Food Chemistry 27 (1988) 225-236 Hydration-related Properties of Caseins at pH 2"0-3"0 B. Mohanty, D. M. Mulvihill & P. F. Fox Department of Food Chemistry, University College, Cork, Republic of Ireland (Received 3 April 1987; revised version received 6 July 1987; accepted 6 July 1987) ABSTRACT Casein is very soluble at pH values < 3"5 and acidic casein solutions are also very heat-stable as they can withstand heating at 150°C for 1 h. The viscosity of acidic" casein solutions increased logarithmically with increasing protein ccncentration and decreased logarithmically with increasing temperature up to 70°C. At similar protein concentrations, acidic casein solutions were much more viscous than sodium caseinate solutions. Sorption isotherms of acidic caseins and sodium caseinate were similar up to Am values of 0"52. Between Aw values of 0"52 and 0.87 sodium caseinate sk, rowed a greater increase in water uptake than acidic caseins, while at Aw volues of 0.87-0.97 water uptake of acidic caseins increased more sharply than that of sodium caseinate, uptake being in the order acidic casein pH 2.0 > acidic casein pH 2"5 > acidic casein pH 3"0 > sodium caseinate pH 7.0. INTRODUCTION In recent years, milk protein products are being marketed with an emphasis on specific functional properties. There are seven major classes of functional milk proteins: acid casein, rennet casein, caseinates, casein-whey protein co- precipitates, whey protein concentrates and isolates and lactalbumin (Mulvihill & Fox, 1983). The functional properties and uses of milk proteins have been reviewed by Southward & Walker (1980), Morr (1982), Evans (1982) and Fox & Mulvihill (1982, 1983). 225 Food Chemistry 0308-8146/88/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain