Microbiological composition of kefir

The microbiota present in kefir and its grains include numerous bacterial species from lactic acid and acetic acid groups, yeasts and filamentous fungi, which develop complex symbiotic associations(3). In this relationship, yeasts produce vitamins, amino acids and other essential growth factors that are important for bacteria. Likewise, the metabolic products of bacteria are used as an energy source for the yeasts. This symbiosis allows the maintenance of stability, so that throughout the fermentation cycle, the microbiological profile of kefir grains and kefir remains unaltered, despite variations in the quality of the milk, microbial contamination, presence of antibiotics and other inhibitory substances(24).

The identification of microbiota present in kefir and its grains is important since it is directly related to the quality of the probiotic product(37). Different methodologies have been applied to study the microbiota of kefir; however, the classical approach of culturing micro-organisms in nutrient media (universal and selective) and identification of isolated cultures is still being performed(38). Nowadays, the understanding of microbial ecology of foods has dramatically changed. The use of a combined approach using culture-dependent and cultureindependent methods, such as functional genomics, transcriptomics, proteomics and metabolomics, are encouraged to understand the behaviour of micro-organisms in foods(39). Using culture-independent methods, including metagenomics, has allowed characterisation of a number of previously unknown micro-organisms in kefir(40). In particular, analysis of the 16S rRNA gene libraries and/or molecular techniques such as denaturing gradient gel electrophoresis are very useful to evaluate and understand the complex microbial populations and diversity of strains from the probiotic kefir(41).

The microbial diversity of kefir described in the literature varies greatly. In our review, we present a complete description of the bacteria and yeasts that have been identified in kefir to date (Table 1). The number of different microbial species in kefir is estimated to be more than 300. The microbial composition of kefir also varies according to microbiological culture medium, origin of kefir grains, different techniques employed during processing, different room temperatures, type and composition of milk used, storage conditions of kefir and kefir grains. Additionally, the amount of grain added to the milk, agitation and incubation temperature can influence the extent of acidification and consequently the microbiological composition of the final fermented milk. Witthuhn et al.(42) observed that the population of bacteria in kefir may vary from 6·4 × 104 to 8·5 × 108 CFU/g and yeasts from 1·5 × 105 to 3·7 × 108 CFU/g. After 24 h of fermentation, kefir presented 108 CFU/ml of Lactobacillus, 105 CFU/ml of Lactococcus, 106 CFU/ml of yeasts and 106 CFU/ml of acetic acid bacteria(43).

According to Lopitz-Otsoa et al.(2), the microbial composition of kefir grains comprised 65 to 80% of Lactobacillus and Lactococcus and the remaining portion was completed by yeasts. Hallé et al.(44) found that 80% of Lactobacillus belonged to Lactobacillus kefiri and the remaining 20% belonged to Lactobacillus paracasei subsp. paracasei, Lactobacillus acidophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus plantarum and Lactobacillus kefiranofaciens.

The diversity of yeasts present in kefir can also be assessed using culture-dependent and culture-independent methods. Yeast species such as Saccharomyces cerevisiae, Saccharomyces unisporus, Candida kefyr, Kluyveromyces marxianus subsp. marxianus, Torulaspora delbrueckii, Pichia fermentans, Kazachastania aerobia, Lachanceae meyersii, Yarrowia lipolytica and Kazachstania unispora are present in kefir and kefir grains in greater numbers(45,46).

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