Desorption isotherms of shelled maize: whole, dehulled and hulls

Water vapour desorption isotherms of whole kernel maize were determined at 40, 50 and 60°C, and those of dehulled kernels and hulls at 50 and 60°C. It was found that while whole and dehulled kernels show similar equilibrium moisture contents, the hull gives lower values. Equilibrium data were modelled with two-parameter equations. It was found that the Henderson equation is best for describing the equilibrium data for whole kernels, while the Halsey equation was more appropriate for hulls. A simple semi-theoretical equation was used to model the effect of temperature on the equilibrium values of whole maize kernels for the three temperatures investigated. BET heats of desorption and the isosteric heat curves were calculated for whole kernels and hull. It was found that the BET heat values correlate quite well with the isosteric heat values at the monolayer moisture content.     

Temperature Dependence of Water Sorption Isotherms of Some Foods

The effect of temperature on sorption isotherms of some foods may be taken into account by considering only the effect of temperature on the monolayer moisture content. Thus, knowing the variation with temperature of the monolayer (data are available in the literature) it is possible to predict the temperature dependence along the isotherm. The physicochemical foundation of this behavior is discussed on the basis of the Hailwood-Horrobin model for water sorption. It is suggested that temperature-induced structural modification in the substrate plays an important role in the observed phenomena.     

Calculation of the variation of the heat of desorption with moisture content on the basis of the BET theory

Data of water desorption on different food products were analysed from the point of view of the BET theory. A plot of log C against 1/T as used for the independent evaluation of E₁-E_L and the pre-exponential a₁b₂/b₁a₂, in the expression for the constant C of the BET theory. It was found that a₁b₂/b₁a₂, may range from 10⁻³ to 10⁻⁶ and the E₁ - E_L values are correspondingly much higher than usual. An equation has been obtained within the framework of the BET theory for the dependence of the net heat of desorption on moisture content. In the present analysis the variation of the monolayer value with temperature is considered by means of a simple empirical equation. The results are compared with the heats of desorption calculated directly by the Clausius-Clapeyron equation.     

Correlation of Water Activity Data in Whole Tomato Concentrates

It is shown that water activity (a_w) data in whole tomato concentrates of varying total solids content may be well correlated using a simple equation derived from knowledge of the a_w lowering behavior of nonelectrolytes. The proposed equation has practical value because it allows a_w, prediction in tomato concentrates (in the a_w range of most practical interest) from measurement of the °Brix value.     

A New Model for Describing the Water Sorption Isotherm of Foods

This work investigated the applicability in the food area of a new sorption equation recently developed by Ferro Fontán et al. (1982). The equation may be written, 1n (γ/a_w) =α (m)^-r, where, γ, α and r are parameters to be determined, m is the moisture content, and a_w is water activity. It was found that the three parameter equation is able to describe the water sorption isotherm of 18 different foods in an extensive range of a_w (up to about 0.95) with only 2–4% error (average) in the predicted moisture contents. Foods examined comprised, among other, oilseeds, starchy foods, and proteins.     

Water Activity of Fresh Foods

This work presents a compilation of water activity (a_w) values for fresh foods namely fruits, vegetables and meats. Water activities were calculated from reported data on the cryoscopic temperature (or initial freezing point) of fruits, vegetables and meats, amounting to eighty six different items. In practically all cases studied the water activity is in the range 0.970–0.996.     

Shape Characterization for a Simple Diffusion Analysis of Air Drying of Grains

This work proposes a simple method to characterize which of the elemental geometries (slab, cylinder or sphere) would be a better approximation for the diffusion analysis of air drying of grains having “nonconventional shapes.” The method is tested with experimental drying data obtained for soybeans, rough rice, wheat and peanuts.