General Information - What are kefir grains composed of?
What are kefir grains composed of?
Kefir grains have a similar shape to the cauliflower. They are elastic, irregular, gelatinous, with an ivory or white colour, and variable size, from 0·3 to 3·5 cm in diameter(16,17) (Fig. 1). In general, kefir grain consists of 4·4% fat, 12·1% ash, 45·7% mucopolysaccharide, 34·3% total protein (27% insoluble, 1·6% soluble and 5·6% free amino acids), vitamins B and K, tryptophan, Ca, P and Mg(18).
The presence of D-glucose and D-galactose in a 1:1 ratio in the complex structure of polysaccharides (kefiran) is responsible for the connection between the micro-organisms in kefir grains(19). Kefiran features include viscosity, water solubility and resistance to bowel enzymic hydrolysis. The production of kefiran is mainly related to the presence of Lactobacillus kefiranofaciens and Lactobacillus kefiri in the grains(2,20).
In kefir grains, the peripheral portion is composed almost exclusively of bacteria, predominantly Bacillus, whereas the inner portion of the grain contains yeasts, and the interface of the inner and outer portions has a mixed composition, where bacteria with long polysaccharide filaments, yeasts and fungi are found(2,19).
The grains can be stored in different ways. When stored at 4°C, they are active for only 8 to 10 d. Lyophilisation or drying at room temperature for 36 to 48 h allows maintenance of the activity for 12 to 18 months(16). Wszolek et al.(21) proposed a conventional method of drying at 33°C or vacuum drying to preserve the grains. However, Garrote et al.(22) observed that freezing at –20°C was the best method for grain preservation. Kefir grains remain stable for many years without losing their activity, if stored under favourable conditions. The process of reconstitution consists of performing successive incubations in milk. The grains slowly re-establish their structure and, subsequently, new kefir grains are formed(23).