FATS AND OILS
Fats are an important component of the diet and is present naturally in many foods. Fats are solid at room temperature while oils are liquid. Fats in the diet can be of two kinds viz., the visible and the invisible fat.
Invisible fats are those present inherently in foods. Example of food containing appreciable quantities of invisible fat include meat, poultry, fish, dairy products, eggs, nuts and seeds.
Visible fats are those fats that are made from these products. They are cooking oils, salad oils, butter, ghee and margarine.
1. They are a concentrated source of energy. One gram of fat contributes 9 kilocalories as against 4 kilocalories contributed by carbohydrates and protein.
2. They are a good source of vitamin A,D,E and K.
3. They provide essential fatty acids which are components of membranes of living cells.
4. They impart special flavour and texture to our foods, thus increasing palatability.
5. They are also used by the body to make prostoglandins involved in a large variety of vital physiological functions.
Oils and fats do not occur free in nature. They occur in animal tissues and in seeds and fruits from which they are isolated.
The extracted oils are crude and contain many constituents like free fatty acids, unsaponifiable matter, gums, waxes, mucilaginous matter, variety of colouring matter, metallic contaminants and undesirable odour producing constituents. In refining these constituents are removed by the following steps:
1. Suspended particles are removed by filtration or centrifugation.
2. Free fatty acids are removed by alkali treatment.
3. Any remaining free fatty acids are removed by neutralisation.
4. Pigments are removed by bleaching using adsorbents like activated earth or carbon and sometimes chemical bleaching agents.
5. The oil is finally deodorized by injecting steam through the heated fat under reduced pressure to obtain refined oil.
HYDROGENATION - VANASPATHI AND
Plant oils contain a large percentage of unsaturated fatty acids and hence have a tendency to become rancid. These unsaturated glycerides in oil can be converted to more saturated glycerides by the addition of hydrogen. This process is known as hydrogenation.
Hydrogenated fat is manufactured from vegetable oils by the addition of molecular hydrogen to the double bonds in the unsaturated fatty acids in the presence of nickel.
The double bonds take up hydrogen and saturated fatty acids are obtained. By this process, liquid fats can be converted to semi solid and solid fats for use as shortening in the preparation of biscuits, cakes and butter substitutes.
Hydrogenation is of great economic importance because it allows oils to be converted into fats, which have better keeping quality.
As hydrogenated fats are prepared from refined deodourised oils, the resulting fats are odourless and colourless and blend well in several food preparations.
Hydrogenated oil in India is known as vanaspathi. It is manufactured by hydrogenating refined groundnut oil or a mixture of groundnut oil with other edible vegetable oils.
According to vanaspathi control order, the melting point of vanaspathi should be between 31 o C and 37 o C and it should contain 5 percent sesame oil and should be fortified with vitamin A.
Margarine is often used as a substitute for butter. It is made from vegetable oils or a mixture of vegetable and animal fat by hydrogenation.
It is then blended with cultured skim milk and salt. The fats most commonly used in the manufacture of margarine are cotton seed oil, soyabean oil, corn oil, groundnut oil, coconut oil and meat fat.
Additional additives may include diacetyl for butter flavour, sodium benzoate for preservation, mono and diglycerides or lecithin for emulsification, yellow colouring matter and vitamin A and D.
Fats and oils undergo certain undesirable changes during storage which result in spoilage. The major spoilage of fats and oils is rancidity.
Rancidity refers to the development of disagreeable odour and flavour in fats and oils owing to specified chemical reaction such as oxidation and hydrolysis.
Hydrolysis is the decomposition of fats into free fatty acid and glycerol by enzymes in the presence of moisture. These free fatty acids released are responsible for the unpleasant flavour and odour.
During oxidation, oxygen is added to the unsaturated linkage and this results in the formation of peroxides. These peroxides decompose to yield aldehyde and ketones which are responsible for the pronounced off flavour. Rancidity may also be caused by the absorption of odour and action of micro organism and enzymes.
Prevention of rancidity :
1. Storage in coloured glass containers prevent oxidation of fats by rays of light.
2. Vacuum packaging retards rancidity by excluding oxygen.
3. Naturally occurring antioxidants like vitamin C, b carotene and vitamin E protect against rancidity.
4. Synthetic antioxidants like butylated hydroxy anisole (BHA), butylated hydroxy toluene (BHT) and propylgallate can also be added to prevent rancidity.
When fats and oils are heated to a high temperature, decomposition of fat occur and finally a point is reached at which visible fumes are given off. This is called smoking point and the temperature is called smoking temperatures of fat.
Smoking temperature is defined as the lowest temperature at which visible fumes consisting of volatile gaseous products of decomposition are evolved.
Factors affecting smoking temperature of fats and oils :
1. The amount of free fatty acids present.
2. The surface of oil exposed while heating.
3. The presence of mono and diglycerides and foreign particles such as flour particles.
Smoking temperature is important for fats used for frying. Fats with low smoke point are not suitable for frying because of the odour and irritating effect of the fumes.
The decomposition products may also give an unpleasant flavour to the food. Hence it is preferable to use fats with relatively high smoking temperatures for frying.
The smoking points of some fats and oils are given
Oil or fat Smoking temperature ( o C)
Soyabean oil 230
Hydrogenated fat 221
Butter fat 208
Groundnut oil 162
Coconut oil 138
ROLE OF FAT / OIL IN COOKERY
1. Fat is used as a medium of cooking in shallow and deep fat frying.
2. Fat improves the texture of food. e.g., cake, biscuit, cookies.
3. Fats help in leavening - in making cake, leavening occurs by incorporating air into the fat during the leavening process.
4. Fat increases smoothness of the product e.g., Halwas, crystalline candies.
5. Fats are shortening agents- one of the most important function of fat is to shorten baked products which otherwise are solid masses firmly held together by strands of gluten.
Fat improves palatability - fat gives taste and flavour to the food.