About Vegetable Oils

Oil Processing

In order to stop spoilage and to extend shelf life, manufacturers claim that nutrients have to be removed -- which causes flavor to be lost as well. All the natural substances that help digest and metabolize the oils are removed, including: phospholipids (including lecithin); phytosterols (which block cholesterol absorption from the intestines); Vitamin E, carotene and their precursors (which protect oils against damage in storage by their antioxidant properties); chlorophyll (which is rich in magnesium); aromatic and volatile compounds; and minerals.

During the processing of oils, natural compounds are altered by heat and chemicals, causing the nutrients to be either converted or removed. Conversion to detrimental substances include trans fatty acids, polymers, cyclic compounds, aldehydes, ketones, epoxides, hydroperoxides, and other compounds not yet identified. Many of these substances are toxic, even in small quantities.

The following are the steps taken to process oils found on supermarket shelves.


  1. Preparation of seeds begins with heating and dehulling, followed by chopping or grinding, to break the cell walls, freeing the oil to make the penetration of solvents into the cells easier. This is accomplished by rolling or flaking, but it is still not enough to release all of the oil. Therefore, a thermal or conditioning step is required. In addition, all oil seeds have enzymes that can influence quality. During processing, the object is to deactivate these enzymes early by means of heat. For example, with canola or rapeseed, the enzyme myrosinase can influence quality because it catalyses hydroysis of glucosinolates to give glucose, sulphate, isothiocyanates, oxazolinine thiones, and other compounds. Some of these compounds act as a catalyst poison during hydrogenation of oil for margarine production.
  2. Extraction from seeds is accomplished either by mechanically pressing or by mixing with such gasoline-like solvents as hexane and heptane (which are lung irritants and nerve depressants). At this stage, if the chemical extraction method is used, the oil is extremely flammable, and some factories have been known to blow up or catch on fire. Later, the oils are steam heated to evaporate the solvents at temperatures around 300°F. Most of the solvents are evaporated, but not all. The primary objective of this step is to produce a clean, crude oil product. Be aware that, at this stage, the oil can now be bottled and sold as "unrefined oil" in health food stores and delicatessens. Oil designated for more refining goes through more processing procedures. After being mashed and cooked for up to two hours at varying temperatures, depending on the seed type, mechanically pressed seeds are subject to additional heating during the "auger" process, where the average temperature reaches about 120°C (248°F) with higher temperatures and pressures producing more oil. At this temperature, however, oil reacts with oxygen more than 100 times faster than at room temperature, producing fatty acid damage. In some cases, after mechanical pressing, the oil is filtered and sold as unrefined oil, but more often, the oil undergoes further refining.
  3. Degumming is a treatment of crude oils and water, salt solutions, dilute acids, or alkalis used in order to remove phosphates, waxes, and other impurities. Caustic soda, (often sodium hydroxide -- commonly known as Drano -- or a mixture of sodium hydroxide and sodium carbonate) is one such substance used to remove free fatty acids that can cause rancidity and decreases the quality of the oil. Alkali solutions combine with the free fatty acids to form soaps and also helps to remove toxic substances that are naturally present in many plants. Temperatures again have reached 75°C (167°F). At this stage, the oil still has its pigmentaion of red, yellow, or greenish hue, which is also deemed undesireable. Degumming converts the phosphatides to hydrated gums which are insoluble in oil and readily separated as sludge. The hydrated gums are vacuum dried for crude lecithin processing. This process also involves the addition of phosphoric acid and water at temperatures of 60°C (140°F).

The industry's rationale for the degumming process is as follows:

    • It is necessary to remove the lecithin, which can cause rancidity of the oil.
    • It satisfies export oil requirments for a product free of impurities that settle out during shipment.
    • Gum removal prior to alkali refining often improves yield because the phosphates can act as emulsifiers in a caustic solution, increasing the neutral oil contained in the soapstock.
    • It substantially decreases refinery waste load because of the lower neutral oil losses and the reduction of gums discharged.
    • It prepares the oil for steam refining. Degummed oil is more suitable to this physical refining technique because of the significant reduction in such nonvolatile impurities as phosphatides and metallic prooxidants.
    • It results in improved acidulation performance. The soapstock from alkali refining is easier to acidulate because of lower emulsifier content, and the acid water has less impact on the wastewater treatment systems. (O'Brien)
  1. Bleaching oils is necessary because they have a strong yellow or reddish pigment that is considered undesirable. In the bleaching process, oils are heated to temperatures of 175°-225°C, for about 4 hours, and mixed with a type of clay substance that will absorb the unwanted pigment. Most of the spent clay is then filtered from the oil. During this phase, some of the polyunsaturated fatty acids may undergo oxidation and toxic peroxides, forming conjugated fatty acids.
  2. Deoderization is done through pressurized steam distillation at temperatures of 240°-270°C (464-518°F) for 30 to 60 minutes, which removes undesirable odors and tastes from the oil. Note: When temperatures go above 150°C (302°F), unsaturated fatty acids become mutagenic. Above 160°C(320°F), trans fatty acids begin to form. Above 200°C (392°F), trans fatty acids multiply substantially, and, above 220°C, the rate of trans fatty acids explodes. Deoderizing reduces the content of many other substances, including residues, toxins, and products of oxidation formed during the bleaching stage, as well as removing sulphur, monoglycerides, sterols, beta carotene, and tocopherols (Vitamin E). The oil is now tasteless and cannot be distinguished from other oils derived from seeds or plants. At this point, despite all of the heating involved, the oils can still be sold as "cold-pressed" since there is no accepted definition of the term. Preservatives are added such as synthetic antioxidants, BHT (butylated hydroxytoluene), BHA (butylated hydroxyanisole, propyl gallate, TBHQ (teriary butyhydroquinone), citric acid, or methylsilicone. A defoamer may also be added to prevent turbidity when refrigerated.
  3. Hydrogenation: After all this, oils not designated for sale, go on to more processing in the form of hydrogenation to make margarines, shortenings, and shortening oils. The hydrogenation of oils converts liquid oils into hard fats by adding hydrogen to the fat molecule. Oils can be hydrogenated to varying degrees, depending on the hardness. The most common forms are shortening, margarines, and the partially hydrogenated fats used for frying and in processed foods. These fats are desirable for its melting point, allowing for high temperature cooking and frying. Hydrogenation involves the artificial saturation of fully refined oils to harden them into spreadable products. All oils sold in supermarkets and convenience stores are processed in the above manner. "ALL" includes safflower, walnut, sunflower, corn, grape seed, soybean, sesame, rice bran, canola, almond, peanut, avocado, and others including blends. Olive oil is the only oil sold on supermarket shelves that is not heated above 150°C. However, it is a poor source of essential fatty acids, containing on average 10% LA and 0.5% LNA.

After the refining process above, oils are put under pressure, using hydrogen gas at temperatures of 120-210°C (248-410°F) in the presence of a metal catalyst (nickel, platinum, or copper) for six to eight hours. A nickel catalyst is actually 50% nickel and 50% aluminum. Remnants of both metals remain in the final products of hydrogenated or partially hydrogenated goods. During complete hydrogenation, all double bonds are saturated with hydrogen. This means there are no unsaturated fatty acids, no w6's, and no w3's. In some partially hydrogenated margarines, the trans fatty acid content can be more than 60%. Partially hydrogenated oils are found in French fries (37.4%), candies (38.6%), and bakery products (33.5%). A completely hydrogenated oil is now a hard fat containing no essential fatty acid activity, causing a nation-wide deficiency of the essential fatty acids.

Cold Pressed Oils:

"Cold pressed" is a term used to describe oils that have been mechanically pressed slowly so that temperatures do not rise above 140°F -- technically speaking. However, it is actually a term searching for a meaning since there are no firm regulations regarding it. Manufacturers claim that if no external heat is applied, the oil qualifies as "cold-pressed," yet these oils can be processed by the same methods as supermarket oils. As seen in the processing method, "cold-pressed" is not an accurate depiction of reality since temperatures can reach well above the minimum "standard" without "external" heat being applied. Machines heat up during mechanical pressing. In Switzerland, cold pressed is defined as those oils which have not exceeded temperatures of 50°C (122°F) from seed to bottle. In North America, there is no such specification. Oils sold as "cold pressed" can be labelled as "unrefined," "unprocessed," "expeller-pressed," etc., and sold in dark containers just like cold-pressed oils that have maintained high quality standards. The secret is in knowing the source and how they produce their oil. A terms to watch for here is "completely protected from light and air during processing" because essential fatty acids absorb sunlight increasing their ability to react with oxygen more than 1000 times opening the door for them to spoil faster, causing free radicals.

Cold pressed oils are more intense in taste and color than refined oils, containing 25-50% more Vitamin E, 45% more beta-sitosterols, and significantly lower levels of trans-fatty acids than refined oils. Unrefining also preserves the essential fatty acids. These oils are usually made from organic seeds since non-organic seeds contain high levels of pesticide residues, which are only partially removed from refined oils The reliable cold-pressed oils are usually sold in health food stores and are to be kept refrigerated.

According to Udo Erasmus, the ideal oil produced should have these steps in mind:

  • Machine parts are made of special metals, avoiding copper, brass, and iron, which catalyze oil breakdown. These metals should not be used in pipe lines or tanks either.
  • Containers should be made of black polyethylene to completely block out light and ultra violet light.
  • Oil is extracted by gravity rather than filters.
  • All containers and lines are flushed with inert gas to clear oxygen.
  • Tin and solder should be rejected since they contain lead.

Labelling trans fatty acids is a nightmare, varying from country to country. In the US, trans fatty acids are lumped together with monosaturated fats with no differentiating between the health-supporting group and the damaging group. Since monosaturated fats are considered a healthy choice, consumers are unaware that they are consuming harmful trans fats along with the healthy monosaturated ones. Trans fatty acids are largely a product of commercial processing called "hydrogenation." This type of fatty acid raises total cholesterol including LDL ("bad" cholesterol) while reducing HDL ("good" cholesterol). This is particularly noted in the French, who consume just as much of the saturated fats as North Americans, but use butter, olive oil, and other vegetable oils for cooking, whereas the majority of the North American diet consists of other types of harmful fats.