PHYSICO-THERMODYNAMIC PROPERTIES OF SOY-GARI

Abstract

The physico-thermodynamic properties of the composite soy-gari were determined to provide information for the prediction of the shelf life and selection of appropriate packaging materials. A split plot in randomized complete block design comprising of three samples (fine–250 µm, medium–500 µm and coarse–1,000 µm), four temperatures (10, 20, 30 and 40C) and seven water activities ranging from 0.371 to 0.937 were used. Moisture adsorption of soy-gari samples were determined gravimetrically – the parameters of the Guggenhiem–Anderson–de Boer, Henderson, Oswin, and the Polynomial models were determined statistically by least square nonlinear and linear regression. The goodness-of-fit of the models was evaluated using percentage root mean square of error (%). The Clausius–Clapeyron equation was used in determining the isosteric heats of desorption. The entropy of sorption was derived from the Gibbs free energy of sorption and related to temperature. A mathematical model proposed by Tsami that expresses the heat of sorption as function of moisture was used to determine the sorption models of the soy-gari samples. There was a highly significant effect (P ≤ 0.05) of water activity (Aw), temperature and particle size on the equilibrium moisture content (EMC) of soy-gari. At all temperatures studied, the EMC increased as the Aw increased. There was increase in sorbed water of soy-gari as temperature decreased at constant Aw. EMC of the samples decreased as the particle size decreased for all the temperatures and water activities studied. The monolayer moisture content did not show a defined trend with temperature change for fine and coarse samples. However, for medium samples the monolayer moisture content decreased with increase in temperature. The plots of isosteric heat show an exponential decay with increase in moisture content approaching the value of heat of evaporation of pure water (ΔHst = 0). The entropy of sorption increased as the moisture content increased and was higher in the coarse than fine soy-gari samples. The moisture adsorption data is useful in determining the permeability of appropriate packaging materials at predetermined shelf life. PRACTICAL APPLICATIONS The formulation of the composite soy-gari, a blend of high carbohydrate traditional food called gari made from cassava and soybean known to be high in protein, was a response to the need for a nutritionally balanced food product for the teeming population in West Africa. This is particularly important as gari is considered to be one of the most common staple foods in the West African subregion. The need to reduce losses of the product under varying packaging and storage conditions makes the study of its physico-thermodynamic properties imperative. Knowledge of moisture adsorption characteristics of soy-gari is important for shelf-life prediction and selection of appropriate packaging materials. It provides valuable information useful for determining the interaction of water and the food material, and the thermodynamics of this interaction.