STRUCTURAL-PROPERTIES OF GELATIN-PECTIN GELS .1. EFFECT OF ETHYLENE-GLYCOL

The role of ethylene glycol (EG) in the gelation mechanisms of acid pi gskin gelatin and high methoxy pectin has been monitored and used as a baseline for the investigation of mixed gelatin-pectin gels in variou s mixed ethylene glycol-water solvents. The addition of EG did not alt er the gelation (t(gel) approximate to 14 degrees C) and melting (t(me l) approximate to 28 degrees C) temperatures of an aqueous gelatin net work, the strength of which, however, was first increased, and then re duced at concentrations of co-solute higher than 30%. The reduction in values of storage modulus (G) was attributed to a decrease in the pro portion of polypeptide chains involved in the formation of junction zo nes. By contrast, increasing levels of ethylene glycol encouraged form ation of pectin gels at high temperatures (e.g. t(gel) was 63 degrees C at 80% EG) which largely retained their structure upon subsequent he ating. The network strength increased rapidly and peaked at 60% co-sol ute followed by a subsequent reduction in storage moduli at conditions of low water activity (60-80% EG). On the basis of a model for gel fo rmation involving a two-step process, it was proposed that ethylene gl ycol promotes the ordered structure of contiguous pectin chains (first stage) but 'dissolves' the hydrophobic clusterings of methyl groups ( second stage). Differential scanning calorimetry demonstrated that the rmodynamic incompatibility between the two polymers is the driving for ce behind the phase behaviour of mixed preparations. Based on the mech anical properties of single components, it was argued that increasing amounts of EG, within the 0-25% range, promote pectin's conformational ordering, which becomes more and more effective in excluding concentr ating up and strengthening the continuous gelatin phase. At higher lev els of co-solute (from 30 to 70%), pectin can form a thermally stable network and during cooling it does so before gelatin's gelation at low er temperatures. Light microscopy work strongly suggests that gelatin also forms a continuous network throughout the body of the sample. The refore, the latter mixtures can be described as phase-separated, bicon tinuous arrangements.