Lactic acid is a weak acid which is produced naturally in the body during metabolism and exercise. It is also produced by bacterial fermentation both naturally and for industrial purposes. In breadmaking, lactic acid is produced in dough with long fermentation times, such as sourdough, by the activity of lactobacilli, and lends a mildly acidic flavour to bread as well as acting as a preservative.
Want to find out more? Look at the chapter on pre-ferments in the book flour and water.
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