Hydration of casein micelles: kinetics and isotherms of water sorption of micellar casein isolated from fresh and heat-treated milk

SUMMARYWater vapour sorption isotherms of casein micelles prepared from raw milk and various heat-treated milks were determined. The equilibrium water contents of the heated preparations were markedly lower than that of the raw-milk casein over the whole range of vapour pressures studied. An analysis of the sorption isotherms in the relative vapour pressure range 0·1–0.45, according to the Brunauer, Emmett & Teller (1938) equation, showed that there were significant differences between preparations in the computed monolayer contents. Differences in the rates of water sorption were also observed between the different preparations.As judged from the amount of absorbed water, the influence of the heating methods could be ranked in the order: HTST (92 °C) ≃ UHT (direct) <UHT (indirect) < HTST (72 °C).

Study of adsorption on metals by the contact potential technique

The change in contact potential between metals due to physical adsorption of water vapour, isopropanol, and ethyl acetate has been studied. The plot of contact potential against relative vapour pressure showed a hysteresis loop, the behaviour varying for different methods of surface preparation. An initial irreversible potential change was always found, due to chemisorption. By studying the dependence of the contact potential on the temperature of one of the metal surfaces it has been possible to determine the change in work function of a gold surface due to physical adsorption.