The people of the
Caucasus in
Central Asia are famous for living well over 100. Most often their
longevity has been attributed to the drinking of kumis (also spelled variously koomis, koumiss or kymyz), which is a kind of
fermented mare's milk. The mare of what
species is often omitted, but, considering the
location and the other
ingredients, it is undoubtedly the
horse.
In the original method of preparation, a container was made from the skin of a horse, which was sewn up so that the hair side was outside. The hair was singed off and the bag smoked and the greased with oats or other grain. Mare's milk was stored in the bag, which was hung outside the yurt. At the end of the summer, the cheese sediment at the bottom was scraped out, dried and put away until spring. At that time fresh mare's milk was added to the dried sediment and mixed with some buttermilk. Then a dried horse tendon was added -- I image for a smoother product. The secret to a great kumis is in the shaking. If the kumis was stored in a horsehide bag, it was again hung outside the yurt. Anyone entering or leaving the yurt would give the bag a good shake. When the liquid was quit bubbly it was ready for drinking.
Curious people of the West can make an approximation of kumis with cow's milk, thickened with a bit of cream. Yeast can be used as the fermenting agent. The type of yeast, of course, will affect the final flavor. The wet yeast used in making bread will help he fermentation along considerably. I would imagine that kefir, a milk beverage now available in some supermarkets, could be used instead of yeast. Flavored kefir should be avoided. Instead of a dried horse tendon, you may want to substitute a raw chuck steak for a touch of authenticity.
While you wait for the fermentation to do its work, store the beverage in sealed bottles in a nice warm place. Whenever you think of it give the bottles a good shake. A word of caution, though: if the kumis is fermented in sealed glass bottles, wrap them in thick bath towels, for I have seen bottles explode in the back of the closet, which could be messy. Bon appetit!