What Are Seed Oils?

Not your grandmother's cooking oil

For most of human history, the fats we cooked with came from animals (tallow, lard, butter) or were mechanically pressed from fatty fruits (olive oil, coconut oil). These fats required minimal processing — render the fat, press the fruit, done. Seed oils are different. Getting oil out of a soybean, corn kernel, or rapeseed requires an industrial process that would be unrecognisable to anyone who lived before the 20th century.

The extraction process

Here's how most seed oils are made, step by step. First, the seeds are cleaned and crushed. Then they're heated to high temperatures — up to 250°F — to begin breaking down the cell structure. The crushed, heated seeds are then washed with hexane, a chemical solvent derived from petroleum. Hexane dissolves the oil out of the seed material. The hexane-oil mixture is then heated again to evaporate off the hexane (mostly — trace amounts remain in the final product).

The seeds are washed with hexane — a petroleum-derived chemical solvent. Then bleached. Then deodorised. Then sold as 'heart-healthy.'

But we're not done. The crude oil is then degummed with phosphoric acid to remove phospholipids. Then it's neutralised with sodium hydroxide (lye) to remove free fatty acids. Then it's bleached — not for colour, but to remove remaining impurities, pigments, and oxidation products. Finally, it's deodorised by heating to over 450°F under a vacuum to strip out the volatile compounds that would otherwise make the oil smell rancid. The result is a clear, odourless, flavourless liquid that bears no resemblance to anything found in nature.

Compare this to butter

Butter: cream from a cow, churned until the fat separates. That's it. Olive oil: olives crushed, oil collected. Tallow: beef fat rendered with heat. Coconut oil: coconut meat pressed. These are all one-step or two-step processes that humans have used for thousands of years. Seed oil production is a multi-step industrial chemical process invented in the early 1900s.

The omega-6 problem

Beyond the processing, seed oils are extremely high in linoleic acid — an omega-6 polyunsaturated fatty acid. In small amounts, omega-6 is essential. But seed oils deliver it in quantities our bodies never evolved to handle. Soybean oil is about 55% linoleic acid. Sunflower oil can be over 65%. Humans evolved on a diet with an omega-6 to omega-3 ratio of roughly 1:1 to 4:1. The modern Western diet, thanks to seed oils, has pushed that ratio to 20:1 or higher.

Humans evolved on an omega-6 to omega-3 ratio of 1:1. Modern diets are at 20:1.

They're in everything

Walk into any supermarket and pick up a random packaged product. Read the ingredients. Chances are you'll find soybean oil, canola oil, sunflower oil, or "vegetable oil" (which is usually soybean). They're in bread, crackers, chips, cookies, salad dressings, mayonnaise, frozen meals, restaurant fryers, and fast food. Avoiding seed oils in a modern food environment requires active effort — which tells you everything about how embedded they've become.