What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (9 page)

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I
t was half air.

Federal regulations specify that the amount of “overrun” in ice cream must be less than 100 percent. Overrun is the increase in volume between the product’s ingredients and the final, air-whipped product. A doubling of volume, known as 100 percent overrun, means that the product is 50 percent air.

Oh, you didn’t know that your ice cream may legally contain as much as 50 percent air? Yep. The amount varies widely among ice creams, but the practice is not considered cheating because air adds smoothness. Without beaten-in air, a brick of ice cream would be almost as hard as a frozen stick of butter. On the other hand, an ice cream containing much more than 50 percent air would strike you as watery and inferior.

Store-bought ice cream, whether soft-serve or regular, must legally weigh no less than 4.5 pounds per gallon, or 18 ounces per quart. Try weighing a quart of your favorite kind. If it weighs about 27 ounces, it contains roughly a 50 percent overrun, or 25 percent air. If it weighs less than 18 ounces, call the cops.

Just for fun, I counted almost two dozen kinds of cheese in the cheese section of my “gourmet” market, and I’d guess there are hundreds more in the world. But two old standbys from my childhood seem to be different from all the others: cream cheese and cottage cheese. When I was a kid I didn’t know anything else. What makes them so different? Are they purely American?

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N
ot purely American, but primarily so.

What cream cheese and cottage cheese have in common, along with French Neufchâtel and a few others, is that they are not aged or ripened. The milk or milk-and-cream mixture is curdled by an acid (usually lactic acid), and the curds are ready to eat as soon as they are separated from the whey.

Variations of cottage cheese, probably the simplest of all cheeses, are made throughout the world, presumably in cottages, although no one seems to know the origin of the name. It has been known as pot cheese, farmer cheese, bonnyclabber (in Ireland), and
Schmierkase
(“spreading cheese” in Pennsylvania Dutch), with several variations in spelling.

In the United States, where cottage cheese is most popular and where it was first manufactured commercially early in the twentieth century, it is made by adding a culture of
Streptococcus lactis
to pasteurized skim or low-fat milk. These bacteria feed on the milk sugar and produce the coagulating lactic acid. Usually, another bacterial culture,
Leuconostoc citrovorum
, which produces flavorful compounds but no acid, is also added. After a fermentation period of several hours, the curds are cooked and some of the water is drained off, leaving loose, crumbly clumps of curd. That’s cottage cheese. If even more water is removed to make a drier product, the product may be called pot cheese. Press the curds into a cake or loaf and it’s called farmer’s cheese.

Because cottage cheese is quite moist (up to 80 percent water), it is very perishable. As a good growth medium for any pathogenic bacteria that may alight upon it, it must be kept refrigerated.

One cup (226 grams) of 2-percent-fat cottage cheese contains 203 calories, while a cup of 1-percent-fat cottage cheese contains 163 calories. Both kinds have a protein content of 28 grams, or 12.4 percent. That’s why cottage cheese has a reputation as a diet food: It’s high in protein, low in fat and carbohydrate.

Cream cheese is quite a different story. It is indeed an American invention, as you might guess from the fact that there seems to be only one brand, named for the city of Philadelphia, Pennsylvania. According to Kraft Foods, which owns the brand, cream cheese originated in 1872 at a dairy in Chester, New York. In 1880 it was branded “Philadelphia” by a New York distributor because at the time Philadelphia had a reputation for high-quality food products. (Pittsburgh cream cheese apparently didn’t make the cut.)

Today’s cream cheese has a minimum fat content of 33 percent and a maximum moisture content of 55 percent. The Philadelphia brand is 34.9 percent fat and contains 810 calories per cup. Its unique creamy-gummy consistency doesn’t happen automatically, however. It is produced by any of several additives, including algin, a thickener derived from seaweed; locust bean gum from the seeds of the carob tree; gum tragacanth, obtained from various Asian and Eastern European plants; and guar gum, derived from the seed of a leguminous shrub. Smoothness has its price.

What does the “%” sign signify on the label of a Brie cheese? I’ve frequently seen cheese for sale marked, for example, “Brie—60%.”

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I
t is the percentage of fat in the cheese, but expressed on what a chemist would call a
dry weight basis
—the percentage of fat in what would remain after all the moisture were removed.

In your example, the famous and ancient (eighth-century) French soft cheese called Brie, after a region east of Paris, can be made from milk and cream mixtures of various butterfat contents, resulting in cheeses containing various percentages of fat. The fat content of most cheeses is detailed on the packaging as a percentage of butterfat in the dry matter of the cheese.

Cheeses contain differing amounts of moisture, even in different batches of the same cheese. So if we want to express the percentage of fat in a cheese—the number of grams of fat per 100 grams of cheese—what should we use as the 100 grams of cheese, moist cheese or dried cheese? Obviously, the result will be more accurate and meaningful if we eliminate the varying amounts of moisture and report the percentage of fat as a percentage of the dry material.

Thus, a sample of the cheese is heated in a laboratory oven to remove all moisture, leaving mostly dried protein and fat. Then the amount of fat is measured and expressed as a percentage of the dry matter. That number will be larger than the percentage of fat in the whole (un-dried) cheese.

For example, let’s say that a whole, moist Brie is 20 percent water. A hundred grams of it would dry down to 80 grams of dry matter. If that 80 grams of dry matter were then found to contain 40 grams of fat, the cheese would be labeled “50% fat” (40/80). But that’s 40 grams of fat in what was originally 100 grams of whole, un-dried cheese. So the whole, moist cheese is actually only 40 percent fat (40/100).

The fat-percent labeling isn’t a dodge to make the cheese seem to have more butterfat than it does. Because the water content of all foods can vary so much from sample to sample, food scientists are in the habit of drying their samples first to eliminate the water and then expressing their composition on a dry weight basis. That’s standard practice in expressing the compositions of many other materials that can have varying moisture content.

I am an apprentice cheese maker. When making Camembert we add
Penicillium candidum
(or some other form of penicillin) to the milk or spray it on the outside. My mother is allergic to penicillin but has never had a reaction to my Camembert. Why? Also, U.S. regulations require that the milk in cheese be pasteurized or that the cheese be aged for at least sixty days. I know pasteurization gets rid of bad bacteria such as listeriosis or brucellosis (which my grandmother called undulant fever), but how does aging get rid of them?

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F
irst, we have to straighten out some terminology. You’re confusing the bacteria with the diseases they cause, the drug with the mold, and the mold with the allergen—the substance that triggers allergic reactions in some people. Here’s the straight scoop:

• 
Penicillin
(not penicillium) is the name of the drug.

• 
Penicillium
(not penicillin) is the genus of the mold that produces the drug.

• 
Listeriosis
and
brucellosis
are diseases caused by bacteria, not the names of the bacteria themselves.

The drug.
The oft-told story of the “wonder drug” penicillin goes back to 1928, when the Scottish physician-bacteriologist Alexander Fleming took a vacation from his work at St. Mary’s Hospital in London. He returned weeks later to find that some spores of the mold
Penicillium notatum
had drifted into his laboratory and settled on one of his cultures of the pathogenic
Staphylococcus aureus
bacteria. (Fleming reportedly ran a rather sloppy lab and habitually left uncovered culture dishes out in the open.)

He noticed that the bacteria refused to grow near where the mold colony was growing, and surmised that the
Penicillium
mold was releasing some kind of antibacterial substance. He named the substance “penicillin” and won a Nobel Prize for it in 1945. (Advice to aspiring Nobel laureates: Keep a sloppy lab and take long vacations.)

Today, penicillin is produced on a large scale by “farming” the mold spores of
Penicillium chrysogenum
, a more prolific penicillin producer than
P. notatum
, in steel tanks, feeding them on “corn steep,” a carbohydrate- and nitrogen-rich waste product of the wet-grinding of corn in making cornstarch.

It’s important to understand what your mother is and is not allergic to. She is allergic to the chemical penicillin itself (formula R-C
9
H
11
N
2
O
4
S, where R represents one of several atomic groupings), not the
P. chrysogenum
mold. The
Penicillium
molds used in cheese making do not generate penicillin, so they pose no problem for anyone who is allergic to the drug.

The molds.
Molds are fungi that grow on moist, warm organic matter. As mycophiles (mushroom lovers) well know, there are good guys and bad guys among the fungi. Even some of the
Penicillium
species produce toxins that may make a food inedible or dangerous. For example, the bluish-green mold that makes your over-the-hill foods look like Chia Pets is a
Penicillium
. But penicillin it is not. So throw away all moldy food, along with any nearby food that may have been exposed to its airborne spores. Don’t run your kitchen like Fleming ran his lab.