Cold-pressed and Virgin Oils
Cold-pressed or virgin oils are optained from unpeeled, in some cases peeled seeds or fruits. It is pressed with a purely mechanical process without the addition of heat.
What remains is the press cake, which still contains 10% to 14% residual fat. After pressing, the oil is filtered to remove seeds or fruit residues.
This gentle extraction process preserves the oil's characteristic flavor, vitamins, unsaturated fatty acids and other desirable secondary plant substances.
This process is obligatory for all ecological oils, but it is also common in the conventional sector.
The native or cold-pressed oil usually has a very good shelf life.
If such an oil is stored in the refrigerator, white streaks will form as the waxes flocculate. This is by no means a deterioration of the oil, but proof that this oil has not been further processed after pressing.
However, there are also native and cold-pressed oils that do not form white streaks when stored in the refrigerator. These oils were "winterized" in the oil mill, i.e. cooled to 8° C for a period of 5-10 hours. During this process, the waxes crystallized and are filtered.
Refining of Cooking Oils (Physical Refining)
When refining organic oils, organic citric acid is used instead of sodium hydroxide. Subsequent deodorization (the oil is washed with steam) removes the free fatty acids up to a maximum of 0.1% residual content. The washing of the edible oil with steam is carried out at a wide range of temperatures. These range from 130° C to a maximum of 220° C.
The advantage of physical refining is a significant reduction in environmental impact.
Fully refined oils are often labeled as RDBW (refined, deodorized, bleached, winterized). Unlike cold-pressed virgin oils, refined oils do not have such a long shelf life.
Furthermore, refined oils are tasteless, nearly colourless, and varieties such as high oleic are extremely heat-resistant, giving them a great advantage when used for hot cooking.
However, desirable secondary plant substances such as vitamins, unsaturated fatty acids, etc. are hardly contained in refined cooking oils.
All organic oils must be processed according to the physical refining system.
Partial refining such as "deodorized" or "weak steaming" is also possible. Therefore, the temperature range can be selected according to customer requirements. In principle it should be noted that the higher the temperature selected or the longer the reaction time, the more neutral the edible oil tastes.
Most cooking oils - except for rapeseed oil, for example - also contain waxes. If these oils are stored cold, wax crystals form and the oil becomes cloudy. At room temperature, the waxes liquefy again and the oil becomes clear again. This natural process can be switched off by winterizing the oil. Therefore, the oil is cooled for a few hours so that the waxes crystallize out. The cold, cloudy oil is filtered, the solid waxes remain in the filter and the oil remains clear even at low storage temperatures.
Extraction of Oils (Exclusively for Conventional Products)
In this method, the oil contained in seeds and fruit is displaced through the use of a chemical solvent (usually hexane). The hexane-oil-mixture is heated to a minimum of 140 degrees, which causes the hexane to evaporate and the plant oil to remain. In place of hexane, the use of crystallized carbon dioxide (CO2) is common as well and more environmentally friendly than hexane extraction. In comparison to press cakes, the by-product – extraction meal – still contains 2% rest fat rendering a higher oil yield.
More than 90% of plant oils worldwide are produced through extraction. However, all of these oils require to be refined or cleaned further before use.
During chemical refining, lecithin is withdrawn from the oil before adding phosphoric acid for settling and filtration. Next, sodium hydroxide is added to the oil, that reacts with the free fatty acids contained within the oil and precipitates in the form of sodium soap. In order to remove colour and all undesirable content such as pesticides, peroxides, or compounds formed during the extraction process, the oil is then bleached with bleaching earth and/or activated carbon. The final step is steaming or deodorization to remove nearly all flavouring agents.