Lactobacillus is one of the most common and most feared beer spoilage microorganisms. It belongs to the group of lactic acid bacteria, which also includes pediococcus. There are many species of lactobacillus, including L brevis, L lindneri, and L delbrueckii. Lactobacillus bacteria are Gram positive and rod shaped. See gram stain. They grow best in environments with a pH value of 4.0 to 5.0 and a temperature of roughly 30°C (85°F). Some species of lactobacillus have a high tolerance for the presence of hop compounds and can survive under anaerobic conditions.

Like brewers yeasts, lactobacillus metabolizes sugars as the main source of energy, but, unlike yeast, it produces lactic acid instead of alcohol. This is a desirable quality for an organism used in making such foods as yogurt, but notable lactic acidity is an off-flavor in most types of beer. Lactobacillus also produces other off-flavors, including diacetyl. Beer with a serious lactobacillus infection will often become hazy.

Although undesirable in most beers, there are a few beer styles for which lactobacillus helps to create part of the beer’s characteristic flavor. These include most Belgian sour beers such as lambic, American sour beers, traditional Berliner weisse, and even Belgian witbier, which traditionally showed at least a tinge of lactic acidity.

Malt generally has large populations of lactobacillus on the husks. Some German breweries will use a small amount of malt to inoculate unhopped wort and sour it. This soured wort can then be used for mash or kettle wort acidification without violating the Reinheitsgebot.

There are various methods available for testing for the presence of lactobacillus in beer. These range from simple plating to rapid polymerase chain reaction. See polymerase chain reaction. Test samples can be taken anywhere from the fermenting vessel to the packaged beer. Testing techniques continue to advance, with the dual goals of quick detection and affordability.

See also beer spoilers, lactic acid, limbic, pediococcus, and sour beers.