Kefir or kephir (// kə-FEER), alternatively milk kefir, or búlgaros, is a fermented milk drink that originated in the north Caucasus Mountains made with kefir "grains", a yeast/bacterial fermentation starter. It is prepared by inoculating cow, goat, or sheep milk with kefir grains. Traditional kefir was made in goatskin bags that were hung near a doorway; the bag would be knocked by anyone passing through to help keep the milk and kefir grains well mixed.
|Alternative names||milk kefir, búlgaros|
|Main ingredients||milk, kefir grains (bacteria, salt, yeasts, proteins, lipids, sugar)|
|Cookbook: Kefir Media: Kefir|
The word kefir, known in Russian since at least 1884, is probably of North Caucasian origin, although some sources see a connection to Turkic köpür (foam) or kef (foam that builds up on the surface of a boiling liquid). Kefir has become the most commonly used term, but other names are found in different geographic regions.
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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.
The kefir grains initiating the fermentation are a combination of lactic acid bacteria and yeasts in a matrix of proteins, lipids, and sugars. This symbiotic culture of bacteria and yeast (or SCOBY) forms "grains" that resemble cauliflower. 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 differ 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.
Kefir grains contain kefiran, a water-soluble polysaccharide, which imparts a creamy texture and feeling in the mouth. The grains range in color from white (the acceptable color of healthy grains) to yellow; the latter is the outcome of leaving the grains in the same milk during fermentation for longer than the optimal 24-hour period, and continually doing so over many batches. Grains may grow to the size of walnuts, and in some cases larger.[clarification needed]
During 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). A portion of lactose is converted to kefiran, which is indigestible by gastric digestion. 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.[page needed]
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. This makes kefir different from yogurt and most other sour milk products where only bacteria ferment the lactose into acids. 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, provided 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.
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.
Variations[clarification needed] 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 for a day or more at room temperature in water with sugar, sometimes with lemon juice and added dry fruit such as figs.
Kefir products contain nutrients in varying amounts from negligible to significant, including dietary minerals, vitamins, essential amino acids, and conjugated linoleic acid, in amounts similar to unfermented cow, goat or sheep milk. At a pH of 4.2 - 4.6, Kefir is composed mainly of water and by-products of the fermentation process, including carbon dioxide and ethanol.
Typical of milk, several dietary minerals are found in kefir, such as calcium, iron, phosphorus, magnesium, potassium, sodium, copper, molybdenum, manganese, and zinc in amounts that have not been standardized to a reputable nutrient database. Also similar to milk, kefir contains vitamins in variable amounts, including vitamin A, vitamin B1 (thiamine), vitamin B2 (riboflavin), vitamin B3 (niacin), vitamin B6 (pyridoxine), vitamin B9 (folic acid), vitamin B12 (cyanocobalamin), vitamin C, vitamin D, and vitamin E. Essential amino acids found in kefir include methionine, cysteine, tryptophan, phenylalanine, tyrosine, leucine, isoleucine, threonine, lysine, and valine, as for any milk product.
Probiotic bacteria found in kefir products include Lactobacillus acidophilus, Bifidobacterium bifidum, Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus helveticus, Lactobacillus kefiranofaciens, Lactococcus lactis, and Leuconostoc species. The significance of probiotic supplements remains unproven. However, a number of recent studies have shown numerous health benefits from the probiotics when consumed as Kefir. These range from cholesterol metabolism and angiotensin-converting enzyme (ACE) inhibition, antimicrobial activity, tumor suppression, increased speed of wound healing, and modulation of the immune system including the alleviation of allergies such as asthma. 
Lactobacilli in kefir may exist in concentrations varying from approximately 1 million to 1 billion colony-forming units per milliliter and are the bacteria responsible for the synthesis of the polysaccharide kefiran.
In addition to bacteria, kefir often contains strains of yeast that can metabolize lactose, such as Kluyveromyces marxianus, Kluyveromyces lactis and Saccharomyces fragilis, as well as strains of yeast that do not metabolize lactose, including Saccharomyces cerevisiae, Torulaspora delbrueckii, and Kazachstania unispora. The nutritional significance of these strains is unknown.
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Production of traditional kefir requires a starter community of kefir grains which are added to milk.
The traditional or artisanal method of making kefir is to add 2–10% kefir grains directly to milk in a sealed goatskin leather bag, which is traditionally agitated one or more times a day. Today the leather bag is replaced with a corrosion-resistant container such as a glass jar. It is not filled to capacity, allowing room for some expansion as the carbon dioxide gas produced causes the liquid level to rise. Non-lightproof containers are stored in the dark to prevent degradation of light-sensitive vitamins. After a period of fermentation lasting around 24 hours, ideally at 20–25 °C (68–77 °F), the grains are strained from the liquid using a corrosion-resistant (stainless steel or plastic) utensil, and kept as the starter for another batch.
The fermented liquid-kefir, which contains live micro-organisms from the grains, may be drunk, used in recipes, or kept aside in a sealed container for many days to undergo a slower secondary fermentation in which bio-synthesis by certain micro-organisms increases the content of folic acid and some other B vitamins, and the kefir becomes sourer. The shelf life, unrefrigerated, is up to thirty days. The grains enlarge in the process of kefir production, and eventually split.
The Russian method permits production of kefir on a larger scale, and uses two fermentations. The first step is to prepare the cultures by incubating milk with 2–3% grains as described. The grains are then removed by filtration and 1–3% of the resulting liquid mother culture is added to milk and fermented for 12 to 18 hours.
Kefir can be made using freeze-dried cultures commonly available in powder form from health food shops. A portion of the resulting kefir can be saved to be used a number of times to propagate further fermentations but ultimately does not form grains, and a fresh culture must be obtained.
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 (flavor, aroma and texture) 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.
Milk sugar is not essential for the synthesis of the polysaccharide that makes up the grains (kefiran), and rice hydrolysate is a suitable alternative medium. Additionally, kefir grains will reproduce when fermenting soy milk, although they will change in appearance and size due to the differing proteins available to them.
Kefir, first consumed in Russia and Central Asian countries for centuries, spread to Eastern and Northern Europe, Japan, the United States, and Western Europe in the early 21st Century. In Chile, where it is known as "yogur de pajaritos" (little birds' yogurt), kefir has been regularly consumed for over a century.
As it contains lactobacillus 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 and Poland. Other variations of kefir soups, such as kefir-based okroshka, and other foods prepared with kefir are popular across the former Soviet Union and Poland. Kefir may be used in place of milk on cereal, granola or milkshakes.
Possible origin of kefir grainsEdit
Other studies indicate small kefir granules may form initially from aggregations of lactobacilli and yeast, followed by a biofilm created by the adherence of additional bacteria and yeasts to the granule exterior.
Other fermented dairy productsEdit
Other fermented beveragesEdit
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