Thermodynamic properties of water sorption isotherms of grape seed

Abstract

A b s t r a c t. In this study the moisture sorption isotherm of grape seed was determined by using a static gravimetric method at 35-65°C and 0.108-0.821 water activity range. The sorption isotherms were found to be typical sigmoid shape of most food materials. Five models including the Brunauer-Emmett-Teller (2-parameter), Guggenheim, Anderson and De Boer (3-parameter), Oswin (2-parameter), and Peleg (4-parameter) models were considered to fit the experimental data. The FerroFontan and Peleg equations (at three temperatures 35, 45, 65°C) having R 2 greater than 0.97 and lower values of standard error of estimate and deviation modulus gave the best fit of the experimental data throughout the entire range of water activity. The net isosteric heat of sorption, calculated by Calusius-Clapeyron equation on experimental data, was found to be a polynomial and exponential function of equilibrium moisture content within the temperature range investigated.K e y w o r d s: moisture sorption isotherm, thermodynamic properties, grape seed INTRODUCTION Grapes (Vitis vinifera) originated in Western Asia and Europe. It accounts for about 16% of the global fruit production. Among the major grape producing countries, Iran with the production of 3 mln t ranks the seventh after Turkey (FAO, 2009). Grape seed, a by-product of grape processing, is a well-known oilseed containing about 8-20% oil, 11% protein, and 10-20% polyphenolic compounds like tannins, sugars, minerals and other substances (Campos et al., 2008).Drying grape seed is a critical processing step in the preparation of the seeds for oil extraction. Moisture reduction is necessary to increase shelf-life and to prevent microbial and chemical deterioration. Drying also facilitates handling conditions and reduces storage costs (García-Pérez et al., 2008). In order to describe the process of drying and its effect on water activity (a w ), knowledge of the relationship between equilibrium moisture content (EMC) and water activity/equilibrium relative humidity (ERH) is essential. This relationship, known as moisture sorption isotherm, depends on structure, composition, as well as pressure and temperature (AmiriChayjan and Esna-Ashari, 2010, Chowdhury et al., 2006). It is useful for modelling, designing and optimization of the drying process. The water sorption isotherms can also be used to calculate the net heat of sorption which provides the energy requirements associated with the sorption behaviour. The differential heat of sorption is an indicator of the state of water adsorbed by the solid particles, and its knowledge is important in designing drying unit operation (Chowdhury et al., 2006;Goula et al., 2008).Numerous mathematical models have been used to describe the moisture sorption behaviour of foods. Van den Berg and Bruin (1981) gave more than 200 equations that had been developed theoretically, semi-theoretically and empirically to model the sorption isotherms of different biological materials. Some of those models are based on theories on the sorption mechanism, ...

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