Fermented foods are having a big moment, prompted in part by a flood of new research on the human microbiome — the ecological community of microorganisms living in the human body — and the benefits of probiotic foods. One of the fastest rising stars on this scene is kefir, an ancient fermented milk beverage believed to have originated in the Caucasus Mountains.
The very first kefir grains — a semi-solid complex of microorganisms, proteins and liposaccharides — probably arose spontaneously. The exact details of kefir's advent remain shrouded in history but it seems likely that kefir was originally discovered by nomads, who used animal skins to carry milk from their sheep or camels. The fresh milk served as a food source for microorganisms present in the animal skins, and a symbiotic colony of bacteria and yeasts (or SCOBY) eventually formed, turning the containers into natural fermentation tanks.
Although it dramatically altered the flavor and texture of fresh milk, fermentation also greatly extended its shelf life without refrigeration. Early kefir consumers learned to strain the grains out of the soured milk and use them to ferment the next batch. These living colonies — gelatinous, walnut-sized blobs resembling cauliflower florets — were carefully nurtured, often passed down through generations.
Kefir is similar to yogurt, but the two differ in several important ways. In both, lactobacillus bacteria digest lactose in milk, producing lactic acid. But kefir also involves yeasts that produce carbon dioxide and ethanol as they multiply, which means that in addition to having the tartness or sourness we associate with yogurt, kefir is also slightly carbonated and contains small amounts of alcohol. Commercially produced kefir is usually processed to remove the alcohol. Home-brewed kefir, however, can range from 0.5-percent to 2-percent alcohol, depending on how long it is allowed to ferment. As a rule of thumb, the fizzier it is, the more alcohol it contains.
Kefir may also be lower in residual lactose than yogurt. In both cases, longer fermentation allows bacteria to digest more lactose. In yogurt, however, increasing lactic acid levels lower the pH, which eventually inhibits the bacteria's activity and slows fermentation. In kefir, yeasts and their by-products buffer some of the lactic acid's acidity, allowing bacteria to continue to work on the lactose. (The bacteria return the favor by creating conditions that are favorable to yeast growth).
Kefir's reputation as a functional food extends back hundreds of years. It was traditionally believed to improve digestion, boost well-being and enhance longevity. More recently, it's been put forth as a treatment for everything from allergies to tuberculosis to heart disease, although evidence for these uses is largely anecdotal.
Modern research confirms kefir's role as a probiotic food. The beneficial microorganisms it contains inhibit the growth of pathological microorganisms (both in foods and in the digestive tract), enhance digestion of other foods, and synthetize valuable nutrients such as B12 and vitamin K. Preliminary research in vitro and in animal models also suggests possible anticarcinogenic, immune-stimulating and cholesterol-lowering effects. But with kefir, researchers face some unique challenges.
The specific health benefits of any probiotic food depends on the particular strains of bacteria or yeasts, and kefir grains host an extremely diverse population. Using electronic microscopy and genome sequencing, researchers have cataloged hundreds of different bacteria and yeasts in traditional kefir products, including numerous strains and subspecies of lactobacilli, streptoccoi, acetobacter and saccharomyces. Many are thought to be unique to kefir; several are even named after it.
Adding to the complexity is the enormous variation in kefirs from different sources. Each colony of kefir grains develops a unique microbial profile, depending on the milk in which it is grown and the ambient microbial environment where fermentation occurs. This, in turn, makes every batch of kefir unique. (Even if I share my colony of grains with you, the kefir I ferment on my counter will not be exactly the same as the kefir you brew in your kitchen.)
Traditionally produced kefir typically contains dozens of different bacteria and yeasts. Commercial producers, on the other hand, use a limited number of carefully selected species to create a more consistent product. But some health benefits ascribed to kefir may depend on the more varied cultures found in traditionally fermented kefir. Fortunately, kefir is exceedingly easy to make at home. Unlike yogurt, which must be incubated between 100 degrees Fahrenheit and 110 degrees Fahrenheit, kefir ferments best at room temperature. Simply pour any type of milk (cow, goat or sheep; non-fat, reduced-fat or whole) over the grains and leave the container on the counter for one to two days.
When the milk has thickened, pass it through a mesh strainer to remove the grains. As long as they have regular access to a fresh food supply (milk), the grains remain viable indefinitely. In fact, you could end up making the kefir faster than you can consume it. Refrigeration slows their activity, and although the grains will survive on their own for a few days between use, they will eventually starve.
Alternatively, you can store the strained grains in a small amount of milk (just enough to cover them) in the refrigerator for up to ten days. When you're ready to use them again, discard the milk they've been stored in and start over with fresh milk.
Kefir can be an acquired taste. If you enjoy drinking buttermilk, you'll probably enjoy unsweetened kefir. For those who find it too sour, it can be flavored with pureed fruit or vanilla extract, or used in smoothies. It can also be used in place of milk or buttermilk in things like muffins and pancakes and to add a sourdough-like tang to breads or pizza crust. Like any probiotic food, however, heating kefir above 110 degrees Fahrenheit or so will kill the beneficial bacteria and yeasts and nullify its probiotic benefits.