The influence of pH on the heat-induced aggregation of model milk protein ingredient systems and model infant formula emulsions stabilized by milk protein ingredients

Abstract

The influence of pH on the heat stability characteristics, at 140 °C, of model milk protein ingredient systems and model infant formulae emulsions prepared using reconstituted skim milk powder (RSMP) and reconstituted electrodialysed whey (REDW) was studied. The heat coagulation time (HCT) of RSMP solutions (0.75–3 g protein 100 ml −1) was influenced by pH and protein concentration while the HCT of REDW solutions (0.75–3 g protein 100 ml −1) was largely unaffected by protein concentration and displayed a broad range of high heat stability at pH values of 6.5–7.4. HCT–pH profiles of blends (1.5 g protein 100 ml −1) of RSMP and REDW with whey protein:casein ( W: C) ratios in the range 100:0–20:80 were determined; each blend displayed a distinct pH range of maximum stability which was dependent on the W: C ratio. The pH region of high stability coincided with the formation of colloidal casein–β-lg complexes upon heating. Two model infant formulae emulsions (3.5 or 4.2 g fat 100 ml −1) with W: C ratios of 60:40 were prepared by homogenizing oil in two RSMP–REDW blends with different levels of added lactose and CaCl 2. Added lactose reduced the heat stability of the blends without altering the HCT–pH profiles while added CaCl 2 shifted the pH of maximum heat stability to more alkaline values. The shape of the HCT–pH profile of the emulsions and corresponding fat-free phases were similar, however, the emulsions were slightly less heat stable. These emulsions had unadjusted pH values of 6.72 and 6.59, respectively, and it was necessary to adjust the pH of these emulsions to >6.9 and >7.2, respectively, values coinciding with high heat stability, otherwise aggregation and/or coagulation occurred on heating (140 °C×80 s). A casein dominant (20:80 W: C ratio) model infant formula emulsion (3.5 g fat 100 ml −1) was stable upon heating at pH values in the range 6.5–7.4.