Aggregation of Proteins in Whey from Raw and Heat-Processed Milk: Formation of Soluble Macroaggregates and Nutritional Consequences

Whey proteins were isolated from raw, pasteurized, UHT and sterilized milk after isoelectric precipitation. Protein macroaggregates were removed from total whey extracts by filtration through a 0.45 μm membrane. The protein content of raw extracts and of the filtrate was monitored by SDS-PAGE and SE-HPLC, and found to decrease as a function of the thermal treatment. The amino acid composition, surface hydrophobicity parameters and disulphide chemical reactivity were also determined; the results obtained indicated that macroaggregates not included in the filtrate were stabilized by hydrophobic interactions and disulphide bonds. Thein vitroprotein digestibility of total whey extracts was progressively impaired as a function of the intensity of the heat treatment of milk, and was lower in total extracts compared to the corresponding filtrates, suggesting that the accessibility to proteolytic enzymes of whey proteins was dependent on the extent of aggregation.