Kefir is a cultured milk beverage which is popular in the Middle East, Eastern Europe, and parts of the Mediterranean and Russia. It is made by inoculating a milk with kefir grains, a mixture of yeasts and bacteria which will sour the milk slightly, creating a drink which is almost like liquid yogurt. Traditional kefir has a tangy, rich flavor which may be altered with the addition of ingredients like pureed fruits and sweeteners, or consumed plain. Some markets and natural food stores stock kefir, and it is also possible to order kefir grains from suppliers to make kefir at home.

The word “kefir” is Russian, and it is probably derived from a Turkish word which means “to froth.” Traditional kefir was made from camel's milk, although milk from other animals can be used as well. People who prefer not to use dairy may also make kefir from plant or nut milks such as soy or almond milk. The beverage is typically made at room temperature, and it takes around 18 hours to mature fully, although in cold climates this may take longer.

When kefir grains are introduced to milk, fermentation begins, changing the chemical structure of the milk. Fermentation creates a characteristically soured flavor, and it can turn the kefir slightly alcoholic in some conditions. As the milk ferments, it thickens, and once the kefir is ready it is consumed or refrigerated. Traditional kefir is not sweet, although many companies package their kefir with sweeteners to make it more appealing.

There are a number of uses for kefir. Some people drink kefir plain because they believe that the cultured milk is good for them, and kefir seems to help with digestion for some people, according to studies on the beverage. Kefir can also be used in smoothies, baked goods, and other foods if a tangy flavor is desired. The beverage generally keeps fairly well under refrigeration for a few days.

For people who are not familiar with kefir, cow's milk kefir is a good place to start, since it has a mild, creamy flavor. Kefir made with sheep or goat milk can be a bit gamy and tangy, because of the hormones that these animals produce. Non-dairy kefir is also commercially available, for people who want to enjoy the flavor and health benefits of kefir without using animal products.

People who want to make their own kefir can order kefir starters through natural food stores or online retailers. These starters come with detailed instructions on culturing kefir, so that people can be confident that they are making a food which is safe to drink. It is important to use sterile containers and stable temperature conditions when producing kefir, to ensure that only good bacteria are allowed to grow in the culture.