What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (8 page)

The label on a container of plain yogurt lists pectin among the ingredients. If they add pectin to thicken it, isn’t it more like a jelly than a yogurt?

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I
n 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 30 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
Lactobacillus 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, walnut- or 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 10 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.

I brought home a quart of Dairy Queen soft-serve ice cream and put it in my freezer. To my surprise, the next day it was just as hard as any ordinary ice cream. I thought the soft serves, like Dairy Queen and frozen yogurt, were special formulas that always had that lovely, voluptuous mouth feel that I like so much. What happened?

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A
(chocolate) Dairy Queen aficionado myself, I tried your experiment twice. Each time, as soon as I got the quart container into the car I measured its temperature by plunging a so-called instant-read thermometer into the middle and waiting for a couple of dozen “instants” until it reached its final reading. On the two occasions it measured 14 and 16°F (
2
10 and
2
9°C). Then, after a couple of days in my freezer, somewhat diminished in quantity by after-dinner “scientific tests,” each quart measured 0°F (
2
18°C). My soft-serve, like yours, had become just as hard as ice cream.

Thus, Dairy Queen and the other soft-serves are nothing but ice cream at a warmer temperature. We love them not only because of their softer, smoother textures but because our palates are more sensitive to flavors at warmer temperatures.

The American Dairy Queen Corp. lists the ingredients of its vanilla product as milk fat and nonfat milk, sugar, corn syrup, whey, mono- and diglycerides, artificial flavor, guar gum, polysorbate-80, carrageenan (the last three are thickeners), and Vitamin A palmitate. Milk fat, the main ingredient, is of course butterfat, and butter is harder at lower temperatures because more of its fat is crystallized. (See p. 83.) The mono- and diglycerides behave similarly to the butterfat’s whole fats (
triglycerides
), while the guar gum and carrageenan thickeners also tend to tighten up at lower temperatures. So it’s no wonder that the Dairy Queen is soft when you buy it but hardens in the freezer.

At Dairy Queen, TCBY, Carvel, and many other franchised and independent stores, rivers of various soft ice cream products, including nonfat ice cream, low-fat ice milk (which perversely contains less milk than ice cream), frozen yogurt, and frozen custard, flow like lava from smug-looking machines that guzzle batches of packaged mixtures dumped into their maws. The machines mix and chill them, adjust their temperature and viscosity, and dispense them in a variety of flavors, even swirling two flavors together for irresolute customers who can’t decide between chocolate and vanilla. (Can there be any question?)

Dairy Queen’s product is a reduced-fat ice cream containing 5 percent fat. TCBY sells both nonfat and 4-percent-fat (billed as “96 percent fat-free”) frozen yogurt. Frozen custard is the smoothest and creamiest of all, and as a consequence the fattiest. (There’s no free lunch.) Typically it contains 10 percent butterfat and a minimum of 1.4 percent egg yolk solids.

Moving up the fat ladder, we come to “ordinary ice cream.” Federal regulations require that ice creams without solid additives such as nuts or candy bits contain at least 10 percent butterfat. The leading fake-Swedish-named brand contains 16 percent butterfat.

Does eating ice cream in hot weather cool you off?

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P
eople do seem to think so. At my local Dairy Queen there is always a long line of people seeking cool after-dinner desserts in the summer, but starting the day after Labor Day, the place is virtually “desserted.”

The answer to your question is in fact, no. After all, we are warm-blooded creatures with thermostats set at 98.6°F (37°C), and eating something cold cannot change that. Our cooling mechanism is purely a surface phenomenon: the evaporation of perspiration from our skins, assisted, when we’re lucky, by a breeze that hastens the process. Putting ice cream into one’s mouth serves only to cool the mouth. You’d do much better by smearing the ice cream all over your body.

According to my calculations, melting a one-inch, 0°F (218°C) ice cube in the mouth would absorb only 1.3 calories of heat. If distributed over the entire body, that amount of heat loss would lower the temperature of a 150-pound person by 0.007°F (0.004°C).

Dogs, however, do cool themselves through the mouth by sticking out their long, wet tongues and panting to evaporate the saliva. You might want to try that; it should be more effective than eating an ice cream cone.

I left about half a cup of unfinished ice cream out of the refrigerator overnight and it melted down into only about a quarter of a cup. Why did it shrink so much?