June 2013

I loved chemistry in High School. It was the one subject that I excelled in without any effort; it made perfect sense to me from the beginning - until in college they added calculus to it, and I was lost (because my brain likes concepts, not numbers).

When I prepared the food science class for our students at the Christian Culinary Academy, I found that the old cliche is still true: cooking is chemistry! Suddenly all those recipes made perfect sense. Why heavy cream becomes fluffy was a no-brainer when I found that amino acids are unraveled in the whipping process and the air-loving part attaches to O2, and voila! the liquid becomes a fluffy, velvety substance that makes cakes possible.

When I realized how easy it is too cook when all the molecules line up, I thought that food science would be a subject that even the most seasoned chef would be interested in, wherefore I decided to write a series of articles on what happens when we heat, freeze, cut and roast our food, in order to give you a glimpse into what really happens in that kitchen of yours.

HEAT 

"Heat" describes the speed of molecules in a substance such as air, or water. The higher the temperature, the faster the molecules are moving and the more energy (or heat) the molecules contain. The fast-moving molecules bump into the slowly-moving molecules and cause them to pick up speed. 

The gaseous molecules in a fire, metal atoms on a skillet, air-molecules in the oven, they all bump into the slow-moving molecules in food, and cause them to pick up, and this creates different chemical reactions in food. As a result, the food can change color, it can lose moisture, or cell-walls can break down (making food more tender).

The important thing is to choose the right kind of heat.

CONDUCTION – heat transferred from molecule to molecule within the food (on the stove).

CONVECTION – heat transferred by the hot air in the oven to the pan, and then from the pan to the food.

RADIANT – The heat emitted by the heating element in the oven and absorbed by the food.

The method decides how the food is cooked, for how the heat makes the molecules move, will decide the end product.

Conduction signifies the transfer of heat from a hotter to a colder region within a food, i.e. the movement of molecules inside a single substance. The exterior begins to cook first, and the heat is conducted into the middle by the movement of molecules (This is why the middle cooks last). Water molecules are much smaller than fat and protein molecules, wherefore they are capable of moving faster and conduct much of the heat. 

Convection is the transfer of heat from a hot liquid (like boiling water or frying oil), or a hot gas (like the air in the oven), to a food. In each case, heat is generated by an external source, such as a stovetop burner, or heating element in an oven.

Radiant heat comes from sun’s rays, grilling, broiling, and even microwaving. Waves of energy interact directly with the molecules in food, causing them to accelerate in speed, and therefore become hotter.

The outside always cooks faster than the inside. If the temperature is too high, the outer layer may become overcooked by the time conduction moves the heat toward the center of the food. This happens because the external moisture will evaporate (water molecules move faster), leaving the surface area vulnerable to becoming very dry.

For example: if you put a steak on a hot pan, it will quickly sear the outside, but the inside will remain raw. If you keep on cooking the steak on the hot pan, you risk burning the outside before the inside has a chance to turn pink (because all the water molecules will have evaporated from the surface before the inside has had a chance to cook). If you, on the other hand, put the steak in the oven, you won't get the flavor on the outside that comes from high heat, but the steak will cook uniformly.

So what to do? Sear the steak on the pan, and finish it off in the oven. That way you will get both the flavor from high heat (via conduction), and the uniform cooking of both the inside and outside of the steak (via convection).

Now that you've produced the perfect steak, let it rest for a moment before sending it to the customer, or eating it yourself. If you cut the steak the moment it comes from the oven, you'll end up with a steak swimming in its own juice, that will taste dry.

Now what is that? 

Meat is mostly water; raw beef is about 75 % water (which is true of humans too), the rest is protein and fat. The proteins in the meat trap the water molecules, wherefore a piece of raw meet will not shed liquid when you cut it. But when you cut meat that has just been cooked, a flood of juices (liquid) will cover the cutting board, because cooking has caused the proteins to release the water molecules. By letting the meat rest for a few minutes (but not too long or it'll turn cold), the water will make it's way back to the spaces the proteins once occupied.

Here's why.

The protein that makes up muscle tissue in raw meat is similar to many bundles of wire. Each wire that represents a single muscle cell is called a muscle fiber. When red meat and poultry are heated, protein molecules begin to chemically bond with each other, causing them to compress and contract, first in diameter, then in length. A single muscle fiber can shrink to as little as half of its original volume during the cooking process, and other proteins dissolve. When the proteins contract and shrink, the liquid trapped is squeezed out, but when the meat rests, the liquid returns to the space once occupied by the now dissolved protein. The dissolved protein holds on to the liquid, and you get a steak that is tender, juicy, and cooked to perfection.

I'd say Bon Appetite to that!

Susanna Krizo 
Editor
Board of Directors, CCI 
Secretary