Proteins recovered from acid whey/skim milk mixtures heated at alkaline pH values

Skim milk and mixtures prepared by combining acid whey with skim milk at volume ratios of 2:1, 1:1, 1:2, 1:3 and 1:4 were adjusted to pH 7.5 and heated at 90°C × 15 min. Protein was isolated from these heated samples by precipitation at pH 4.6 and it was found that 65% of the whey protein was recovered in each case. Non‐recovered proteins included the proteose peptones and small quantities of β‐lactoglobulin, α‐lactalbumin and bovine serum albumin. The solubility of these isolates, which contained from 10–25% whey protein, decreased to > 95% when the whey protein exceeded ˜16%. Further characterization of the isolate, prepared from the 1:1 volume ratio of acid whey and skim milk, showed that ˜50% of the whey protein was insoluble, bound to casein and non‐functional while the other ˜50% was complexed with casein and was soluble. The addition of a reducing agent suggests that sulphydryl bonding alone is not responsible for complex formation.