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Every yogurt needs two bacteria
The two bacteria
The two are Streptococcus thermophilus and Lactobacillus bulgaricus. They essentially eat the sugars in milk and produce lactic acid, which is what ferments it.
The lactic acid does two jobs. It gives yogurt its trademark sour taste, and it makes the proteins in the milk clump together, which gives yogurt its gelatinous texture.
Making yogurt
With both strains in hand, making yogurt is easy: add them to some warm milk and let the mixture sit and ferment overnight, maybe up to 24 hours.
Different styles of yogurt use different ratios of the two bacteria to adjust flavor and texture, but both are always in there. Bulgarian yogurt tends to have a higher proportion of Lactobacillus bulgaricus.
How do the two bacteria help each other?
Researchers call the relationship protocooperation. Lactobacillus bulgaricus breaks down milk proteins into amino acids, including valine, that Streptococcus thermophilus needs to grow. In return, S. thermophilus releases formic acid and carbon dioxide, which stimulate L. bulgaricus. Together the pair ferments milk faster than either one alone.
Why does yogurt need warm milk?
Both species grow best at temperatures well above room temperature, around 108 to 115 degrees Fahrenheit. Streptococcus thermophilus gets its name from Greek roots meaning heat-loving. In a refrigerator the bacteria slow to a crawl, which is why finished yogurt keeps for weeks.
Why is milk heated before it is cultured?
Commercial makers heat milk to about 185 degrees Fahrenheit before adding the bacteria. The heat unfolds the whey proteins, and they bind to casein. That bound network holds more water, so the finished yogurt sets firmer and weeps less liquid.
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