Application of Solution Thermodynamics to the Water Sorption Isotherms of Food Materials

Abstract

Solution thermodynamics was applied to the water sorption isotherms of foods. First, we corrected Le Ma:~uer's theory. By this correction, it was found that the same thermodynamic equations were applicable to both water sorption data measured at constant total pressure (P) and to those measured under the particular vapor pressure, P = Pw' for a food material. Second, the change in the water sorption behavior of rrce flour by defatting and extrusion cooking was analyzed by solution thermodynamics. It was confirmed that the principle of solution thermodynamics is more appropriate than that of conventional adsorption thermodynamics for evaluating the affinity of food for water. The enhanced affinity of rice flour for water by defatting and extrusion was quantitatively evaluated by solution thermodynamics. where Pw is the vapor pressure. V' is the solution volume, Vw is the water volume, V aO is thc adsorbent volume in a dry state, and W is the watcr content, i.e., mw/m a . If V'~Vw + V aO ' the second term on the right side of Eq. (5) is negligible. Equation (5) can then be approximated to f a w LlG>=: -R'T°W/awdawwhere R' is the modified gas constant (= 0.462 [kJ jkg of water' K]), the reference kilogram here being water. Parameter LlG: is given as follows:where m a is the mass of the adsorbent, m w is the mass of water, P: is the chemical potential of water in the solution, P; is the chemical potential of the adsorbent in the solution, P: is the chemical potential of pure water at the same temperature (T) and pressure (P) as those of the solution, and 11a* is the chemical potential of a pure solid at Tand P. The relationship between parameter LlG: and water activity a w is ) AnalyticalSolution thermodynamics. 20) When solution thermodynamics are applied to water sorption isotherms, the condensed phase formed by water sorption to the adsorbent is taken as the solution. The reference state for water is the liquid form (hereafter taken to be water) under the solution temperature and pressure, that for the adsorbent being a pure solid under the solution temperature and pressure. In addition. since the data in sorption experiments are expressed as mass rather than moles, thermodynamic functions are expressed by mass units. Details of the derivation are shown in the appendix.(1) Gibbs free energy The parameters, LlG: and LlG;, and the change in integral Gibbs free energy, LlG', are defined by It is of great importance to understand the water sorption behavior of foods for controlling food quality. Water sorption isotherms are influenced by many factors such as the food composition, affinity of food for water, and the surface structure of the food. It would be possible to roughly estimate the state of water just by the shape of the water sorption isotherm. Further quantitative information on the interaction between food and water can be obtained by thermodynamic analysis. A conventional thermodynamic analysis (hereafter referred to as adsorption thermodynamics) of water sorption isotherms 1 -17) estimates only the the...

Analysis of Water Sorption Behavior of Native and Denatured Proteins by Solution Thermodynamics

“…(5) by using the water sorption isotherm. Parameter LtG: can be evaluated analytically, if the water sorption isotherm fits well with the GAB (Guggenheim, Anderson, and de Boer) equation as in the studies of Le Maguer 1 ) and Kumagai et al 2) However, the water sorption isotherms obtained in this study did not fit well with the GAB equation, especially in the region of high water activity. Therefore, parameter LtG!…”

“…(7) can be used when the measurement is done at constant pressure, i.e., with inert gas. However, as Kumagai et al 2) have discussed, the second term on the right side of Eq. (6) is negligible if the swelling ratio, VS/( Vw + V aO ), is about unity.…”

“…Le Maguer 1) has applied solution thermodynamics to the water sorption isotherms of potato starch. Kumagai et al 2) corrected the errors in the theory of Le Maguer's paper and applied the modified theory to the water sorption isotherms of defatted and extruded rice flour samples. They clarified the physical meaning of "integral Gibbs free energy," showing that it was the appropriate parameter for evaluating the affinity of food for water.…”

“…9) However, the water sorption behavior of proteins has not been analyzed by solution thermodynamics, although their affinity for water cannot be evaluated by adsorption thermodynamics, especially at high water content. 2) In this study, we measured the water sorption isotherms of native and denatured ovalbumin samples, whose structural change were evaluated by several methods (the degree of denaturation~ the amount of disulfide bonds, surface hydrophobicity, and solubility), and applied solution thermodynamics to the water sorption isotherms.…”

Analysis of Water Sorption Isotherms of Superabsorbent Polymers by Solution Thermodynamics

“…Le Maguer 12 ) has applied solution thermodynamics to the water sorption isotherms of potato starch. Kumagai et al 13) have corrected the errors in the theory of Le Maguer's paper and applied the modified theory to the water sorption isotherms of defatted and extruded rice flour: they have shown that the "integral Gibbs free energy" physically means the change in Gibbs free energy by water sorption and that it was an appropriate parameter for evaluating the affinity of food for water. As a result, the enhanced affinity of rice flour for water after defatting and extruding was quantitatively evaluated by solution thermodynamics.…”