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Dairy foods. In the dairy industry, many products result from fermentation by microorganisms in milk and the products of milk. For example, buttermilk results from the souring of low-fat milk by lactic acid. The flavor is due to substances such as diacetyl and acetaldehyde, which are produced by species of Streptococcus, Leuconostoc, and Lactobacillus as they grow.

A fermented milk product with a puddinglike consistency is yogurt. Two bacteria,Streptococcus thermophilus and Lactobacillus bulgaricus, are essential to its production. After the milk has been heated to achieve evaporation, the bacteria are added, and the condensed milk is set aside at a warm temperature to produce the yogurt. Sour cream is produced in a similar way, using cream as a starter material.

The protein portion of the milk, the casein, is used to produce cheese and cheese products. Precipitated from the milk, the protein curd is an unripened cheesesuch as cottage cheese. The leftover liquid, the whey, can be used to make cheese foods.

When the cheese is allowed to ripen through the activity of various microorganisms, various cheeses are produced. Soft cheeses, such as Camembert, do not spoil rapidly. Camembert cheese is a product of the growth of the fungus Penicillium camemberti. Hard cheeses have less water and are ripened with bacteria or fungi. Swiss cheese is ripened by various bacteria, including species of Propionibacterium, which produces gas holes in the cheese. Bleu cheese is produced by Penicillium roqueforti, which produces veins within the cheese as it grows.

Other fermented foods. Other fermented foods are also the product of microbial action. Sauerkraut, for example is produced by Leuconostoc and Lactobacillusspecies growing within shredded cabbage. Cucumbers are fermented by these same microorganisms to produce pickles.

Bread. Bread is still another product of microbial action. Flour, water, salt, and yeast are used to make the dough. The yeast most often used is Saccharomyces cerevisiae. This organism ferments the carbohydrates in the dough and produces carbon dioxide, which causes the dough to rise and creates the soft texture of bread. Unleavened bread is bread that contains no yeast. Sourdough bread can be made by using lactic acid bacteria to contribute a sour flavor to the dough.