You already know that applying heat to food changes its appearance, flavor and texture, so why do you need to know any more than that? The answer is that not all the effects of heating are positive and a basic understanding of the changes taking place will help you preserve nutrition and avoid some of the downsides.
Proteins
When proteins are heated they begin to thicken, which is why egg whites turn opaque, and then to harden. Over-heating protein, then, will make it tough and unpalatable.
However this characteristic is also useful since it gives us hard-boiled eggs, roast pork crackling and the crust on bread to name but a few.
Carbohydrates
These are basically starches and sugars, both of which behave differently when heated.
Starch grains are coated in a cellulose 'husk' which, when heated in moist conditions such as a sauce, will soften and swell. At or near boiling point this husk bursts, releasing the starch which in turn thickens the liquid it is in.
Sugar treated in the same way forms a syrup with the liquid which will eventually caramelize if heating continues. It then rapidly burns and turns to ash.
Dry heat, on the other hand, turns starch to dextrin, which is a kind of sugar, and drives of water. We use this principle to make toast, for example. Sugar will simply caramelize quickly and then burn.
Fats
When you heat fat, water is given off and the fat melts. Once the water has all gone a faint blue haze can be seen which is the 'cooking point' but may sometimes be referred to, somewhat inaccurately, as the smoke point.
The true smoke point occurs at the upper tolerance of the fat to heat, when its molecules break down sufficiently for it to ignite spontaneously. Oil that is smoking is dangerous and must be removed from the heat.
Vitamins
Some vitamins survive the cooking process very well. Others can be lost simply by being immersed in water. Understanding how each behaves will give you any number of clues as to how best to treat vegetables and fruit when cooking.
Vitamin A and carotene are not affected by either boiling or steaming. There is the same amount of both in, say, a boiled carrot as there is in a raw one.
Vitamin D survives all methods of cooking and does not dissolve in water.
Thiamine is soluble in water and is also partially destroyed by high temperature cooking and by the use of such things as baking powder. A 50% loss of B1 is not unusual.
Riboflavin, known as B2, also leaches out in water and is dissolved during cooking in liquids.
Niacin is stable when heated but does oxidise when exposed to air. It also dissolves in liquid to some extent.
Vitamin C is the most delicate of all the main nutrients. It easily dissolves in both the washing and cooking water and is destroyed by excessive heat. This is one of the reasons that good cooks reserve the cooking liquid of potatoes, for example, and use it to make gravy or savoury sauces.
Vitamin C also oxidises to form a substance with no nutritional value, which is one very good reason for boiling potatoes with the lid on. And if that wasn't enough to cope with, foods rich in Vitamin C also carry an associate enzyme that begins to destroy it on exposure to air. The good news is that the enzyme activity is quickly destroyed by boiling.
The most important thing to bear in mind is not to soak vegetables in water for extended periods, allowing leaching to take place. In addition, always subject your food to the shortest possible cooking time in order to preserve as many nutrients as possible.
