Kinetics of Cooking

Everyone has cooked something at one time or another, from trying to make coffee to making a complete Thanksgiving dinner. You may have wondered, as you stood over your boiling pot of water, why it takes any time at all to cook the food. Why can't you just have instantly cooked potatoes every time? And, you may have also wondered why you needed to get the food hot in order to cook it. Well, both of these questions can be answered through chemical engineering concepts. Cooking is based on kinetics, heat transfer, and mass transfer. You need heat to make the uncooked food become cooked food. If you don't add heat, then you won't be able to make your baked potato soft or your hard-boiled egg hard! When you increase the temperature of the food, you increase the speed of the molecules in a food. The greater their speed, the more they collide. These collisions between the molecules can lead to changes in molecular structures by creating new molecules. These new molecules have different characteristic colors, flavors, and textures from the original ones. The formation of new molecules is called a chemical reaction. A chemical reaction is necessary to break down the proteins present in an uncooked egg. A different chemical reaction is required to change the structure of the starch present in an uncooked potato.

Chemical reactions have a set speed of progress at any condition. There is very little you can do to speed up a reaction without adding heat to raise the temperature. Even so, since hard-boiling eggs occurs in a pot of boiling water, adding heat to the pot will only cause the water to boil faster, but not cook the egg any faster. This is because the boiling temperature of water will not change unless either something is added to the water (such as salt) or moving closer to sea level. Adding salt to water will increase the boiling temperature of the water. This is similar to the addition of salt to the snow and ice on the roadways in the wintertime. Salt, when present in water, lowers the freezing point (causing the snow and ice to melt) and raises the boiling point. At sea level, the atmospheric pressure is greater and therefore, the water must reach a higher temperature before it can boil. For more information about this effect of pressure, see the Pressure Effects page. The rate of a chemical reaction can be measured by its chemical reaction rate constant. In general, the chemical reaction rate constant doubles for each 20^oF rise in temperature.

Before we look at an example of kinetics we should look at the consept of energy. After investigating the basics of energy we will look at kinetics using the example of bread making