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SynopsisA model for the hydration behavior of human stratum corneum has been developed from measurements on in vitro samples isolated in a manner which minimized tissue treatment and trauma. Water sorpt~ion/desorption rate meslsurements as a function of water activity, temperature, and tissue integrity are reported. These data, together with thermodynamic data (infrared and nmr results reported earlier) are consistent with a model in which rapidly sorbed/desorbed water (ca. 0.5 mg water/mg stratum corneum) is associated with ("bound" by) the tissue, while slowly sorbed/desorbed "free" water (up to 12 mg water/mg stratum corneum) is kinetically restricted and probably intracellular in location. Both equilibrium water binding and desorption kinetic data suggest structural changes of this cellular water barrier upon hydration. Evidence for hysteresis in water sorption isotherms (reported by others) could not be reproduced.

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The predictions of previously developed models for the hydration, gelation, and solubility of soy protein isolate regarding the effect of a chemical modification (acetylation) of the protein have been borne out by experiment. Acetylation decreased water binding, increased solubility in the pH 4.5-7 range, and decreased gel strength of soy protein isolate. A shift in molecular weight distribution of the protein to lower molecular weight species also occurred. This work shows that the functional properties (in a food use context) of soy proteins can be altered by chemical modification and that the effect of a particular chemical modification of these properties can be predicted.