Milky microbes: the making of kefir

Foods containing live active cultures have been touted as beneficial for digestion and immune health. Yogurt, particularly Greek yogurt, has risen to the top of the probiotic trend in recent years. With the uptick of microbiome research and news lately, foods that allegedly improve the gut microbial community has acquired quite a following. Now another fermented milk product named kefir (not to be confused with kaffir lime) is slowly gaining in popularity in our health-conscious world. Though today, many may not know what kefir is or even how to pronounce it (“keh-FEAR”), kefir is sure to attract mainstream attention in the coming years.

Google search trend for “kefir” in the last decade.

Kefir’s history
Kefir has a rich history dating back thousands of years and is thought to have originated in the northern Caucasus, the region of current-day Russia between the Black Sea and the Caspian Sea. The name kefir is believed to be derived from “kef,” a Turkish word meaning “good feeling.” Kefir has become a prized possession in the Caucasus and was initially kept secret from the rest of the world. Eventually, with some espionage and scheming, kefir eventually spread through Russia. Throughout time, many have believed that this concoction holds special powers: a gift from the gods, an elixir of life, a miracle drink.

Even Elie Metchnikoff, who was awarded the 1908 Nobel Prize in Physiology or Medicine for the discovery of phagocytosis, had theories about fermented milk. He was fascinated by the longevity of people in the northern Caucasus and hypothesized that the lactic acid bacteria in fermented milk products were the cause. He wondered about the beneficial effects of lactic acid bacteria and attempted to alter his own intestinal microbiota by drinking fermented milk he prepared himself from lactobacilli isolates. Metchnikoff noticed health benefits from drinking kefir and documented his work in The prolongation of life; optimistic studies. For a deeper discussion on Elie Metchnikoff’s life and work (and to get a glimpse of late 19th-early 20th century science, check out R. B. Vaughan’s The Romantic Rationalist: A Study of Elie Metchnikoff.

Kefir chemistry
Kefir and yogurt, both made fermented from milk, have similar tastes. Tart, sour, and tangy. To my taste buds, kefir is like an intensified form of yogurt, effervescent and wonderfully tart. While yogurt is fermented with just bacteria, kefir is fermented with a more diverse community of bacteria and yeast. As a result the chemistry between the two products differ and contribute to the differences in taste and texture. Almost all of the lactose in the milk is metabolized to lactic acid by the lactic acid bacteria that dominate kefir making it possible for lactose intolerant individuals to drink kefir without ill effects. Lactose is also broken down into kefiran, a water-soluble polysaccharide that contributes to kefir’s creamy texture. Many other acids formed by the bacteria (ex: pyruvic acid, citric acid, amino acids) also contribute to the flavor. Towards the end of the fermentation process, yeasts break lactose down to ethanol and carbon dioxide giving kefir its bubbly characteristic. You can’t get drunk on kefir however; the alcohol concentration in kefir is usually approximately 0.2-0.3 percent.

Making kefir
Unlike other fermentations such as beer and wine which can take weeks to years, kefir fermentation only takes 12-24 hours. Fermentation requires starter cultures called kefir grains which resemble miniature cauliflower. These grains contain bacteria and yeast that are encased in a matrix of protein, lipids and sugars. Making kefir is pretty simple: obtain grains, drop them in milk, and let it sit at room temperature for 12-24 hours. After this incubation is complete, the grains can be transferred to fresh milk to make the next batch or grains can be covered in a little milk for storage in the fridge. Kefir grains will grow in size as they are propagated from batch to batch making it easy to share or keep an extra stash of grains incase of mess-ups.

Because we move kefir grains from one batch to a fresh batch of milk, it only seems logical that the kefir itself can be used as the microbial inoculum for the next batch. However, store-bought kefir has gone through a lot of trauma before making it to the grocery store shelf. Kefir is often times pasteurized prior to reaching the shelves. Prolonged storage and low temperatures can kill off many of the microbes in kefir and limit the diversity of microbes making subsequent batches diverge away from the ideal kefir microbial communities. Furthermore, store-bought kefir is made with about 10 strains of bacteria and yeasts whereas homemade kefir is made approximately 50 strains!

Three years ago I had no idea what kefir is. Now I have it everyday in my breakfast and am currently waiting for my first batch of kefir grains to arrive in the mail (today!). Please stay tuned for next week’s Foodie Friday involving a more in-depth story linking the microbes in kefir to flavor components.

CC BY-NC-ND 4.0 Milky microbes: the making of kefir by Jennifer Tsang is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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