Cooking and Preserving Nutrition
Cooking as practiced in India:
Wet cooking: boiling, steaming or pressure cooking
Dry cooking: frying, roasting and baking.
Cooking and the Loss of Nutrients
The loss of nutrients on cooking depends on the temperature, duration of cooking and the nutrient involved.
The loss of nutrients during the ordinary cooking process is not as much as it is generally believed. Simple cooking practices cause little loss of carbohydrate, protein or fat in cereals, pulses and meat.
Facts to remember when cooking different types of foods
Some protein may be lost if vegetables are cooked in water containing salt and the cooking water is discarded. There causes considerable loss of minerals, especially Sodium, Potassium and Calcium due to leaching. It is therefore advisable either to cook in a minimum amount of water or to use the cooking water in soups and gravies.
Root vegetables do not suffer much loss of nutrients by either wet or dry method of cooking since the outer skin prevents leaching out of nutrients. It is therefore recommended that the root vegetables are cooked with their skin. Peeling can happen after cooking.
Cutting vegetables into small pieces and exposing them to air before cooking may result in loss of vitamins particularly vitamin C. It is advisable to cut in larger pieces and cook for a short period in boiling water. Steaming however prevents losses due to leaching.
Rice has a special problem with regard to loss of nutrients, i.e. minerals and vitamins, during cooking since its normal nutrient content is low.
Certain amount of minerals and vitamins are lost even during preliminary washing before cooking. Hence washing with a large quantity of water and repeated washing must be avoided. Washing may remove as much as 40% of the thiamine and nicotinic acid. Rice must be washed, if washing is necessary, with minimum amount of water.
Boiling rice in excess water and discarding the gruel (kanjee) also results in loss of some amount of minerals and vitamins. However, loss due to discarding of kanjee is not as much as during washing. Cook in just sufficient amount of water so that all the water is absorbed and no kanjee is discarded. However, if paddy is parboiled and that rice is used, loss of vitamin and minerals due to washing and discarding of kanjee is much less. Parboiling diffuses nutrients into the grain and a protective gelatinised starch coating is formed on the grain preventing leaching.
Vitamins, especially those of water-soluble B group, show a greater loss during cooking. Vitamin A and carotene are not lost during wet cooking however; cooking in acid media with tamarind and other acids has a protective effect against vitamins.
Vitamin C is lost during exposure of cut vegetables. Cooked vegetables when exposed to atmosphere before serving may also result in loss of vitamin C. It is preferable to cook vegetables in minimum amount of water with the vessel covered with a lid and to consume it as soon as possible.
Eggs appear to suffer little loss in nutritive value during conventional cooking.
Effects of Heating
If fat is repeatedly heated during frying it may end up containing toxic substances, due to peroxidation and rancidity. Use a minimum quantity of fat for frying and avoid using it over and over again.
Excessive heating may compromise nutritive value of proteins, particularly if the heating is done in presence of free reducing sugars since in this process amino acid lysine of proteins combine with carbohydrate rendering lysine unavailable. Excessive heating of milk, which contains lactose and other foods with jaggery, may be undesirable since this may lead to browning and reduce the protein quality. Even storing for prolonged periods, foods like milk products under moist condition may lead to browning.
Cooking has also some beneficial effects. It improves the appearance and palatability of foods and confers new flavours. Cooing also destroys harmful food borne micro organisms. Digestibility of several foods, particularly of starch improves on cooking. During cooking starch swells and cell walls burst making it easy for the enzymes in the digestive juices to have access to starch and digest it. Cooking eggs helps in destroying avidin which is known to bind biotin and make it unavailable. Some legumes contain trypsin inhibitors, particularly Soya bean. Protein digestibility is reduced by trypsin inhibitor. Heating or autoclaving the legumes help destroy the trypsin inhibitor and improve protein quality. Similarly duck eggs also contain a trypsin inhibitor in the white portion. Prolonged boiling or autoclaving can destroy this inhibitor. In general, quality of proteins improves after cooking since their access to enzyme and their digestibility improves.
The above material was excerpted with permission from Nutritive Value of Indian Foods by G Gopalan, BV Rama Sastri and SC Balasubramanian, Published by National Institute of Nutrition, ICMR, Hyderabad - 500007