What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (20 page)

I’ve been reading and hearing a lot of buzz lately about how easily most vegetable oils become rancid and the possible harm that can cause. From what I understand, oils become rancid, or oxidized, when exposed to heat or oxygen or light, creating free radicals that wreak havoc in the body. How can I avoid oxidized oils?

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R
ancid
is a sort of catchall word that comes from the Latin
rancidus
, meaning rank or stinky. The word has no explicit chemical meaning. It is used loosely to mean bad-smelling and -tasting, most often in reference to over-the-hill fats. There are several things you can do to avoid fat rancidity. But first, let’s sort out the concepts in your question, because oxidation is only one of the ways in which fats can spoil.

Fats and oils can become rancid either by reacting with oxygen (
oxidative rancidity
) or by reacting with water (
hydrolytic rancidity
). Also, when oils are heated to high temperatures, as in deep-frying, they can change chemically in other wicked ways.

Here, then, is a highly condensed version of “Fats Gone Wild.”

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Oxidative rancidity:
This happens primarily to fats containing unsaturated fatty acids. Helped along by heat, light, trace metals, or certain enzymes, unsaturated fatty acids react with oxygen in the air, producing highly reactive peroxides and free radicals that, as you say, can wreak havoc in our bodies. (See “Those home-wrecking radicals,” p. 175.)

Whether the free radicals in rancid oils persist long enough to be harmful when the oil is ingested is debatable. But many of the stable end-products of the free-radical-driven reactions are bad-smelling chemicals called aldehydes and ketones, so rancid oils are unpleasant to ingest at the very least.

Many vegetable oils contain natural antioxidants—free-radical killers—called
tocopherols
; if they didn’t, they wouldn’t stay fresh as long as they do. Animal fats, on the other hand, which contain mostly saturated fatty acids, are not as susceptible to oxidative rancidity. That’s why lard, for example, can be kept nearly forever without going rancid.

To foil oxidative rancidity, then, you should protect your vegetable oils from the oxygen, heat, and light that spark the free-radical reactions. Keep the bottle tightly sealed in a cool, dark place. Even refrigerating your oils isn’t a bad idea if you don’t use them very often. Because every oil is a mixture of fats with different freezing (solidification) temperatures, some of them may freeze in the fridge, but they will liquefy again on warming.

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Hydrolytic rancidity:
This is caused by
hydrolysis
: the reaction of a fat with water, whether the fat is saturated or unsaturated. The reaction is helped along by heat or by enzymes called lipases.

When water reacts with a fat, the fat molecule splits into its glycerol portion and its fatty acid portions. Free fatty acids generally smell pretty bad, especially the lightweight (small-molecule) ones that can float off easily into the air toward our noses. The predominant bad-smelling, small-molecule fatty acid in rancid butter is butyric acid, whose other common habitat is unwashed armpits.

Butter is ideal prey for both oxidative and hydrolytic rancidity: oxidative because 32 percent of its fatty acids are unsaturated, and hydrolytic because it contains about 18 percent water, distributed in the form of microscopic droplets throughout the matrix of fat. Defensive tactics against rancidity in butter must therefore include protection from both oxygen and heat, inasmuch as heat accelerates both types of rancidity. These considerations dictate both airtight wrapping and refrigeration.

•
 Rancidity from deep-frying:
When a piece of food is plunged into very hot fat, the water on its surface—and all foods contain water—can react with the fat to hydrolyze it, freeing its fatty acids. As the fat is used and perhaps reused, the free fatty acids build up, negatively affecting its flavor and that of anything fried in it.

But that isn’t the worst of it. When the fatty acids are split off from a fat molecule, what remains is the glycerol portion of the molecule. Upon further heating, the glycerol decomposes into a highly irritating, acrid gas aptly named
acrolein
. At about the same time, the fatty acids decompose, producing smoke. The longer the fat is heated, the more free fatty acids it will contain and the more likely it will be to smoke at progressively lower temperatures. Another thing that happens to long-used deep-frying oil is polymerization: the free fatty acids combine to form large molecules that darken and thicken the oil until it is almost syrupy.

The moral of the frying story is that to minimize the production of foul-tasting fatty acids, lung-clogging smoke, eye-watering acrolein, and other possibly carcinogenic compounds, use your deep-frying oil only once or at most twice. I dispose of mine by pouring it into an empty food can, freezing it, and discarding it with my solid garbage.

Other than the difference in calories, how does light olive oil differ from “regular”?

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W
hoa there! Light olive oil does not contain fewer calories than other olive oils or, for that matter, any edible oils. As far as calories are concerned, an oil is an oil, all oils are fats, and all fats give us approximately 9 calories of energy per gram.

The word
light
(or the nonword
lite
) is thrown around by food manufacturers to mean anything they want it to mean, including virtually nothing. In the case of olive oil, however, all you have to do is look at the bottle and you know that in this case it means light in color, and almost certainly in flavor.

We use vegetable oils in the kitchen mainly for sautéing, frying, and dressing salads. These functions are primarily physical rather than chemical in nature. In frying, for example, oil acts as an inert liquid that allows us to cook foods very quickly at a temperature much higher than that of boiling water. On salads, oil helps the dressing to stick; it “cuts” the acid; it carries other flavors, such as those of garlic and herbs; and it imparts unctuousness—a smooth, flowing mouth feel. These qualities are all largely physical, not chemical, and any relatively flavorless oil such as canola or corn oil can do those jobs.

Perhaps without realizing it, then, we may be tempted to think of kitchen oils in purely mechanical rather than flavor-related terms. As a result, many Americans prefer oils that are relatively tasteless—in fact, the blander the better—and colorless (or at most pale yellow). The olive oil producers oblige those customers by making a decolorized and deodorized version. They decolorize the oil, which is often not of top quality to begin with, by adsorbing the colored substances onto a fine clay, and they deodorize it (odor being a big part of flavor) by treating with high-pressure steam, in much the same way in which seed oils are refined. (See “That’s oil, folks,” p. 159.)

But that’s really a shame, because natural olive oils, ranging in color from yellow to gray-green, possess a remarkable diversity of rich flavors and aromas that cooks prize for various culinary uses. Unlike most other kitchen oils, olive oil contributes its own flavor to whatever is cooked in it or dressed with it. It is a flavor ingredient in itself, not just a medium for carrying other flavors. That’s why most Mediterranean cooking, which uses olive oil almost exclusively, is so flavorful. (The garlic also helps.)

The flavors of olive oils, like those of wines, vary with the country of origin, the variety of fruit (there are some fifty different species in general cultivation), the local soil and climate, how the groves are cultivated, when the olives are harvested, and how they are processed. The predominant flavor notes in olive oils include fruity, green, fatty, grassy, sweet, bitter, and astringent.

The chemistry of olive oils can be discussed from at least two standpoints: what are they made of in general, and what are their flavor and aroma compounds in particular? I won’t burden you with the polysyllabic names or formulas of all these chemicals, but I do want to point out a few of the ones that are of particular relevance to the qualities we value most highly in olive oils.

In view of the wide variation in olive oils, no exact analyses of their fatty acids can be stated, despite the supposedly exact figures quoted in many food publications. The oleic acid content, for example, can range anywhere from 55 to 83 percent. Nevertheless, some average values, not to be taken too literally, are shown in Table 3. Not listed are about a dozen other fatty acids that are present in minor amounts.

When olives are crushed to express their oil, they release enzymes (
lipoxygenases
) that oxidize some of the polyunsaturated fatty acids to produce a wide variety of volatile aromatic compounds, including aldehydes, esters
,
and alcohols. More than one hundred volatile compounds have been identified in the olives’ aromas alone, and chemists know the details of how most of them are formed.

The acidities of olive oils—that is, the percentages of free oleic acid molecules broken off from fat molecules—have been much discussed. Acidity is easy to measure, and quality-control examiners have long used the degree of acidity, along with several other characteristics, as a gauge of quality. Supposedly, the more free acid, the harsher the flavor and the lower the quality. In the fall of 2003, the European Union reduced the maximum permitted acidity of EU-produced extra-virgin olive oils from 1.0 to 0.8 percent. Nevertheless, it has recently been shown that acidity and flavor quality are not necessarily directly related to each other.

According to regulations of the EU and the International Olive Oil Council, here are the characteristics of the several grades of olive oil, in order of decreasing quality.

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Extra-virgin olive oil
is virgin olive oil (see following grade) that meets strict composition and flavor characteristics. It’s the top o’ the heap. Obtained from perfect olives that have been crushed as soon as possible after harvest (their flavor deteriorates quickly) and processed without the use of heat or steam, extra-virgin oil exhibits the ideal flavor and aroma of its variety. It must contain less than 0.8 percent of free fatty acids. Extra-virgin oils are sometimes called “cold-pressed,” but that term is being phased out as pointless; olive oil presses don’t need to be cooled and are rarely, if ever, heated.

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Virgin olive oil
must contain 100 percent olive oil from olives of one or more varieties. It must be obtained only by pressing, washing, decanting, centrifuging, and filtering, or certain other processes that do not alter its natural state. No additives, colorants, flavorings, or any other foreign matter may be added.

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Pure olive oil or 100% olive oil
is virgin olive oil that has been blended with
refined
olive oil: oil that has been further processed with steam to remove off flavors and acids. But it still contains nothing but olive-derived ingredients.

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Light or extra-light olive oil
is typically a blend of virgin and highly refined oils, most of whose colors, off flavors, and (for that matter) “on flavors” have been removed.

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Pomace oil:
The oft-quoted statement that extra-virgin oils come from the “first pressing” of the olives is baloney. Olives are pressed only once. But more oil may later be extracted from the pressed pulp, skins, and stones (the pomace), which still contains some 4 to 10 percent oil. It can be extracted by a combination of pressure, heat, and chemical solvents, yielding what is called pomace oil, the bottom of the barrel in quality. You won’t even find it in most grocery stores.