Vegetable Oils- Classification & its Extraction
Classification of Vegetable Oils
One the basis of their ability to absorb oxygen from atmosphere vegetable oils are usually classified into the following four main group :
Drying oils –
These oils readily absorb atmospheric oxygen when exposed and form a dry thin elastic film. They are fairly rich in unsaturated fatty acids, especially linoleic and linolenic acids, and have a high iodine number (more than 130). Drying oils are particularly used in paint and varnish industries. Linseed oil (Linum usitatissimurn, Linaceae), safflower oil (Carthamus tinctorius, Asteraceae), poppy oil (Papaver somniferum, Papaveraceae). Soyabean oil (Glycine max, Fabaceae), and tung oil (Aleurites fordii, Euphorbiaceae) are some important oils belonging to this category.
Semi-Drying oils –
These are intermediate between drying and non-drying oils. They absorb atmospheric oxygen slowly and produce a soft film even after along exposure. They are fairly rich in linoleic acid. Their iodine number varies from 100 to 130. They are mainly used for edible purposes, as illuminant and in making soaps and candles. Some common semi-drying oils are cotton seed oil (Gossypium spp., Malvaceae), sesame oil (Sesamum indicum, Pedaliaceae), rape and mustard oils (Brassica spp., Brassicaceae), sunflower oil (Helianthus annuus, Asteracene) and corn oil (Zea mays, Poaceae).
Non-Drying oils –
These oils do not absorb atmospheric oxygen and as such are incapable of forming elastic films even after long exposure. They are fairly rich in saturated acids and oleic acids and either contain small amounts or no linoleic and linolenic acids. Their iodine number is less than 100. They are particularly useful in the manufacture of soaps, as lubricants and as food. Some important non-drying oils are ground nut oil (Arachis hypogaea, Fabaceae), castor oil (Ricinus communis, Euphoriaceae), and olive oil (Olea europaea, Oleaceae).
Fats are solid or semi-solid at ordinary temperature. They are fairly rich in saturated fatty acids, and have a very low iodine number (8-50). They are edible and are also useful in the manufacture of soap and candies. Some important vegetable fats are coconut oil (Cocos nucifera, Arecaceae), cocoa butter (Theobroma cacco, S.erculiaceae) and mohua fat (Madhuca indica, apotaceae).
Extraction of Oils
Oils and fats are usually contained as insoluble droplets or deposits within the cell of plant tissue, especially of the seed. Prior to oil extraction, the seed coat is usually removed with the help of specially designed decorticating machines and then the seed kernel is reduced to a paste. The oil is extracted by mechanical expression or solvent extraction.
Mechanical expression –
Expression is the most important method of oil extraction. It involves application of pressure to the oil-bearing tissues to squeeze out the fat. It is usually done by hydraulic pressing or screw pressing. Expression may be of hot or cold type. Hot expression involves pre-cooking of the kernel, usually with steam, to facilitate oil flow, whereas cold expression involves mere pressing of the kernel without steam. The oil is then passed through filter presses to refine it.
Solvent extraction –
By this process even very low proportion of oil can be extracted. A number of solvents like gasoline, benzene, carbon disulphide, petroleum ether and chlorinated hydrocarbons are used for the purpose. Fatty oils are freed from extracting solvents by distillation. The oil thus obtained is subjected to a refining process to remove impurities. Solvent extraction is often practiced on the press cake left after expression to remove the small percentage of oil left.
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