All cultured (also called fermented) milk products have varying amounts of lactic acid, which gives them their pleasingly tart, slightly tangy flavor. There’s a difference between milk product fermentation and yeast fermentation that some people confuse.

Milk product fermentation is by bacteria that produce lactic acid, while yeast (a completely different microorganism) convert sugar to alcohol in such things as bread dough, brewing beer and wine.

Product	Remarks
Yogurt	Two different cultures of lactic acid-producing bacteria ferment it. May start with whole, low-fat or non-fat milk. Fermented to 0.9% acidity (pH 4.4). Slow fermentation at cooler temperature results in smoother, creamier texture, more costly product.
Sour cream	Two sets of live cultures added to light cream. One culture ferments, second culture produces flavor. Fermented to 0.5% acidity.
Buttermilk	Same culture ferments it as sour cream but instead of cream, low-fat milk is used. Fermented to 0.8% acidity.

Today’s yogurt comes in mind-boggling array of flavors. Processors add fruit purée or fruit syrup (15 to 18 percent) either leaving it on the bottom of the container before culturing (sundae-style) or they quickly blend it into cultured yogurt just before chilling (Swiss style). Stabilizers, that also thicken it, make up about half a percent of commercial yogurt.

You can get fooled into thinking that nonfat yogurt is your perfect diet food, but the high sugar content ups the calories considerably. The amount of sugar ranges from 7 to 15 percent, but in some brands it is as high as 25 percent, twice the amount than in a can of soda. If your goal is diet food, you are better off to buy unflavored yogurt, then add your own sweetener or flavorings.

Frozen yogurt is simply Swiss-style yogurt that the processor quickly freezes. It comes in packages like ice cream and you serve it like ice cream. Other cultured products less commonly available are sour half-and-half, which is a lowerfat sour cream, and crème fraiche, that cooks use like cream in French marinades or sauces, where they prefer a thicker consistency and slightly tart flavor. Crème fraiche is easy to make at home.

Start with heavy cream, inoculate it by adding a little cultured sour cream or buttermilk, and let the mixture ferment for a day at room temperature until thickened. The result is just barely sour, with about 0.2 percent lactic acid.

Two interesting cultured products that never made it to North America are kefir and koumiss. Both of these originated with the nomads in the Steppes of Central Asia around the year 1000. The kefir you find in health-food stores is a beverage that bears no resemblance whatsoever to the original, only the name is the same.

In both kefir and koumiss, two cultures ferment simultaneously, a lactic acid-producing bacteria and an alcohol-producing yeast that live in symbiotic relationship. The result is a sour, tangy alcoholic beverage that Russians and some Eastern Europeans are very fond of. It fizzes like beer and is mildly intoxicating.

The alcohol content is fairly low, ranging from 1 to 2.5 percent, much lower than beer. The acid content is 0.7 to 1.8 percent, quite a bit more tart than our yogurt. The difference between kefir and koumiss is what they begin with. They produce kefir from cow, goat or sheep milk, and koumiss from mare’s milk (though originally, before horses, the nomads used camel’s milk).

Large herds of mares graze peacefully in Russia like

cows in Wisconsin, and farmers on these horse dairies get up as early as Wisconsin farmers do to milk their herd of horses. Due to shortages of mare’s milk now, some Russian processors switched to cow’s milk to make koumiss. Even if they use cow’s milk for both koumiss and kefir, different live cultures produce the two, and they taste different.

There’s nothing like a sixpack of ice-cold koumiss on a hot summer day! Natives in the Himalayas use another fermented milk drink similar to kefir called airan.

They make this from the milk of nak (the female companion of a yak). It is hard to find airan in North America, but it is supposedly an unusual-flavored, somewhat fatty beverage that takes acquired taste buds to love