In principle, and in many countries of the world, yogurt is very simple. It is made by adding certain strains of beneficial living bacteria to the milk from a cow, goat, or sheep. 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. But in the modern, mechanized American food industry, nothing is ever that simple.
Whole milk with all of its fat intact will produce a thick, rich yogurt. But there is a large consumer demand for low-fat yogurt. So to avoid a thin, runny product, manufacturers may add a thickener or stabilizer: milk solids, pectin (a water-soluble carbohydrate obtained mostly from fruits) or a small amount of gelatin.
Yogurt has been made for centuries in Eastern Europe and the Middle East, but only relatively recently has it taken hold in the United States, where it has a flaunted though unproven reputation for keeping us healthy, slim, and fit. Commercial yogurt manufacturers fight the calorie wars by making their products from low-fat or nonfat milk so that they can plaster those consumer-pleasing words across their labels. But because many Americans don’t like the taste of plain yogurt, most of our yogurt products are doctored with sugar or fruit preserves, and up goes the calorie count anyway.
Does eating yogurt help you lose weight? Sure, if you eat it instead of stevedore-sized lunches and afternoon candy-bar snacks. But forget the come-hither adjectives on the containers and read the government-mandated Nutrition Facts chart; it gives the actual number of calories per serving.
Among the thousands of species of bacteria and molds there are bad guys and good guys, just as there were in the old westerns. The bad guys can make us sick, but we welcome the good guys and use them to produce a galaxy of wonderful foods, from yogurt to hundreds of cheeses, beers, and wines.
The first step in making yogurt is to kill off any pathogenic bacteria (the ones wearing very, very tiny black hats) that may be lurking in the milk. Ordinary pasteurization, which consists of heating the milk either to 161°F ( 72°C) for 15 seconds or to 145°F (63°C) for 30 minutes, would do the job, but yogurt makers generally use higher temperatures: 203°F (95°C ) for 10 minutes or 185°F (85°C) for 3o minutes. The higher temperatures help to thicken the product’s texture by coagulating some of the milk proteins. The pasteurized milk is then cooled to 109°F (43°C), a nice, comfortable temperature at which the good guys can flourish.
The white-hat bacteria used in making yogurt are Lactobacillus bulgaricus (LB) and Streptococcus thermophilus (ST), mixed in equal amounts. (Other bacteria, such as L actobacillus acidophilus, may be added as well.) The LB and ST have a unique symbiotic relationship. While they dine together on the milk’s lactose, the LB also breaks down proteins into amino acids (the building blocks of proteins) and peptides (two or more amino acids bound together) that the ST can eat. In turn, the ST produces carbon dioxide gas that stimulates the growth of the LB.
Among the main flavor chemicals these bacteria produce are lactic acid, acetic acid (the vinegar acid), and acetaldehyde, a tart, walnutor green-apple-flavored compound also created 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.
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 moguls kill them off with heat. In that case, the label will probably say “Heat-treated after culturing.”
Some people believe that eating the live bacteria somehow makes them healthier, but there is no convincing scientific evidence of that. Nevertheless, if you prefer yogurt whose bacteria are still alive and kicking, look for something like “Contains active (or living) yogurt cultures” on the label. Better yet, look for the National Yogurt Association’s LAC (“Live and Active Cultures”) seal. That means that when manufactured, the product contained at least i0 million bacteria per gram, or over 2 billion in an 8-ounce cup. A sobering thought.
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 the yogurt home and eat it.
It is possible 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 supposition hasn’t been sufficiently investigated.
In the conversion of milk into yogurt, the bacteria-produced acids act on the milk’s protein, which is mostly casein, to make its tiny, widely dispersed globules (its micelles) come together into a solid mass. This happens when the bacteria have acted long enough to produce a certain level of acidity. For casein, that’s a pH of 4.6, its so-called isoelectric point, at which the micelles (pronounced MY-cells) lose their mutually repulsive electric charges and can stick together. What one observes when that acidity level 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.
