THE EVERY DAY CHEMISTRY OF COOKING

When you’re cooking, you’re a chemist! Every time you follow or modify a recipe, you are experimenting with acids and bases, emulsions and suspensions, gels and foams. In your kitchen you denature proteins, crystallize compounds, react enzymes with substrates, and nurture desired microbial life while suppressing harmful microbes. And unlike in a laboratory, you can eat your experiments to verify your hypotheses. In CULINARY REACTIONS, author Simon Field explores the chemistry behind the recipes you follow every day. How does altering the ratio of flour, sugar, yeast,  salt, butter, and water affect how high bread  rises? Why is whipped cream made with nitrous oxide rather than the more common carbon dioxide? And why does Hollandaise sauce call for “clarified” butter? This easy-to-follow primer even includes recipes to demonstrate the concepts being discussed, including Whipped Creamsicle Topping (a foam), Cherry Dream Cheese (a protein gel), and Lemonade with Chameleon Eggs (an acid indicator). It even shows you how to extract DNA from a Halloween pumpkin. You’ll never look at your graduated cylinders, Bunsen burners, and beakers—er, measuring cups, stovetop burners, and mixing bowls—the same way again.

Proteins

Proteins have many effects on food and cooking, from stabilizing foams and emulsions to firming up gels. Knowing a little bit more detail about proteins will help you design and modify recipes and fix things that go wrong. Amino Acids Proteins are made up of amino acids. An amino acid has a central carbon, with a carboxyl group (COOH) at one end and an amine group acid has a central carbon, with a carboxyl group (COOH) at one end and an amine group (NH2) at the other end. The simplest amino acid is glycine. Amino acids can join together by joining end to end and losing a water molecule—the OH at the left and the H at the right. For example, two glycines can join to form diglycine.
There are about 22 different amino acids found in the proteins that make up our bodies and the foods we eat. These amino acids differ from one another in one particular way: the things that are attached to the carbon right next to the nitrogen. In glycine, there is just a hydrogen atom there.

Denaturing Proteins 

In their natural state, proteins like egg albumin and milk casein are soluble in water. Most of their hydrogen bond-forming parts are tucked inside the folded structure of the protein, making them unavailable for forming bonds with other molecules. They are all the same shape, so they all have the same properties and can form crystals. There are several mechanisms that destroy these properties. Heat, acids, strong alkalis, alcohol, urea, salicylate, and ultraviolet light are among the more common ways that proteins become denatured. A denatured protein unfolds as many of the hydrogen bonds that preserve the three-dimensional structure of the protein are broken. Instead of a uniform solution of molecules that are all the same shape, in a denatured protein, the molecules can take a staggering number of different shapes (on the order of 1,020 different shapes, depending on the size of the protein molecule).
Like snowflakes, few if any of the molecules will have the same shape, and they will no longer form regular crystals. The unfolded molecules also have more bond-forming areas exposed on the outside, so they form bonds with one another and coagulate. They become insoluble in water. You have seen that surface effects cause proteins to denature. When you beat egg whites or whip cream, the proteins unfold as their hydrophobic parts rearrange to avoid water in favor of air or fat. The unfolded proteins can then bond to one another to create stabilizing protein films that keep the new forms in the desired shape. In cooking, you control the denaturing of proteins in several ways. You can control the temperature, you can control the acidity, you can use copper bowls to beat the egg white and catalyze the formation of disulfide bonds in the proteins, and you can control the fat or air content when you beat the proteins. For example, when beating egg whites, it is important to keep fats out of the egg whites.
A little bit of oil or egg yolk can prevent the foam from stabilizing, as the air competes with fat for the hydrophobic parts of the molecule. Because proteins have both acidic parts and basic parts, the acidity of the solution they are in changes their behavior.
Acids release protons (hydrogen nuclei) and bases accept protons. In an acidic solution, the basic parts of the protein accept protons from the acidic solution and become positively charged. The positive charges repel one another and the protein molecules are less likely to combine with one another. In a basic solution, the acidic parts of the protein lose a proton and become negatively charged. This also results in repulsion between the protein molecules, and combination is reduced. Charged areas of the protein interact with water molecules because water is a polar molecule, with one end negative and one end positive. These charged ends are attracted to opposite charges on the protein. Whether a protein forms a gel is thus affected by the acidity of the water it is dissolved in.

Milk 

Nearly 80 percent of the proteins in milk are casein proteins. Caseins have a lot of the amino acid proline, which has a side chain that causes proteins to bend wherever there is a proline. This makes the proteins unlikely to stack into regular, orderly secondary structures. In addition, caseins have no disulfide bonds, and so they have little tertiary structure. This means that the hydrophobic parts of the molecule are open and exposed (not tucked inside a ball). All of this combines to give caseins interesting properties. The hydrophobic parts end up migrating together, and the hydrophilic parts arrange themselves to the outside, facing the water. These tiny, hairy balls of protein are called micelles. Caseins bind together with calcium and phosphorus. As a nutrient for the young mammal that needs to build bones, this is a useful property. Without the caseins, calcium phosphate would not be soluble.
In the basic solution of milk, the hydrophilic parts of caseins become negatively charged and repel one another. This allows milk to stay liquid. But caseins clot in the stomach, due to acids that counteract the negative charges and enzymes that cut the proteins into smaller pieces. This clotting makes the proteins stay longer in the stomach, releasing amino acids slowly, which aids digestion and absorption of the protein. In the stomach of young mammals, enzymes cut off part of the water-soluble casein (kappa-casein) that has the negative charges that keep the micelles apart. In making cheese, these enzymes, extracted from the stomachs of young calves, are used to make the caseins clot together into a solid.

Eggs 

The proteins in eggs largely determine the characteristics of foods that contain eggs. Understanding the different properties of these proteins can be helpful when cooking or creating new dishes. When you crack an egg into a pan, you can immediately see three parts. There is the yolk, a thin watery white, and a thick gelled white. Egg white contains several mucoproteins, in which the protein is attached to carbohydrates. In the egg, these serve as nutrients for the growing embryo, and as support and protection.
Over half the protein in egg white is of one type: ovalbumin. It denatures at 176°F (80°C), forming the solid white mass you see at breakfast. The next most prevalent protein in egg white is ovotransferin, also called conalbumin, which makes up about 12 percent of the proteins in the white of the egg. It denatures at a lower temperature, about 145°F (63°C). The third most prevalent protein is ovomucin, at 11 percent. It is found close to the yolk, mixed with other proteins, thickening them. When you crack an egg into a frying pan, the thin part of the egg white has less ovomucin, and the thick part of the white has two to four times as much. Ovomucin is the main gelling agent in egg white. The first protein to denature when heating an egg white is the ovotransferin. As it unfolds, it binds not only to other unfolded ovotransferin molecules but to other proteins that are not yet denatured.
The ovomucin molecules, which do not coagulate with heat by themselves, can thus be incorporated into a strong gel with the ovotransferin and ovalbumin. The remaining proteins make up less than a quarter of the protein in egg white, but some of them bear mentioning here. Avidin makes up a very small portion of the egg white (less than a 1/10 of a percent), but it binds very tightly to the essential nutrient biotin (vitamin B7), making the biotin unavailable as a food source. This effect is destroyed when the protein is denatured by heat or beating, but it can be a problem in a diet that contains a lot of raw egg white.

Meat 

Raw meat is tough because each tiny packet of muscle fibers is surrounded by a tough sheet of connective tissue. This is the same tissue that, when boiled, makes gelatin. When meat is cooked, the tough connective tissue denatures and becomes soft gelatin. The proteins in the muscle fibers also denature. Enzymes in the tissue no longer function when they are denatured, so cooked meat will keep longer than raw meat. If the meat is overcooked, the water in the fiber bundles boils and the gelatin bag holding them bursts, and the meat dries out. At high temperatures, the proteins also undergo further denaturing and cross-linking, making the meat tough again. Crisp bacon is an excellent example of this. A thick, juicy steak would be inedible if cooked to the hardness of bacon.

Enzymes 

Enzymes in foods are often a problem for food storage. As cells break open, the enzymes inside them leak out and react with other parts of the food. This causes brown soft spots in fruits and vegetables, and it makes meats smell and taste bad. The damaged parts also invite decay microorganisms. Denaturing the enzymes can help to preserve the food. The heat of cooking is one familiar way to denature enzymes, but proteins can also be denatured by acids, strong alkalis, desiccation, or salt.

Shortening

When wheat flour and water are mixed and kneaded, sheets of gluten are formed. With further kneading, these sheets stick together into larger and larger sheets. But if oils are added, the hydrophobic amino acids in the gluten attach to the fat, so that they are not available to form bonds with other gluten molecules. This changes the nature of the dough, making it more tender and less able to trap bubbles of leavening gasses. The result is a more cakelike, less breadlike structure.

Glutamate 

One particular amino acid has a strong effect on the taste of foods. That is glutamic acid, and salts of it are called glutamates. Besides being an abundant neuro-transmitter in the brain, glutamate activates sensors on the tongue that detect savory protein-rich foods. Meats, poultry, fish, cheese, and soy sauce are rich sources of glutamate. The commercial form of pure glutamate is monosodium glutamate, MSG, which is an additive in many foods.

Cheese 

Camembert

Cheese is made from milk that has been curdled by the addition of acids and an enzyme from the stomach of calves called rennet. The acid can come from almost any food source, but for the most part it is produced by bacteria that convert the milk sugar lactose into lactic acid. Yogurt is also produced this way. Cheese can be made without rennet, but the enzyme makes the curds stronger and more rubbery. Rennet allows the milk to curdle with less acid, which in turn allows flavor-producing bacteria to colonize the curd. Cheeses made with rennet will melt easily, while cheeses made with acid alone remain solid at high temperatures. The curds are salted and moisture pressed out so the product will not be as easily attacked by bacteria as raw milk would. Thus making cheese is a way of preserving milk.


By Simon Quellen Field in the book "Culinary Reactions : The Everyday Chemistry of Cooking" Published by Chicago Review Press, Incorporated, Chicago, U.S.A, 2012, excerpts from p. 64 to 71. Adapted and illustrated to be posted by Leopoldo Costa.