Influence of the degree of esterification on the emulsifying performance of conjugates formed between whey protein isolate and citrus pectin

Conjugates were prepared from whey protein isolate (WPI) and pectins in a dry heating process (80 degrees C, 79% RH). Citrus pectins with different degree of esterification (DE) were employed: low methylesterified (LMCP, DE = 34%), high methylesterified (HMCP, DE = 72%), very high methylesterified (VHMCP, DE = 84%). SDS-PAGE of heat treated WPI-pectin samples showed typical patterns of conjugate formation with substances of M-w > 100 kDa being detected first after 6 h of reaction time. Fluorescence intensity measurements of conjugate solutions indicated a maximum for WPI-HMCP and WPI-VHMCP after 15 -18 h of reaction time. However, the fluorescence intensity of WPI-LMCP solutions increased continuously reaching the highest values of the three mixtures. Zeta potential measurements of conjugate solutions exhibited the opposite behavior. A minimum was found for WPI-HMCP. The zeta potential of WPI-LMCP solutions decreased monotonously reaching values of around -50 mV. In emulsification experiments, a significant reduction of the emulsion's Sauter mean diameter d(3,2) was found when conjugates were employed at pH 5.5 and 7. The smallest droplet sizes in the hundred nanometer range were obtained for WPI-LMCP conjugate emulsions. Linking fluorescence measurements of conjugate solutions to emulsion d3,2 revealed that 1.) the stabilizing mechanism of WPI-pectin conjugates is mostly steric, 2.) the conjugate yield is the main factor dominating emulsion droplet size and 3.) the conjugate yield is highest when low esterified pectin is used. (C) 2015 Elsevier Ltd. All rights reserved.