Development and breakdown of structure in yoghurt studied by oscillatory rheological measurements

Yoghurt was produced from heat treated skim milk in situ in a rheometer. The development of set yoghurt structure was found to depend on the pH and not on the rate of pH decrease. At pH 4.25, 44C, 0.1 Hz, within the linear viscoelastic region yoghurt had a storage modulus of -230 Pa, and a 1055 angle of -13. High strain, up to 0.8, oscillation was used to break down the in situ gel structure at varying temperatures. Over a 3D-min period, apparent storage modulus decreased by 1-2 orders of magnitude. The decrease in gel storage modulus was found to occur at least on 2 timescales, the proportion of fast and slow breakdown varying with strain and temperature. The estimated relative residual modulus showed little variation in the examined tempe rature and pH ranges but was strongly dependent on strain and to a lesser extent on frequency of the oscillatory shear.