Sour Milk

sour milk an ambiguous term, which can mean either milk which has ‘turned’ by mischance or milk which has been deliberately soured in a controlled manner. Either can correctly be described as fermented milk; see fermentation.

If raw, unpasteurized milk is left in a warm place, the lactic acid-producing bacteria which are always naturally present in it will grow and will convert its lactose (milk sugar) into lactic acid. The process is a kind of fermentation. It produces a sour taste which may be pleasant if the fermentation is not allowed to go too far (when it would cause curdling).