What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (18 page)

Why do the brown, bruised parts of fruits often taste sweeter than the other parts?

....

T
hink about this: If you smashed all the bottles of chemicals in a chemistry lab with a baseball bat, you wouldn’t be surprised at any unusual chemical reactions that occured as their spilled contents ran together on the floor, would you?

Well, plants are made up of remarkably packaged, exquisitely organized little “bottles of chemicals” called cells. When physical damage is done to a fruit, the cells are broken open and the chemicals that were previously isolated from one another in different parts of the cells spill out and mix.

When you bruise or cut into an apple, pear, or avocado, for example, the damaged flesh soon turns brown from the action of oxidizing enzymes called polyphenol oxidases, which are released from their captivity as soon as the cell walls are broken. These enzymes act upon the fruit’s phenols, a large group of antioxidant compounds responsible for flavor, color, and many other characteristics of our edible plants, sending them along a chemical path leading to a variety of large molecules (polymers), many of which are brown in color.

This so-called enzymatic browning—to distinguish it from both caramelization and Maillard browning (see pp. 296–97)—can be minimized by deactivating the enzymes with heat (in other words, cook those apples promptly) or with an acid. Lemon and lime juices are the most acidic substances in our kitchens, more acidic than vinegar.

Instead of destroying the oxidation-encouraging enzyme, we can cut off the oxygen to the cells, for example by covering the cut surface of the fruit with plastic wrap. Or we could treat it with any of a variety of chemical compounds that inhibit oxidation, such as sulfur dioxide, ascorbic acid (vitamin C), or citric acid in the form of (again) lemon juice.

In some fruits, the enzyme-driven browning reactions do produce sweet sugars. But in others, including apples, sour acids or bitter flavors are produced.

So don’t physically abuse your fruits in an effort to make them sweeter. Uninjured fruits always look and taste best.

Banana for banana, does a sweeter one have more calories than a dull-tasting one? As they ripen, they definitely do get sweeter, but can they produce more calories just by sitting around, or is that creating energy?

....

Y
ou answered your own question. Yes, that would be creating calories, and calories are energy. Energy can come only from other forms of energy (heat, mechanical, electrical, and so on) or from matter via
E
=
mc
2
. If a banana knew how to convert matter into energy, as uranium does, we could make atomic “B-bombs” out of them.

I can guess what you’re thinking, though. More sugar, more calories, right? But where is that sugar coming from? As the fruit ripens, starches are being broken down into sugars, and both starch and sugar—in fact, all digestible carbohydrates—give us the same 4 calories of energy per gram when we metabolize them. It’s a calorie-for-calorie wash. It doesn’t matter whether the sugar molecules are still tied together as starch molecules or free as a bird.

So you can’t run a power plant on ripening bananas unless you set fire to an awful lot of them—not an easy job because their flesh is 75 percent water. Even if dried first, they would burn to release only 400 calories per pound of original banana. Compare that with a pound of coal, which burns to release 3,000 calories, or a pound of uranium, which can release 21 million calories.

The only way bananas could solve an energy problem would be for a tired athlete to carbo-load by eating a whole bunch of them at 27 grams of carbohydrate per banana.

I bought some big, green bananas in a Latin American grocery store and put them on my kitchen windowsill to ripen. When they eventually turned yellow, I tried to eat one, but it was tough and tasted like chalk. What kind of bananas were they?

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T
hey weren’t bananas; they were plantains, tropical fruits closely related to bananas—both members of the genus
Musa
—but much starchier and containing much less sugar when ripe. They’re also known as cooking bananas, which is a clear tip-off that they’re not meant to be eaten raw.

Plantains are a staple in Africa and especially in Latin America, where they are known as
plátanos
in Spanish. In Puerto Rico, for example,
plátanos
are made into a variety of crunchy appetizers, including
tostones
(round slices of green plantain, flattened, fried, and garlic-salted) and
arañitas
or “little spiders” (fried, ragged pancakes of shredded plantain). Soft, ripe plantains, called
amarillos
(“yellows”) are baked with butter, brown sugar, and cinnamon in a Caribbean version of Bananas Foster. (See following recipe.)

And by the way, the old windowsill-ripening ploy is sill-y. It was originally intended as a dependably sunny spot, but picked fruits don’t need sunlight to ripen.

Can you comment on the chemical similarity between edible oils (olive, sunflower, etc.) and inedible oils used as lubricants? Is there some chemical quality that makes a substance an “oil”?

....

O
nly the fact that they are liquids whose molecules don’t stick together very strongly, so they can slide easily past one another. That’s what makes them slippery. But chemically speaking, these two kinds of oils are quite different. And thereby hangs an embarrassing tale.

A few decades ago I was teaching graduate-level chemistry in Spanish (what chutzpah!) at a university in Venezuela, utilizing the vestiges of my high school Spanish augmented by a few sojourns in Mexico and a six-month residence in Puerto Rico. One day in class I was puzzled by the ripple of smirks that swept the room whenever I referred to the product of Venezuela’s petroleum industry as
aceite
, which my dictionary had told me was Spanish for oil. A sympathetic student eventually took me aside and explained that
aceite
refers only to edible oils, most commonly the oil obtained from olives, or
aceitunas
. The word I should have been using was
petróleo
. I had inadvertantly been talking about Venezuelans pumping olive oil out of the ground! In the United States we use the same word,
oil
, for both. (I was partly vindicated years later by an understanding Spaniard, who pointed out that as soon as crude petroleum is refined into motor oil or machine oil, it is indeed referred to as
aceite
. At least in Spain.)

Petroleum is a mishmash of hundreds of hydrocarbons—compounds of nothing but carbon and hydrogen—which can be separated by distillation or broken down (“cracked”) and refined into hundreds of products from gasoline to Vaseline, not to mention the thousands of synthetic petrochemicals that chemists can create once they get their hands on the raw material.

Hydrocarbons play only minor roles in living things. In fact, it might be said that petroleum consists of once-living plant and animal matter that has had all the life squeezed out of it. Petroleum-derived oils are therefore inert as far as our food metabolism is concerned; they are indigestible. A dose of mineral oil, a highly purified petroleum product, for example, passes straight through the body unchanged, lubricating the entire digestive tract along the way and acting as a laxative.

Although the edible oils that we obtain from plants do contain small amounts of hydrocarbons, they are predominantly triglycerides. Triglyceride molecules are largely similar to hydrocarbon molecules, but in addition to the long chains of carbon and hydrogen atoms, each of the triglyceride molecules also contains six oxygen atoms at one end. And changing the components or structure of a molecule even slightly can make a huge difference in its chemical and physiological properties.

Along with proteins and carbohydrates, triglycerides in the form of either liquid oils or solid fats make up the vital triumvirate of food components. Triglycerides are broken down in our bodies to produce energy, but in excessive amounts they are converted into what are known indecorously as love handles and spare tires.

The 1.4 billion gallons of salad and cooking oils that we consume each year in the United States (that’s 33 million petroleum barrels’ worth) must be processed before they are pure enough and acceptable to our finicky palates. Here are some of the typical cosmetic treatments that vegetable oil may suffer before it lands on your grocery shelf labeled “all-natural” and “100 percent pure”:

•
 First, most of the oil may be squeezed (“expressed” or “expelled” or “expeller-pressed”) out of the seeds by machines called expellers, which can handle as much as 30 tons of sunflower seeds, for example, in a single day. Expellers are screw-driven presses that use friction and pressure to squeeze the oil out of the seeds. The friction can heat the mass of oil and pulp to temperatures of 140 to 210°F (60 to 99°C), although water-cooled expellers are also used to make the “cold-pressed” oils that are preferred by “natural-food” and raw-food enthusiasts.

•
 More often, the oil is dissolved out of (extracted from) the ground-up seeds into hexane, a volatile hydrocarbon liquid that dissolves the oil and is later evaporated off by being heated to 212°F (100°C). Since hexane boils at only 156°F (69°C), there should be virtually none left in the finished oil, although traces of up to 25 parts per million can often be detected. That’s why health-food stores brag about selling “expeller-pressed” and “cold-pressed” oils that have never been in contact with hexane. They’re more expensive, however, because their supply is limited; hexane extraction gets a lot more oil out of the seeds than pressing does.

•
 The crude pressed or extracted oil may then be degummed by the addition of a small amount of water and/or citric acid, which precipitates certain gummy chemicals called phosphatides.

•
 Next, the oil is treated with an alkali (usually sodium hydroxide, or lye), which removes any remaining phosphatides, proteins, and mucilaginous compounds and, most important, neutralizes any free fatty acids, which have unpleasant flavors. The reaction of the alkali with the fatty acids produces soap (yes, soap), which is removed by a later washing with hot water.

•
 If the oil has an undesirable color it is then bleached, not by Clorox, but by finely divided clays or activated charcoal, which adsorb molecules of pigmented impurities.