What Einstein Told His Cook (9 page)

There were a few interesting sidelights to this experiment that I’ll record for you science buffs. (The rest of you may go on to the next question.)

First, it turned out that the conductivities of the salt waters after simmering with potato were slightly higher than—not lower than—those of the untreated waters. So potatoes alone must contribute some electrical conductivity to the water in which they are boiled. That took me by surprise, because on first blush one would think that only starch comes out of the potatoes into the water, and starch doesn’t conduct electricity. But potatoes contain a lot of potassium, about two-tenths of a percent in fact, and potassium compounds do conduct electricity, just as sodium compounds do. At any rate, I corrected for that effect by subtracting the potato’s conductivity contribution from the conductivities of the potato-simmered salt waters.

Second, if, in spite of the tight cover and gentle simmering, any substantial amount of water had been lost from the pots by evaporation while cooking the potatoes, the conductivity of the water would have gone up, not down, and no such effect was found after correcting for the conductivity provided by the potato itself.

I think it’s an airtight case, don’t you?

HOLD THE SALT

Why does a recipe tell me to use unsalted butter, and then later to add salt?

I
t sounds silly, but there’s a reason.

A quarter-pound stick of typical salted butter may contain 1½ to 3 grams, or up to half a teaspoon, of salt. Different brands and regional products may contain very different amounts. When you’re following a carefully formulated recipe, especially one that uses a lot of butter, you can’t afford to play Russian roulette with something as important as salt. That’s why serious, high-quality recipes will specify unsalted or “sweet” butter and leave the salt for a separate seasoning step.

Many chefs prefer unsalted butter also because it is often of higher quality. Salt is added partially for its preservative effect, and butter that is used promptly, as in a restaurant kitchen, doesn’t need it. Also, in unsalted butter any “off”-flavors, such as incipient rancidity, are more readily detected.

T
HE THREE MAJOR COMPONENTS
of our foods are proteins, carbohydrates, and fats. But judging by the flood of ink spent on fats in newspapers, magazines, and official dietary guidelines these days, one might think that fat is the only one we need be concerned about—not about eating enough of this essential nutrient, but about eating too much and/or the wrong kinds.

There are two major concerns: the caloric content of all fats, which is about nine calories per gram, compared with only four calories per gram of either protein or carbohydrate; and the unhealthful effects of eating certain kinds of fat.

I am not a nutritionist and am therefore not qualified to address the health aspects of various fats—not that even the experts can agree on many issues. Instead, I will focus on what fats are and how we use them. Understanding these basics should enable you to interpret and evaluate that flood of ink more intelligently.

ON FATS AND ACIDS

I
have read this kind of inaccurate writing probably far longer than you have. In fact, as a chemist I cannot help but harbor the suspicion that many writers just don’t know the difference. And there is indeed a difference.

Every molecule of fat incorporates three molecules of fatty acids. The fatty acids may be either saturated or unsaturated, and they thereby impart those qualities to the fat as a whole.

First, let’s see what a fatty acid is.

Fatty acids are the acids that are found as components of fats. They are members of a larger family that chemists call carboxylic acids. As acids go, they are very weak—unlike sulfuric acid, for example, which is the highly corrosive battery acid in your car.

A fatty acid molecule consists of a long chain of as many as sixteen or eighteen (or more) carbon atoms, each one of which carries a pair of hydrogen atoms. (Techspeak: The chain is made up of CH
₂
groups.) If the chain contains its full complement of hydrogen atoms, the fatty acid is said to be saturated (with hydrogen). But if somewhere along the chain one pair of hydrogen atoms is missing, the fatty acid is said to be monounsaturated. If two or more pairs of hydrogen atoms are missing, it is said to be polyunsaturated. (Actually, one hydrogen atom is missing from each of two adjacent carbon atoms, but let’s not quibble.)

Some common fatty acids are stearic acid (saturated), oleic acid (monounsaturated), and linoleic and linolenic acids (polyunsaturated).

To chemists, and apparently to our bodies as well, the exact positions of the unsaturated parts of the fatty acid molecules (Techspeak: the double bonds) matter. You’ve heard that the “omega-3” fatty acids found in fatty fish may play a role in preventing coronary heart disease and strokes? Well, “omega-3” is the chemist’s way of telling exactly how far the first missing pair of hydrogen atoms (the first double bond) is from the end of the polyunsaturated molecule: it is three places from the end. (Omega is the last letter—the end—of the Greek alphabet.)

Representation of a fat (triglyceride) molecule, showing three fatty acid chains attached to a glycerol molecule at left. (Hydrogen atoms are not shown.) The top two fatty acid chains are saturated; the bottom one is monounsaturated—that is, it contains one double bond.

Fatty acids are generally bad-tasting and foul-smelling chemicals. Fortunately, they don’t usually exist in foods in their free, yucky forms. They are tamed by being chemically fastened to a chemical called glycerol, in the ratio of three fatty acid molecules to each glycerol molecule.
Three fatty acid molecules tied to a glycerol molecule constitute one molecule of fat.
Chemists draw the fat molecule’s structure schematically on paper as a short flagpole (the glycerol molecule) with three long banners (the fatty acids) flying from it. They call the resulting molecule a triglyceride (
tri-
indicating that it contains
three
fatty acids), but its common name is simply a “fat” because by far the majority of natural fat molecules are triglycerides.

The fatty acids (I’ll call them FA’s) in any given fat molecule can be all of the same kind or any combination of different kinds. For example, they might be two saturated FA’s plus one polyunsaturated FA, or they might be one monounsaturated FA plus one polyunsaturated FA plus one saturated FA, or all three might be polyunsaturated FA’s.

Any real-life animal or vegetable fat is a mixture of many different fat molecules containing various combinations of FA’s. In general, shorter-chain and less saturated FA’s make softer fats, while longer-chain and more saturated FA’s make harder fats. That’s because in an unsaturated FA, wherever a pair of hydrogen atoms is missing (Techspeak: wherever there is a double bond), the FA molecule has a kink in it. As a result, the fat molecules can’t pack together as tightly to make a hard, solid structure, and the fat is likely to be more liquid than solid. Therefore, predominantly saturated animal fats tend to be solids, while predominantly unsaturated vegetable fats tend to be liquids. When you read that a certain olive oil, for example, is 70 percent monounsaturated, 15 percent saturated and 15 percent polyunsaturated, it means that those are the percentages of the three kinds of FA’s, added up over all the different fat molecules in the oil. We don’t care how the FA’s are distributed among the fat molecules, because
it is only the relative amounts of the three kinds of FA’s, added up over the whole mixture of fat molecules, that determine the healthful or unhealthful qualities
. The glycerol portions of all the fat molecules aren’t nutritionally important and just go along for the ride. The so-called essential fatty acids are those FA’s that the body needs in order to manufacture the important hormones called prostaglandins.

While we’re talking about fatty acids and triglycerides, let’s straighten out some other fat-related terms you may have heard.

Monoglycerides and diglycerides are like triglycerides but, as you may guess, have only one (mono-) or two (di-) FA molecules attached to the glycerol molecule. They exist in very minor amounts along with the triglycerides in all natural fats, and their FA’s are incorporated into the saturation/unsaturation profiles of the fats. They are also used as emulsifiers (substances that help oil and water to mix) in many prepared foods. But are they considered fats themselves? Sort of. Triglycerides are broken down into mono-and diglycerides during digestion, so their nutritional effects are essentially the same.

Finally, there is the word
lipid
, from the Greek
lipos
, meaning fat. But we use the word much more broadly than that. Lipid is a catchall term for anything and everything in living things that’s oily, fatty, or oil-loving, including not only mono-, di-and triglycerides but such other chemicals as phosphatides, sterols, and fat-soluble vitamins. When your blood chemistry report comes back from the medical lab it may contain a
lipid panel
, listing not only the amount of triglycerides (fat blood isn’t good) but also the amounts of the various forms of cholesterol, which is a fatty alcohol.

WHAT CAN BE DONE
to minimize the confusion between “fats” and “fatty acids” in food writing?

First of all, we have to recognize that although the word
fat
strictly means a specific kind of chemical—a triglyceride, as distinguished from a protein or a carbohydrate—in common usage the word
fat
is used to refer to mixtures of fats, such as butter, lard, peanut oil, and so on. (Each of these products is referred to as “a fat” in the diet.) There is little a reader can do about that ambiguity, except to try to determine whether the word is being used in the context of a specific chemical substance or a category of food.

Second, we can implore food writers to be more careful about switching indiscriminately back and forth between “fat” and “fatty acid.” Here are some suggestions:

The relative saturation and unsaturation of a fatty food can be expressed without using either term. For example, we can just say that it is
x
percent saturated,
y
percent monounsaturated, and
z
percent polyunsaturated, without adding the object (fatty acid) that these adjectives in truth modify.

Instead of saying, as I have seen many times, “a saturated (or unsaturated) fat,” which is meaningless, we should say “a fat high in saturates (or high in unsaturates)” or “a highly saturated (or highly unsaturated) fat.” Those are shorthand ways of saying “high in saturated (or unsaturated) fatty acids.”

In general, the less often the term fatty acid is used the better, because people already understand the term
fat
(or think they do), and that word is less intimidating. But if individual fatty acids must be discussed, the term should be defined the first time it is used as something like “the building blocks of fats.”

WHEN GOOD FATS GO BAD

What makes fats turn rancid?

F
ree fatty acids. That is, fatty acid molecules that have been broken off from their fat molecules. Most fatty acids are foul-smelling and bad-tasting chemicals, and it doesn’t take much of them to give a fatty food an off flavor.

There are two main ways in which the fatty acids can become disconnected: the fat’s reaction with water (hydrolysis) and its reaction with oxygen (oxidation).

You might think that fats and oils won’t react with water because they are so loathe to mix. But given time, enzymes that are naturally present in many fatty foods can make the reaction happen. (Tech-speak: They catalyze the hydrolysis.) So foods like butter and nuts can turn rancid by hydrolysis simply by being stored for a long time. Butter is particularly vulnerable because it contains short-chain fatty acids, and these smaller molecules can fly off into the air more easily (Techspeak: They are more volatile) and produce a bad smell. In rancid butter, butyric acid is the main culprit.

High temperatures also speed up the rancidity of an oil by hydrolysis, such as when wet foods are deep-fried in it. That’s one reason why deep-frying oil begins to smell bad when overused.

The second major cause of rancidity, oxidation, happens most readily in fats containing unsaturated fatty acids, with polyunsaturates being oxidized more readily than monounsaturates. The oxidation is speeded up (catalyzed) by heat, light, and trace amounts of metals, which may be present from the machinery that processed the food. Preservatives such as ethylenediaminetetraacetic acid, mercifully nicknamed EDTA, prevent metal-catalyzed oxidation by imprisoning (sequestering) the metal atoms.

Moral: Because rancidity reactions are catalyzed by heat and light, cooking oils and other fatty foods should be kept in a cool, dark place. Now you know why the labels always tell you that.

ENOUGH IS ENOUGH

On food labels I often see “partially hydrogenated” vegetable oil. What is hydrogenation, and if it’s so good why don’t they go all the way with it?

O
ils are hydrogenated, that is, hydrogen atoms are forced into their molecules under pressure to make them more saturated, because saturated fats are thicker—more solid and less liquid—than unsaturated fats. The hydrogen atoms fill in hydrogen-poor gaps (Techspeak: double bonds, which are more rigid than single bonds) in the oil molecules, and that makes them more flexible. They can then pack together more closely and stick to each other more tightly, so they won’t flow as easily. Result: The fat becomes thicker, less liquid and more solid.

If the oils in your margarine hadn’t been partially hydrogenated, you’d be pouring it instead of spreading it. But partial hydrogenation may fill in only about 20 percent of the missing hydrogen atoms in the molecules. If your margarine were 100 percent hydrogenated, it would be like trying to spread candle wax on your toast.

Unfortunately, saturated fats are less healthful than unsaturated fats. Food manufacturers therefore walk a tightrope between minimum hydrogenation for health and enough hydrogenation to produce desirable textures.

FAT MATH

N
o, all fats fall into those three categories.

I had never noticed the funny arithmetic you mention, but as soon as I received your question I ran to my pantry and grabbed a box of Nabisco Wheat Thins. Here’s what I saw in the Nutrition Facts panel for the amounts of fat per serving: “Total Fat 6g. Saturated Fat 1g. Polyunsaturated Fat 0g. Monounsaturated Fat 2g.”