Lactobacillus (plural lactobacilli), also known as lactic acid bacteria or LAB, is a 'friendly' (i.e. non-pathogenic or disease-causing) type of bacteria naturally present in the environment. In pre-ferments and bread doughs left to ferment for relatively long periods (in practice typically 12 hours or longer), lactobacilli will start to ferment sugars in the flour, and will produce lactic acid and sometimes acetic acid as a byproduct. These acids act as a mild preservative to keep pathogenic bacteria and other microorganisms at bay, and also lend a characteristic tangy flavour to bread; the acids produced by lactobacilli are what give sourdough bread it's distinctive taste. Lactobacilli produce very little gas during fermentation and so are not a leavening agent; they exist and ferment alongside yeast.
The fermentative properties of lactobacilli are also used to make products such as cheese, yoghurt, sauerkraut and kombucha, among others.
Want to find out more? Look at the chapter on pre-ferments in the book flour and water.
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