Understanding kefir production

Kefir is a traditional fermented dairy product originating from the Caucasus and Eastern Europe. It is becoming a popular dairy drink in many countries including western countries. It is also considered a probiotic dairy product due to the presence of beneficial microorganisms that boost human health. Kefir production is initiated by the addition of a starter culture or kefir grains to milk. Kefir grains, which have complex microbial communities, are usually replaced with a starter culture in commercial production systems for better control of the final kefir due to the reduced number of microorganisms in starter cultures compared to grains. However, the quality of kefir produced by starter cultures is believed by some to be less than kefir produced with kefir grains. The microorganisms used have a direct influence on the quality of the final kefir during fermentation as do factors such as milk type and production parameters. Understanding kefir production, particularly the roles of kefir microorganisms and milk type is, thus, crucial for consistently producing good quality kefir.
Given the growing interest of kefir as a functional fermented milk product, the first study aimed to determine the suitability of using kefir grains for producing consistent kefir drinks by evaluating the changes in the microbial communities of kefir grains and their corresponding kefirs over time using high-throughput amplicon sequencing. It was found that Lactobacillus kefiranofaciens and Lentilactobacillus kefiri dominated the grains whereas Lactococcus lactis dominated kefir. The communities were more variable in the kefir than in the kefir grains with the relative abundance of Lactobacillus kefiranofaciens and Lactococcus lactis fluctuating over time. Kazachstania turicensis and Torulaspora delbruekii were the dominant fungal species in the kefir grain communities although the ratio between the two species varied significantly. The findings of the study suggest that using kefir grains to produce consistent kefir drinks might be hindered due to the changes in the microbial communities over time, highlighting the need for future studies to explore the effect these changes have on the production of flavour and aroma compounds in kefir.
The bacterial communities and volatile organic compounds (VOCs) of cow and goat kefirs produced using the same kefir grains were characterised to understand factors influencing kefir quality. The VOC profiles of both kefirs varied significantly. The bacterial communities were similar in both cow and goat kefirs with Lactococcus lactis dominating both kefirs followed by Lactobacillus kefiranofaciens, Pseudomonas putida, and Leuconostoc mesenteroides. As the same grains were used to produce the kefirs and similar bacterial communities were found in both kefirs, it seems likely that the different VOC profiles were due to differences in the milk.
The influence of different kefir grains on the production of VOCs and sensory attributes was then studied using kefir grains from two different sources. The microbiome of both kefir grains and their kefirs were similar at the genus level based on high-throughput amplicon sequencing with Lactobacillus spp. dominating both grains and Lactococcus spp. dominating the kefirs. Kluyveromyces and Kazachstania were the dominant fungal genera in both kefir grains and kefirs. The VOCs were similar in kefir A and B, with carboxylic acids, particularly octanoic acid and acetic acid accounting for the highest levels of VOCs. However, some compounds (hydrocarbons and one ketone) were detected in kefir B only. The sensory evaluation of kefir A and B was similar. The results suggest that although the differences in the VOCs of kefir A and B could be associated with the differences in the number of sequence reads for the dominant genera in kefir A and B, these differences did not influence the panel’s sensory perception of the kefirs. The final part of the thesis aimed to determine the influence of both milk type and microorganisms on kefir quality. The effect of three different cow milks (Full-cream milk, non-homogenised milk, and high-pressure pasteurised milk) on kefir produced using three different and diverse kefir grains and starter cultures was examined with and without fortification with Lactobacillus kefiranofaciens. It was found that the VOCs of the kefirs from the three different milks did not vary significantly when fermented by the same grain or starter culture. However, the VOCs of kefirs produced from the same milk type (e.g., full cream) using the different kefir grains and starter culture were significantly different. The total titratable acidity and pH but not exopolysaccharide production varied among kefirs from different milk types and among kefirs produced with different microorganisms.
The addition of Lactobacillus kefiranofaciens to the starter culture resulted in an increase of the number of the VOCs produced in full cream milk kefir and a change in the relative abundance of some VOCs compared to kefir produced without the addition of Lactobacillus kefiranofaciens. Additionally, full cream milk fermented with single and double strains contained fewer VOCs compared to using kefir grains or starter culture indicating that the interactions in microbial communities are important for the production of the full range of VOCs in kefir. This study concludes that milk type is important for the physicochemical parameters of kefir such as TTA and pH but does not influence the VOC profiles of the final kefir. The formation of VOCs is most influenced by the microbial communities used to ferment the milk.
This thesis concludes that using high-throughput amplicon sequencing is a robust technique for revealing the entire microbial communities of kefir grains and kefir including the minor populations. It also provides evidence that kefir grains, and their microbial communities, may not be suitable for consistent kefir production due to changes over time and that a starter culture containing the most important microorganisms is needed to consistently produce kefir with similar quality as from kefir grains. The microorganisms used to ferment milk as well as the type and processing of the milk have a significant influence on the characteristics of kefir that may affect consumer perception of kefir quality and ‘appeal’. Further investigation is required to improve our understanding of the relationship between these factors, but they are important considerations for the commercial production of kefir where a consistent, high-quality product is desired.