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Abstract
When a modified starch was added to yogurt milk and the mixture fermented at 43°C, the viscosity of the stirred yogurt increased, but the yogurts developed a grainy texture. Lowering the fermentation temperature from 43°C to 35°C increased the fermentation time, decreased graininess and whey drainage, but decreased the viscosity of stirred yogurt with added starch. However, stirred skim milk yogurt with 12% milk solids non-fat (MSNF) and 1% w/w added starch made by fermentation at 35°C had a higher viscosity and was more resistant to whey drainage than yogurt with 14% MSNF without starch made by fermentation at 43°C. Sensory evaluation indicated that the yogurts with added starch fermented at 35°C were significantly more acceptable than those fermented at 43°C.
Aust. J. Daily Technol. 58, 228-232
Introduction
Milk proteins have gelling, thickening and water-binding properties, which are of importance to the development of yogurt texture. Milk intended for yogurt manufacture is generally heat-treated to temperatures of 80-90°C for 5 to 30 minutes. During this heat treatment, the whey proteins denature and form complexes with the caseins. Denaturation of whey proteins also increases their ability to hold water (Augustin et al. 1999; Dannenberg and Kessler 1988a,b). Acidification of the milk through the action of starter cultures results in the formation of a gel.
Starches have been used to modify the texture of yogurts because of their ability to thicken, gel and retain water. The incorporation of starch increases the viscosity of yogurts (Nielsen et al. 1991). Doreau (1993) reported that the use of specialty starches improved the texture, viscosity and stability of yogurts. However, Thomasow and Hoffman (1978) concluded that the addition of starch to skim milk had an undesirable effect on the taste of the yogurt. McGlinchey (1997) reported that the use of a new specialised native starch in the manufacture of low-fat yogurt resulted in yogurts with a desired texture and viscosity.
One variable available to the yogurt manufacturer to alter is the temperature at which the yogurt is fermented. Changes in the incubation temperature influence the growth of the starter micro-organisms, with reduced temperatures leading to slower acid production (DeBrabandere and DeBaerdemaeker 1999, Haque et al. 2001). Alteration of the fermentation conditions has also been shown to be a...