Take home message

• Kefir is not well defined and there is a big difference between kefir made from commercial cultures (sometimes with, sometimes without yeasts/fungi) and kefir made traditionally with the kefir grains.
• With traditional kefir, the way the community of micro-organisms work together, grow and develop leads to not only the composition being different in terms of bacteria and yeasts, but also other substances ending up in the kefir drink.
• Commercial kefir is much more like yoghurt, while grain-based kefir really has its own ‘kefir composition’.

Kefir from SCOBY versus kefir made of a commercial starter?

German colleagues (Nejafati et al., 2022) looked at not only the microbial composition of kefir, but also the metabolic products of the bacteria and yeasts present in the various kefirs, the group of volatile organic compounds (VOCs). They mention about 20 volatile organic compounds such as ethanol, butyric acid, acetone and acetic acid, belonging to the ketones and aldehydes. They are among the flavourings of a product.

A distinction was made between traditional kefir, made from kefir grains or also called SCOBY, and commercial kefir, as made by the various big dairies. This commercial kefir is often made with a kefir starter, of which the selection of bacteria and yeasts is not always clear. Characteristic bacteria found in traditional kefir are Lactobacillus kefiranofaciens and Lactobacillus kefir. However, you won’t find these in the commercial fermented milk, which often consists of Lactococcus lactis and Leuconostoc. Something similar applies to the yeasts in kefir, where commercial kefirs mostly use Debaryomyces hansenii instead of SCOBY-type yeasts like Kluyveromyces marxianus or Kazakhstania species.

Surprisingly, the researchers find, that the group of traditional kefirs is very similar in terms of VOCs, but very different from the group of commercial kefirs. Moreover, the VOC composition is more like that of two analysed yoghurts and to milk, than to that of traditional kefir. For instance, the traditional kefir increased the ethanol (alcohol) concentration, which is absent in commercial kefirs, yoghurts and milk.

The research further assessed how strongly milk sugar (lactose) is broken down.

Table. Different contents in milk (n=1), traditional kefir after 24h or 48h fermentation (n=5) and commercial kefir (n=3)

	Lactose (g/L)	Glucose (mg/L)	Galactose (mg/L)	Lactate (g/L)	Acetate (mg/L)	Glycerol (mg/L)
Milk	47	98	0	0	0	0
Traditional kefir 24h	37	166	215	2	358	199
Traditional kefir 48h	21	32	21	4	781	412
Commercial kefir	42	45	605	7	713	26

The table shows a few things. The lactose in milk is broken down more intense in traditional kefir than in commercial kefir, and more lactose disappears, if the kefir-processing takes 2 days instead of 1 day. The microorganisms convert lactose (12 Carbon) into glucose (6C) and galactose (6C), and these sugars each consist of a ring of six carbon atoms. Glucose is the main energy source for bacteria and many fungi. Galactose is used in traditional kefir to build kefiran, the exopolysaccharide skeleton, the coral reef of kefir. Galactose is also high in yoghurt because there are no bacteria present to consume galactose. This is is also the case in commercial kefir. Lactate is lacking in normal, sweet milk, but increases in kefirs. Traditional kefir would have less lactate than commercial kefir, because yeasts like Kluyveromyces marxianus use lactate as an energy source, and therfor the lactate concentration goes down. This does not happen in commercial kefir. Acetic acid as a metabolic product runs parallel to the lactate amount. The high glycerol content is explained by the activity of yeasts, which are present in traditional kefir.

In short: kefirs differ by whether the producte is made from kefir grains or a commercial starter. The process duration also affects the outcome of traditional kefir, and only in the second 24 hours of fermentation, the impact of the yeasts will be found.

Literature

• Nejati, F., Capitain, C. C., Krause, J. L., Kang, G. U., Riedel, R., Chang, H. D., … & Neubauer, P. (2022). Traditional Grain-Based vs. Commercial Milk Kefirs, How Different Are They?. Applied Sciences, 12(8), 3838.