Structure and surface properties of the serum heat-induced protein aggregates isolated from heated skim milk

Serum heat-induced milk protein aggregates were isolated from heated skim milk. Protein analysis showed that the aggregates were essentially composed of whey protein and κκ-casein linked mainly by disulfide bridges. The hydrodynamic diameter of the aggregates, as measured by dynamic light scattering (DLS), was about 70 nm. Multi-angle DLS analysis and electron microscopy indicated that they were almost of spherical shape. Measurement of the turbidity, size and zeta potential of the aggregates in milk permeate at pH 2–9 showed that their apparent isoelectric point was ∼4.5. Surface charge in milk permeate at pH 7.0 was −17 mV. 8-anilino-1-naphthalene sulphonic acid (ANS)-binding indicated that surface hydrophobicity of the aggregates was higher than those of native casein micelles, which suggested that their precipitation on acidification may be initiated before reaching pH 4.5.