Thermodynamic incompatibility and microstructure of milk protein/locust bean gum/sucrose systems

Abstract

Phase equilibria of milk protein/locust bean gum systems in high levels of sucrose were investigated to understand the behaviour of these components in diary products and also to look at the particular effect of sucrose. Two milk protein systems were investigated, skimmed milk (SMP) and native phosphocaseinate (PCN). The main advantage of using this latter sample is that it is representative of the casein micelles of milk and allows understanding of what happens in a model system with controlled sugar and salt content. At a temperature of 5°C, pH 6.8, and at an ionic strength of 0.08 M, these mixtures tend to phase separate because of high incompatibility. The similarity between the behaviour of SMP/LBG and micellar casein/LBG systems is evident, suggesting that the incompatibility between skimmed milk and locust bean gum is largely attributable to the incompatibility of micellar casein and locust bean gum. Ternary phase diagrams, in the absence or presence of 20%, 30% or 40% (w/w) sucrose, were established and compared. Sucrose has only a slight influence on the thermodynamic compatibility. The addition of sucrose leads to a concentration effect on the protein phase and dilution of the locust bean gum phase. Some rheological and turbidity measurements, as well as observations using confocal laser scanning microscopy and electron microscopy, were carried out and demonstrated that for the concentrations, temperatures and times investigated, there is no interaction between the components. The microstructures of mixtures have also been investigated in order to study the concentration effect of each biopolymer as well as the sucrose effect. It was clearly demonstrated that the microstructure is driven by the phase volume ratio and this probably leads to a difference in mechanical properties.