Heat-induced Aggregation of Casein in Reconstituted Skimmed Milk at pH 3.30 – 3.55

Abstract

Fewer studies on the effects of heat treatment on casein under acidic conditions have been reported compared with those under neutral conditions. This study aims to clarify the effect of heat treatment under acidic conditions on the properties and functions of milk proteins. The turbidity of reconstituted skimmed milk increased with increasing temperature (70 – 90°C) and pH (3.40 – 3.55). Suspensions of heated samples were stable and did not sediment at low-speed centrifugation. Mean particle diameters of heat-induced particles were 200 – 680 nm. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis and ion-exchange high-performance liquid chromatography revealed that heat-induced colloidal particles comprised α_S1-casein and β-casein. α_S2-Casein did not participate in aggregate formation. The proportion of α_S1-casein and β-casein did not change regardless of temperature, whereas the proportion of β-casein was dependent upon pH. The results showed that heat-sensitivity of casein constituents at pH 3.30 – 3.55 is markedly different from that under neutral conditions, and some caseins aggregate into stable colloidal particles upon heating.