Partial denaturation and renaturation of β-lactoglobulin at airwater interfaces

Abstract

Foaming in protein solutions is an important phenomenon which may be either beneficial or harmful. In the manufacture of therapeutic proteins or of food products foams are typically produced during fermentations or in pumping operations and may cause handling difficulties. Conversely, the development of a stable foam is essential for whipped food products. Cryo-scanning electron microscopy of whipped cream [l] showed that the air-water interface is stabilised initially by a 4 nm thick protein layer, as in skimmed milk foams [2]. When air or nitrogen is bubbled through skimmed milk or whey this protein layer, or membrane around the air bubbles, persists even after the has burst and the foam collapsed. In dairy foams this bubble material is sufficiently stable to be collected by centrifugation and washed by repeated suspension and recentrifugation with weakly acidic or neutral buffers. Adjusting pH to 8.5-9.0) or adding dissociating agents (8M urea or 6M guanidine HCI rapidly solubilised the ghost pellets. Microscopic examination [B.E. Brooker, personal communication] showed that while ghosts formed in as,or @-casein solutions, they were transient and redissolved spontaneously. However when N, was bubbled through whey or total whey protein solutions at pH values 5 to 7 they rapidly became turbid and centrifugation gave a ghost pellet. In whey the four major proteins @-lg, a-la, BSA and IgA are present at approx. 4, 2, 1 and 0.5 mg/ml respectively. Using 5 mg/ml solutions of each protein in 50 mM sodium phthalate buffer pH 5.0 it was found that all foamed readily but turbidity