We can thank microbes for the tangy flavor and the effervescent creamy texture of kefir. This fermented milk product relies on dozens of bacteria and yeasts to convert the lactose and other compounds found in milk to small molecules that contribute to the taste and texture of kefir. As we know get to know more about the microbiology of food, food scientists may be able to streamline production, improve health benefits, and customize the flavors of fermented foods.
So what are these helpful microbes and how do they contribute to the fermentation of kefir?
Scientists from University College Cork sought to address such questions by analyzing kefir grains (mini-cauliflower shaped globs of bacteria and yeast that perform the fermentation) and their fermentation products. The group obtained three kefir grains from distinct geographical locations (France, Ireland and the United Kingdom) and began to ferment milk in the lab.
The scientists first identified the microbes present in the kefir grains. In all three grains, more than 92% of the microbes were Lactobacillus (lactic acid producing bacteria). Since the kefir grains disperse microbes into the milk during fermentation, the team also characterize the microbes in the milk at different stages of the fermentation process. They sampled the kefir at 0 hours, 8 hours, and 24 hours after fermentation was initiated. Notably, microbes from all three kefir samples followed the same pattern of microbial succession: Lactobacillus dominated at the early stages of fermentation but as fermentation progressed, Leuconostoc and Aceobacter (acetic acid producing bacteria) took over.
By establishing what microbes are present in the kefir, the researchers could determine whether the microbes were responsible for producing different compounds and flavors. To do this, they first turned to the genes. Could the genes in the microbes predict the flavor compounds found in the milk? Using metagenomic shotgun sequencing, they found that genes involved in carbohydrate metabolism and fatty acid biosynthesis were most prevalent at 8 hours. By 24 hours, genes involved in amino acid metabolism were most common. Similar genes are present in cheese and kimchi possibly pointing to shared fermentation mechanisms among different foods.
Next, the researchers profiled the volatile compounds in the kefir after 0, 8, and 24 hours of fermentation. The group identified 39 volatile compounds that potentially contribute the flavor of kefir. Armed with microbial species profiles, gene profiles, and volatile compound profiles, the team could correlated the volatile compounds produced with the microbes in the kefir. The researchers saw the following correlations between microbe and flavor:
A. pasteurianus: vinegar flavors
L. kefiranofaciens: cheesy flavors
L. mesenteroides: buttery flavors
L. kefiranofaciens and S. cerevisiae: fruity flavors
As interest is growing in food microbiology, both scientists and the public can reap the benefits from studies such as this. Scientists can use the simpler, less complex microbial communities on food such as kefir or cheese to study microbial interactions that may be relevant in more complicated environments such as the gut or the soil. We may also one day be able to customize fermentations with different mixes of microbes and different fermentation conditions to produce various fermented product that satisfies all of our taste buds.