 2002). The spreading pressure (Φ), or surface potential, represents the surface free energy of adsorption and can be regarded as the difference in the surface tension between bare sorption sites in the solid and sorbed molecules (Al-Muhtaseb, McMinn, & Magee, 2004). The nonlinear relationship between spreading pressure and water activity for cereal grains and starchy materials was determined by Tolaba, Suárez, and Viollaz (1995).
The objectives of this study were to obtain MSI data of pestil produced in the laboratory, to evaluate the best MSI equation to fit the experimental data and to determine the thermodynamic functions (net isosteric heat, differential entropy, spreading pressure, net integral enthalpy, and net integral entropy) in relation to moisture sorption in the pestil.
Section snippets
Preparation of pestil and thickness measurement
Grapes (dökülgen), for pestil production with initial total soluble solids (°Brix) of 20 g/100 g, were obtained from a farm in Gaziantep in September 2003 and stored at 4 ± 0.5 °C. Prior to pestil preparation, grapes were taken out of storage and processed. All grapes used for pestil making were from the same batch. Gaziantep white earth used for fruit juice clarification was obtained from grape producers in Gaziantep. All chemicals used were of reagent grade and deionised water was used for all
Sorption isotherms
Moisture content of pestil was found to be 10.51 ± 0.5%. The thickness of the pestil samples was 0.60 ± 0.05 mm. The appearance of pestil was light yellow with a smooth surface. The experimental adsorption isotherms obtained at 15, 25, and 35 °C are shown in Fig. 1. The sorption isotherms of pestil showed type III behaviour according to the BET classification. This is probably due to the high amount of sugar and carbohydrate (∼85%) (Maskan et al., 2002). At water activities higher than 0.65, there is
Conclusions
- 1.
Moisture sorption isotherms of pestil were evaluated using the isopiestic method at 15, 25, and 35 °C. Changing temperature was not significant for sorption isotherms of pestil (p > 0.05). Opposite temperature effect was observed at water activities >0.8 for the samples stored at 35 °C.
- 2.
GAB, Oswin, Halsey, and Peleg equations were applied to sorption data. Except for Oswin, all of them gave an acceptable fit of the experimental data. The Halsey equation was used for predicting the water activity of
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