Processing of Oils


Processing of Oils

Several different steps and methods are used in the processing of oils. The quality, flavour and nutritional content of oils vary greatly according to which processes are used.

Extraction Methods

How an oil is extracted affects the nutritional quality of the oil because heat, light and oxygen can destroy nutrients. Here are the most commonly used extraction methods:

Expeller/Mechanical Pressing A chemical-free mechanical process that extracts oil using a screw type machine. Friction generates temperatures that may reach or exceed 49 degrees Celsius (120 degrees Fahrenheit) depending on the hardness of the seed, grain or nut.

Cold/Fresh Pressing Term used for expeller pressing at temperatures below 49°C. No external heat is applied during extraction. The heat is generated only when pressure builds up around the screw by the accumulation of oil seeds (friction). There is no refining process following the extraction. Extra Virgin Olive Oil, Virgin Oils and high quality EFA Oils are extracted in this manner. The term “Fresh Pressed” is used in North America, as “Cold Pressed” is an official designation in European countries, the definition of which North American manufacturers does not adhere to stringently.

Vacuum Extraction Vacuum expeller process system that extracts oils in a non-oxygen and light-free atmosphere at temperatures as low as 21°C (or 70°F).

Solvent Extraction Oils are extracted chemically with petroleum solvents such as Hexane, which destroy the oil’s nutritional value. Solvent extraction damages the integrity of oils and creates trans fatty acids in the process.


Refining removes an oils’ minor ingredients or impurities. This ensures oils perform well (not burn) when used for high heat cooking applications. Impurities may include phytosterols, chlorophyll, flavour molecules, colour molecules, lecithin, antioxidants, and other oil-soluble beneficial molecules.

There are two methods of refining:

Mass Market solvent extracted oils use refining processes that include very high temperatures (of up to 260°C or 500°F) for processing and deodorizing. They are defoamed with methyl silicone, washed in a caustic solution of lye and soap (that removes essential fatty acids), bleached with hydrochloric or sulfuric acid, and deodorized with chemicals like BHT, BHA.  They are also degummed, a process which removes impurities. Refining exposes delicate oils to high heat, oxygen, and moisture, causing oxidation and rancidity. This process, resulting in colourless and nearly tasteless oil, contributes to the formation of free radicals (see definition below). Refined oils are used in high temperature cooking, since being free of impurities they have a high smoking point.

A growing number of natural oil companies refine fresh pressed oils for higher temperature culinary uses. These companies do not use any harsh chemicals or excessive temperatures (over 121°C or 250°F) in their refining process. They claim these oils are free of free radicals from processing.  Naturally refined oils tend to keep some of their natural flavour as well.

Nevertheless, both processes subject oils to several steps beyond extraction and involve enough heat to denature the oils and destroy valuable nutrients. About refined cooking oils, Udo Erasmus, Oil researcher and author states:

“In addition to the removal of beneficial nutrients, 0.5 to 1% of the oil molecules are changed into substances that have never been present in nature, do not fit into the very precise molecular architecture of the human (or animal) body, and therefore have highly toxic effects on life’s biochemical processes.”

The highest quality oils with the most nutritional value are unrefined. Only unrefined oils have the nutrients needed to sustain life. They also retain their full flavour, whereas chemically refined oils are virtually odourless and tasteless.

  • The processing method of an oil determines its quality and nutrient profile.
  • The least processed oils are the most beneficial for human nutrition.
  • Fresh Expeller pressing leaves oils in their most natural and nutritional state.
  • Refined oils have been denatured and are lacking essential nutrients.

Hydrogenation and Partial Hydrogenation

Hydrogenation is the process of changing liquid oil into one which is solid at room temperature. Margarine and shortening are products that have been hydrogenated.  Highly inexpensive, inferior quality polyunsaturated oils, already rancid (see definition below) from the solvent or high heat extraction process, are mixed with nickel or aluminium compounds and subjected to hydrogen gas at high pressure and temperature, then emulsified, deodorized by high temperature steaming, bleached and mixed with colour and flavour so the resulting product resembles butter. The process is used to make an oil that provides a longer shelf life for baked products, provide longer fry-life for cooking oils, and provide a certain texture.

Hydrogenated oils block the utilization of EFAs by the body, leading to a host of health issues. Hydrogenated oils can be damaging to the immune system and detrimental to proper cellular function. Unlike saturated butter or virgin coconut oil, hydrogenated oils contain high levels of trans fatty acids. Until the 1970’s, food producers used coconut or palm oils. The American obesity epidemic began when hydrogenated and partially hydrogenated vegetable oil became the processed food oil of choice. Monoglycerides and di-glycerides are oil products made from hydrogenated fats.

Rancid Oils and Free Radicals

Rancid oil is one that contains many free radicals. These are molecules with unstable, reactive, unpaired electrons, which invade the body causing negative chemical reactions and damaging tissues. They form when oil is subject to excess heat, oxygen or moisture such as during processing and cooking. Free radical damage is one of the major causes of premature aging and wrinkles and has been shown to be a major contributor to autoimmune diseases and other health issues.

Trans Fatty Acids

A trans fat is an otherwise normal fatty acid that has been radically changed by hydrogenation and partial hydrogenation. Unlike other dietary fats, trans fatty acids are neither required nor beneficial for health. Trans refers to the formation of the oil molecule, which in a natural state is in a cis formation. Trans fats have their hydrogen atoms across from each other on the carbon chain whereas in cis formation they are on the same side. Trans fats are toxic to the body and as the digestive system does not recognize them as such, they are absorbed in to the body and incorporated into cellular membranes, as cis formation fats would be. This disturbs the functionality of the cells, as the fatty acids need to be in specific configurations for metabolic processes to occur. Most fatty acids in the trans configuration are not found in nature and are the result of human processing. Eating trans fats greatly increases the risk of coronary heart disease and is disruptive to immune function. Health authorities worldwide recommend that consumption of trans fat be reduced to trace amounts at best.

Note: There are in nature, a small amount of naturally occurring trans fats. Natural trans fats are created in the stomachs of ruminant animals such as cattle, sheep, goats, etc. and make their way into the fat stores and milk of the animals. Natural trans fats in the diet have been thought to have some potential benefit to aid in both muscle building and fat loss efforts. Conjugated Linoleic Acid (CLA) is one of these naturally occurring trans fats.

However, the quality of natural trans fats in the meat and dairy of ruminant animals is greatly reduced by mass-production methods of farming and their grain and soy-heavy diets. Meat and dairy from grass-fed, free-range animals always have a much higher quantity and quality of these fats.

Some commercially available deep-fried products such as potato chips claim to be “Trans Fat Free”. These products use high heat refined cooking oils, and are not heated beyond the oils smoke point (see definition below). Although they are free of free radical causing trans fatty acids, these highly denatured oils will still contain the damaged fat molecules that Udo Erasmus speaks of above.

  • Hydrogenation and Partial Hydrogenation damages important fatty acid structure creating trans fatty acids, this makes these oils toxic to the body.
  • Free radicals are produced in oils that have been exposed to light, heat and oxygen. Most commercial cooking oils contain free radicals.

 Oil Descriptions

Extra-virgin: This designation is for olive oil only. It means that the oil contains no more than 0.8% acidity. It is the highest quality olive oil. Extra-Virgin olive oil is from the first cold pressing of the olives after they have been picked. There can be no refined oil in an Extra-Virgin olive oil.

Virgin: This term applies to olive oil as well as to other oils. It means the oil has been extracted by mechanical means (cold or expeller pressed) and has not been refined. When applied to olive oil, it also means that the acidity level is less than 2%. It’s the second best grade of olive oil. The process of making virgin oils is very simple. Cleaned seeds and nuts are expeller pressed at a low speed in a small screw press. The oil obtained may or may not be filtered and bottled, ideally in dark containers. It is not a term widely used in North America. Cold or Fresh Pressed  Oils are Virgin Oils.

Unfiltered: While some oils are filtered to remove the ‘impurities’ from the oil, some are not. Unfiltered oils may contain soluble and insoluble fibres such as lignins (found in large quantities in flax seeds) and other beneficial ingredients.

Pure: This term only means that the oil in the container comes from only one type of seed or nut. For example, pure sunflower oil means that the oil only comes from sunflower seeds, not a blend of sunflower with other oils. It doesn’t guarantee that the oil is fresh pressed or unrefined.

Extra-light: Oil is fat, even if it comes from a vegetable source. Thus, it cannot be light in calories. All oils contain 9 calories per gram. When the term “light” is on a label, it usually means that the oil is light in taste. If an olive oil is light in taste and colour, it probably has been refined in part or totally.

Unrefined: Means that the oil was not refined and it is in its natural state – not de-gummed, bleached, deodorised, or hydrogenated. The best oils are unrefined, and taste like the seed from which they were mechanically/expeller pressed without solvents. They have been pressed, bottled, stored, and transported without light, heat, or oxygen entering the process, and are consumed fresh. Ideally the pressing date or expiry date should be printed on the label along with information on whether the seeds have been organically grown.

How to recognize cold or fresh pressed oil from refined

A fresh pressed oil of good quality must be bottled in dark containers and an expiry date should be written on the label. If the oil is translucent and odourless, it was refined. Fresh pressed oil should smell and taste of the seeds, nuts or fruits from which it was extracted.

Storage Tips

The packaging and storage of oils can affect their quality. Heat, oxygen and light promote rancidity. Unrefined oils should be stored in cool, dark places and unrefined oils that are high in Omega-3 fatty acids should be stored in the refrigerator. Refrigeration slows down light and oxygen destruction. Oil can also be frozen to prolong its life. Polyunsaturated oils containing Omega 3 and 6 will keep 6-9 months after pressing date (check expiry date), but only up to 6 weeks once opened. Other unrefined oils will keep from 9-24 months if stored properly. These oils contain natural antioxidants that help prevent oxidation and keep them from going rancid over a certain period of time. Refined or heavily processed oils will last 24-36 months or longer.