What Einstein Kept Under His Hat: Secrets of Science in the Kitchen (43 page)

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Enzyme inhibitors
slow spoilage by enzyme-driven reactions in foods. Sulfites (again) inhibit enzymatic degradation reactions in fruits such as raisins and dried apricots. Acids, such as ascorbic acid and the citric acid in lemon juice, deactivate enzymes, including the enzyme phenolase, which makes apples and potatoes start turning brown as soon as they are cut.

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Sequestrants,
also known as chelating agents, gobble up atoms of trace metals such as iron and copper that catalyze (accelerate) oxidation reactions and cause discoloration. The most widely used chelating agent is EDTA or
ethylenediamine tetraacetic acid
(pronounced ETH-ill-een-DYE-a-meen- . . . oh, never mind). Other sequestrants are polyphosphates and citric acid.

Okay, so some of these chemical names are hard to pronounce. But contrary to the opinions of some, that doesn’t make them evil. They’re all added in tiny amounts regulated by the FDA, and nobody eats them by the spoonful.

Your alternative to eating foods containing preservatives is to visit the farm or farmers’ market every day for fresh meat and produce. Also, make your own cream, preserves, pickles, cheese, wine, potato chips, cereals, and olive oil, being sure to consume them before they go bad.

And welcome to the eighteenth century.

When we travel abroad and return to the United States, we’re not allowed to bring home plants or foods for public health reasons. But aren’t these items sterilized when they pass through the airport security X-ray machines?

....

N
o. Airport security X-rays are not nearly intense enough to kill insects, parasites, and the like. The radiations used to sterilize and preserve foods are millions of times as intense.

I wonder why aluminum cookware and utensils become discolored and seemingly corroded in my dishwasher. An aluminum mesh strainer went particularly fast. Is this because there is an acid condition in the soap or water?

....

N
o, it’s not an acid. It’s the chemical opposite of an acid: an alkali, known to chemists more accurately as a base.

Most dishwash
er
detergents for machines, as opposed to the dishwash
ing
detergents for hand-washing dishes, contain the highly alkaline compound sodium carbonate, also known as good old-fashioned washing soda—
not
baking soda, which is sodium
bi
carbonate.

Alkaline chemicals are needed in the dishwasher because they gobble up grease, transforming it into soap. A soap is one of a class of chemical compounds formed by the action of an alkali on a fat. A detergent, on the other hand, is a more modern synthetic compound specifically designed to do soap’s cleaning chores. That difference doesn’t stop many people from calling all of today’s household detergents “soaps” anyway.

But I digress.

We tend to think that if a chemical is attacking and dissolving a metal, it must be an acid. And that’s generally true; a strong enough acid could devour a Humvee and spit out the tires. But aluminum is an unusual metal in that it is attacked by both acids and alkalis. (It is
amphoteric
.) So the alkaline sodium carbonate in the dishwasher detergent does indeed attack aluminum, at the very least eating deeply enough into the surface to make it dull and pewter-gray with aluminum compounds. For this reason most manufacturers of quality aluminum cookware advise against putting it in the dishwasher.

Worse yet, some dishwasher detergents contain potassium hydroxide or sodium hydroxide (lye), which are much stronger alkalis than sodium carbonate and will literally eat into your aluminum utensils. That’s probably what converted your mesh strainer into a basketball hoop. If you still want to wash your aluminum cookware in the dishwasher, scan the labels of the dishwasher detergents in your supermarket and choose one that contains neither potassium or sodium hydroxide nor sodium carbonate. They do exist.

There’s a second aluminum-damaging phenomenon going on in your dishwasher if the aluminum utensil happens to be touching another metal, which will most likely be stainless steel. Whenever any two different metals, in this case aluminum and what is essentially iron, are in contact while immersed in an electrically conducting liquid, an electrical (more properly, an
electrolytic
) reaction takes place that attacks one of the two metals, in this case the aluminum, corroding its surface and dulling it. So if you insist on washing an aluminum utensil in the dishwasher, make sure it isn’t touching any other kind of metal.

I’m confused about emulsions. Some recipes tell me I’m emulsifying certain ingredients, but all I can see is that I’m merely blending them. Is there some special trick I’m missing?

....

N
o, but I feel your pain. The word
emulsify
is too often misused as a synonym for
blend
or
thicken
. It is not. Restaurant menus love to call any thick sauce an emulsion. It is not. Chefs like to say they’re emulsifying a sauce by using a
roux
. They’re not.

Many substances, including flour, cornstarch, gelatin, pectin, okra, egg, and even pureed banana, will thicken a soup, custard, jam, gravy, or sauce. But when you use them you are not emulsifying anything. An emulsion is a very specific kind of mixture of two liquids that don’t ordinarily mix, one suspended in the form of tiny droplets within the other.

The prototypical kitchen example of a true emulsion is mayonnaise, in which the mutual loathing between oil and water (the latter existing within the vinegar or lemon juice and egg white) has been overcome by two things: the brute force of being beaten together, and the action of a special chemical ingredient called an emulsifier. Only when this combination of physical and chemical powers is operating will oil and water mix and stay mixed in the form of a true emulsion.

As on that blind date we’ve all suffered through at one time or another, there are simply no attractive forces between a water molecule and an oil molecule. So even if you shake a bottle of vegetable oil and vinegar until they appear to have coalesced into a homogeneous whole, they will sooner or later, usually sooner, separate into two distinct layers. You will have failed to make a stable emulsion.

At most, you will have made a colloidal suspension, in which the oil has been broken down into such tiny microdroplets or globules that they are kept suspended in the vinegar by the constant bombardment of water molecules from all directions. But this marriage is doomed to failure. No matter how much muscle power you put into shaking your vinaigrette dressing, even with the assistance of a governor of California, the oil globules will eventually bump into one another and reunite into a coherent, separate layer. Again, no permanent emulsion.

We can foil the reunification of the oil globules by adding a secret ingredient called an emulsifying agent or an emulsifier. Emulsifiers are made of snakelike molecules that have long, oil-loving (
lipophilic
) tails and water-loving (
hydrophilic
) heads. Their lipophilic tails burrow into the oil globules, leaving their hydrophilic heads sticking out like thousands of cloves studding a ham. The “cloves” attract a cloak of water molecules because they contain positive and negative charges that pull on the water molecules’ slightly negative and positive parts. (Water molecules are
dipoles
.) The resulting cloak of water molecules disguises the oil droplet as a water lover and prevents other oil droplets from attempting to unite with it. Because the emulsifier cloaks all the oil droplets in this way, they will not coalesce even if they bump into one another. They remain individually suspended. Now we have a true emulsion.

Where can we find those secret agents called emulsifiers? An excellent one is lecithin, a phosphorus-containing, fatlike chemical (a
phospholipid
) found in egg yolks. The phosphorus ends of its molecules are hydrophilic and their other ends are lipophilic. In mayonnaise, they emulsify the oil and vinegar into a permanently stable, homogeneous sauce.

How emulsifier molecules make oil and water compatible. The zigzag, fat-loving (lipophilic) tails of the emulsifier molecules penetrate the oil globule, leaving their water-loving (hydrophilic) “eyes” sticking out, thereby giving the globule a water-loving surface.

Because we make mayonnaise from a small amount of vinegar or lemon juice (water) and a large amount—about eight times as much—of oil, it may be hard to believe that all that oil has been crowded into that small amount of water. Many people are thus led to believe that mayonnaise is a suspension of tiny water droplets in oil, rather than a suspension of tiny oil droplets in water. But in fact there are so many oil globules that they aren’t actually suspended in the vinegar so much as coated with a very thin film of water, as in a bucket of wet peas, which tend to stick together because of the water’s surface tension. That’s why mayonnaise is so thick.

It is true, however, that a suspension of tiny water droplets in oil—the opposite of mayonnaise—is also classified as an emulsion. Butter and margarine, for example, are emulsions of water in oil.

When making mayonnaise with a whisk, we must dribble the oil into the acid-and-egg mixture very slowly to ensure that every added bit of oil is promptly reduced to colloidal-sized globules. If the oil drops are too big to remain suspended, they will coalesce while floating up to the top, where they will form a separate layer, defeating the entire purpose. After a while the oil can be added a little faster, because relatively large droplets will be quickly surrounded by millions of already emulsifier-coated colloidal globules that will “insulate” them from one another, keeping them apart temporarily until they themselves get whacked down to colloidal size.

When making mayonnaise in a blender, on the other hand, we can add a small amount of oil directly to the vinegar-and-egg mixture just before turning on the machine. The blender blades are much quicker than a whisk at chopping the oil globules down to colloidal size—so quick that the globules don’t have time to coalesce.

Commercial dressings and other foods contain a wide variety of emulsifiers to keep their complex mixtures of carbohydrates, fats, proteins, and water together in a stable form. Some of the emulsifiers that you may see on ingredient labels are mono- and diglycerides, polyglycerol esters, propylene glycol esters, and sugar esters of fatty acids. And, of course, lecithin.

Foods are often thickened by substances such as gelatin, starches, and gums, including agar, acacia, xanthin, and carrageenan. But thickeners aren’t emulsifiers. Thickeners work by making the watery part so viscous that even relatively large oil droplets can’t rise through it to coalesce into a layer.

Any old trick to keep a food homogeneous. Who wants to eat oily puddles mixed with globs of watery paste?