According to the American Heritage Dictionary Online http://education.yahoo.com/reference/dictionary/entry/fermentation, fermentation is:
Any of a group of chemical reactions induced by living or nonliving ferments that split complex organic compounds into relatively simple substances.
Probably the most common usage of fermentation is with food. The basic five reasons it's used with food are the following:
1. Fermentation increases the food value of a product by changing it to simpler forms that the body can use. Amino acids, EFA's, vitamins, and minerals are just some of the nutrients released when certain foods are fermented.
2. Fermentation enhances a food's natural appeal. The taste buds are stimulated by sharp, sour, and aromatic foods that are fermented. This creates a more diverse diet.
3. Fermentation aids human digestion. It chelates heavy metals into forms the body can use and neutralizes harmful bacterias, toxins, and anti-nutrients.
4. Fermentation preserves food by changing the sugars to ethanol. Common by-products of fermentation are lactic acid, alcohol, acetic acid, and alkaline.
5. Fermented foods require less cooking time and fuel consumption.
Let's take, for example, the making of sourdough bread. In the normal process of making bread, the yeast grows and releases carbon dioxide, which makes the bread rise. During baking, the carbon dioxide bubbles break, leaving small holes in the bread which make it light and fluffy. The final bread product no longer contains carbon dioxide. But, in sourdough bread making, the carbon dioxide released by the growing yeast is trapped by an airtight cover and reenters the yeast sponge where it changes and converts the natural sugars/starches in the wheat. This anaerobic environment is where fermentation thrives the best. Some sourdough starters take days to form and are quite pungent in taste due to this fermentation process. I'm not a chemist, so this is the best I can do to explain this.
Fermentation is a vital, natural process in nature. Man has learned to use it and benefit from it.