Structural properties of gelatin—pectin gels. Part I: Effect of ethylene glycol

Abstract

The role of ethylene glycol (EG) in the gelation mechanisms of acid pigskin gelatin and high methoxy pectin has been monitored and used as a baseline for the investigation of mixed gelatin—pectin gels in various mixed ethylene glycol—water solvents. The addition of EG did not alter the gelation (t gel ≈ 14°C) and melting (t mel ≈ 28°C) temperatures of an aqueous gelatin network, the strength of which, however, was first increased, and then reduced at concentrations of co-solute higher than 30%. The reduction in values of storage modulus (G) was attributed to a decrease in the proportion of polypeptide chains involved in the formation of junction zones. By contrast, increasing levels of ethylene glycol encouraged formation of pectin gels at high temperatures ( e.g. t gel was 63° Cat 80% EG) which largely retained their structure upon subsequent heating. The network strength increased rapidly and peaked at 60% co-solute 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 formation involving a two-step process, it was proposed that ethylene glycol promotes the ordered structure of contiguous pectin chains (first stage) but ‘dissolves’ he hydrophobic clusterings of methyl groups (second stage). Differential scanning calorimetry demonstrated that thermodynamic incompatibility between the two polymers is the driving force behind the phase behaviour of mixed preparations. Based on the mechanical 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, concentrating up and strengthening the continuous gelatin phase. At higher levels of co-solute (from 30 to 70%), pectin can form a thermally stable network and during cooling it does so before gelatins gelation at lower temperatures. Light microscopy work strongly suggests that gelatin also forms a continuous network throughout the body of the sample. Therefore, the latter mixtures can be described as phase-separated, bicontinuous arrangements.