The effect of heating on β-lactoglobulin–chitosan mixtures as influenced by pH and ionic strength

Work was undertaken to examine the effect of pH, ionic strength and temperature on the protein–polysaccharide complexation between the milk protein 1% (w/w) β-lactoglobulin (β-lg) and 0.1% (w/w) chitosan (mw=75–150 kDa). The zeta potential of mixtures of β-lg and chitosan changed from +34.5 mV at pH 4.0 to a point of zero charge at pH 7.4. At pH 5.5 where 56% of the β-lg was insoluble in the presence of chitosan a mean particle size of 42.1 nm was obtained, which was increased compared to β-lg alone (6.2 nm, 3.5% insolubility). This indicated the presence of both soluble and insoluble complexes of β-lg with chitosan. Maximum insolubilisation of β-lg (85.3%) by chitosan was observed at pH 6.2. Levels of chitosan-bound protein were reduced in the presence of 100 mM NaCl, however, the pH-binding curve was extended into the more acidic region, which is indicative of charge screening effects by NaCl. The insolubilisation of β-lg with heating (78 °C for 10 min) in the pH range 4.0–5.1 was significantly reduced in the presence of chitosan. Aggregation of β-lg was evidenced with heating at pH 4.0, by a particle size of 243 nm, while in the presence of chitosan, no protein aggregation was observed. In contrast, aggregation/denaturation of β-lg in the presence/absence of NaCl was significantly increased in the pH range 5.5–7.0 by the complexation with chitosan. Results have shown the ability of chitosan to both enhance and inhibit the aggregation/denaturation of β-lg depending on ionic strength and pH conditions.