Influence of heat impact in reconstituted skim milk on the properties of yoghurt fermented by ropy or non-ropy starter cultures.

The paper describes studies on the influence of heat impact in reconstituted skim milk on chemical and functional properties of yoghurt products. Reconstituted skim milk was heated for 20 min at 85 degrees C, 90 degrees C, or 95 degrees C. Ropy (producing exopolysaccharides, EPS) or non-ropy strains of S. thermophilus and L. delbrueckii subsp. bulgaricus were used as starter culture for yoghurt manufacture. The studies have shown that the fermentation times decreased with increasing heat impact when the ropy starter culture was used, while they remained to a far extent unchanged if the non-ropy starter culture was applied. The lactic acid contents of the yoghurt products were in the same range when the milk was heated at 85 degrees C or 90 degrees C, while they were different when milk was heated at 95 degrees C. There was a tendency visible that an increase in preheating leads to increased L(+)- and decreased D(-)-lactic acid contents if the non-ropy culture was applied. Using the ropy culture, it was vice versa. A slightly decrease in proteolysis with increasing heat impact was to be noted with both starter cultures. Concerning the relation of proteolysis to acidification, the fermentation process could be subdivided into three sections with different slopes if the non-ropy starter culture was used, while a linear relation was found if the ropy starter culture was applied. Regarding final product characteristics it was found that the functional properties of yoghurt decreased with increasing heat impact when the ropy starter culture was applied, while they remained to a far extent unchanged when the non-ropy starter culture was used. It can be concluded from these studies that a preheating of milk at a temperature of 85 degrees C (20 min) is optimal in regard to final yoghurt product characteristics.