The short answer

Vegetable oil is made by extracting oil from plant seeds or fruits and then refining it to remove impurities. Plant seed vegetable oil goes through a particularly meticulous process due to the difficulty of extracting oil from seeds and the additional refinement needed.

The long answer

Let's begin with everyone's favorite: definitions!

Vegetable oil is defined as oil that is made from the fruits or seeds of plants, including olives, palm, soybeans, rapeseeds, sunflower, corn, and many more. Unlike animal oil which is made from lard and mineral oil which is made from petroleum, vegetable oil just means it was made from plants.

Historically, olive oil and palm oil have been the most popular plants from which to extract oil. This is because these oils come from pressing the fruits, making the oil easy to extract and the crude oil (i.e. unprocessed oil) has a nice taste and quality.

Getting oils from seeds is a lot more challenging. Simply pressing the seeds doesn't get as much oil as pressing fruits like olives or palm. And seeds contain impurities like waxes, flavors, free fatty acids, and phospholipids which essentially requires refinement to make it palatable and shelf-stable.

But over time, new technologies have come around and allowed for the efficient extraction and refinement of plant seed oils. Outside of olive oil, early vegetable oil extraction methods likely only obtained around 10% of the oil available in a seed. But today's extraction methods can extract up to 98%.

Since we colloquially refer to plant seed oil as "vegetable oil," that's how I'll refer to it going forward.

How is vegetable oil made?

There is a multi-step process followed to extract the most oil from plant seeds and make the oil shelf-stable and appetizing to consumers.

Phase 1: Oil extraction

Step 1: Cleaning

The seeds undergo several cleaning processes. They are run over magnets to remove any traces of metal, they are screened to remove dirt and other impurities, and their husks or shells are removed. Then the cleaned, dehulled seeds are ground into a meal to be pressed.

Step 2: Pressing

The seed meal is then heated up to help with the extraction of the oil. The hot seed meal is fed into a press that steadily increases the pressure to get as much oil out as possible in this step.

Step 3: Solvent extraction

Since only a portion of oil can be extracted by pressing seeds alone, solvents are introduced. When the solvent is added to the pressed "oil cake," it dissolves and extracts the oil.

Interestingly, the solvent that is most commonly used to extract vegetable oil is hexane, which itself is a product of crude oil, or petroleum. Hexane is also used as a base in paint thinners and as a cleaning agent in furniture manufacturing.

The solvent is then distilled out of the mixture to meet food safety standards, and the remains of the oil cake are then sold on as animal feed. At this stage, the oil is considered to be "crude" since it has not yet been refined.

Phase 2: Refinement

Now that the oil has been extracted we enter the refining stages. The following stages are done to remove impurities from the crude oil that make its appearance, taste, odor, and physical quality undesirable for a consumer.

Step 4: Degumming

The first step in refining the crude seed oil is degumming. Depending on the type of seeds used, the gums will either have hydratable or nonhydratable phospholipids. That means the gums will either able to be removed with water or require a small quantity of acid to be removed, such as phosphoric or citric acid. The oil is spun at high speeds in a centrifugal device to separate the gums and other impurities from the oil.

Step 5: Neutralizing

Crude seed oil also contains free fatty acids which can impact the oil's taste and shelf life. That's why the oil is then neutralized to remove these acids.

The oil is treated with an alkali solution, typically sodium hydroxide, which reacts with the free fatty acids to form soap stock. The soap stock is then removed and goes into further production to be used in a variety of applications, including animal feed, soap production, biofuel, lubricants, and fertilizers.

Step 6: Washing and drying

After neutralization, the oil contains remnants of the alkali solution and soap stock, which need to be removed to ensure purity and quality. The oil is washed in water at a very high temperature to separate the impurities.

Then the water-washed oil is dried in a vacuum dryer until the moisture level falls below 0.1%, preventing microbial growth and further oxidation, both of which could degrade the oil over time.

Step 7: Bleaching

At this stage, the oil likely has a dark color and residual impurities. The oil is mixed with bleaching clays or activated carbon, which adsorb the colorants and impurities. This mixture is then held at a specific temperature to facilitate the adsorption process. After bleaching, the oil is filtered to remove the adsorbent along with the trapped impurities.

Step 8: Dewaxing

Some types of vegetable oil contain wax, like sunflower or corn germ oil. Waxes give the oil a cloudy appearance, so they are removed.

The oil is gradually cooled so that the waxes crystallize. Then the cooled oil is passed through a filter to separate the wax from the oil. The waxes then go on to be used to produce vegetable shortening.

Step 9: Deodorizing

Apparently vegetable oil does not taste or smell very pleasant, so the last step is to deodorize the oil to remove the taste. It also helps extend the shelf life of the product. However, nutrients such as vitamins and antioxidants are unfortunately also removed during this step.

The oil is heated to a high temperature and then a very high vacuum is applied, which lowers the boiling points of the unwanted components. This allows these taste and smell components to be removed without excessively heating the oil and harming its quality.

Then steam is injected into the oil in order to carry away the undesirable components, which vaporize at lower temperatures than the oil. The steam carrying the taste and odor components condenses and is separated from the oil. Once the oil is cooled, it is ready to be bottled and shipped out to consumers.

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Sources

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