3. Processing of raw materials:

Extraction of vegetable oils may be the result of simple mechanical stress and in this case it may be designated as compression, expression or expelling, or of a combination of such a treatment with a solvent-laden step. That is the general case, but solvent-free exceptions deserve special mention, as they are bound to yield oils which, not having withstood any untoward chemical or physical stimulus are named virgin oils, supposedly reflecting the composition of the naturally occurring vegetable lipid phase.

Inactivation of potentially damaging lipid breakdown catalysts, namely enzymes must precede the mechanical operation freeing the lipid from cellular enclosures. For this purpose it usual to use steam or hot water in what amounts to a blanching operation and will necessarily mean contact of a water phase with the expelled oil.

The nature of the liquid mixture is such that phase separation may become a real problem, and advantage must be taken of the enhanced separability occurring at higher temperature. One consequence of this technology is that a small percentage of hydrolysis is wont to occur, yielding free fatty acids and diglycerides as impurities.

Virgin oils are those which result from extraction operated without chemicals, and not subject to purification beyond physical phase separation at moderate temperatures.

Though virgin oil from seeds are known, commercially important virgin oils are obtained from fruit. Olive oil originating in the Mediterranean area and palm oil traditionally obtained in equatorial Africa are god examples of virgin fruit oils.

Olive oil, obtained from the expression of olives, is a highly monounsaturated oil. Composition may include as little as 0.5 % stearic acid and as high as 80% oleic, with palmitic and linoleic also relatively important. Prime material may be sold as "Extra-virgin" when less than 1% acidity (as free oleic acid), or as "Virgin" olive oil if titrating to less than 3.3% oleic acid. Higher acidity olive oils are not generally sold before refining. Virgin olive oil commands a market value which is ca. five times that of refined vegetable oils, and enjoys an upsurge of demand, as a consequence of its healthy image.

For high quality virgin oil it is of great importance that fruit does not suffer stress, both mechanical or chemical. Endogenous lipoxygenase and lipase enzymes are abundant at the fruit surface and must not be allowed contact with the oil for they will initiate oxidative and hydrolytic processes, and the reaction products will mostly remain in the oil and become responsible for its instability. Oxidation will increase PV (peroxide value) and hydrolysis will increase acidity, both contributing to a decrease in conservation of oil. It is therefore usual to pick fruit carefully avoiding damage to its skin, to expell the oil soon after harvest minimizing contact time and after decanting to keep this oil in tight containers away from oxygen and light.

Other virgin oils pale in importance when compared with olive oil. Palm oil which is deeply orange coloured due to the presence of important quantities of b-carotene is an important food item in equatorial Africa, but as an industrial commodity originates nowadays mainly from South East Asia.