Effects of Heat on the Nutrients in Foods
To understand the way in which cooking affects the digestibility of food, it is necessary to know the effects of heat upon the main nutrients.
Of the carbohydrates in food, starch is the most affected by heat. If water is used in cooking, the starch grains absorb water and swell and eventually rupture the cellulose walls of the cells. The starch is then said to be gelatinous.
Dry heat converts starch into a soluble form, eventually into dextrin. This is responsible for the brown colour and slightly sweet taste of baked cereals foods, e.g., the crust of bread.
Cellulose is not digested in man, but must be broken down in order for the cell contents to be digested. Cooking softens the cellulose, but it is the swelling of the starch grains inside which is largely responsible for breaking in the cellulose.
The effect of heat on proteins is to coagulate them. This takes place at a temperature of about 600C. Above that temperature, the protein of most animal foods, with the exception of eggs, shrinks and become slightly less digestible. Vegetable proteins also coagulate on cooking, but they are usually found along with starch with wells in moist heat and bursts the cellulose covering. Cooking thus makes vegetable protein more digestible.
Fat is not affected by heat so much as protein and carbohydrate. As it is heated, fat melts, but at high temperatures it begins to decompose(break down into elements, decay) and may be irritating to the stomach.
Minerals are little affecting by cooking, but may be lost by solubility in water.
Vitamins vary in their stability in cooking. This has been dealt with in detail under the individual vitamins, but may be summarized briefly….
- Vitamins A and D and Nicotinic Acid are little affected by home cooking processes.
- Thiamine, Riboflavin and Vitamin K are destroyed in the presence of alkali, e.g., soda bi carbonate, and may also be lost by solubility in water.
- Vitamin c is the most unstable of the unknown vitamins and can be easily lost or destroyed in food preparation.