The aggregative stability of β-lactoglobulin in glassy mixtures with sucrose, trehalose and dextran

The aim of this study was to investigate the effect of the addition of different carbohydrates on the thermally induced aggregation of a model globular protein, β-lactoglobulin (BLG), in the glass state. Amorphous mixtures of BLG with trehalose, sucrose and dextran were prepared by freeze-drying, their glass behaviour was characterised using calorimetry and thermally induced aggregation was measured using size exclusion chromatography. Pure BLG shows increasing levels of aggregation when heated in the temperature range 70-100°C for 48-144 h. The addition of the disaccharides sucrose and trehalose both resulted in a decrease in aggregation rate which approached negligible rates at 50 wt.% carbohydrate. The effect of dextran addition was similar to that of the disaccharides when preparations containing 9wt.% carbohydrate were heated at 70°C for 2 days. However, when the concentration exceeded 23 wt.%, the reaction temperature was 70°C or above or the reaction time was longer than 48 h, the addition of the polysaccharide did not protect the protein from thermally induced aggregation, suggesting that protein-polymer phase separation could have occurred during freeze-drying. Overall the results support the proposal that one aspect of carbohydrate additive functionality is as a diluent with the added condition that the carbohydrate remains miscible with the protein during processing.