Thermodynamic Incompatibility of Food Macromolecules By Vladimir B. Tolstoguzov

INSTITUTE OF FOOD SUBSTANCES OF THE RUSSIAN ACADEMY OF SCIENCES, VAVILOV STREET 28, MOSCOW 117813, GSP-1, RUSSIA

1 Introduction This paper will discuss aspects of the thermodynamic incompatibility of proteins and polysaccharides. Why is this incompatibility so important for the functionality of food biopolymers? We will take up this question, as well as the reasons why the phenomenon has many technological impli- cations.

2 Thermodynamic Incompatibility of Food Macromolecules Figure 1 shows two phase diagrams for mixed solutions of the 11s broad bean globulin (called legumin) plus dextran and skimmed milk protein plus carboxymethylcellulose (CMC). In the composition region lying outside the binodal curve, aqueous biopolymer solutions are completely miscible. The region lying within the binodal curve represents compositions of two-phase systems. This means that, on mixing aqueous solutions of different bio- polymers, the mixture breaks down into two liquid phases and a water-in- water (W/W) emulsion can be formed. 1-5 The upper and lower branches of the binodals represent compositions of the co-existing phases. Binodal branches are joined together at the critical point. The thin lines are tie-lines. They connect the points corresponding to the compositions of the co-existing phases. Each phase mainly contains one of the biopolymers. Thermodynamic incompatibility of biopolymers is observed under condi- tions of weaker attractive forces between macromolecules of different types. As a result each macromolecule shows preference for being sur- rounded by its own type. The investigation of the affinity of biopolymers for each other and for the solvent (water) in dilute solution by the light-scattering technique enables the phase behaviour of their mixed concentrated solutions to be predicted.