Rheology of Set Type and Stirred Type Yoghurt: Build-up, Break-down and Recovery; The Effects of pH, Temperature and Starter

Abstract

Skim milk was heat treated (100°C/5min) and fermented with ropy and non-ropy commencai starters, in beaker or in rheometer cup. Yoghurt properties were followed by dynamic rheometry (Bohlin Universal Rheometer) and by penetrometry (Stevens LFRA Texture Analyzer) and the measuring methods were compared.

Set yoghurt storage modulus depended on pH but not on fermentation time. A maximum in loss angle was observed during the gel development. Yield stress increased with falling pH but yield strain was pH-independent.

Set yoghurt could be broken down in the rheometer, with no observed slip, by large strain oscillating movement. The amount of residual structure as measured by storage modulus was generally proportional to its initial value. The relative breakdown could be controlled by the choice of strain and frequence but was independent of pH and temperature. Break-down kinetics were complex, 4 to 5 constants being needed to describe the time course of the storage modulus. No general difference was observed between the structure breakdown of yoghurts containing exocellular polysaccharides and standard non-ropy starters.

After moderate break-down levels, the character of gel as measured by the loss angle was rapidly restored. The recovery of storage modulus resembled breakdown in reverse; — suffer gels recovered more of their stiffness, there was an initial fast increase but the total increase occurred on several timescales.