Predicting the phase diagram for a gelatin-pectin system

Abstract

It is a recognised phenomenon that mixtures of biopolymers in aqueous solution may result in the formation of a two-phase system. Increasingly biopolymers are being used together in food production. Therefore i t is important to understand the conditions and concentration limits that result in phase separation. Gelatin and pectin are important food polymers having the functional properties of gelling, binding and thickening agents [ I ] Hence for the gelatin-pectin system the theoretical treatment of Edmond and Ogston based on excluded volumes [2] has been explored making ube of direct measurements made on the individual biopolymers using sedimentation equilibrium, membrane osmometry and light scattering with the intention of finding the most suitable method for the prediction of the gelatin-pectin phase diagram. In previous work [3], we demonstrated that the phase diagram prediction using weight average molecular weights (MW) and virial coefficients determined from sedimentation equilibrium did not result in a good comparison with the measured binodal. Preliminary indications are that we need to measure number average MWs and virial coefficients as the binodal equation [ 2 ] for predicting the binodal is derived from osmotic pressure formulations, and in this case the MW is always a number average [4]. In this report we present data on number average molecular weights and virial coefficients using osmotic pressure measurements and light scattering. An industrial low-methoxy pectin (Hercules) and a gelatin (Extraco, 2.50 Bloom) were used. The gelatin and pectin were dissolved in 0.5M NaCl and adjusted to pH 5.5 for the osmotic measurements while for the light scattering measurements they were dissolved in O.IM NaCl and adjusted to pH 5.5. The reduced osmotic pressure for both biopolymers was measured over a range of concentrations and is shown in figure 1. The MW and virial coefficients were calculated from the data using the Fisher c-matrix method 141.