Kefir is made from 'grains' containing both bacteria and yeast, while yogurt uses only bacteria.

Kefir grains are a prime example of a SCOBY, which stands for a Symbiotic Culture of Bacteria and Yeast. These grains consist of a matrix of proteins, lipids, and a unique soluble polysaccharide called kefiran. Research published in the journal 'Frontiers in Microbiology' indicates that these grains can contain over 50 microbial species, including Lactobacillus kefiri and yeasts like Saccharomyces kefir.The fermentation process for kefir is distinct because it involves both lactic acid fermentation and alcoholic fermentation. While yogurt bacteria typically only convert lactose into lactic acid, the yeasts in kefir also convert sugars into trace amounts of ethanol and carbon dioxide. This dual process results in the signature effervescent texture and a alcohol content that usually ranges from 0.02% to 2%.Historically, kefir originated in the North Caucasus Mountains, where local tribes discovered that milk stored in goatskin bags would ferment into a powerful probiotic drink. Unlike yogurt, which is a thermophilic ferment requiring heat, kefir is mesophilic and ferments at room temperature. Modern studies by institutions like the University of Sao Paulo suggest that the complex microbial diversity in kefir provides a wider range of gut health benefits compared to the more limited bacterial profile of traditional yogurt.