To understand the way in which cooking affects the digestibility of food, it is necessary to know the effects of cooking on different types of food.
Cereals consist mainly of starch with a little protein enclosed within a cell wall of cellulose. The method of cooking used must be adequate to break the cellulose so that the digestive juices can reach the starch.
Boiling of cereals can cause the starch grain to swell and burst the cellulose. Rise, e.g., absorbs twice it’s own weight of water during cooking, and the cooked weight is therefore three time the dry weight.
If much water is used in cooking cereals, the thiamine will be lost from for by solubility in water. Thiamine will be destroyed if soda bi carbonate is added during the cooking.
The protein of cereals is also coagulated on cooking and depends on the breaking of the cellulose for digestion.
Well cooked cereals are thus more easily digestible then in the raw state
Pulses contain less carbohydrate but more protein than cereals but the effect of cooking is much the same. Pulses also contain some carbohydrate which is not digestible and an antitrypsin substance which prevents the action of trypsin. The undigested carbohydrate may be broken down by bacteria in the large intestine and the cause flatulence. The antitrypsin substance is destroyed by adequate boiling so that is very important that pulses and dhals should be properly cooked.
Green Leafy Vegetables
Green leafy vegetables provide little calorie value but give much of the mineral and vitamin contain of normal diet. They are usually cooked by boiling but there is almost invariably from loss of food and value in the process. If much water is used, some of the minerals, thiamine and vitamin C will be lost by solubility. Thiamine may also be destroyed if soda bi carbonate is used in the cooking. The vitamin A and carotene contain is not much affected by home method of cookery.
Other vegetables vary in food content but, like green leafy vegetables, contribute minerals and vitamins to the diet. The same precautions should be used in preparation and cooking to preserve the food value.
Tubers, e.g., potato and yam, contain a higher proportion of starch than other vegetables and must be adequately cooked so that the starch grains swell and burst the cell walls. They should not be soaked in water for a long time unless they are old. Nutrients in tubers are retained by cooking with the skin but if they are old, strong flavour is retained also. New potatoes and carrots should be scraped rather than peeled to preserve the mineral layer. It is preferable to bake, steam or boil the tubers slowly as rapid boiling breaks the skin.
With the exception of fruits which are good source of vitamin C, e.g., citrus fruits, guava and nellikai, fruits are eaten more for their sweetness and flavours than for their nutritive value. Fruits contain of 85-90 % water with a little carbohydrate as cellulose and sugar. The proteins, fats and minerals contain very low.
The majority of fruits can be eaten raw, and if fresh will supply vitamin C to the body. Cooking makes fruits more digestible by softening the cellulose, but usually result in the loss of sugar by solubility in water. If the fruit is cooked by stewing and the juice eaten along with it, the sugar loss is very little but there will still be loss of vitamin C.
Meat, Fish and Liver
These are all protein foods of animal origin which contain also variable amount of fat.
When meat is cooked, there is a reduction in the water contain, and this occurs even when the meat is boiled. There is also coagulation of the protein, softening of the fibres by conversion of the connective tissue to gelatine, and the fat melts. If meat is cooked by roasting or grilling, the protein on the surface coagulates quickly, sealing the juices inside the meat.
Some of the mineral salts may be lost in process by boiling or stewing, but this is of little important, if liquid is also used as soup or gravy.
Prolonged cooking, especially by dry heat as in roasting, cause the protein to become hard and ingestible.
There are three main kinds of fish….
- White fish, with flesh which is opaque and white when cooked, containing protein but very little fat. It regarded as the most easily digestible kind of fish.
- Oily fish, with flesh which is a pinking-brown colour when cooked, containing a higher proportion of fat than white fish and as a result, is less easily digestible.
- Shell fish, e.g., prawns, crabs, with flesh which is very dense and tough and difficult to digest.
The cooking of fish improves the digestibility in the same way as meat. Fish has, on the whole, less flavour than meat, but can be made very tasty by serving an appetising sauce with it.
Liver and kidney is compact organ containing mainly protein with very little fat connective tissue. Because of they are close texture they are rather difficult to digest unless well –cooked and finely chopped or carefully chewed.
Milk is often termed a perfect food, but this is not strictly true because it lacks two essential nutrients, iron and vitamin C. Milk is, however, one of the most easily digestible and nutritious foods.
Boiling of milk cause little change in the food value though it brings about some change in taste. If the milk is kept at boiling point for some time, to sterilize it, than the taste is quite changed, and there may be some loss of vitamin content. Pasteurization does not alter or change the taste and vitamin content much.
When milk is taken, it curd less as soon as it reaches the stomach, due to the action of rennin. Boiled milk curdless more slowly and give a less dense clot than raw milk, and is more easy to digest.
Milk drunk rapidly produces smaller cud and is more easily digested than milk drunk slowly or sipped. Milk mixed with some other food, particularly cooked with cereals, is more easily digestible than plain milk because the starch helps to break of the curds.
Cetric acid or sodium citrate added to the milk precipitates some of the calcium and produce ales dense and more easily digestible clot. Sodium citrate added in the proportion of 1 grain per 30 ml. Of milk does not alter the taste, but increases the digestibility.
Eggs contain protein which coagulates on heating. Raw eggs are not well digested unless beaten or mixed with other food but the digestibility of cook egg depends on the method of cooking.
Lightly boiled, poached, or scrambled eggs are the most easily digestible. If fried, or made into omelette, the extra fat used in cooking decreases the digestibility of the egg.
If an egg is to be cooked until both the white and yolk are solid, and it should be kept in water at about 780C. For 10-15 minutes. But not allow to boil. Hard boiling makes the protein dense and leathery and difficult to digest.
Except for modifying the digestibility of protein, there is little change in the nutritive value of eggs on cooking. There may be some loss of thiamine.