In Fermented Produce

• Vegetables and fruits are preserved in many cultures through lactic fermentation. The produce is salted or brined to extract its juices and create a suitable environment for the lactobacilli and other bacteria. Famous examples of the technique include European sauerkraut and Korean kimchi, as well as various other traditional Western and Asian pickles. In these foods, the lactic fermentation creates a wide range of new and subtle flavor molecules, both sweet and savory, as well as the mouth-puckering tartness of lactic acid. Some foods also produce the stronger acetic acid, through a secondary bacterial process. Inexpensive pickles are made by adding acetic acid directly to the vegetables, but these lack the complex flavors of naturally fermented pickles.

In Fermented Meats and Fish

• Lactic fermentation is a less common method of preserving meats and fish, but it's a powerful one. The lactic fermentation process breaks up and reconstructs amino acids in the proteins, creating powerfully savory flavor molecules along with the lactic acid's tang. Fermented fish produces Thai fish sauce and English Worcestershire sauce, as well as the cherished liquamen of the ancient Romans. A number of traditional sausages, such as dry Italian salami and French "saucissons secs," also harness lactic fermentation for preservation purposes. When properly cured, the sausages are food safe at room temperatures, with a clean tang from the lactic acid and rich, savory flavors created from the interaction of the meat's amino acids and the lactic fermentation.

In Sourdough Bread

• The distinctive flavor of sourdough bread is the result of a complex interaction between the flour, wild yeasts and a range of beneficial bacteria. The yeasts consume natural sugars in the wheat and produce alcohol, which in turn is digested by the bacteria and converted to lactic acid. This in turn discourages all but the acid-tolerant sourdough yeasts from growing in the bread. The lactic acid gives sourdough bread its distinctive tang, while enzymes in the flour break down its carbohydrates and produce a range of complex and distinctive secondary flavors. Bakers regulate the degree of lactic acid in their dough by regularly discarding part of the starter and replacing it with fresh flour and water.

In Fermented Dairy Products

• Lactic acid derives its name from the fact that it's commonly found in fermented dairy products. A variety of bacteria in the Lactococcus and Lactobacillus families occur naturally in milk, and when they're given favorable conditions -- usually warmth -- they quickly convert its lactose to lactic acid. That inhibits the growth of spoilage bacteria, helping preserve the milk. The acidity also congeals the milk's proteins, helping produce foods including yogurt and sour cream. These vary in tartness, depending how much lactic acid the bacteria produce. The same bacteria contribute to the flavor of fine cheeses as well, providing the characteristic sharp acidity of a well-aged cheese.