Souring of milk is a chemical process primarily caused by the fermentation of lactose (milk sugar) into lactic acid. This process is carried out by lactic acid bacteria, most notably Lactobacillus and Streptococcus. While many of these bacteria are "facultative anaerobes" (meaning they can survive with or without oxygen), the actual process of souring—fermentation—is an anaerobic process. The production of lactic acid lowers the milk's pH, giving it a sour taste and causing the proteins to coagulate or curdle.

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