Proteolysis, protein distribution and stability of UHT milk during storage at room temperature

Proteolytic degradation and distribution of caseins and whey proteins between the soluble and colloidal phases were studied in six batches of commercial UHT milk (three skim and three whole milks) during storage at 25 ± 2 °C. For that purpose, at 30 day intervals, milk samples were ultracentrifuged and the pellets and supernatants analysed by capillary electrophoresis and SDS-PAGE. Samples were also visually examined for signs of gelation. Extensive proteolytic degradation of the micellar fractions and severe changes in the electrophoretic pattern of the proteins present in the serum fractions were observed in all the batches. A higher proportion of denatured whey proteins not attached to the micelle surface was found in the skim milk samples as compared with the whole milk samples that could provide less resistance against gelation. In addition to β-Lg, para-κ-casein was also found in the serum fraction. A high proteolytic activity against κ-casein could be responsible for the hydrolysis of serum-liberated κ-casein or could have enhanced the liberation of β-Lg–para-κ-casein complexes through proteolysis of micellar κ-casein.