What are the important changes in nutrients during cooking

When food is subjected to heat, many changes occur; there is some destruction of proteins, lipids, and vitamins, which is detrimental to the nutritional value of food. However, there are also some beneficial changes. Cooking is required if we have to obtain the maximum nutritive value of some foods and maintain a safe and wholesome food supply. Heat treatment is also one of the most common and effective methods of food preservation and may be used alone or in combination with other preservation techniques. Some of the changes that take place in the constituents of food during cooking are discussed below.

Changes in Proteins:

The principal effect of heat on protein is denaturation. This results in the destruction of microorganisms and inactivation of microbial and natural enzymes within the food. Cooking also des- troys the toxic proteins and peptides, enzyme inhibitors, antivitamins, and other natural toxicants in food, which can seriously affect their nutritive value. Legumes contain trypsin inhibitors, haemagglutinins, and other toxic substances that affect the digestibility and availability of sulphur-containing amino acids. These are destroyed by heat. Approximately 40% of the growth-depressing effect of uncooked soyabeans is due to the presence of trypsin inhibitors. Of the remaining proteins of soyabeans, digestibility increases after cooking. Cereal grains also contain trypsin inhibitors and natural toxicants. Heat destroys these antimetabolites in rice, wheat, and oats, but has little effect on other cereals.

Cooking can also result in the interaction of protein with nonprotein components of the food system; there can be interaction of protein with carbohydrate or lipid oxidation products. There can also be interprotein and intraprotein reactions in the presence or absence of oxygen. These changes in cooking result in the nutritional unavailability of proteins.

There is significant loss of lysine and the sulphur-containing amino acid cysteine after heating proteins. On prolonged heating, tryptophan, methionine, and the basic amino acids are also lost. Charring and the presence of off-odours during cooking is due to the destruction of amino acids and proteins. These changes affect the palatability of the final product.

Interaction between the free amino groups of proteins with reducing sugars or carbonyl groups formed by lipid oxidation results in nonenzymatic browning (Maillard browning). This reaction is of importance because it is responsible for many of the specific tastes, aromas, and colours of foods. In this reaction, the alfa-amino group of the essential amino acid lysine reacts with carbonyl groups of sugar and fatty-acid oxidation compounds, which results in a decrease in the nutritive value of proteins.

Changes in Carbohydrates:

Monosaccharides, oligo, and polysaccharides undergo many transformations when cooked in an aqueous medium. The sugars are subjected to degradation and epimerization, and of over 100 compounds that are formed by such transformations, hydroxymethyl furfural is the most important compound. This compound and furfural are also products of nonenzymic browning reactions. But these compounds have no adverse effects.

Starch molecules, which are the main source of calories in many diets, when heated in an aqueous or moist environment, swell and rupture, and this permits greater enzymatic digestion by enzymes like amylases Cooking thus increases the digestibility of carbohydrates. Starch, when subjected to dry heat at a temperature of 200°C or higher, breaks down, resulting in the formation of dextrin and volatile compounds like furfural and hydroxymethyl furfural

Changes in Lipids(Fats):

Lipids undergo hydrolytic, oxidative, polymeric, or other degradative changes that modify not only the physical properties of the lipid but also their biological properties when heated. The hydrolytic and oxidative changes result in rancidity. Hydrolytic rancidity is catalyzed in food at high temperatures and pressure in an aqueous medium in the presence of acids, alkalis, and lipolytic enzymes (lipases). Hydrolytic rancidity by itself does not bring about any significant change in the nutritive value of the food. However, the objectionable flavour imparted by free fatty acids lowers the consumption of food.

Oxidative rancidity is responsible for more losses in the quality and nutritive value of lipids than any other change. This rancidity results in the formation of hydroperoxides as primary products owing to the attack of oxygen on the unsaturated centres of lipids. The products of oxidation exhibit strong unpleasant flavours even when present in extremely low amounts. The oxidation of a fat at high temperature, in addition to volatile and nonvolatile compounds associated with lipid oxidation at normal temperature, gives isomerization products (trans- and conjugated double bond products), cyclic compounds, dimers and polymers. While lipids oxidized at normal temperatures exhibit no toxicity, thermally oxidized lipids have shown various detrimental effects. But the lipid or food system would be unpalatable long before the concentration of the toxic compound reached a hazardous level.

In addition to the effect on the biological properties of lipids, thermal effects bring about physical and chemical changes also. In sauteing and shallow fat frying, the quantity of oil used is small, cooking time is short and there is generally no reuse of fat or oil and thus there is little concern over the nutritional effects of lipids absorbed from such cooking. In contrast, there has been a great deal of concern over deep fat fried foods. If the deep fat frying is continuous, oxidative changes are small because the fat absorbed by the food is constantly replaced. In discontinuous deep fat frying, as is common in homes, there is liberation of fatty acids due to the addition of water to the oil from the food, decreased unsaturation and increase in peroxides, conjugated double bonds and polymers. Such fats absorbed by foods could be toxic when consumed.

Changes in Vitamins and Minerals:

These are lost primarily by leaching, oxidation of the water-soluble nutrients, and thermal destruction. The loss of water-soluble vitamins ranges from 0 to 60% as a result of leaching and thermal destruction. In addition to losses by water and heat, ascorbic acid is lost by oxidation due to exposure to air of food during cooking. Vitamin A and carotene are water insoluble and, as such, are not lost as a result of leaching, and their destruction due to oxidation is very slight. Frying and roasting cause their loss up to 40-60%. Minerals are also lost on account of leaching, and their losses are smaller (0 to 35%).

Changes in Colour:

Colour factors in foods, such as anthocyanins, carotenoids, chlorophylls, myoglobin, etc., are affected by heat. In addition to heat, the acidity or alkalinity of the cooking medium, oxygen, and presence or absence of metals also contribute to colour changes when heated. In some cases, the colour changes that take place in foods on cooking are desirable (as in baking) while in some other cases the changes may be undesirable (as in the prolonged cooking of cabbage). The cooking condition should be so organized as to obtain the desired colour qualities in the cooked food.