Bovine milk whey proteins were treated with porcine stomach pepsin, bovine chymosin, human uropepsin, and gastric juice from rat stomach at various pH values. Although R-lactalbumin and bovine serum albumin were hydrolyzed by these proteases, -lactoglobulin ( -LG) was not hydrolyzed by any pepsin samples at any pH in vitro. Heat-denatured whey proteins, including heated -LG, were easily hydrolyzed by pepsins. Native -LG injected into the stomach of a rat was not digested in stomach in vivo, whereas heat-denatured -LG was digested in the stomach of a rat. Native -LG as well as the heated sample was hydrolyzed by both commercially available porcine pancreatin and the rat pancreatic juice collected from rat intestine in vitro. Native -LG injected into the stomach was digested in the intestine in vivo as well as heat-denatured -LG. The body weights of rats fed heated -LG increased more rapidly than those of rats fed native -LG.

Whey protein solutions at pH 3.5 elicited an astringent taste sensation. The astringency of whey protein isolate (WPI), the process whey protein (PWP) that was prepared by heating WPI at pH 7.0, and the process whey protein prepared at pH 3.5 (aPWP) were adjusted to pH 3.5 and evaluated by 2 sensory analyses (the threshold method and the scalar scoring method) and an instrumental analysis (taste sensor method). The taste-stimulating effects of bovine and porcine gelatin were also evaluated. The threshold value of astringency of WPI, PWP, and aPWP was 1.5, 1.0, and 0.7 mg/mL, respectively, whereas the gelatins did not give definite astringency. It was confirmed by the scalar scoring method that the astringency of these proteins increased with the increase in protein concentration, and these proteins elicited strong astringency at 10 mg/mL under acidic conditions. On the other hand, the astringency was not elicited at pH 3.5 by 2 types of gelatin. A taste sensor gave specific values for whey proteins at pH 3.5, which corresponded well to those obtained by the sensory analysis. Elicitation of astringency induced by whey protein under acidic conditions would be caused by aggregation and precipitation of protein molecules in the mouth.

beta-Lactoglobulin was purified from whey protein concentrate by a combination of pepsin treatment and membrane filtration. Porcine pepsin was added to whey protein (1:200, wt/wt), and the mixture was then incubated at pH 2.0 and 37 degrees C for 60 min. The protein fraction was collected by ammonium sulfate precipitation, and the precipitate was either dialyzed against water using a dialysis membrane (20-kDa pore size) or filtered using an UF membrane (30-kDa pore size). The beta-LG did not differ from standard beta-LG as measured by chromatography, SDS-PAGE, native PAGE, differential scanning calorimetry, or UV spectrum. Based on the results, a simplified procedure was developed, consisting of pepsin treatment and UF, to purify beta-LG directly from whey.