Fermentation occurs when a microorganism, such as yeast or bacteria, converts carbohydrates into another chemical. It is one of the first chemical reactions human beings ever observed. Between 10,000 and 15,000 years ago, fermentation helped people make the transition to farming. Today, it is used for fuel as well as food.
Yogurt is made from fermented milk. People began raising dairy animals, such as goats and sheep, around 5000 B.C. Archaeologists believe people probably discovered yogurt by accident when milk stored in gourds or animal skins turned sour. Yogurt is created through the combined action of the bacteria Streptococcus lactis and Lactobacillus bulgaricus or Lactobacillus acidophilus. These bacteria consume the sugar in the milk called lactose, curdle the milk and create the chemicals lactic acid and acetaldehyde, which give yogurt its distinctive taste.
Alcoholic beverages are created when yeast gives off ethyl alcohol and carbon dioxide as by-products of sugar consumption. According to brewer Stephen Snyder, ancient people probably discovered alcohol by accident when stored grain mixed with yeast and water. Today, manufacturers create beer, wine, mead and their derivatives, such as whiskey and brandy, by combining water and yeast with sources of sugar, including grain, grapes, berries, rice and honey. According to Snyder, alcohol, carbon dioxide and high pH levels make these beverages inhospitable to bacteria, providing a safe source of liquid when clean water isn't available. In addition, sorghum beers from Africa provide a rich source of vitamin B to people with limited food supplies.
Cucumbers, other fruit and even meat can be preserved through pickling. Historians believe pickling is at least 4,000 years old. Modern pickle manufacturers ferment their cucumbers in a tank containing brine made from 90 percent water and 10 percent salt. The manufacturer stores the cucumbers in the brine for about five weeks. During storage, bacteria break down the cucumber's sugar and create lactic acid, which gives pickles their sour flavor. Pickles created this way will last several months.
When a baker makes a loaf of bread, he adds yeast and sugar to the dough. When the baker sets the dough aside to rise, the yeast consumes the sugar and gives off carbon dioxide. The carbon dioxide gives the bread bulk and texture that it wouldn't have otherwise. Some breads, such as sourdough or rye, are made with sourdough starter, or dough that has been fermented overnight. Bacteria in the starter give the dough a stretchy texture, while lactic acid gives the bread a distinctive sour flavor.
Gasohol is a fuel made from gasoline and alcohol, such as ethanol or methanol. Since 1998, many cars sold in the United States have been designed so that their owners can fuel them with E85, a gasohol mixture containing 85 percent ethanol and 15 percent gasoline. Gasohol reduces some air pollution by burning more coolly, slowly and completely than gasoline. However, it also vaporizes quickly and can add ozone to the air in warm weather. In the upper atmosphere, ozone protects life from ultraviolet radiation; in the lower atmosphere, it is a component of smog.