The bateria used for fermentation - many of them probiotics - live naturally in milk and on the surfaces of grains, vegetables, and fruits. They're called lactic acid bacteria because during fermentation, they produce lactic acid as well as other chemicals that counter the bacteria responsible for spoilage. Lactic acid also accounts for the tart flavor that characterizes fermented foods.
The health benefits of fermentation come from two sources. First, probiotic bacteria proliferate as food ferments. When we eat foods that contain live probiotics, we're de3livering an army to our digestive tract - friendly warriors that will do battle against harmful microbes and boost our immune system. Fermented foods also contain what I call "metabiotics," helpful metabolic byproducts from the bacteria. When probiotic bacteria digest fiber and other nutrients, they produce waste products that are actually beneficial to our health.
In addition, many fermented foods are a source of prebiotics - compounds that make the digestive tract more hospitable to probiotic microbes. One example is lactoferrin, a protein found in whey (the liquid part of curdled milk) that has antimicrobial properties. Lactoferrin is contained in yogurt and in other fermented dairy products that include whey. Other exables of prebiotics in fermented foods are the fiber and antioxidents in pickled vegetables. You'll hear more about prebiotics in Chapter 12.
Probiotic bacteria are living organisms. If they're exposed to heat, or other adverse conditions, they will die. In contrast, prebiotics and metabiotics are usually more stable under adverse conditions. For instance, if you add yogurt to a sauce and simmer it, the probiotic bacteria will not survive - but the prebiotics and metabiotics will remain.
