- The active culture used in most yogurt consists of two bacteria in a symbiotic relationship, which acidify the milk and produce the characteristically thick yogurt texture. When the warm milk is inoculated, the heat-loving Streptococcus takes the lead in converting its sugars to acid. As the Streptococcus creates an increasingly acidic environment, it slows down and Lactobacillus Bulgaricus becomes more important.
- Lactobacillus Bulgaricus was identified and named by Dr. Stamen Grigorov, a Bulgarian researcher, in 1905. It is hardier and more acid-tolerant than its streptococcus counterpart, and remains active until the yogurt reaches its final level of acidification, usually around one percent. The combination of the S. thermophilus and L. bulgaricus ferments the milk more quickly than either of the bacteria alone.
- The two bacteria described previously are the only ones necessary to the production of yogurt, but others are commonly added for their suspected health benefits. The most common is Lactobacillus Acidophilus, but several other strains of Lactobacilli and Bifodobacteria may also be used. Most of these are derived from other fermented milk products, and are chosen for their ability to survive in the human digestive tract.
- Russian immunologist Ilya Metchnikoff, a Nobel laureate for his pioneering study of white blood cells, was intrigued by Grigorov's study of yogurt. He theorized that yogurt played a role in the unusual longevity claimed for some ethnic groups in the Balkans and the Caucasus, where yogurt remains a staple. His influence was responsible for yogurt's reputation as a health food. Modern research has determined that many Lactobacilli and Bifidobacteria can colonize the human digestive tract, producing various health benefits. These additional cultures, collectively described as "probiotic," are often added to contemporary yogurt products.
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