Fermentacion En El Yogurt

YOGURT
FERMENTATION PROCESS
During the manufacture of yoghurt, the heat-treated milk is cooled to the incubation temperature of the starter culture (S. thermophilus and L. delbrueckii subsp.bulgaricus) and, in general, the milk is fermented at 42-44 ºC, that is, the optimum growth condition for the mixed culture - the short incubation method. In some cases the incubation period can be as shortas 3 h, assuming that the starter culture (2 - 3%) is an active one and the ratio between the rods and the cocci is well balanced. However, the longer incubation method, (i.e. overnight) can be usedand the incubation conditions are 30 ºC for around 16±18 h, or until the desired acidity is reached (Hrabova and Hylmar, 1987; see also Merlo, 2000; Rodgers, 2001). Zabadi (i.e. Egyptian equivalent ofyoghurt) has been manufactured at 30 ºC or 35 ºC with minimum syneresis, improved firmness and smoothness and with a pleasant flavour (Mehanna, 1991). However, Cho-Ah-Ying et al. (1990) producedyogurt using different strains of thermophilic starter cultures (i.e. EPS producer and non EPS-producer), and fermenting the milk at 38 ºC and 43 ºC; they concluded that the temperature of incubation hadsignificantly affected only one sensory character (e.g. texture), and overall the yoghurts made at 38 ºC had the tendency to gain higher scores.

It is evident that the incubation temperature canaffect the characteristic of the yogurt gel. If 42ºC is the typical fermentation temperature for yoghurt, using a lower incubation temperature (e.g. 40 ºC) will lead to slightly longer gelation timebut, on the positive side, the advantages are: (a) the product is firmer and more viscous, (b) the gel is less prone to syneresis, and/or (c) less lumpy/grainy defects on stirring the coagulum during thecooling stage (Robinson, 1981; Lucey, 2002; Lee and Lucey, 2003). At a lower incubation temperature, the casein particles increase in size because of a reduction in hydrophobic interactions which,...