Heat may be transferred to the food during cooking by conduction, convection, radiation or by the energy of microwaves-electronic heat transfer.
Water or steam and air or fat or combination of these are used as cooking media. Moist heat involves water and steam.
Air or fat are used in dry heat. Foods can also be cooked by microwaves. Classification of cooking methods is given.
Heat Methods:
Boiling:
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Boiling is cooking roods by just immersing them in water at 100°C and maintaining the water at that temperature till the food is tender.
Water is said to be boiling when large bubbles are seen rising constantly on the surface of the liquid and then breaking rapidly.
Water receives heat by conduction through the sides of the utensil in which the food is cooked and passes on the heat by convection currents which equalize the temperature and become vigorous when boiling commences.
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Boiling point of water is 100°C and alter at high altitudes and in the presence of electrolytes.
When foods are cooked by boiling, the food should be brought to a vigorous boil first and the heat is then turned down, as violent boiling throughout tends to break the food.
The temperature of the water cannot be increased any further after it begins to boil and continued vigorous boiling only results in excessive evaporation of water and waste of fuel. And foods are likely to get burnt at the bottom and form a dry crust at the top.
Foods may be boiled in any liquid which is bubbling at the surface such as stock, milk, juices or syrups.
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Foods that are cooked by boiling are rice, eggs, dais, potatoes, meat, sago and beetroot. Boiling can be done with excess amount of water (eggs, potatoes) or with sufficient water (dal, upma).
Boiling is generally used in combination with simmering or other methods, e.g., cooking rice, vegetables or dal.
Advantages:
(i) It is the most simple method. It does not require special skill and equipment.
(ii) Soluble starches can be removed and rice grains are separated.
(iii) Protein gets denatured, starch gets gelatinized and collagen gets hydrolysed.
(iv) Uniform cooking can be done.
Disadvantages:
Loss of nutrients:
If excess water is used in cooking and the water is a discarded 30-70% water soluble nutrient like vitamin C may be lost.
To prevent this type of losses, cooked water should be used in soups, rasam, sambhar and dhal. Some protein may be lost if vegetables are cooked in water containing salt and the cooking water is discarded.
There is considerable loss of minerals especially sodium, potassium and calcium due to leaching.
Loss of colours:
Water-soluble pigments, like betanin from beetroot may be lost. Beetroot should be cooked along with the skin to prevent the loss of colour.
Time consuming:
Boiling may take time and fuel may get wasted.
Loss of flavour and texture:
Boiled foods are not considered tasty because flavour compounds are leached into the water. Over-boiling of food may make the food mushy.
Simmering :
When foods are cooked in a pan with a well-fitting lid at temperature just below the boiling point 82-99°C of the liquid in which they are immersed the process is known as simmering.
It is a useful method when foods have to be cooked for a long time to make it tender as in the case of cheaper cuts of meat, fish, cooking custards, kheer, vegetables and carrot halwa. This method is also employed in making soups and stocks.
Advantages:
(i) Foods get cooked thoroughly.
(ii) Scorching or burning is prevented.
(iii) Losses due to leaching are minimum.
Disadvantages:
(i) There is loss of heat-sensitive nutrients, due to long period of cooking.
(ii) Takes more time and more fuel is required.
Poaching:
This involves cooking in the minimum amount of liquid at a temperature of 80-85°C that is below the boiling point.
Foods generally poached are eggs, fish and fruits. For poaching eggs, the addition of little salt or vinegar to the cooking liquid lowers the temperature of coagulation. Eggs get cooked quickly by poaching.
Advantages:
(i) Very quick method of cooking.
(ii) Easily digestible since no fat is used.
Disadvantages:
(i) It is bland in taste.
(ii) Water-soluble nutrients may be leached into the water.
Stewing:
This is a gentle method of cooking in a pan with a tight-fitting lid, using small quantities of liquid to cover only half the food.
The food above the liquid is cooked by the steam generated within the pan. The liquid is brought to a boiling point and then the heat applied is reduced to maintain the cooking at simmering temperature i.e., 98°C.
Stewing is a slow method of cooking taking from 2 to 4 hours depending upon the nature and volume of the foods being stewed.
This method is generally used for cooking cheaper cuts of meats along with some root vegetables and legumes all put in the same cooking pot and cooked in stock or water.
The larger cooking time and lower temperatures enable tougher meat fibres to become soft.
The cooking of meat and vegetables together makes the dish attractive, and nutritious since no liquid is discarded. Apples can also be cooked by this method.
Advantages:
(i) Loss of nutrients by leaching does not take place.
(ii) Flavour is retained e.g., in making oondhya the vegetables are stewed by which flavour is retained.
Disadvantage:
This process is time consuming.
Steaming:
This method requires the food to be cooked in steam. This is generated from vigorously boiling water or liquid in a pan so that the food is completely surrounded by steam and not in contact with the water or liquid.
The water should be boiled before the food is placed in the steamer. Here the food gets cooked at 100°C.
Steaming is generally done in special equipment designated for the purpose e.g., Idli cooker and Rukmani cooker. There are three types of steamings.
Wet steaming:
Here the steam is in direct contact with the food e.g., idli.
Dry steaming:
Here double boiler is used for cooking the food.
Double boiling is cooking in a container over hot or boiling water. This process is used for such preparations as sauces and custards where temperatures below boiling point are desirable.
The food is placed in an utensil which is kept in another utensil containing water. When the water is heated or boiled the food gets cooked.
Waterless cooking:
In steaming, food is cooked by steam from added water while in waterless cooking the steam originates from the food itself.
Cooking food wrapped in an aluminium foil is another form of waterless cooking. In this case, there is an advantage of preventing the transmission of the flavour from or to the sealed food.
Recipes made by steaming are idli, dhokla, rice or ragi puttu, idiappam, appam, kolukattai, undralu and custards. Puttu made from fish or prawn is also made by steaming.
Advantages:
(i) It does not require constant attention.
(ii) Nutritive value is maintained because there is no leaching and cooking time is less.
(iii) Easily digestible since not much fat is added. It is good for children and patients.
(iv) There is less chance for burning and scorching or overcooking.
(v) In double boilers sudden increase in temperature in making custards and overflow of milk can be avoided.
(vi) Texture of food is better and becomes light and fluffy.
(vii) Steamed foods have good flavour.
Disadvantages:
(i) Special equipment is required.
(ii) Many foods cannot be prepared by this method, e.g., cooking whole grains.