Kefir and kefir grains : diversity of chemical properties and microflora
Year: 2012
Type: dissertation
Abstract: Kefir grains contain bacteria and yeasts embedded in a protein and polysaccharide matrix. They constitute the primary starter culture for kefir, a fermented milk drink with a complex sensory profile containing high amounts of lactic acid, ethanol, CO2, acetaldehyde and diacetyl. In commercial production of kefir in Norway, seven kefir grain cultures of Russian and Romanian origin are used in combination. These kefir grain cultures are subcultured several times every week, and have been treated similarly, yet separately for more than 50 years. Consequently, they constitute a unique material for investigations of treatment influence on microbiota, and microflora investigations were performed using a polyphasic approach of culture-dependent and –independent microbiological methods.\nEnumeration on selective growth media showed that lactococci, lactobacilli and yeasts were all found in similar amounts in all kefir grain cultures, log 7.9±0.1, log 7.7±0.2, and log 7.6±0.1 cfu g-1 of kefir grain, respectively. Using culture-independent microbiology methods, it was shown that the dominant microflora in all seven kefir grain cultures were the same, consisting of Lactobacillus (Lb.) kefiranofaciens, Kluyveromyces marxianus and Kazachstania spp. 16S rRNA gene sequencing of 245 bacteria isolates demonstrated in addition the presence of a secondary microflora mainly consisting of lactic acid bacteria, and whose composition varied somewhat between the different kefir grain cultures. Bacterial isolates found to belong to the same species of lactic acid bacteria were strain typed by rep-PCR. The results showed no correlation between rep-PCR phylogroups and kefir grain culture, however, a strong correlation was found between the phylogroups and the growth medium used for isolation.\nIn the commercial production of large volumes of kefir, direct fermentation with kefir grains is impractical as this would demand a correspondingly large quantity of grains. In addition, it would give a product with a disrupted gel and a bulging carton due to high production of CO2. A stepwise fermentation process is therefore employed, in which the kefir grain fermentate is used for production of a bulk starter. as starter for further inoculations. Kefir grain fermentate and the final kefir commercial product were analysed for microbial content and composition. In the fresh fermented kefir, lactococci and lactobacilli were both present in levels of log 8 cfu mL-1, whereas yeasts were present in lower amounts of log 3.3 cfu mL-1. After 4 weeks of storage, lactococci and lactobacilli showed significant decrease (P<0.01) by 2 and 3 log units, respectively, and remained at this level also after 8 weeks of storage. Conversely, yeast numbers continuously increased during storage, reaching log 5 cfu mL-1 after 8 weeks of refrigerated storage. By analyses of total DNA of the fermentates, Lactococcus lactis was found both in kefir and kefir grain fermentates, and in addition Lb. rhamnosus, Lb. kefiri and Leuconostoc spp. could be demonstrated in kefir. None of the fermentates formed detectable amounts of PCR products when using yeast primers.\nTo reduce the sequencing load when employing denaturing gradient gel electrophoresis (DGGE) for microbial community investigations of dairy products, an additional approach of identification was proposed. By use of high-resolution melt (HMR) analysis and comparison of DGGE band melting profiles to those of known reference strains, rapid species-level identification of lactic acid bacteria can be achieved. \nIt was of interest to investigate whether kefir grains could be replaced by a pure culture starter of lactic acid bacteria and yeast isolates therefrom. The products of single- and co-culture inoculums of microbial isolates from kefir grains incubated in milk were analysed with emphasis on aromatic volatile compounds, organic acids and carbohydrates. The inoculum compositions were of proportions corresponding to the amounts earlier determined to be present in kefir grains. The results showed that the metabolite profiles of the kefir and kefir grain fermentates contained significantly more aroma- and flavour contributing components than the single- and co-culture fermentates, underlining the crucial role of kefir grains for the manufacure of authentic kefir.\nAssessment of free amino acids content in kefir during regfrigerated storage showed reduction of glutamic acid during storage, and a consequent increase in its decarboxylation product γ- aminobutyric acid (GABA). GABA has earlier been found to have blood-pressure-lowering effect in mild hypertensives when consumed in fermented milk in amounts of 10 mg daily over a 12-week period. After two weeks of storage, the amount of kefir necessary for a 10 mg intake of GABA would be 220 g.
Author: Grønnevik, Heidi Yvonne
Language: English
Subfield: Food Science
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W7048166492