Interactions of whey proteins with milk fat globule membrane proteins during heat treatment of whole milk

Abstract

The association of β-lactoglobulin (β-Lg) and α-lactalbumin (α-La) with milk fat globule membrane (MFGM), when whole milk was heated in the temperature range 60-95°C, was investigated using one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) under reducing and non-reducing conditions. In SDS-PAGE under reducing conditions, β-Lg was observed in MFGM material isolated from milk heated at ≥60°C for 10 min; small amounts of α-La and κ-casein were also observed in the MFGM material of milk heated at ≥65°C for 10 min and ≥75°C for 10 min, respectively. However, these proteins were not observed in SDS-PAGE under non-reducing conditions. Two-dimensional SDS-PAGE of MFGM material isolated from heated milk showed that the protein complexes that remained at the top of the non-reducing gel (first dimension) were resolved into β-Lg, α-La and the major original MFGM proteins in the reducing gel (second dimension). These results indicate that β-Lg and α-La associated with MFGM proteins via disulfide bonds during the heat treatment of whole milk. The amounts of β-Lg and α-La that associated increased with an increase in the temperature up to 80°C, and then remained almost constant. These maximum values for β-Lg and α-La were ∼1.0 mg·g-1 fat and ∼0.2 mg·g-1 fat, respectively. Of the major original MFGM proteins, xanthine oxidase and butyrophilin were not affected by the heat treatment of whole milk, whereas PAS 7 was heat labile and PAS 6 decreased to some extent during heating.