Why Is Milk White? (31 page)

Not everyone does. But the idea is to release the gas bubbles that have nucleated on the inside surface of the can, so the bubbles float to the top and pop.

If there are numerous bubbles already formed on the inside of the can, when the pressure in the can is released, these bubble grow in size immediately, becoming many times their previous size. At the same time, they act as surface area for more carbon dioxide gas to form, making the bubble bigger still.

If enough of the now-large bubbles form low in the can, they make the contents of the can (the soda inside) increase in volume, causing it to spill out of the can. If the top of the can has not been quickly and fully opened, a stream of bubbly soda can shoot out of the narrow opening, making a sticky mess, usually in the direction of the person opening the can.

Tapping on the can or setting it on the table with a thump will dislodge the bubbles from the inner side of the can, where they accumulate at the top and break. Now when the can is opened, only the gas escapes from the initially small opening. The gas makes much less of a mess when it hits the face of the person looking for a cool drink.

Besides carbonic acid and water, soda water often contains sodium compounds such as sodium bicarbonate (baking soda), which are slightly alkaline. The alkaline salts counteract the acidity of the carbonated water and make it taste slightly salty, like the mineral water it was originally made to resemble. It is these sodium salts that give soda its name.

In flavored sodas, sugar or other sweeteners change how the tongue reacts to the acid, in the same way that adding sugar to lemon juice makes lemonade taste far less sour than the lemon. In fact, so much sugar is added that additional acids are needed
to add the refreshing hint of tartness we like. This is why many soft drinks (especially colas) add phosphoric acid to the drink. Citric acid is also often added, especially in citrus-flavored drinks. Another common additive for tartness is malic acid, the molecule that makes apples tart.

Benzoic acid or sodium benzoate is often added, since it prevents bacteria and molds from being able to ferment sugars and spoil the flavors in the drink.

To maintain the levels of acidity, a buffering agent such as sodium phosphate is sometimes added. This keeps the acidity at a fixed level, so the product tastes the same all the time, even as the carbonic acid levels get low as the carbon dioxide bubbles form and break.

Caffeine is often added as a stimulant drug. Naturally present in cola and other plant-derived flavorings (and in coffee and tea), caffeine was one of the main reasons to drink cola drinks when they were first invented.

Caramel color and FD&C colors are also added to many soft drinks, just to make them look more appetizing.

No. Many bottled waters have an expiration date on them. This is not because they will go bad but because some places have laws that say every drink has to have an expiration date. Even if you don't live in such a place, the bottling company wants to be able to sell the water in those places, so they print a date on the bottle. It may also help their sales if people think they have to buy new bottles of water instead of drinking the older ones in the back of the refrigerator.

Plastic bottles will gradually release volatile molecules from the plastic. Most of these will escape into the air, but some of them will get into the water. It is unlikely that you would be able to taste them in the water, however, and it is not clear that they would accumulate over time instead of being reabsorbed in the plastic
once they reach equilibrium levels in the water. Glass bottles don't have volatile molecules to leach into the water.

An unopened bottle will not allow algae or bacteria to grow in it. Once the bottle has been opened, however, it is possible that algae spores could make their way into it, and with enough sunlight, you could get a green film on the inside of the bottle.

The same carbon dioxide bubbles that make sodas fizzy make cookies rise. In bread, those carbon dioxide bubbles are made by yeast, a tiny microbe that eats sugar and makes alcohol and carbon dioxide. In cookies, the bubbles are formed by a reaction between baking soda (sodium bicarbonate) and an acid.

The acid can be from something tart in the recipe, such as lemon juice or other fruit juices, vinegar, or buttermilk. Or it can be an acid produced when water is added to powdered tartaric acid (cream of tartar) or monocalcium phosphate. These are the powders that, along with baking soda, make baking powder.

Baking soda by itself will decompose to release carbon dioxide as the temperature of the cookie rises. Steam will also enter the bubbles and expand them. Air that was beaten into the cookie dough will also expand as the cookies get hot. All of these things combine to help make the cookies rise in the oven.

When the cookies cool, however, those hot gases will contract again. If the cookie has not been chemically changed by the heat, it will fall and get flat again. The chemical changes in the cookie that help it keep its fluffy texture and shape after it cools are mostly reactions of proteins. As proteins are heated, their carefully formed three-dimensional structures unwind (denature), and the untangled strings and sheets of protein can form bonds with other proteins. The proteins join up into big nets and sheets that hold the bubbles in place, and as the hot gases cool and try to contract, the proteins hold their shape, so air from the outside is sucked in.

The same protein denaturing that keeps cookies fluffy happens when you cook an egg. The white of an egg is a transparent gel before cooking. The proteins are dissolved in water, but they have electrical charges on their surfaces that make them repel one another. This gives the liquid its gel form and keeps much of the egg white in a high puddle around the egg yolk.

There are several different proteins in the egg white. Some are more gelled than others. If you crack an egg, you will see a runny, almost watery protein solution on the outside, and a firmer gel of egg white toward the center.

As you heat the egg white, some of the proteins denature sooner than others (at a lower temperature). As the proteins unfold, the electrical charges that were on the inside are available to form bonds on the outside. The proteins take up more space as they get bigger and start to bond with one another.

These larger proteins scatter light more effectively than they did when all the molecules were smaller than a wavelength of visible light. Instead of a transparent gel, the egg proteins become as white as clouds, which scatter light in the same way. As the temperature rises, more of the proteins denature and bond together. The proteins in the yolk are the last to solidify, at the highest temperatures.

Jelly feels squishy because of electricity. You just learned about the gel in an egg white, in which the proteins were kept separate by their electrical charges on the outside of the molecule. Jelly is made of water in which huge molecules are dissolved, much like the protein molecules in egg white.

The molecules in jelly that make it firm are made of sugars that are all bonded together into huge, tangled molecules made of many thousands of sugar units. Unlike starch, which is somewhat similar
but made of only the sugar glucose, pectins contain several different kinds of sugars. Plants use pectins as building materials, to give shape and strength to their cell walls.

Pectins react to heat in a way that is almost the opposite of proteins. Whereas proteins unfold and connect to one another as the temperature rises, pectins do not. Instead, as the temperature gets warmer, the pectins bounce around against one another more and lose their rigidity.

When fruit and fruit juices that contain pectin are heated, the pectins leave the cell walls and dissolve in the water around them. When the water cools, the pectins stop jostling and settle down next to one another but are still kept apart by their electrical charges. If there is enough sugar in the juice, the pectins firm up.

The result is a firm gel that you can poke, jiggle, and squish with your fingers or your tongue. The molecules are not strongly bound together, so the jelly is not a solid. But sugar and the large molecules of pectin lock up the water molecules around them, so they don't flow like water. The result is jelly.

Fruits are a way for plants to spread their seeds. To attract animals and birds to the seeds and get them to spread the seeds, the plant makes the fruit sweet, soft, and fragrant and usually changes the color to indicate that this fruit is ready to have its seeds scattered. Unripe fruits are not sweet, generally not fragrant, and remain a different color, so they are not picked before the seeds are ready.

During ripening, stored starches are converted to sugar. This makes the fruit sweet, and it also makes it softer. The pectin in the cell walls breaks down, further softening the fruit. The chlorophyll breaks down, allowing the yellows, reds, and oranges to show through. Anthocyanins are produced to get purples and reds, while also acting as sunscreens and antioxidants to keep the fruit tasting good.

The hormone that triggers ripening is a small molecule called ethylene. Commercial fruit growers sometimes pick the fruit
(especially bananas) while it is still green and hard, so it can handle transportation without bruising. They then add ethylene gas to the containers of fruit to start the ripening process. By the time the fruit is in the market, it is nearly ripe.

Hydrogen bonds make honey sticky. Earlier you learned about hydrogen bonds (
page 153
). Unlike other some bonds, hydrogen bonds form between molecules instead of between atoms in a compound. The attraction between honey and your fingers is due to hydrogen bonds in the honey attracting molecules in your fingers.

Honey is mostly water, fructose, and glucose. The last two are simple sugars, also known as monosaccharides. These three ingredients make up 86 percent of honey, with other sugars making up the bulk of the rest. Sugars bind well to water. Almost all of the 17 percent of honey that is water is bonded (with hydrogen bonds) to sugar, so that little water remains to support bacteria or molds. This is why honey needs no refrigeration.

The hydrogen bonds between the sugars and the water make the honey viscous, so it pours slowly and feels thick. Those same bonds make it stick to your fingers. But water also bonds to your fingers with hydrogen bonds but doesn't feel sticky. What is the difference?

When your fingers get wet, they do tend to stick to one another a little bit more than when they are dry. But water does not stick very well to itself, so when you pull your wet fingers apart, a little water remains on each finger. The water sticks to your fingers more than it sticks to itself.

Honey sticks to itself far more than water does. You can see this when you pour the two liquids out of a cup. The honey takes a long time to pour out.

When the honey sticks to your fingers, it also sticks to itself. You have to use more force to pull your fingers apart than if they were just wet with water. We call that feeling “sticky.”

You already know about why snow and ice melt (
page 169
) and why chocolate melts (
page 228
). Butter is similar, but a little bit different.

All of them melt when a rise in temperature shifts the equilibrium between melting and solidifying. Butter, like chocolate, contains a mixture of fats. But butter has a wider mix of different fats and doesn't melt at a particular temperature all at once. Butter softens over a range of temperatures, until finally a temperature is reached where it is almost entirely liquid.

Butter is different from ice in another way. Butter is an emulsion of tiny water droplets inside of the fat. Each droplet is completely surrounded by fat, and the fat forms a continuous coating over all of the water droplets. Like soap bubbles and the bubbles in bread and cookies, a
surfactant
molecule—in this case, milk protein—keeps one end in the water and the other end in the fat, stabilizing the droplet and preventing it from joining other droplets. This keeps the butter from separating into a layer of water and a layer of fat.

When the butter melts, the droplets are freed, and they do join together. You can see the layer of water under the layer of melted fat when you melt butter in a pot.

Fake butter, called margarine, is a bunch of tiny water droplets surrounded by a continuous coating of fat—just like butter. But the fat in this case comes from plants instead of a cow. Some early margarines used fat from beef, seals, whales, and fish, but modern margarine is mostly plant-based. Some butter substitutes mix in some cream from cows to make it taste more like butter.

As with butter, the emulsifying agent is milk proteins: skim milk. Vegetable fats tend to be unsaturated to some degree and thus have low melting points, making them oils. To raise the melting
point, the oils are saturated with hydrogen using catalysts. This is called
hydrogenation,
and the result is hydrogenated vegetable oil.

Partially hydrogenated vegetable oils used to be used in margarine and butter substitutes, but when oils are heated in the process of hydrogenating, some harmful trans fats are produced. If the fat is fully hydrogenated, so that every carbon atom has the maximum number of hydrogens, then there is no trans fat. Fully hydrogenated (saturated) fats are now used, mixed with oils to get the proper hardness and softening range.

Other emulsifiers, such as lecithin, are sometimes used along with the skim milk. Colors are added (usually annatto or carotene) to get the right yellow color (uncolored margarine is white). Sometimes the milk is cultured with yogurt bacteria to get a stronger buttery flavor.