What is kefir?

Kefir, a fermented dairy drink, is similar to yogurt, but distinct in several interesting ways.

Yogurt is made from the bacterial fermentation of milk — bacteria metabolize a sugar in milk called lactose, producing lactic acid. This then acts on proteins in the milk, in effect curdling it on a microscopic scale, which gives yogurt its texture and slight sourness.

The microorganisms involved in kefir are different: There are yeasts, in addition to various types of bacteria. They grow in little clumps called grains, which in most commercial products are not much bigger than rice grains, but can be much larger. Each grain is a complicated mixture of proteins, fats, and sugars from the milk as well as the bacteria and yeast, which live together symbiotically. Together, the yeast and bacteria make a gel called kefiran, which is made of chains of sugar molecules. That contributes a lot to the texture of kefir.

The bacteria are similar to those that ferment yogurt. The yeasts turn some of the milk sugars into alcohol and carbon dioxide, much as yeasts do in beer, so kefir is typically slightly alcoholic (about 0.5 percent) and somewhat fizzy.

Making kefir is straightforward. The basic idea is to put milk in an acid-resistant, nonmetallic container, add a few percent by volume of kefir grains to provide the bacteria and yeast, and let it sit at room temperature. Waiting overnight is long enough. Once you’ve got some kefir going, you can use it to seed the next batch.

Kefir does not have to made with cow’s milk; one can find varieties made from the milk of goats and sheep, or even from coconut or soy “milks.’’ And there are kefir-like grains called tibicos that can ferment sugary water (sometimes with fruit added) to make “water kefirs.’’ These are more like soda pop. Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.