Effect of fermentation temperature on the rheology of set and stirred yogurt

The effect of fermentation temperature (over the range 37–46°C) on the formation and rheology of yogurt has been studied using reconstituted skimmed milk (13.9% solids) pre-heated over a period of 20 min to ~87.5°C and fermented with either a ropy or non-ropy starter culture. Fermentation was arrested by cooling to 5°C when the pH reached 4.5. Stirred samples were prepared at 4 l scale, using a carefully defined and reproducible stirring regime; set-style samples were produced by fermentation in retail cartons (~140 ml per carton); both were characterised after storage for 2 days at 5°C; the time-course of structure formation was monitored by in situ fermentation in an oscillatory rheometer. For both cultures, increasing fermentation temperature gave a progressive increase in: (i) the rate of pH reduction and structure formation; (ii) the pH at which the onset of gelation (abrupt increase in G′) occurred; (iii) gel strength from compression testing of set-style samples; (iv) viscosity of stirred samples, as characterised by the time taken for a fixed volume to flow out from a standard funnel; and (v) solid-like character (G′) of the stirred samples. The increased rate of fermentation is attributed to increased metabolic activity of the lactic acid bacteria; the onset of network formation at higher pH (i.e. with less suppression of electrostatic repulsion between casein micelles) and the enhanced rheology of the resulting yogurt, both before and after stirring, are attributed to a progressive increase in the extent and strength of hydrophobic association as fermentation temperature is raised. Plots of G′ and G″ vs time showed an abrupt decrease in slope in the early stages of gelation, and at longer times the moduli recorded using horizontal (cone-and-plate) geometry were much lower than those obtained using a vertical (cup-and-bob) configuration. We attribute these effects to slippage, triggered by network contraction, sedimentation and syneresis, and suggest that extreme caution should be exercised in quantitative interpretation of moduli obtained from measurements of casein networks under oscillatory or unidirectional shear.