Kefir

Kefir has a long history in Russia, where it originated, but is now manufactured and consumed in several other parts of the world as well (1,13), though the per capita consumption remains high in Russia at 4 to 5 kilograms. The name kefir is derived from the Turkish term kef or from kefy in the Caucusus region implying pleasant taste (4). Kefir is a viscous pourable liquid, with a smooth, slightly foamy body and whitish color. It is yeasty, acidic, mildly alcoholic, refreshing, and slightly effervescent. Kefir is generally made from cow's milk but in the early years it was also made from goat's and sheep's milk in leather sacks. It perhaps still is in some rural areas. Industrial manufacture of kefir began in Russia in 1930 (14).

Like many other milk products, kefir ranges in fat content from nonfat to 3.5% but it is in many ways a unique product in that it entails acid and alcohol fermentation. Alcohol fermentation is the result of the addition of yeasts in the form of kefir grains. Because of the multiple fermentation process, the resulting product possesses flavor that is characterized by a balance of lactic acid (0.8%), diacetyl, acetaldehyde, and ethanol (1%). Diacetyl and acetaldehyde in a 3:1 ratio provide optimum flavor. The complex flavor is rounded out by fizz from carbon dioxide that is also produced during fermentation.

Kefir is manufactured using one of two main procedures. In the traditional process, kefir grains are employed for fermentation. Another method involves the use of lyophilized concentrates of cultures. In the latter method, kefir grains are not used.

A. Manufacture of Kefir with Grains

Kefir may be manufactured with grains in one of two ways (1). In the traditional system, kefir grains are added to cow's milk that has been heated to 85°C for 30 min and cooled to 22 °C. Incubation is for 24 hr at 22 °C with occasional stirring. The grains are sieved when they rise to the surface due to carbon dioxide production, and ripening for alcohol production continues at 10-15°C for 24 hr.

In another process, a kefir starter is first obtained from the grains for fermentation. Here, freeze-dried kefir grains are rehydrated in a sterilized 0.9% sodium chloride solution at 20 ° C for 5 hr. The grains are then washed with sterile water and added in a 1:30 ratio to skim milk that has been heated to 95°C for 30 min and cooled to 25°C. After incubation for a day, the grains are sieved and the process of fermentation in skim milk is repeated two more times. After the final fermentation, the grains are sieved and the skim milk without the grains is then added as the kefir culture at the rate of 5% to milk that has been heated to 85 ° C for 30 min and adjusted to 22°C. Fermentation ensues for 12 hr (pH 4.5 to 4.6) followed by ripening for 1 to 3 days at 8 to 10°C. During ripening, yeast fermentation occurs.

The ratio of kefir grains to milk affects the microflora of the starter. If a relatively large proportion of grains are added to milk (e.g., ratio of 1:10), the proportion of lactococci and yeasts is lower than when the proportion is smaller (1:30 to 1:50) because of the rapid accumulation of lactic acid. The optimum development of all groups of organisms in the grains takes place at a ratio of 1:50 (14).

B. Manufacture of Kefir from Concentrated Culture

Kefir may also be produced with a concentrated culture that is prepared from isolates obtained from kefir grains (1,15). In the actual manufacturing process, no grains are used, as the organisms have already been isolated as a concentrated culture. This culture consists of 75% homofermentative lactococci, 24% citric acid-fermenting lactococci, 0.5% lactoba-cilli, and 0.1% Candida kefir yeasts. A bulk starter made from this concentrated culture is added to heated milk (94°C, 5 min, cooled to 22°C) at the rate of 1%. Incubation is for 1822 hr until pH is 4.5 to 4.6.

C. Kefir Grains

Kefir grains are the most important components of kefir manufacture (Fig. 2). The concept of kefir grains is believed to have originated in the accidental discovery of kefir many hundreds of years ago in the Caucasus region. These grains vary in size from a wheat grain to the size of a walnut. They are of whitish to yellowish color and gelatinous and irregular in

Pencil Drawings Semi Trucks
Figure 2 Kefir grains. (Courtesy F. V. Kosikowski LLC.)

shape and with a rough surface. The dominant microflora of kefir grains are listed in Table 3 and consist of mesophilic, homofermentative, and heterofermentative lactococci, hetero-fermentative lactobacilli, lactose-fermenting and lactose-nonfermenting yeasts, and acetic acid bacteria. The microbial ecology within the grains depends on the origin and method of cultivation of the grains (4,16-18). In one study, 120 strains of lactobacilli were isolated from kefir grains. The predominant strain was Lactobacillus kefirogranum (19). These groups of bacteria when purified do not grow at all or grow very slowly in milk. In fact, it has been demonstrated that manufacture of good-quality kefir by using isolated bacteria is difficult (20). Thus, the symbiosis within kefir grains is critical in optimal growth for kefir production. Koroleva has described such symbiosis (14): proteolysis and vitamin production by yeasts and acetic acid bacteria stimulates lactic acid bacteria, which in turn utilize lactose to promote growth of lactose-nonfermenting yeasts. The metabolites of growth of lactic acid bacteria also check the growth and alcohol production by yeasts.

Newer strains from kefir grains such as Lactobacillus kefiranofaciens (21) and Saccharomyces turicensis (22) have been isolated. It is conceivable that as new microbiological techniques become available, newer strains will be identified and the microbial ecology of kefir grains will be better understood.

Kefir grains may be contaminated with coliforms, bacilli, micrococci, and mold, which will rapidly spoil the product manufactured with such grains. According to Koroleva, a good quality kefir culture prepared in skim milk should contain 108-109 lactococci, 107-108 leuconostocs, 105 thermophilic lactobacilli, 102-103 mesophilic lactobacilli, 105-106 yeasts, and 105-106 acetic acid bacteria per milliliter (23).

Kefir grains consist of approximately 24% polysaccharide called kefiran, which consists of glucose and galactose in equal proportion and is not easily attacked by enzymes. Various lactobacilli such as Lactobacillus kefiranofaciens (24) and others (25,26) that are present in the grains produce these polysaccharides. The polysaccharide content of kefir is approximately 0.2-0.7% and provides a slightly ropy texture to the final product.

Table 3 The Microflora of Kefir Grains

Species

Yeasts

Candida kefir

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