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Specific evaporation energy of bound water in tomato flesh

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The article presents the results of a thermodynamic analysis of the moisture removal process from tomato pulp and the determination of the specific energy of evaporation of bound water based on the hygroscopic characteristics of the raw material. The object of study was the pulp of ripe plum-type tomatoes, pre-dried by radiation-convective variable drying to a residual moisture content of Wобр.= 0.064–0.078 kg/kg. The hygroscopic properties were studied using the static tensometric method of Van Bammelen with sulfuric acid solutions of various concentrations. Based on experimental data, sorption isotherms were build, describing the dependence of water activity Aw on equilibrium moisture content at temperatures T = 293 and 313 K. Through zone-wise approximation of empirical data, a functional relationship between water activity Aw and the moisture content and temperature of tomato pulp was derived. For the thermodynamic analysis, the Gibbs–Helmholtz equation was applied, from which functional dependencies of the free, bound, and internal energy of moisture were obtained, as well as the total specific thermal energy of water vaporization from tomato pulp in the hygroscopic region. It was found that the shape of the isotherms corresponds to S-shaped (Type II according to BET) behavior, typical of colloidal-capillary food materials. The highest specific thermal costs for moisture removal are observed at low moisture content, where strongly bound moisture predominates. Based on the analysis of sorption isotherms for the product under study, the hygroscopic moisture content of tomato pulp was determined for various temperatures: Wg1 = 0.540 kg/kg for T = 293 K and Wg2 = 0.521 kg/kg for T = 313 K. The results obtained can be used in the design and optimization of convective and radiation-convective dryers for tomato raw materials.

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