Kefir, alternatively milk kefir, or búlgaros, is a fermented milk drink made with kefir "grains" (a yeast/bacterial fermentation starter) and has its origins in the north Caucasus Mountains. It is prepared by inoculating cow, goat, or sheep milk with kefir grains.
Kefir grains are a combination of lactic acid bacteria and yeasts in a matrix of proteins, lipids, and sugars, and this symbiotic matrix, (or SCOBY) forms "grains" that resemble cauliflower. For this reason, a complex and highly variable community of lactic acid bacteria and yeasts can be found in these grains although some predominate; Lactobacillus species are always present. Even successive batches of kefir may be different to each other due to factors such as the kefir grains rising out of the milk while fermenting, or curds forming around the grains, as well as room temperature.
Several varieties of probiotic bacteria are found in kefir products such as Lactobacillus acidophilus, Bifidobacterium bifidum, Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus helveticus, Lactobacillus kefiranofaciens, Lactococcus lactis, and Leuconostoc species. Although the significance of probiotic content to nutrition or health remains unproven. Lactobacilli in kefir may exist in concentrations varying from approximately 1 million-1 billion colony-forming units per milliliter and are the bacteria responsible for the synthesis of the water-soluble polysaccharide kefiran.
In addition to bacteria, kefir often contains strains of yeast that can metabolize lactose, such as Kluyveromyces marxianus and Kluyveromyces lactis, as well as strains of yeast that do not metabolize lactose, including Saccharomyces cerevisiae, Torulaspora delbrueckii, and Kazachstania unispora; however, the nutritional significance of these strains is unknown.
During the fermentation, changes in composition of nutrients and other ingredients occur. Lactose, the sugar present in milk, is broken down mostly to lactic acid (25%) by the lactic acid bacteria, which results in acidification of the product. Propionibacteria further break down some of the lactic acid into propionic acid (these bacteria also carry out the same fermentation in Swiss Cheese). Other substances that contribute to the flavor of kefir are pyruvic acid, acetic acid, diacetyl and acetoin (both of which contribute a "buttery" flavor), citric acid, acetaldehyde and amino acids resulting from protein breakdown.
The slow-acting yeasts, late in the fermentation process, break lactose down into ethanol and carbon dioxide: depending on the process, ethanol concentration can be as high as 1–2% (achieved by small-scale dairies early in the 20th century), with the kefir having a bubbly appearance and carbonated taste: most modern processes, which use shorter fermentation times, result in much lower ethanol concentrations of 0.2–0.3%.
As a result of the fermentation, very little lactose remains in kefir. People with lactose intolerance are able to tolerate kefir, providing the number of live bacteria present in this beverage consumed is high enough (i.e., fermentation has proceeded for adequate time). It has also been shown that fermented milk products have a slower transit time than milk, which may further improve lactose digestion. A 2003 study found that consumption of the polysaccharide kefiran by human adults with lactose intolerance led to a significant decrease in flatulence.
Production of traditional kefir requires a starter community of kefir grains which are added to the liquid one wishes to ferment.
The traditional, or artisanal, method of making kefir is achieved by directly adding kefir grains (2–10%) to milk in a covered acid proof container which is traditionally agitated one or more times a day. It is not filled to capacity, allowing room for some expansion as the kefiran and carbon dioxide gas produced causes the liquid level to rise. If the container is not light proof it should be stored in the dark to prevent degradation of vitamins and inhibition of the culture. After a period of fermentation lasting around 24 hours, ideally at 20–25 °C (68–77 °F), the grains are removed from the liquid by straining using a non-corrosive straining utensil which can be stainless steel or food grade plastic and reserved as the starter for a fresh amount of liquid.
The fermented liquid which contains live microflora from the grain, may now be consumed as a beverage, used in recipes, or kept aside for several days to undergo a slower secondary fermentation which further thickens and sours the liquid. Without refrigeration, the shelf life is up to thirty days. The grains will enlarge in the process of kefir production, and eventually split.
For the preparation of the present factory-produced kefir, the so-called kefir mild, kefir grains are no longer used, but a precise composed mixture of different bacteria and yeast, allowing the flavor to be kept constant.
Traditional kefir is fermented at ambient temperatures, generally overnight. Fermentation of the lactose yields a sour, carbonated, slightly alcoholic beverage, with a consistency and taste similar to thin yogurt.
Variations that thrive in various other liquids exist, and they vary markedly from kefir in both appearance and microbial composition. Water kefir (or tibicos) is grown in water with sugar (sometimes with added dry fruit such as figs, and lemon juice) for a day or more at room temperature.
Kefir is a popular drink across Eastern and Northern Europe. It was consumed in Russia and Central Asian countries for centuries, but is now becoming popular in Japan, the United States and Europe. In Chile, where it is known as "yogurt de pajaritos" (little bird's yogurt), kefir has been regularly consumed for over a century; it might have been introduced by one of the various waves of migrants from the former Ottoman Empire and migrants from Eastern Europe.
The alleged health benefits of kefir have recently been popularized in North America, Australia, and the United Kingdom, and kefir can be found in pasteurized form in many stores and supermarkets.
Kefir grains will ferment the milk from most mammals, and will continue to grow in such milk. Typical milks used include cow, goat, and sheep, each with varying organoleptic and nutritional qualities. Raw milk has been traditionally used.
Kefir grains will also ferment milk substitutes such as soy milk, rice milk, and coconut milk, as well as other sugary liquids including fruit juice, coconut water, beer wort and ginger beer. However, the kefir grains may cease growing if the medium used does not contain all the growth factors required by the bacteria.
As it contains lactobacilli bacteria, kefir can be used to make a sourdough bread. It is also useful as a buttermilk substitute in baking. Kefir is one of the main ingredients in cold borscht in Lithuania. Other variations of kefir soups and foods prepared with kefir are popular across the former Soviet Union and Poland. Kefir may be used in place of milk on cereal or granola.
» Homemade Mommy: "5 Ways to Use Kefir That You've Never Heard Of"
» Leite et al., 2013: "Microbiological, technological and therapeutic properties of kefir"