Emulsifying and foaming properties of beta-lactoglobulin modified by heat treatment

Abstract

The effects of heat treatment on emulsifying properties of beta-lactoglobulin were studied in order to compare them with previous studies on foaming properties. Both of them are closely linked to the structural changes on the protein. Aliquots from 5.5% (w/v) beta-lactoglobulin solution in 20 mM phosphate buffer pH 6.8 were heated at 85 ºC for different time periods, from 1 to 15 min. Protein solubilities were measured for unheated and heated beta-lactoglobulin samples. Protein-stabilized O/W emulsions were prepared with these samples and corn oil. Droplet size distribution in the emulsions and emulsifying activity index were determined for each system, as parameters of the emulsifying ability of the protein. Emulsion stability was estimated from three different methods: backscattering, determination of the remaining protein concentration after creaming and monitoring the oiling off process. With the assayed methodology, heat treatment of beta-lactoglobulin led to different effects on foaming and emulsifying properties of the protein, depending on the time of heating. For shorter times of heating, both foamability and foam stability improved, while emulsifying properties diminished. After 10 min of heating at 85 °C, both foaming and emulsifying properties diminished. Formation time scales, as well as size of the sedimentable aggregates and their steric effect on the interfacial film, play an important role in explaining these differences between foaming and emulsifying properties.