What are the important scientific methods of food preparation?

Food preparation is much more than a science; it is an art: for it is linked with the total cultural pattern of people. Food preparation requires a sense of discrimination in the blending of flavours as well as of textures, colours and shapes. Food preparation, like any skill, requires a considerable amount of practice in order to achieve a high quality product with efficient use of time, money and material. The principles of food preparation are based upon the physical and chemical characteristics of the various food groups.

I. The following principles of cooking need to be borne in mind.

i. All foods must be cooked in a way which ‘keeps the flavour in’.

This ensures that the flavour or aroma of the food is retained. It stimulates the secretion of the digestive juices and aids in effective digestion and assimilation of foods. For example, the flavour of fish or vegetables may be retained by frying them with a protective covering of batter.

ii. Sometimes the flavour of the food is ‘drawn out’ into the gravy or broth.

This too aids digestion and brings about maximum absorp­tion of nutrients. Meat when cooked over a slow heat produces an aroma which makes the broth more palatable.

iii. The preservation of the maximum nutritive value can be ensured by using the correct methods of cooking, suited to the par­ticular foods.

For this, we need to know the effects of heat on different nutrients.

Wet cooking breaks up the starch cells in food, making it softer and more accessible to starch-splitting enzymes. Starch gelatinises at a temperature below the boiling point of water. Dry heat converts starches into dextrin, a midway product between starch and sugar. Sugar when heated melts in its own water of crystallisation. It is converted first into barley sugar and then into caramel sugar, both of which have entirely different properties in comparison with the first sugar used.

The different proteins present in meat are collagen in connective tissue, fibrin in blood clot, and myosin found in muscle fiber. Milk protein is known as caesinogen, while legumin is found in pulses. Wheat protein, gluten, is a mixture of two proteins-gliadin and glutenin. These when mixed with water give the characteristic sticki­ness to dough. The proteins found in rice, barley and maize are known as oryzenin, hordenin and zein respectively. Albumin forms the major protein content of egg and has the highest biological value.

Proteins get coagulated by both dry and wet methods of cooking. They are changed into gelatin on moderate heating. Gelatin is soluble in water and is conveniently absorbed. Proteins tend to harden, shrink and become indigestible when heated strongly, as is seen in hard­boiled eggs and roasted meat.

Fats are not generally much affected by heat. Partial decomposi­tion may take place if they are subjected to very high temperatures for a really long time. The fatty substances thus produced have an acrid odour and irritate the digestive tract.

Vitamins C and B are heat-labile and are destroyed by heat. The fat-soluble vitamins are only destroyed at a very high temperature. Substantial loss of minerals occurs when foods are cooked in large quantities of water. This loss is further increased if the water is thrown away after cooking vegetables and other foods.

II. There are several reasons for cooking food

i. Cooking improves the flavour and palatability of foods, e.g. meat and potatoes. The use of moderate amounts of spices and condiments adds taste and stimulates the flow of digestive juices.

ii. Cooking brings about physical and chemical changes in the food whereby colour, texture and appearance may be improved. This increases palatability, acceptability and the digestibility of the food.

iii. The same food cooked in different ways provides variety in the diet. For example potatoes may be eaten in many forms-boiled, baked, fried, in parathas, mixed with other vegetables in cutlets-to avoid monotony.

iv. The cooking of food makes it more digestible. It softens the food making it easily chewed, masticated and digested without undue strain on the digestive system. Cooking makes raw rice, pulses and meats more digestible.

v. Cooking en chances the availability of some nutrients. For example, trypsin inhibitor present in protein foods is destroyed by cooking. This makes the trypsin freely available to the body. Similarly, starch is more easily available to the body after cooking.

vi Cooking destroys bacteria, thus making the food safe for consumption. This also improves the keeping quality of the food. Boiling sterilizes milk to some extent, making it safe to drink. Boiled milk can also be kept for a longer time without getting spoilt.

Different, traditional and modern methods of cooking are discussed below:

(A) Dry Heat

Cooking by dry heat includes broiling, baking and roasting.

Broiling or Grilling

Broiling food is the most ancient and primitive method of cooking. It is the simplest method of preparing food. In this, the food is exposed directly to fierce red heat either of a gas flame, electric wires or live coals. It is necessary to have strong heat to begin-with. In broiling or grilling the fire should be free from smoke, or ash, otherwise the food will become sooty. A little salt sprinkled on the fire will keep it clean.

Pan broiling can also be done by using a hot metal pan or top of the stove, with just enough fat to keep the food from sticking; fat is poured off as it accumulates. In this method of cooking, there is little loss of vitamins and the food has a good flavour.

Baking

Baking is cooking by dry heat in an enclosed box called an ‘oven’. It can be an electric oven or gas oven. The enclosed air in the oven is heated and that cooks the food. To obtain good results, it is necessary to place food properly in the oven and have an evenly distributed temperature throughout. Biscuits, bread, cakes, vegetable pies, baked puddings are some examples of baked preparation.

Earthen Oven

An earthen oven or tandoor is a little different from the ordinary oven. The earthen oven is comparatively cheap, big and round in shape. First it is heated up with the help of fire or electricity. Then ‘chapaties’ or ‘nans’ are made and pasted on the inside walls of the oven. Meat preparations like seekh kababs, tandoori fish and chicken are some examples of food cooked in a tandoor.

Baking is rather a slow method of cooking, but has the advantage that large quantities of food can be cooked and the foods are cooked evenly. There is not much loss of nutrients. The food is easily digestible.

Roasting

Roasting is cooking over an open fire in a dry medium so that all surfaces of food are equally heated. Examples of roasting are brinjals for burtha, appalam and phulka. Sometimes the food is not brought into direct contact with fire as in roasting grams, peanuts, sweet potatoes, puffed rice. This is done by putting, sand in a deep frying pan (karahi) and when the sand is very hot, the peanut or gram or corn may be put in it. Sometimes roasting is done by using a little fat as medium; examples of such foods are roasted groundnuts and corn. In this method very little fat is used, from time to time. It requires skill and careful control of heat source also. It is a quick method of cooking but some of the vitamin contents of food are destroyed as food cooked comes directly in touch with fire.

(B) Moist Heat

Cooking by moist heat includes boiling, stewing, braising and steaming.

Boiling

This is a method of cooking food with the help of water. The food is completely immersed in water and boils at 100° C. The bubbles break rapidly on the surface of the water. Note here that boiling violently does not cook the food faster. In fact it wastes fuel, breaks the food and spoils its appearance. The quick evaporation of water can result in the food getting burnt. Boiling is specially adapted for cooking root vegetables, cereals, pulses and meat which by reason of their size, thickness and toughness require thorough cooking in a medium. The advantages of boiling are that it is an easy method and does not require constant attention. Food cooked by this method is always recommended for patients.

There are, however, certain disad­vantages of cooking food by boiling. Some valuable nutrients and flavours are lost when the cooking water is discarded. Boiling affects the colour of the vegetables also. To get good results, the housewife should use just the right amount of water for boiling the food. The size of the pan may be selected according to the amount of food to be boiled. The lid of the container should be closed tightly. The foodstuff should be boiled with the skin to conserve the nutrients. In case of green vegetables, the pan should be uncovered for the first five minutes. Mostly, vegetables are simmered in a small amount of water. If boiled in too much water rapidly, the water evaporates quickly and the food is cooked insufficiently.

Stewing

This is another method of cooking food in water like boiling. In stewing, the cooking is carried out in a small amount of liquid, in a covered pan, over mild heart, below the boiling point of water for a long period. The cooking utensil is tightly covered to avoid evaporation. Stewing is advantageous in that it does not need constant and frequent attention stewing is suitable only for cooking tough foods such as meat, pulses and dried vegetables to make them tender and digestible. Since foodstuffs are cooked in covered pans and the juices are retained as gravy, stewed foods are nourishing.

Protein is coagulated without over-hardening. Soluble vitamins, other nutrients and flavoring constituents pass into water which is served also. However, some of the valuable contents of the food, chiefly Vitamin C, are destroyed because of the slow process of cooking. To get good results the containers should be tightly closed. Food to be stewed should be cut in small pieces to get it cooked well. The use of water should be limited and lastly the heat provided should be slow and steady to avoid over cooking.

Braising

Braising combine’s two methods, roasting-cum-stew­ing. This is the most common method used in everyday life. The vegetable and meat is first made slightly brown in hot fat and then cooked in little water. By doing so there is no loss of nutrients as moisture is entirely used up.

Steaming

Steaming is also a method of cooking foods with water, but in this case, the food does not come into direct contact with water. It can be done by two ways-direct and indirect. In the direct method of steaming, the steam is applied directly to the food which is placed in a perforated rack over boiling water in a pan which is tightly covered with a lid-as in making idlis, or steamed fish.

In the indirect method, the food-is packed into a vessel with a lid. The vessel is then immersed in another vessel of boiling water (double boiler). The heat for cooking the food is supplied by the boiling water all round the immersed inner vessel, as in the making of puddings. This indirect method of cooking can be used to advantage for heating up food. The food cooked by this method is light and easily digestible. Steamed food can be served to a patient.

There is little loss of nutrients and the flavour is also not lost. The food does not lose its shape as the food grains do not shrink due to evaporation of water. It can be made economical if several compartments are used at the same time as in a pressure cooker.

Cooking under Pressure

A pressure cooker is a device which is commonly used in this method. Food is placed in a sealed container with very little water and cooked by the pressure of steam. The pressure of ‘weight’ (5, 10 and 15 lbs) is used for various foodstuffs, which maintains the temperature at about 120° C inside the container. The ‘weight’ regulates the pressure by preventing the escape of steam. It is a useful device now-a-days as it saves fuel and time. The food can be cooked rapidly. Different dishes of the meal can be prepared at a time. There is a loss of Vitamin C and other soluble vitamins and mierals.

The pressure cooker and its various components should be thoroughly cleaned after use. The cooker should not be filled more than 2/3 of its capacity. The lid used should be air-tight. The ‘weight’ should be put on when there is complete pressure inside and it starts making a hissing sound. The time should be checked properly and after the food is cooked, it should be cooled slowly. No attempt should be made to remove the ‘weight’ till the pressure cooker has fully cooled.

(C) Frying

In frying the food is cooked in hot fat. Fat has a very much higher boiling point than water. Frying is a quick method of cooking because of the high temperature that is used. It needs continuous careful attention. Frying can be done by different ways:

(i) Sauteing.

In this, a small quantity of fat is used, which is just sufficient to be absorbed by the food cooked in it. The food is turned frequently.

(ii) Shallow Frying.

Sufficient quantity of fat is used in the pan and food is turned to cook both sides equally as it is done in the case of paratha, omlette, pancake, and tikki. The fat should be drained by using absorbent paper before serving.

(iii) Deep Frying.

Deep fat frying is done in a deep saucepan or kadai which contains excess quantity of fat or oil as to so immerse the food fried. Potato chips, bonda, pakoras and puris are deep fat fried preparations.

For satisfactory results, food should be fried in fats and oils heated to 320° C. At this temperature a faint blue fume or smoke rises from the fat or oil. If the food is introduced into the hot fat or oil before the blue fume rises, then the fat will penetrate into the food and make it sodden, soggy and greasy. The fat should not be overheated, as over­heating causes browning and burning of the foods outside, while the interior may remain uncooked.

The food is cooked rapidly and evenly. Fried food is delicious to eat and appears pleasant looking. The disadvantages of fried food are that the food becomes heavy and is difficult to digest. It loses its vitamins and, therefore, is not so nutritious to eat.

(D) Solar Cooking

It is a method of cooking food by converting the solar energy into heat energy. A solar cooker is placed at such an angle that the mirror reflects the sun’s rays into the food placed in the container. The containers are blackened to absorb the maximum of the sun’s heat. This is an economical method of cooking as it saves fuel and electricity. The solar cookers are sold at subsidised rates.

(E) Infra-red Radiation or Microwave Cookery

It is a new advanced method of cooking which is not very commonly used. The equipment is still too expensive for household use. The food to be cooked in placed in an electronic oven where it is exposed to the penetration of microwaves produced by a magnetron tube. The microwaves cause agitation of the molecules within the food so that heat is generated. Cooking time is shortened to ten times less than may be needed by conventional methods.

The disadvantages of microwave cookery are that food is only cooked in small quantities. The food is unattractive to look at, as it does not brown the food. Time has to be adjusted carefully to avoid over-cooking. The flavor and nutritive values of vegetables are good in comparison with other methods of cooking.

Changes in Food during Preparation-Colour, Texture and Flavor

However, nutritious a meal is, it needs to be attractive in appear­ance and flavor if it is to be eaten and the nutrients made use of. It must stimulate the appetite. The art of food preparation is the art of skilful combination of colour, texture and flavor to please the eyes, the nose and the palate.

Colour.

Food undergoes many colour changes during cooking, some of which enhance the desirability of the product whereas others do not. Fruits and vegetables have attractive colours but during cooking, the colour is changed due to the pH of the cooking medium. Chlorophyll, the green pigment of the plant foods is affected due to heat. The green colour of the leafy vegetables is changed to olive green and then to brown in the long run, especially when an acid medium is used. Sometimes baking soda (alkali) is used to maintain the green colour, but it destroys the Vitamin C and thiamine content of the food. It is advisable to maintain the good colour by cooking the green leafy vegetables in an uncovered utensil or leaving the pan uncovered for the first few minutes of cooking.

Carotenoides, the red pigments present in peaches and carrots, leach out into the cooking medium making the food look pale in appearance. The colour remains red in a slightly acid medium but turns blue, in an alkaline medium. The important feature of the carotenoids is that they are organic compounds with unsaturated chains. This property is destroyed by oxidation in air or hydrogena­tion.

Oxidation is responsible for loss in colour. During the ageing of kneaded flour or bleaching by oxidising agents, the colour is bleached producing white flour which in turn makes the bread white. The linkage of certain amino acids in flour with sugar forms brown crusts on bread and cakes after heating-dextrinization. Dextrin is a midway product between starch and sugar.

The pale yellow colour of onions, asparagus and apple is due to flavonoid pigments. Alkaline tap water is liable to cause them to turn into a deep yellow-brown colour on cooking which can be prevented by adding an acid such as cream of tartar or a little lemon juice. Yellowing of rice can also be prevented in this way.

The colour of meat is due to a red pigment in muscle called myoglobin and also due to blood pigment called hemoglobin. The red colour of meat is changed to pink, grey, or brown upon exposure to heat.

Texture:

Some people are very sensitive to the texture or physical state of the food they eat. One gets irritated when one comes across grains of sand stones or pebbles or any uncooked food particles under the teeth. Raw food will go through many processes or stages during preparation. The food is cleaned and cooked before it is consumed. Proteins coagulate on heating; egg white coagulates and becomes solid when heated. When milk is heated a scum is formed on cooling; this is due to the protein coagulation. The addition of citric acid in hot milk coagulates it very fast by separating casein, the protein part of milk. Coagulation is better at low temperature. Overheating makes the substance stringy and tough.

Not all proteins coagulate on heating. Collagen and elastin, the two important insoluable proteins in meat, become tough on cooking by heat; so in order to make the connective tissues of meat tender, the, meat should be cooked by moist heat.

The texture of cereals, fruits and vegetables is related to the cellulose fibres. The cellulose becomes soft on cooking but loses its shape on overcooking. In case of cereals cooking gelatinizes the starch, softens and breaks down the cellulose framework of the plant. When starch is cooked using by dry heat, it darkens in colour as the starch is converted into dextrins which are more easily digested than starch.

However, excessive heating causes starch to char. These changes can be observed during the toasting of bread. If starchy food is cooked by moist heat, is grains absorb moisture and swell and burst or gelatinise and a thickish paste is obtained. It is because of this that maida, cornflour or flour is used as a thickening agent. When sugar is cooked by using dry heat it melts and becomes dark brown in colour and is known as caramel. On further heating it chars and burns.

The use of fats brings tenderness in texture. The quality of cakes and biscuits is adjudged by their texture. Fat increases the fermenting Power of baking powder in the cake batter and improves the final tenderness. It gives strength to the batter and decreases its tendency to collapse under its own weight before gluten and egg-white (both proteins) set. Fat also prevents the formation of a continuous gluten system and produces a low breaking strength which is known as shortening power. This makes the cakes and biscuits easy to bite.

Flavour:

Cooking not only improves the colour and texture of foods but brings out new flavours. Flavour is sensed by taste and smell, the two important sense organs of the. Good flavoured food encourages formation of saliva in the mouth which is helpful in digesting food. However ill-cooked or overcooked food loses its natural flavour as well as taste. Physical and chemical changes are responsible for the changes in flavour which are brought about during cooking processes.

The roasting and grinding of coffee seeds pro­duces a change in the flavour. Raw meat and fish always have an un­favourable flavour judging by the smell and taste; but the odorous components of the food are lost on heating. Spices and essences are used in cooking to improve the flavour. It is better to use these ingredients as late as possible with minimum heating to conserve the essential oils. Highly flavoured foods should be wrapped in aluminum foil to retain the maximum inherent flavour, which is lost when it is cooked unwrapped.