Re: What causes the tart flavour in yoghurt?

Basically, yogurt is made from the fermentation of dairy ingredients (milk, powder milk, etc.) by a bacterial culture that contains the two following species: Lactobacillus bulgaricus and Streptococcus thermophilus. There can be other ingredients, other species of bacteria and other treatments. The flavor of yogurt is the result of the action of the bacteria on the ingredients. There is a good description of that in "The Sensory Evaluation of Dairy Products" by Bodyfelt et al., Van Nostrans Reinhold, 1988: Plain yogurt is characterized generally as having a tart or sour, somewhat green apple-like flavor, due to the levels of lactic acid (0.9-1.2%) and acetaldehyde (5-40 ppm) present. Acetaldehyde is readily produced by both L. bulgaricus and S. thermophilus, and is the chemical compound most responsible for providing characteristic "yogurt flavor". I hope that answer your question.