Vegetable oil is extracted from seeds and fruits through two main methods: mechanical pressing, which physically squeezes oil out, and solvent extraction, which uses a chemical to dissolve and separate the oil. Most cooking oils you buy at the grocery store go through one or both of these processes, followed by refining steps that produce the clear, neutral-tasting oil you’re used to.
Not Every Vegetable Actually Contains Oil
The term “vegetable oil” is a bit misleading. Most vegetable oils come from seeds and nuts, not from vegetables like broccoli or carrots. Soybeans, sunflower seeds, canola (rapeseed), and corn germ are the workhorses of the industry. Olives and palm fruit are the main exceptions, where oil comes from the fleshy fruit itself rather than a seed.
Oil content varies dramatically between crops. Canola seeds are nearly 49% oil by weight, sunflower seeds about 41%, and soybeans closer to 20%. That difference in oil content determines which extraction method works best. High-oil seeds like canola and sunflower respond well to mechanical pressing alone, while lower-oil crops like soybeans almost always require solvent extraction to get enough oil out economically.
Preparing Seeds for Extraction
Raw seeds can’t just be dumped into a press. They go through several preparation steps that make a significant difference in how much oil you can recover. Most oilseeds follow a similar sequence: cleaning, drying, dehulling, size reduction, flaking, and tempering.
Cleaning removes plant stems, sticks, leaves, and other debris that could decompose during storage and degrade oil quality. Dehulling strips the outer husk or shell, which matters because hulls are abrasive (they wear down equipment) and they absorb oil, reducing total yield. After dehulling, seeds are cracked into smaller pieces using corrugated roller mills, then flattened into thin flakes. Flaking ruptures the cell walls inside the seed, making it far easier for oil to escape during pressing or solvent washing.
Before extraction, flaked seeds are often tempered: heated to around 149°F with steam while adjusting moisture content. This conditioning step softens the flakes and improves how efficiently oil flows out during the next stage.
Mechanical Pressing: Squeezing Oil Out
Mechanical pressing is the oldest and most intuitive method. A screw press (also called an expeller) works like a giant, powerful meat grinder. Seeds enter one end of a barrel-shaped cage, and a rotating worm shaft pushes them forward. The shaft is designed so the space inside gradually shrinks from inlet to outlet, compressing the seed material more and more tightly. Oil is forced out through narrow gaps between the bars of the cage, while the remaining solids exit as a compressed “press cake.”
Modern screw presses use multiple pressure sections along the shaft, with conical rings that create zones of high compression alternating with brief relief sections. These relief zones allow the compressed material to shift and reform new tiny channels, letting trapped oil drain out more completely. The drier the seeds going in, the higher the friction and pressure inside the press, which extracts more oil but also limits how fast the press can run.
Small-scale presses handle as little as 3 kg of seeds per hour, while industrial presses process far more. Mechanical pressing alone typically leaves 6 to 14% of the original oil behind in the press cake, depending on the seed type and press design.
Cold-Pressed Oil: What the Label Means
Cold-pressed oils are produced using mechanical pressing at lower temperatures, preserving more of the oil’s natural flavor, color, and nutrients. The friction inside a screw press naturally generates heat, so producing a true cold-pressed oil requires slower pressing speeds and careful equipment design to keep temperatures in check. These oils retain more vitamin E, polyphenols, and plant sterols compared to refined versions, but they also have a lower smoke point. Unrefined sunflower oil, for example, starts breaking down around 320°F, while its refined counterpart handles 450°F. That makes cold-pressed oils better suited for salad dressings, drizzling, and low-heat cooking.
Solvent Extraction: Washing Oil Out Chemically
For crops with lower oil content, or to recover the oil left behind after mechanical pressing, the industry turns to solvent extraction. The most widely used solvent is n-hexane, a petroleum-derived chemical that dissolves oil efficiently and is easy to remove afterward.
The process starts with prepared seed flakes (or partially pressed cake from an expeller). These are washed repeatedly with hexane, which dissolves the oil and carries it away as a mixture called “miscella.” The hexane is then separated from the oil through evaporation and distillation. Because hexane boils at just 63 to 69°F above room temperature, it evaporates readily and can be captured, condensed, and reused.
Solvent extraction is remarkably efficient, pulling out nearly all the oil that mechanical pressing leaves behind. The combination of first pressing, then solvent-extracting the press cake, is standard practice for many large-scale operations.
Is There Solvent Left in Your Oil?
After distillation, trace amounts of hexane can remain. European regulations cap hexane residues in finished oils at 1 mg per kilogram, a level considered negligible from a health standpoint. The evaporation and refining steps that follow extraction drive off the vast majority of solvent long before the oil reaches a bottle. Defatted protein products and flours, which undergo less processing, are allowed slightly higher residue limits of 10 mg/kg.
Refining: From Crude to Cooking Oil
Whether pressed or solvent-extracted, crude oil straight from the seed is dark, strongly flavored, and contains compounds that shorten shelf life. Refining transforms it into the stable, neutral oil most recipes call for. The process involves several distinct steps.
Degumming removes phospholipids and other gummy substances by mixing the crude oil with water or a mild acid, which causes these compounds to clump together so they can be separated out. Neutralizing treats the oil with an alkali solution to remove free fatty acids that cause rancidity and off-flavors. Bleaching passes the oil through an absorbent clay that strips out pigments, residual soap, and trace metals. Finally, deodorizing heats the oil under vacuum to very high temperatures, driving off volatile compounds responsible for any remaining taste or smell.
The result is what the industry calls RBD oil: refined, bleached, and deodorized. It’s the clear, mild-tasting product you see on store shelves. This process does remove some beneficial nutrients along with the impurities, which is the trade-off for a higher smoke point and longer shelf life. Refined avocado oil, for instance, can handle temperatures of 480 to 520°F before breaking down, compared to 350 to 400°F for the unrefined version.
How It Works for Olive Oil
Olive oil follows a different path because the oil comes from fruit flesh rather than a dry seed. Olives are crushed into a paste (traditionally between stone wheels, now more commonly with steel hammer mills), and the paste is slowly mixed to allow small oil droplets to merge into larger ones. A centrifuge then spins the paste at high speed, separating the oil from the water and solid pulp. Extra virgin olive oil skips the chemical refining steps entirely, which is why it retains its distinctive flavor, color, and higher nutrient content but has a lower smoke point than refined olive oil.
Pressing Oil at Home
Small countertop oil presses are available for home use, typically rated to process 3 to 6 kg of seeds per hour. They work on the same screw-press principle as industrial expellers, just scaled down. Sunflower seeds, sesame seeds, flax, and peanuts are popular choices for home pressing because of their relatively high oil content. Expect lower extraction efficiency compared to industrial equipment, meaning more oil stays trapped in the leftover cake. The oil you get is unrefined, so it will have a stronger flavor and shorter shelf life. Store it in a dark glass bottle in the refrigerator and use it within a few weeks for the best quality.