Kefir is transformed with constancy

Flames. A method of preserving and preparing food, appreciated for the flavors it develops from raw ingredients, fermentation is omnipresent in food: sourdough bread, wine, beer, chocolate, sauerkraut, pickles, surströmming (fermented herring ), yogurt… From the same barrel, milk kefir is similar to yogurt in taste and by a composition rich in lactic acid, while remaining liquid, slightly pearly. At least the finished product, consumed as a drink, is a liquid. However, kefir is divided into two fractions: the liquid phase on the one hand, and the kefir grains on the other hand, which grow in size throughout the fermentation, then are collected and inoculated in fresh milk to reproduce the surgery. These whitish, cauliflower-shaped grains are made up of around forty species of lactic acid bacteria and yeasts, held together by a gelled matrix.

At the European Molecular Biology Laboratory (EMBL, Heidelberg, Germany), Sonja Blasche, Yongkyu Kim, Kiran Raosaheb Patil and their collaborators studied the transformations of kefir during the ninety hours of its fermentation, taking into account both variations in chemical composition and fluctuations in the microbial ecosystem that accompany them. Their results are the subject ofan article published in the February issue of Nature Microbiology.

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By characterizing the microscopic flora of kefirs of different geographical origins, they confirmed that they were populated by lactobacilli, leuconostocci and lactococci cohabiting with other less abundant microbial species. But the main contribution of their work comes from monitoring the proportion of each species over time. They accurately describe how the liquid fraction of kefir is successively colonized by different species, which take turns as its chemical composition changes.

Two scenarios

In the first twenty hours, a few pioneer bacteria thus prepare a niche for microorganisms which will then take off. For example, Lactococcus lactis consumes citrate and breaks down casein in milk, releasing amino acids and minerals. It also ferments lactose into lactic acid, which acidifies the medium and becomes a nutrient resource for later acidophilic species such as Acetobacter fabarum. Other species exchange the molecules they produce, which allows their simultaneous proliferation; Leuconostoc mesenteroides thus supplies lactic acid Lactobacillus kefiranofaciens, while the latter provides it with a source of amino acids.

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Kefir is transformed with constancy