Probiotics are defined as "living microorganisms which, on ingestion in sufficient numbers, exert health benefits beyond inherent basic nutrition." This classifies yogurt as a probiotic. The custard-like texture, tart taste, and probiotic nature of yogurt comes from its characteristic bacteria that ferment the milk, Lactobacillus bulgaricus, which converts lactose (milk sugar) into lactic acid, and Streptococcus thermophilus.
The lactic acid, an end product of metabolism, curdles the milk protein and gives yogurt its characteristic tart taste. Other lactobacillus strains, L. casei, L. acidophilus, plus the Bifidobacterium spp. may be added for their probiotic properties. The health-promoting benefits of yogurt lie in two categories, nutrition enhancement and immune modulation.
These "friendly" probiotic bacteria grow at the expense of pathogenic bacteria in the intestines and are associated with health benefits for many gastrointestinal diseases, including lactose intolerance, constipation, diarrheal diseases, colon cancer, inflammatory bowel disease, infections and allergies. Yogurt with these bacteria has such a solid reputation for promoting the growth of healthy bacteria in the colon that it is often recommended to patients following antibiotic treatments.
There are several proposed mechanisms by which lactic acid bacteria displace pathogenic bacteria. These include lowering the pH (increasing the acidity) of the intestines, production of compounds damaging to the invading microorganisms, and competition for nutrients and living space.
Of course, the trick in getting the benefits of yogurt is to assure that the live bacteria reach the intestines so that they have a chance to flourish. The first obstacle the live bacteria must overcome is the highly acidic condition of the stomach. Stomach acids and protein-digesting enzymes make short work of most bacteria before they pass into the intestines, which is a necessary line of defense. When one consumes a very large number of bacteria in yogurt, some bacteria survive and pass through the stomach. But even that is no guarantee of their survival in the rest of the digestive tract. The yogurt bacteria must compete for nutrients and attachment sites with the normal intestinal bacteria.
|This segment of a larger diagram from AboutYogurt.com shows the Live and Active Cultures seal and describes the criteria processors must meet in order to use the seal. Image courtesy of the National Yogurt Assn.
The National Yogurt Association has criteria for "live and active cultures." The finished yogurt product must contain at least 1 billion live lactic acid bacteria per gram at the time of manufacture, and the cultures must stay active throughout the stated shelf life. If yogurt is heat-treated after the culturing process, the active cultures are destroyed.
Over half of the world's population is intolerant to lactose, the sugar in milk, to some degree. Also, it's natural as we age to lose the ability to produce the enzyme lactase, which splits lactose to its two constituent sugars, glucose and galactose.
During yogurt fermentation about 20 to 30 percent of the lactose is split into these absorbable monosaccharides. Still more lactose is converted to lactic acid. If the cultures are live, there is the added benefit that the yogurt bacteria may express some functional lactase, which further reduces the content of lactose in the final product. Hence, people who are lactose-intolerant can generally enjoy some yogurt, even when they cannot tolerate milk, making yogurt a good source of dairy protein and vitamin D for them.
The protein in yogurt may be more digestible than that of the original milk, since yogurt bacteria contain protein-digesting enzymes. This action continues throughout the life of the yogurt if the cultures are live. Casein, the solid portion of the protein, is reduced to finer and more digestible particles.
Milk fat can also undergo changes during the fermentation process. The fat-digesting enzymes in the bacteria release some free fatty acids from the milk fat. More importantly, the level of conjugated linoleic acids (CLA) may also be enhanced. CLAs are reported to have some positive effects on the immune system and may be anticarcinogenic.
Possibly the most important mechanism by which yogurt may affect health is through the interaction of lactic acid bacteria with the mucosal lining of intestines. The intestinal lining is an important part of the immune system, which defends against ingested pathogens. This defensive system is known as the gut-associated lymphoid tissue (GALT).
In the small intestine, lymphoid tissue is organized into a collection known as Peyer's Patches. Stimulation of the GALT results in the production of IgA, the main immunoglobulin or response protein in the gut. Serum IgA guards against the colonization of pathogenic bacteria in the gut as well as penetration of pathogenic antigens. Eighty percent of all plasma cells, the source of IgA, are located in the intestinal mucosa. Just how intestinal bacteria influence mucosal B cells to become IgA-stimulating plasma cells is not known.
There are other ways in which lactic acid bacteria many be protective. In animal studies, lactic acid bacteria have been shown to stimulate production and activity of the inherent cells that destroy invading foreign cells. Other studies suggest that lactic acid bacteria cause the modification of molecules called cytokines - small proteins that affect communications between cells or influence the behavior of cells. Cytokines are produced by T cells involved in immunity.
All this makes it easy to understand the allure of live-cultured yogurt as a beneficial ingredient for foods and beverages. Expect more and more manufacturers to join the culture club as ways are found to keep yogurt bacteria alive throughout the food processing chain.