Re: What makes milk turn sour?

Milk is often naturally contaminated with bacteria called Lactobacillus. When the milk is prepared to be sent to the bottling company it is pasturized. This process helps to kill a large amount of this bacteria and make the milk suitable for storage and drinking. However, since this does not kill all the bacteria, those left will reproduce. Eventually the bacteria in the milk will produce enough of their waste products to sour the milk. The waste product is an acid which gives milk its soured smell and taste.