Stonyfield Farm's yogurt label lists among the ingredients "naturally milled sugar" and pectin. What is naturally milled sugar? And if the company adds pectin to thicken it, isn't it more like a jelly than a yogurt?

Naturally milled sugar is sugar, period. Stonyfield Farm is an environmentally conscientious company whose products appeal to customers who have an aversion to foods that are produced by any supernatural process.

Yogurt is fascinating stuff. In essence, it's just milk (cow's, goat's, sheep's or any handy mammal's) to which certain living bacteria have been added. The bacteria feed on the lactose (milk sugar) in the milk, metabolizing it into lactic acid and other interestingly flavored chemicals, some of which (the acids among them) coagulate or curdle the milk's protein into a thick gel.

Yogurt has been made for centuries in Eastern Europe and the Middle East, but has only relatively recently taken hold in the United States, where it has a flaunted though unproven reputation for keeping us healthy, slim and fit. Because many Americans don't like the taste of pure yogurt, most American yogurt products are doctored with sugar, and up goes the calorie count. The yogurt manufacturers ultimately triumph in the calorie conflict by making the yogurt from low-fat or nonfat milk, which allows them to plaster those consumer-friendly words across their labels.

Does eating yogurt help you lose weight? Of course it does, if you eat it instead of your stevedore lunches and afternoon candy bars. Just forget the come-hither adjectives on the labels and read the Nutrition Facts chart, which gives the actual number of calories per serving.

White Hats and Black Hats

Just as in the old westerns, there are good guys and bad guys among bacteria. In making yogurt, the first step is to kill off any black hats that may be lurking in the milk. Ordinary pasteurization isn't effective enough, because even a few surviving bad guys will have a great opportunity later to multiply along with the good guys. So the milk is pasteurized at a high temperature for a long time: either 30 minutes at 85 degrees Celsius (117 degrees Fahrenheit) or 10 minutes at 95 degrees C (203 F), whereas ordinary pasteurization heats the milk to only 63 C or 145 F. The milk is then cooled to 43 C (109 F), a nice, comfortable temperature at which the good guys can flourish.

The white-hat pardners used in making yogurt are Lactobacillus bulgaricus (LB) and Streptococcus thermophilus (ST), mixed in equal amounts. (Other bacteria, such as Lactobacillus acidophilus, may be added as well.) The LB and ST have a unique symbiotic relationship. While dining together on the milk's lactose, the LB also breaks down proteins into peptides and amino acids that the ST can eat, while the ST produces carbon dioxide gas that stimulates the growth of the LB.

Among the main flavor products they produce are lactic acid, acetic acid (the vinegar acid), and acetaldehyde, a tart, walnut- or green-apple-flavored chemical produced also during the fermentation of wine and beer.

It's the lactic, acetic and other acids that do the trick of thickening the milk into yogurt's creamy consistency. Acids act on the milk's protein, which is mostly casein, to make its tiny -- about four-millionths of an inch -- widely dispersed globules (Techspeak: micelles) come together and stick into a solid mass. This happens when the bacteria have acted long enough to produce a certain level of acidity. (For casein, it's a pH of 4.6, its so-called isoelectric point.) What we observe when that acidity is reached is that the milk coagulates or curdles into curds and whey. Down at the yogurt works, they then homogenize the curds, whey and milk fat into a single smooth texture.

The more fat there is in the milk, the thicker the yogurt's texture will be. So to avoid runny low-fat or nonfat yogurt, a thickener or stabilizer may be added: either milk solids, or pectin -- a water-soluble carbohydrate obtained mostly from fruits -- or a small amount of gelatin, which is made by boiling down animal bones and skins. (Most yogurts containing gelatin will specify "kosher gelatin" on their labels: no pig parts.)

In what must be the quintessence of ingratitude, as soon as the bacteria have done their job by producing just the right flavor and texture, most yogurt-manufacturing moguls kill them off with heat. In that case, the label will say "heat-treated after culturing." The labels of yogurts that contain still-live and active bacteria (to which many people attribute yogurt's reputation for healthfulness) will say something like "contains active [or living] yogurt cultures" and will probably bear the National Yogurt Association's Live & Active Cultures seal.

But don't be fooled by a yogurt label that says "made with active cultures." Of course they were active originally, or they wouldn't have transformed the milk into yogurt. The question is whether they're still alive when you take it home and eat it. Some people believe that the live bacteria somehow make them healthier. But the jury is still out on that one.

It is possible, however, that people who are mildly lactose-intolerant and have trouble digesting dairy products may be able to tolerate yogurt because the bacteria have already gobbled up most of the lactose. If the bacteria are ingested still alive, they may be able to survive our digestive processes and continue their lactose scavenging in our digestive tracts. But that jury, too, is still deliberating.

Labelingo: Perspicacious readers Brent Auble of McLean and Lori Fraind of Sterling have independently noticed that the cover of the tin of Altoids Citrus Sours boasts that it contains "Natural Flavor with other Natural Flavor." The list of ingredients, however, lists "natural flavor" only once. One wonders which one it means.

Seriously: The first "natural flavor" means that it came from a citrus fruit, rather than a chemical laboratory. The second "natural flavor" refers to the Food and Drug Administration's broader definition: a flavoring substance derived from any edible plant or animal material, regardless of by what convoluted route. Altoids, apparently, contain both kinds of natural flavors.

(Have you noticed any silly things on food labels? Send your Labelingo contributions, along with your name and town, to Food 101, Food Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or to the e-mail address below.)

Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Cook: Kitchen Science Explained" (W.W. Norton, $25.95). He can be reached at wolke@pitt.edu.