Abstract
Hygroscopic equilibrium curves or sorption isotherms are important in order to define dehydration limits of the product, estimate moisture content alterations under environment conditions and to acquire moisture content values for safe storage. This work aimed to determine desorption isotherms of cucumber seeds stored at different temperature (10, 20, 30, 40 and 50 ºC) and relative humidity (0.11 to 0.96) and the thermodynamic properties of this process. Mathematical models were fitted to experimental data in order to represent the agricultural products hygroscopicity. Net isosteric heat of desorption was performed based on the Clausius-Clapeyron thermodynamic relation and Gibbs free energy through the Van't Hoff equation. Based on statistical parameters, Modified Henderson model was the one that best represented hygroscopicity of cucumber seeds. Equilibrium moisture content of cucumber seeds decreased with temperature increment at a constant value of relative humidity. Differential enthalpy values, representing the energy required to remove moisture from the cucumber seeds, ranged from 2923 to 2469 kJ. kg-1. Values of differential entropy and Gibbs free energy decreased with moisture content increase. Enthalpy-entropy compensation theory is valid for the sorption of cucumber seeds, in which the water sorption mechanism in cucumber seeds can be considered to be enthalpy controlled.
Highlights
Cucumber (Cucumis sativus L.) is a vegetable of great importance in Brazil due to social and economic value, consumed as salad or canned vegetables
Cucumber seeds with initial moisture content of 38.0% (w.b.) were used for the desorption process
R2 and the standard deviation of the estimate (SDE) are not sufficient to prove the fitting quality of a model, requiring the analysis of mean relative error (MRE) and residual plots
Summary
Cucumber (Cucumis sativus L.) is a vegetable of great importance in Brazil due to social and economic value, consumed as salad or canned vegetables. Because of its increasing production and aggregated value of cucumber seeds, it is important to study new technologies and problems solution regarding post-harvest procedures. Among these procedures, drying and storage are widely known and used. In order to accomplish these post-harvest operations, knowledge of the relationship between temperature and relative humidity of the air is required. Higher probability of insect and microorganisms attack at the seeds is observed, allowing toxin appearance. Physiological quality of the seeds is devalued, hardening or even not feasible its commercialization
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