THE EFFECT OF LOW HEATING TEMPERATURES ON CASEIN WHEY PROTEINS

In the work, a comparative analysis of the fractional composition and resistance to thermal denaturation of proteins of two samples of casein wheys was carried out: the laboratory one, which was obtained at a temperature of 18°C, and the production one – at a temperature of 46°C. In both cases, wheys were separated after isoelectric precipitation of casein complex proteins with hydrochloric acid. Express electrophoresis in a polyacrylamide gel showed an identical qualitative composition of major and minor proteins fractions in both samples. Also, no differences were found in the relative content of the main protein fractions of wheys – β-lactoglobulin (β-lg), α-lactalbumin (α-la), serum albumin (BSA) and immunoglobulins (Ig). To compare resistance to heat denaturation, samples of laboratory and production wheys were heated and held for 5 min at temperatures from 60°C to 100°C with an interval of 5°C. The precipitate of denatured proteins was isolated by centrifugation. The concentration of proteins in the supernatant was determined spectrophotometrically and the qualitative and quantitative composition of the main protein fractions was analysed by express electrophoresis. As a result, it was established that the concentration of undenatured protein in both wheys does not coincide in the temperature range of 70–90°C. Laboratory whey proteins were more resistant to heat denaturation. Moreover, these differences can be detected only with short-term (5 min) thermal denaturation. Long-term high-temperature heating (30 min) does not allow them to be detected. Electrophoretic analysis of the relative content of the main protein fractions showed that changes in stability occur due to two fractions – β-lg and BSA. The relative concentration of these fractions in the undenatured residue from production whey is ˂ 3%, and in the residue from laboratory whey > 8% for BSA and > 24% for β-lg when heated to 90°C for 5 min. Long-term exposure of both wheys at 20°C before investigating their resistance to thermal denaturation may indicate that some of the changes in the stability of production whey proteins caused by preheating to 46°C are irreversible.