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 protem-polysaccharide complexation between the milk protein I % (w/w) beta-lactoglobulin (beta-1g) and 0. 1 % (w/w) chitosan (mw = 75-150 kDa). The zeta potential of mixtures of beta-1g 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 beta-1g was insoluble in the presence of chitosan a mean particle size of 42.1 nm was obtained, which was increased compared to beta-1g alone (6.2 nm, 3.5% insolubility). This indicated the presence of both soluble and insoluble complexes of beta-1g with chitosan. Maximum insolubilisation of [beta-1g (85.3%) by chitosan was observed at pH 6.2. Levels of chitosan-bound protein were reduced in the presence of 100mM 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 beta-1g with heating (78 degrees C for 10 min) in the pH range 4.0-5.1 was significantly reduced in the presence of chitosan. Aggregation of beta-1g 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 beta-1g 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 fl-Ig depending on ionic strength and pH conditions. (C) 2007 Elsevier Ltd. All rights reserved.