Most vegetable oil starts as seeds, primarily soybeans or canola, that are cleaned, crushed, and processed through a combination of mechanical pressing and chemical extraction. The bottle labeled “vegetable oil” at the grocery store is almost always soybean oil, though canola, corn, sunflower, and safflower oils are also common. The journey from seed to clear, neutral-tasting cooking oil involves several distinct stages, and the finished product looks nothing like what comes out of the seed.
Cleaning, Hulling, and Preparing the Seeds
Raw seeds arrive at processing facilities carrying dirt, stems, stones, and other debris. The first step is cleaning and sorting to remove anything that isn’t seed. After cleaning, the outer hulls are removed, since they contain very little oil and would absorb some of what you’re trying to extract. The de-hulled seeds are then cracked or flaked into thin pieces to increase surface area, making it easier to get the oil out. Some facilities also heat or “condition” the flakes with steam at this stage, which softens the cell walls and helps the oil flow more freely.
Pressing: The First Extraction
The prepared seed flakes are fed into a mechanical press, essentially a giant screw that squeezes the material under intense pressure. This initial pressing extracts a significant portion of the oil, but it can’t get everything. The pressed material that comes out the other side, called “press cake,” still contains a meaningful amount of oil that would be wasted without further processing.
Temperature matters here, and it creates real differences between oil types you see on store shelves. Cold-pressed oils are extracted at temperatures that stay below 122°F, which preserves more of the natural flavor and nutrients but yields less oil. Expeller-pressed oils use the same mechanical screw approach but can reach temperatures above 140°F from the friction and pressure alone. Both methods are purely mechanical, with no chemicals involved, which is why these labels appeal to consumers looking for minimally processed options.
Solvent Extraction With Hexane
The vast majority of commercial vegetable oil goes through a chemical extraction step after pressing. The press cake is washed with hexane, a petroleum-derived solvent that dissolves the remaining oil out of the solid material. Fresh solvent passes through material that has already been mostly extracted, then gets reused on material with more oil still in it. This countercurrent approach is highly efficient: solvent extraction leaves behind only about 0.35% to 0.60% of the original oil in soybean flakes.
For canola, the combination of pressing and solvent extraction captures nearly all the oil. After pressing removes the bulk, solvent extraction pulls out an additional 13.7% of the raw seed’s weight in oil, dropping the remaining oil content from around 20% down to just 0.8%.
The hexane then needs to be removed from the oil. The mixture of oil and solvent (called miscella) is heated to evaporate the hexane, which has a relatively low boiling point of about 143°F when steam is present. The recovered hexane is recycled back into the system. In the European Union, the maximum allowable hexane residue in finished oil is 1 mg/kg. The U.S. FDA doesn’t specify a maximum residue limit for hexane in oil, though commercial refining processes remove nearly all of it through the heating steps that follow.
Refining, Bleaching, and Deodorizing
Crude oil straight from extraction is dark, smelly, and contains compounds that would shorten its shelf life. The refining process transforms it into the clear, neutral oil consumers expect. This happens in three main stages, often referred to collectively as RBD processing.
Refining
The first stage neutralizes free fatty acids that would make the oil taste harsh and go rancid faster. It also removes phosphorus-containing compounds called phospholipids. The oil is filtered to pull out remaining solid particles, leaving a cleaner but still discolored product.
Bleaching
Despite the name, this step doesn’t use liquid bleach. Instead, the oil is mixed with a special type of clay called bleaching clay, which absorbs pigments, residual soaps from the refining step, and other unwanted compounds. The clay is then filtered out, taking the color with it.
Deodorizing
This is the most aggressive step. Steam is passed through the oil at temperatures between 360°F and 465°F (180–240°C) under a strong vacuum. At these temperatures, the compounds responsible for off-flavors and odors, including aldehydes, ketones, and short-chain fatty acids, vaporize and get pulled away. What remains is the mild, nearly flavorless oil that works as an all-purpose cooking fat.
The high heat of deodorization does come with a trade-off. Research on canola oil has shown that this step can raise the level of trans fats from an initial 0.1–0.3% in crude oil to as much as 5% in the refined product. European quality standards cap trans fat content at 1.0% for refined edible oils, which pushes manufacturers to optimize deodorization temperatures and times.
What Refining Does to Nutrients
Raw seeds contain beneficial compounds like tocopherols (a form of vitamin E), plant sterols, and polyphenols. Refining strips a significant portion of these out. Studies on sunflower, rapeseed, and soybean oils found that the full refining process destroys 10–36% of tocopherols and 6–52% of plant sterols. Polyphenols take the biggest hit: 93–98% are lost during refining.
This is the main nutritional argument for cold-pressed or expeller-pressed oils. Because they skip the high-heat chemical refining steps, they retain more of these naturally occurring antioxidants. The trade-off is a shorter shelf life, stronger flavor, and a higher price point.
What Happens to the Leftover Seed Material
Oil extraction produces enormous quantities of a protein-rich byproduct called seed meal or cake. Globally, the primary use for this material is animal feed, since it’s packed with protein, vitamins, and minerals. Soybean meal, in particular, is one of the most important protein sources in livestock nutrition worldwide.
Seed meals have increasingly found their way into human food products as well. The protein can be isolated and used in plant-based meat alternatives, gluten-free baked goods, breakfast cereals, and dietary supplements. Some seed meals serve as substrates for growing mushrooms, producing organic acids and enzymes, or manufacturing cosmetic and pharmaceutical ingredients. At its simplest, the leftover cake works as plant compost, returning nutrients to the soil with no additional processing required.
Cold-Pressed vs. Conventional Oil
The process described above, pressing followed by solvent extraction and full RBD refining, is how the vast majority of vegetable oil is made. It maximizes yield and creates a product with a long shelf life, high smoke point, and neutral taste. Cold-pressed and expeller-pressed oils skip the solvent step entirely and undergo minimal or no refining. They cost more because mechanical pressing alone recovers less oil from each batch of seeds. You’ll typically find them in smaller bottles at higher prices, marketed for salad dressings or finishing rather than deep frying.
The clear, inexpensive vegetable oil most people cook with has been through every stage: cleaning, pressing, solvent extraction, refining, bleaching, and deodorizing. Each step removes something, whether it’s solid debris, color, flavor, odor, or nutrients, until what remains is a highly processed but versatile cooking fat designed to stay out of the way of whatever you’re making.