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

Phase equilibria of milk protein/locust bean gum systems in high levels of sucrose were investigated to understand the behaviour of these components i n diary products and also to look at the particular effect of sucrose. Two milk protein systems were investigated, skimmed milk (SMP) and native phosp hocaseinate (PCN). The main advantage of using this latter sample is that i t 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 degrees C, pH 6.8, and at an ionic strength of 0.08 M , these mixtures tend to phase separate because of high incompatibility. Th e similarity between the behaviour of SMP/LBG and micellar casein/LBG syste ms 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 pres ence of 20%, 30% or 40% (w/w) sucrose, were established and compared. Sucro se has only a slight influence on the thermodynamic compatibility. The addi tion of sucrose leads to a concentration effect on the protein phase and di lution of the locust bean gum phase. Some rheological and turbidity measure ments, as well as observations using confocal laser scanning microscopy and electron microscopy, were carried out and demonstrated that for the concen trations, temperatures and times investigated, there is no interaction betw een the components. The microstructures of mixtures have also been investig ated 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. (C) 1999 Elsevier Science Ltd. All rights reserv ed.