Take home message

• Grain-based kefir is an intriguing ecosystem of a biofilm, bacteria and yeasts. Like a coral reef, it maintains itself rather stably; it grows and lives.
• The base of all kefir grains is apparently very similar, but from kefir-to-kefir drink, from continent to continent, the species found in small amounts in the grains are different.
• Different in terms of species, but with probably the same functionality.

Symbiocene thinking

Through the Dutch TV-programme ‘Tegenlicht’, it became clear, that a new way of thinking, a philosophy, is needed to leave the Anthropocene era behind us. We are living in the Anthropocene, the human-dominated technological age. Australian environmental philosopher Glenn Albre sought new concepts to express our feelings associated with experiencing nature. Unfortunately, many of these feelings stem from human destruction of nature and the environment, urbanisation and artificial ways of living and feeding.

The answer to the Anthropocene should be the Symbiocene. This Post-Antropocene should free us from future environmental damage, mountains of waste, floating plastics, or non-degradable agents (PFAS, DDT). The way to the Symbiocene is through imitating and copying natural processes, understanding ecosystem complexity and establishing symbiotic relationships with the nature around us. Harmony as a key concept.

The kefiran-reef as a metaphor

Kefir drink is made from (raw) milk and kefir grains. Kefir grains are small granules formed from natural substances. It is the milk proteins and sugars that build the matrix of the grains. Kefiran is a biodegradable biofilm, a biopolymer, on, in or between a diversity of bacteria and fungi can settle down. In this, Lactobacillus kefiranofaciens is the main builder of the kefiran skeleton. Kefiran is an example of the new Symbiocene, as a complex, degradable natural product is used to make new materials, including packaging materials or new medicines (Radhouani et al., 2018).

Together, the kefir grains form a kind of coral reef. Coral is probably one of the most imaginative ecosystems, characterised, like kefir grains, by the presence of species that build their own skeleton. In corals, this is done by the lime-secreting polyps. They make the reef grow. An inner and outer shell is created, in and on which all kinds of other organisms can survive, and a complex community of living organisms is present. The reef of kefir grains is the kefiran. Bacteria and fungi grow on the inside and outside of the kefiran reef.

One kefir is not the other

When you compare kefir grains from different countries, there are sometimes big differences. One is yellower, more cauliflower-like in texture, the other flatter, more elastic and tougher. Why this is so is anyone’s guess. From region to region and probably even from household to household, kefir is made a little differently. This may be due to the milk used (cow, goat, sheep), whether the milk is heated, the culture temperature, the rhythm of inoculation, whether the seeved grains are rinsed, and the hygiene standard of the kefir maker (m/f). Yet it seems, the bacteria that builds the kefiran reef is the same for all kefirs: Lactobacillus kefiranofaciens. In total, as many as 50 species of bacteria are found in kefir grains. On the outside of kefir grains, there are large amounts of Lactococcus bacteria, different strains of L. lactis for example. In milk, they develop quickly and initiate the process of lactic acid formation. As the L. lactis multiply, these bacteria then spread freely through the milk and are no longer bound to the kefir grains.

Importantly, however, the 50 kefir bacterial species belong to a limited number of functional kefir characteristics and are therefore functionally interchangeable as species. The bacteria in kefir grains fall into three main groups: (1) the skeleton builders, (2) the true lactic acid formers and (3) the part-time lactic acid formers c.q. flavourers. The group of lactic acid-forming bacteria is therefore very heterogeneous, when it comes to shape (rods, rounds, clusters), yes/no spore formation and lifestyle.

Functionality of lactic acid-forming bacteria in kefir

Lactic acid-forming bacteria convert milk sugar into lactic acid in two steps: lactose (12 Carbon) is split into glucose (6C) and galactose (6C) using the enzyme lactase (also called beta-galactosidase). 6C sugars are split into lactic acid (3C). So, from one molecule of lactose (12C), 4 molecules of lactic acid (3C) are formed. Various yeasts use the lactic acid (3C) as a food source to make ethanol (or alcohol) (2C) from it. From 2 lactic acid molecules (3C), 3 ethanol molecules (2C) are formed.

There are also acetate-building bacteria, the acetic acid formers. Acetic acid also consists of 2C compounds. As a further decomposition product, carbon dioxide gas (CO₂) is formed, a typical feature of anaerobic digestion (Table 1).

TTable 1. Functions of bacteria in kefir grains: biological and physiological properties (derived from Fan et al., 2022)

Name	Function	How	Amount
Lactobacillus
Lb. kefiranofaciens	Skeleton builder of kefiran	Kefiran is created from milk sugar and milk protein	>90%
Lb. kefiri			Enkele %%
Lactococcus	Mesophyll lactic acidifier, homofermentative	Rapid conversion of lactose into lactic acid
L. lactis
L. cremoris
Leuconostoc	Mesophyll lactic acidifier, heterofermentative	In addition to lactic acid, carbonic acid, ethanol and volatile fatty acids are also produced
Enterococcus	Mesophyll lactic acidifier
E. faecalis
Streptococcus	Thermophyle lactic acidifier
S. thermopilus

Wang et al., (2020 and 2021) looked at the difference in kefir grains from three continents. Lactobacillus kefiranofaciens is what connect the different grains (ca 90% of all bacteria). Local differences and further differences between kefir grains are in the last 10%. Bacterial species that perform a function in one kefir may switch places with other species in kefir of a different origin.

In terms of yeasts and fungi, three dominant genera emerge: Saccharomyces, Kazachstania and Kluyveromyces. They make up about 90% of the yeast population, though in different proportions and dominance of species. For kefir grains, up to about 25 different species are described. They fall into two functional groups: (4) those that can digest lactose and (5) those that cannot and therfor digest the lactic acid (= lactate), (Fan et al., 2022).

When kefir grains are added to milk, a fermentation process starts. In this process, bacteria and yeasts play ball with each other and symbiosis therefore occurs. Bacteria form lactic acid and thus lower the pH of the product. With the disappearance of their food source (lactose) and increasing acidification, the growth of lactic acid-forming bacteria comes to an end. Yeasts like Saccharomyces cerevisiae use lactic acid as a food source, break it down, making the product less acidic. As a result, this yeast allows lactic acid-fermenting bacteria to grow again, which is visible in a further reduction of lactose sugars.

Fazit

We can learn from the ecosystems of kefir and kefir grains, how they live and survive in symbiosis. Together, bacteria and yeasts form an alliance, which is relatively stable over time. The basis of kefir is formed by a biofilm, the kefiran. On it and within it grow some important functional groups of bacteria and fungi, which are exchanged from kefir to kefir and from place to place by species, which are locally significant, yet apparently perform very similar functions compared to kefir from another continent or place.

Literature

• Albrecht G. (2022) Earth emotions. New words for a new world
• Fan, D., Stoyanova, L. G., & Netrusov, A. I. (2022). Microbiome and Metabiotic Properties of Kefir Grains and Kefirs Based on Them. Microbiology, 91(4), 339-355.
• Radhouani, H., Gonçalves, C., Maia, F. R., Oliveira, J. M., & Reis, R. L. (2018). Kefiran biopolymer: Evaluation of its physicochemical and biological properties. Journal of Bioactive and Compatible Polymers, 33(5), 461-478.
• Wang, H., Wang, C., & Guo, M. (2020). Autogenic successions of bacteria and fungi in kefir grains from different origins when sub-cultured in goat milk. Food Research International, 138, 109784.
• Wang, H., Sun, X., Song, X., & Guo, M. (2021). Effects of kefir grains from different origins on proteolysis and volatile profile of goat milk kefir. Food Chemistry, 339, 128099.