How is cooking oil refined?

Browse through the internet and you will find loads of blogs, reels, and videos talking about how one should completely avoid refined vegetable oils and use cold pressed oils only instead, for a host of reasons that rang from containing harmful chemicals to being loaded with trans fats. Makes one wonder why cooking oil manufacturers would put in the time, efforts, resources, and money to process & refine oil at all, doesn’t it? How much truth is there in all these? Are refined cooking oils as dangerous as they are made out to be? Are cold pressed oils as safe and healthy as they are made out to be? Let’s do some tehkeekaat, shall we?

Before we get into the merits and demerits of the oil refining process, let’s explore what is this process of refining for edible/cooking oil.

Photo by cottonbro studio

How long have we been refining edible oil?

Mankind has been refining oils for consumption for thousands of years. In earlier times, people would use the Sun, fire, or even the heat from an oven to heat the oily plant products until the plants exuded the oils which would then be collected. Historical records have been found that confirm that the Chinese and the Japanese extracted soy oil as early as 2000 BC and south Europeans have been extracting olive oil as early as 3000 BC. In the Americas, peanuts and sunflower seeds were roasted, then beaten to a crush them and obtain a paste, which was then boiled in water, causing the oil in them to rise to the surface, which was then skimmed off. Africans have grated and beaten the palm kernels and coconut meat to get a pulp which was then boiled in water to release oils.

A lot of cooking oils that we know today did not exist in the earlier times. For instance, corn oil became available only since the 1960s. Cottonseed oil, watermelon seed oil, grapeseed oil, etc. also became available only recently, as extraction technology improved. Until then, most of these seeds were considered a waste and were not used for oil extraction.

With time, there was a need to extract more oil from the raw material. Once extracted, the oil had to be filtered and processed to enhance its shelf life and make it more fit for consumption.

Photo by khats cassim

How is cooking oil refined?

Step 1: Cleaning

The first step in the manufacturing of refined edible oil is cleaning and grinding. The oil containing raw material – seeds/pulp/fruit/etc. is passed over a magnetic surface to remove trace metals. The matter is then dehulled, deskinned, etc. and all extraneous matter is removed. Fro instance, if cottonseeds are being used, then the lint should be removed from the ginned seeds . If corn kernels are being used, then they are milled to separate the germ. So, this process would depend on what the raw material is.

Step 2: Grinding

Once the cleaning is done, the raw material is ground to a coarse meal which helps increase the surface area for extraction of the oils when the pressing will take place. The crushing could be done using mechanical grooved rollers or hammer mills to get the meal with a proper, uniform consistency. The meal thus obtained is heated to help extract the oil from it. The heating helps increase the extraction from the raw material. But in this process, there will also be more impurities that get extracted, which would need to be removed to make the oil fit for human consumption.

Step 3: Extraction by Pressing

The next step is pressing. The heated coarse meal that is obtained in the previous step is continuously fed to an oil press, usually a screw press, which would increase the pressure progressively as the meal passes through a slotted barrel-like structure. The oil oozes out of the slots in the barrel from where it is collected. This oil is highly impure and not yet fit for consumption.

Step 4: Extraction by Solvents

A large part of the oil gets extracted in this step, however, there is still a considerable amount of oil present in the coarse meal, which can be extracted using specific solvents. Sometimes, some raw materials are not pressed at all and go straight to solvent extraction, as is the case with soybean oil. Soybean have very little oil that can extracted with pressing so it goes straight to solvent extraction. In other cases, where the initial screw press extraction takes place, the oil cake obtained after pressing is then exposed to specific solvents to maximize extraction. A volatile hydrocarbon is commonly used for this purpose. The most popular solvent for this is hexane. Hexane will dissolve the remaining oil present in the oil cake. The resulting solution of hexane and the oil matter is distilled to extract out the oil from it.

Step 5: Solvent Recovery

Once the solvent extraction is done and the oil is recovered from the solvent, any traces of the solvent should be removed from the oil too. Since the solvent used is highly volatile, more than 90% of the solvent will simply evaporate and be separated from the oil by itself. As it evaporates, it is collected and re-used. To retrieve the remaining 10%, a stripping column is used. The oil is boiled using steam, and the remaining solvent in the oil evaporates, moving upwards, where it is condensed, collected and ready for re-use.

Step 6: Refining

Next in the process is a step called refining. The oil is refined to remove the color, odor, and bitterness from the oil. The oil is heated to a temperature between 40 to 85 degree Celsius and an alkaline substance like sodium hydroxide or sodium carbonate is mixed with it. The undesirable free fatty acids in the oil would combine with the sodium hydroide/carbonate, forming soap. This is the standard saponification process. All traces of this soap matter are separated out using centrifugation. After centrifugation, the oil is washed too, to remove any soapy matter that might have been left behind. The final cooking oil we receive is completely free of all soapy matter and all solvent matter. This is the first step of refining.

Step 7: Degumming & Neutralization

The next step in refining oil is de-gumming or neutralization. De-gumming is an important step to eliminate impurities like phosphatides or gums. These phosphatides cause high refining losses and if they go into the final product, once they decompose, they will darken the oil a lot as they lack thermal stability. The oil is treated to very hot water, about 85 to 95 degree Celsius, or to steam, or even acidic water. The gums (phosphatide compounds) precipitate out by hydrating (in case of degumming by water) and are separated from the oil. For neutralization, the oil is treated to acidic water, then sodium hydroxide or carbonate is added, thereby neutralizing the phosphatide compounds and removing them from the oil.

When soybean oil is being manufactured, a very important byproduct of the degumming process is Lecithin – Soy Lecithin. This is a vegetarian version of the egg lecithin. Lecithin is industrially valuable, and is a common ingredient in chocolates, vegetarian mayonnaise, cosmetics, soap, paint, plastics, etc.

Step 8: Adsorption

The process is more commonly called bleaching, but that can be a scary term. The actual process happening is adsorption and absorption for improving the color of oil and removing the pigmented matter from the oil. Factually speaking, nobody would buy oil that would have colorful specks inside it, would they? To have a pure, clear appearance, and to make it safe for consumption, the oil must be cleared by removing the pigmented matter from the oil. This is done using substances like fuller’s earth, activated charcoal, activated clay, etc. This step would also remove residual phosphatides. Oil that is not intended for high heat applications, say it is to be used in salads, drizzles, dips, etc. is winterized. Winterization is a process in which the oil is rapidly chilled to low temperatures, causing the waxes present in the oil to solidify, which can then be effectively removed. These waxes are not added to the oil at any point, they are present in the oil or have formed during the processing so far, and need to be removed from the oil to make it fit for consumption. Cooking oil has to be liquid at ambient temperature and these waxes are impurities in the oil that must be removed.

Step 9: Deodorization

We all know our cooking oil to smell and taste a certain way. Any departure from that would not be acceptable to consumers. At this point in oil processing, the oil still has some impurities that would give it an off odor. Deodorization of oil will remove odoriferous matter, free fatty acids and other unwanted compounds which can lead to oxidation and spoilage of the oil or hamper its quality. First, steam is passed over the oil at about 225 to 250 degree Celsius, causing the volatile compounds to evaporate & distill out. Sometimes, a small amount of citric acid, just about 0.01% might be added to the oil to function as an anti-oxidant, protecting the oil against going rancid by oxidation and inactivating any trace metals, all of which would shorten the shelf life of the cooking oil.

Once the oil is refined and purified, it is now good to package and sell, and is ready for cooking and consuming.

Photo by RDNE Stock project

Why is cooking oil refined?

Some vegetable oils like olive, peanut, coconut, and sometimes even sunflower & sesame seeds can be cold pressed. Cold pressing oils requires minimal processing and produces a light, flavorful oil. Cold pressed oils are good for some purposes, like using in salad dressings. However, cold pressed oils should not be used for all cooking. Also, most oil sources are not suitable for cold pressing as doing so would leave behind undesirable trace elements in the oil which would cause the oil to have off odors, bitter tasting, and/or dark in color.

The oil you buy from the shop shelves has to meet certain legal standards set by the Food Safety and Standards Authority of India. These standards could vary slightly for every oil (depending on source and whether it is a single product oil or a blended oil, etc.). However, the following are some of the most important factors specified by FSSAI which every edible oil manufacturer has to meet:

  • Butyro-refractometer reading at 40C
  • Refractive index
  • Saponification value
  • Iodine value
  • Unsaponifiable matter
  • Bellier test
  • Hydrocyanic acid content
  • Argemone oil content (should be zero)
  • Hexane content (should not be more than 5 ppm)

For any oil to be classified as a cold pressed or kachhi ghani oil, the natural allyl isocyanate content in the oil cannot be less than 0.20% by weight.

Apart from these, there could other parameters defined for individual type of oils, whose limits and standards need to be adhered to.

Also, refined vegetable oils are required to be fortified with oil-soluble vitamins – Vitamin A and Vitamin D. Deficiencies of both these vitamins are very common, so consuming refined edible oil is beneficial in this aspect that one gets some amount of Vitamin A & D through it.

Photo by Anna Tarazevich

Is refined cooking oil harmful?


Refined vegetable oils, when consumed in moderation, would not be as dangerous. Avoid repeated usage of used oils, especially the oil that has already been used once for frying. Using oils that are high in saturated fats might work better for frying. For regular cooking, the usual refined vegetable oils are fine. Cold pressed oils can be used for lower heat cooking tasks and for raw use. Using blended oils that offer the best of multiple different oil sources would also be good.

Does refined cooking oil contain trans fats?

Why don’t you discover the answer to this yourself? Whenever you are in the market, pick a bottle or container or pouch of any edible oil, turn the packet around and check the nutrition label. Just about all food products, including edible oils, are required to disclose the trans fat content in it, so you should see the amount very clearly mentioned there.

What I can tell you for sure is, refined vegetable oils are NOT loaded with trans fats, DO NOT have impurities, ARE safe to consume, are refined and processed to make it fit for consumption and make them stable for use, while also extending their shelf lives. When consumed in moderation, they provide us with the required fats and vitamins, and are not the devils they are made out to be.

Photo by Geraud pfeiffer

Penny for your thoughts!