Fermented Milks

• The word "buttermilk" originally applied to the liquid byproduct of butter production. Most modern buttermilk, however, owes its sour taste to the fermentation activities of Streptococcus lactus bacteria. The bacteria convert lactose, a sugar in milk, into lactic acid.
  
  Lactobacillus acidophilus works similarly to create acidophilus milk. Acidophilus milk can provide health benefits to people with intestinal problems, but has a highly acidic taste.
  
  The Russian drink kefir relies on multiple species of bacteria and yeast for its tangy flavor. Clumps of bacteria and yeast known as kefir grains are added to milk to create kefir. The microorganisms produce a bubbly, slightly alcoholic-tasting drink.
  
  

Yogurt

• Lactobacillus bulgaricus and Streptococcus thermophilus ferment milk to produce yogurt -- it's the fermentation of lactose to lactic acid that results in yogurt's unique flavor and texture. The bacteria incubate in the milk for about four hours before the mixture is either pasteurized or chilled. Chilled yogurt may be labeled as having "active cultures" -- the cold stops bacterial growth, but does not kill the bacteria as the heat of pasteurization does.
  
  

Cheeses

• Penicillin produces more than antibiotics -- the fungus also plays a role in the production of cheeses such as Roquefort and bleu cheese. Various species of Lactobacillus and Streptococcus, along with other bacteria, yeasts and fungi, interact with rennet during cheese production. Specific combinations of microorganisms create the many different types of cheeses. For example, Propionibacterium shermanii creates the characteristic holes of Swiss cheese.
  
  

Creams

• The production of sour cream closely resembles that of yogurt, except that the Streptococcus and Lactobacillus species ferment cream rather than milk. Creme fraiche, French for "fresh cream," is similar to sour cream, but with a richer flavor and thicker texture, as well as a higher percentage of fat.
  
  

Butter

• In 1889, the American Association for the Advancement of Science reported that butter made from "ripened" -- fermented -- cream churned more easily and kept longer than butter made from fresh cream. Though sweet cream butter is the main type of butter produced in the United States today, ripened cream butter is still produced as well.