Take home message

• Cronobacter sakazakii and Escherichia coli are Gram-negative bacteria, belonging to the Enterobacteriaceae family. Both bacteria or strains of them can cause a food infection, which you obviously want to avoid.
• In kefir, these bacteria have no chance to grow out, especially in raw milk kefir.

Fermentation rather than pasteurisation

An important goal of pasteurisation of milk is to reduce the germ count, both total and that of specific bacteria, such as E.coli. Usually, pasteurisation is said to be ‘successful’ when the mesophilic germ count is reduced by 5-6log10. Converted, this means that after pasteurisation the germ count in milk of 1,000,000 cfu/ml (6log10), then after pasteurisation it is less than 1log10 or 10 cfu/ml. Usually germ counts in raw milk are much lower, often rather max 3-4log10. E.coli is only a part of the total plate count and does not exceed 1-2log10 in good raw milk. Theoretically, you cannot lower a milk with 4log10 by 5-6log10 steps, there are not that many bacteria in the milk. Of course, there is always a small residual of bacteria left, of a level that is usually considered ‘safe’.

By fermenting milk, you create a sharp drop in pH in a short time (usually within 12-15 hours): the milk acidifies due to the bacterial conversion of lactose.

Kefir supresses gram-negative bacterial-growth

Spanish researchers made two types of kefir, both from raw milk and pasteurised milk (Aguirre-Ramirez et al., 2024). The first type of kefir contains only lactic acid bacteria (9 species or strains), while the second additionally contains the yeast Kluyveromyces marxianus. Strictly speaking, the first kefir is not true kefir, as moulds and yeasts are an essential part of kefir.

Both pathogens were cultured and diluted to a concentration of about 4-5log10 cfu/ml. In a 1:1 ratio, the bacteria were mixed with the kefirs. After 24 hours of incubation at 37^o C, the amount of E.coli and C.sakazakii bacteria were counted. The trial was initiated with fresh kefir and with kefir stored in the refrigerator for 7 days.

Regarding suppression of growth of C.sakazakii, there is no difference, not immediately after fermentation or after 1 week of storage at 4^o C (Fig 1). Shown is as an example the effects of the 1-week-old kefir. The control (without inhibitory species) grows 9log10 cfu/ml of C.sakazakii bacteria, while the kefir species all gave a 6-7log10 reduction, as did the positive control treated with the whey protein lactoferrin (yellow bar), Lactoferrin was used as a control because it has a proven suppressive effect.

Fig. 1. The difference in growth of C.sakazakii in an untreated control (C), a control with proven suppression (lactoferrin = LF), two kefirs from raw milk (RMK) and two from pasteurised milk (PMK). Kefir-B contains only lactic acid bacteria, kefir-C additionally the yeast Kluyveromyces. The asterisks indicate the significance of the reduction compared to C. (Origin: Aguirre-Ramirez et al., 2024).

Only kefir from raw milk suppresses growth of E.coli

The results for E.coli growth show a clear difference between kefir from raw milk or from pasteurised milk (Fig 2). Lactoferrin and both raw milk kefirs give a reduction of almost 5log10 (yellow bar), while there is no reduction at all, when the kefir is made from pasteurised milk. However, it does not matter if the kefir contains yeast. The authors cannot state, why the differences arise between kefir made from raw milk or pasteurised milk. However, other studies also found a suppression of E.coli growth if heated milk was used.

Fig. 2. The difference in growth of E.coli in an untreated control (C), a control with proven suppression (lactoferrin = LF), two kefirs from raw milk (RMK) and two from pasteurised milk (PMK). Kefir-B contains only lactic acid bacteria, kefir-C additionally the yeast Kluyveromyces. The asterisks indicate the significance of the reduction compared to C. (Origin: Aguirre-Ramirez et al., 2024).

Conclusion

The fermentation of milk into kefir means that unwanted gram-negative bacteria, such as E.coli strains, cannot grow in kefir. There can also be differences due to heating of milk, with effects of bacterial inhibition being lost in kefir from pasteurised milk.

The log10 reduction by fermentation into kefir matches the desired reduction achieved by pasteurisation, which is almost 5log10. From this point of view, kefir can be understood as a ‘GAS’ product: Generally Accepted as Safe, even if it is made from raw milk.

Literature

• Aguirre‐Ramírez, D., Abad, I., Pinilla, E., Pérez, M. D., Grasa, L., & Sánchez, L. (2024) Antibacterial activity and antioxidant capacity of dairy kefir beverages. International Journal of Dairy Technology. doi: 10.1111/1471-0307.13153