REFINED OILS
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:
- Suspended particles are removed by filtration or centrifugation.
- Free fatty acids are removed by alkali treatment.
- Any remaining free fatty acids are removed by neutralisation.
- Pigments are removed by bleaching using adsorbents like activated earth or carbon and sometimes chemical bleaching agents.
- The oil is finally deodorized by injecting steam through the heated fat under reduced pressure to obtain refined oil.
HYDROGENATION –
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.
Vanaspathi:
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°C and 37° C and it should contain 5 percent sesame oil and should be fortified with vitamin A.
Margarine :
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.
Winterization : some cooking oil become cloudy when they are stored in the reftrigerator. This occurs because some of the triglyceride molecules in the oil have higher melting points than other molecules in the mixture and crystallize or become solid at the lower temperature. In manufacturing oils intended to be used primarily for the making of salad dressings, a winterizing process is applied. In this process, the temperature of the oil is lowered to a point at which the higher melting triglycerides crystallize. Then the oil is filtered to remove the crystal the remaining oil has a lower melting point and does not crystallize at refrigerator temperature. It is referred to as salad oil. Removing the solid particles is called winterization
Leavening : When fats are creamed with sugars as in preparation of cake, butters, they help to incorporate air into the batter making it tight and foamy. On baking the protein films coagulate enclosing the air and making the end products light in texture and soft in mouthfeel. In biscuits since foaming is not required the fat is rubbed into the flour gently to provide slight aeration to impart a light crisp texture.
Shortening : Fats provide a crumbly texture to food by foaming a film around the starch and protein particles in flours preventing the formation of long gluten strands in them. This accomplished when fat is rubbed into a flour gently using the tips of the fingers only. Such treatment to flours before dough formation makes products soft and tender or short after cooking. This effect is called shortening and is used for making biscuits, cakes pie bases, pastry products etc. where a golden color and crisp texture is required through baking and frying.