Improved heat stability of protein solutions and O/W emulsions upon dry heat treatment of whey protein isolate in the presence of low-methoxyl pectin

Abstract

Whereas Whey Protein Isolate may be used as an effective emulsifier, it suffers from limited stability upon heating. In this research, the effect of combining Whey Protein Isolate (WPI) with Low Methoxyl Pectin (LMP) on the heat stability of WPI was investigated. This was accomplished either by simple mixing or by conjugate formation by dry heat treatment. WPI-LMP mixtures and conjugates were prepared at a WPI to LMP ratio of 1:0, 4:1, 2:1, and 1:1. Conjugates were prepared by means of dry heat treatment at a temperature of 60°C and 74% relative humidity by incubation for up to 16 days.The pH effect and brown color development of the WPI-LMP conjugates upon incubation was monitored. SDS-Page, free amino group determination, as well as diffusion coefficient analysis, all confirmed the formation of new compounds with high molecular weight. The heat stability of the conjugates was then tested and compared to the native WPI. Upon 2min of heating at 80°C and pH 6.5, the solubility of WPI was reduced by approximately 50% due to thermal denaturation and subsequent aggregation. However, dry heat treatment of the WPI-LMP mixtures highly improved the heat stability of WPI: as the incubation time was increased, the residual protein solubility upon heating of the WPI became higher. Upon 16 days of incubation, the protein solubility of the heated WPI-LMP conjugates was comparable to that of the conjugates before heating. Considering the emulsifying properties, it was found that WPI-LMP conjugates produced smaller oil droplet compared to either native WPI, dry heated WPI, or mixtures of WPI-LMP which were not subjected to conjugation by dry heat treatment. Heating the emulsions at 80°C for 10 and 20min revealed that WPI-LMP conjugates stabilized emulsions exhibited excellent stability towards heat: whereas pronounced aggregation and gelation occurred in emulsions stabilized by WPI or mixtures with LMP, the conjugate stabilized emulsions retained their original viscosity and particle size.