Library›Plants and preparation›Cold Pressing Expelling and Heat Why the Same Seed Gives Different Oils

Cold Pressing Expelling and Heat Why the Same Seed Gives Different Oils

Cold Pressing, Expelling and Heat: Why the Same Seed Gives Different Oils covers the mechanical side of extraction — pressing, expelling, and what heat does on the way — and why the same raw material yields oils of completely different character, yield, shelf life and price depending on how it was taken out. It is the companion to Laboratory Equipment and Extraction Engineering, which covers the solvent and distillation side; nothing here repeats that page. Alpha.

1. The trade-off that governs everything

Every extraction method sits somewhere on one line:

• More heat and more pressure → more yield, worse quality.
• Less heat and less pressure → less yield, better quality, higher price per litre.

There is no method that wins both ends. A mill choosing "cold pressed" is accepting a much lower yield in exchange for flavour, aroma, colour and intact minor compounds. A mill choosing heat and solvent is accepting degradation in exchange for getting nearly everything out.

What heat actually destroys, in rough order of fragility:

• Aroma volatiles and thiols — gone first, often below 40 °C. See A Field Guide to Smell Molecules: Classes, Key Odorants and the Reactions That Make Them.
• Vitamin E (tocopherols), polyphenols, carotenoids — degrade progressively with time × temperature.
• Unsaturated fatty acids — oxidise; the more double bonds, the faster.
• Enzymes — denatured, which is sometimes the point (see §5).

Heat also creates things: Maillard and caramelisation products that give toasted sesame and roasted pumpkin-seed oil their entire character. Heat is not simply damage; it is a different product.

2. Cold pressing and what the words legally mean

• Cold pressed — pressed without applied external heat. Friction still raises the temperature, so "cold" does not mean cold.
• In the EU, for olive oil, "cold extraction" is defined: below 27 °C for the malaxation and extraction stage. That is a real legal definition with a number attached.
• For most other oils, and in the United States, "cold pressed" is not legally defined at all. A seed oil labelled cold pressed may have reached 60–90 °C from screw friction alone. The term is largely marketing outside the olive-oil rules.
• "Expeller pressed" means a continuous screw press with no solvent. It is honest about being warm — typically 60–100 °C from friction. It is not the same as cold pressed, and is often the more truthful label.
• "Virgin" and "extra virgin" are grades, not methods, defined for olive oil by chemistry (free acidity, peroxide value) and by a sensory panel finding zero defects. "Extra virgin" is a quality result; "cold pressed" is a process claim.
• "Unrefined" / "crude" — pressed and filtered only. "Refined" — subsequently degummed, neutralised, bleached and deodorised (RBD), which strips colour, flavour and most minor compounds but gives a neutral, high-smoke-point, long-life oil.

3. The press types

• Hydraulic / batch press. Material wrapped and squeezed between plates. Slow, low temperature, low throughput, high quality. The traditional method, and still used for premium nut and seed oils.
• Screw press (expeller). A continuous auger forces material down a tapering barrel against a choke. High throughput, but the friction generates real heat; many are jacketed for cooling. The workhorse of the industry.
• Two-stage pressing. A gentle first press for the premium oil, then a harder or heated second press of the same cake for a lower grade. This is why one harvest yields two products.
• Ram / wedge presses and the old stone mills. Historic; the edge-runner mill crushing olives under a rolling stone is still in use and still produces good oil.
• Centrifugal separation (olive, avocado) — the fruit paste is malaxed and then spun; the oil separates by density rather than being squeezed. A two-phase decanter yields oil and wet pomace; a three-phase adds water, gets slightly more oil, and washes out polyphenols into the wastewater. Two-phase gives better oil and less effluent.
• Residual oil tells the story: a good screw press leaves 6–12% oil in the cake. Solvent extraction takes it to under 1%. That gap is why commodity oil is solvent-extracted and why pressed oil costs more.

4. Pressing versus the other routes

• Solvent (hexane) extraction — nearly complete yield, then the solvent is stripped and the oil refined. Economically dominant for soy, canola and sunflower. Covered in Laboratory Equipment and Extraction Engineering.
• Supercritical CO₂ — tunable, cold enough to preserve fragile compounds, no residual solvent, capital-intensive.
• Steam distillation — an entirely different product. It takes the volatile fraction only and leaves the fat behind, so you get an essential oil, not a fixed oil. Pressing lemon peel gives expressed lemon oil; steaming it gives a different, flatter lemon oil, because pressing keeps compounds steam destroys.
• Expression for citrus specifically — citrus peel oil is cold-expressed (scarified and pressed) because the aroma cannot survive distillation. This is also why expressed citrus oils carry phototoxic furocoumarins such as bergapten, which distillation would have left behind.
• Enfleurage and maceration — fat absorbs the aroma from flowers too delicate for any of the above.

5. When heat is the right answer

Not everything should be cold.

• Toasted sesame oil and roasted pumpkin-seed oil are defined by roasting. Cold-pressed versions are different products, not better ones.
• Conditioning / cooking the seed before pressing ruptures oil cells, lowers viscosity and raises yield substantially. It is standard practice for a reason.
• Deactivating enzymes. Many seeds carry lipase, which splits the oil into free fatty acids the moment the cell is broken — raising acidity and ruining shelf life. Heat kills the lipase. Rice bran oil is the classic case: it must be stabilised quickly or it degrades fast.
• Myrosinase in mustard and rapeseed, and alliinase in garlic, likewise act only after crushing. Whether you want them active is a product decision, not a quality one.
• Toxins. Some press cakes need heat: cottonseed carries gossypol, castor carries ricin (not in the oil, but in the cake). Heat treatment is a safety step.

6. Handling, storage and rancidity

The best oil can be ruined after pressing.

• Two rancidity routes. Oxidative — oxygen attacks double bonds, giving the stale, cardboard, painty smell. Hydrolytic — water and lipase split the fat, giving free fatty acids and, in coconut and butter, the distinctly sickly short-chain note.
• The four enemies: oxygen, light, heat, and metal ions (copper and iron catalyse oxidation).
• So: dark glass or tin, full containers, cool and dark, and used promptly. Unrefined oils have much shorter shelf lives than refined ones, which is the hidden cost of buying them.
• The numbers that quantify it: peroxide value (early oxidation), free fatty acid / acid value (hydrolysis), p-anisidine (secondary, later oxidation). Totox combines them. A seller who can quote these knows their product.
• Smoke point is mostly a function of free fatty acids and fine particles, which is why unrefined oils smoke low and refined ones smoke high — and why smoke point drops as an oil ages or is reused.

7. Judging an oil

• Yield claimed versus method claimed. If a cold-pressed oil is cheap and abundant, one of the two claims is doing work it cannot support.
• Colour and cloudiness indicate unrefined — not automatically better, but informative.
• Harvest or press date matters more than a "best before" date for unrefined oil.
• The lab numbers in §6 beat every adjective on the label.
• Olive oil fraud is extensive and well documented; grade claims on olive oil in particular should be treated with scepticism unless there is panel testing behind them.

8. Where this connects

• The solvent, distillation and closed-loop side: Laboratory Equipment and Extraction Engineering.
• What the extracted aroma actually consists of: A Field Guide to Smell Molecules: Classes, Key Odorants and the Reactions That Make Them.
• Concentrating and redepositing a resin rather than pressing it: Standardized Extracts, Enhanced Leaf and the Perfumery of Botanical Aroma.
• An oil pressed as a pesticide rather than a food: Neem, Botanical Pesticides and What Actually Works.

Sources

• International Olive Council, trade standard on olive oils and olive-pomace oils (grades, acidity, peroxide value, panel test); EU Regulation 1308/2013 and 29/2012 on cold extraction below 27 °C.
• Codex Alimentarius Standard 210 for named vegetable oils.
• Gunstone F. D., Vegetable Oils in Food Technology (2nd ed., 2011).
• Shahidi F. (ed.), Bailey's Industrial Oil and Fat Products (6th ed., 2005) — pressing, conditioning, refining and residual-oil figures.
• Frankel E. N., Lipid Oxidation (2nd ed., 2012) — peroxide value, anisidine, Totox.
• Rice-bran lipase stabilisation and gossypol/ricin cake-treatment literature in the above references.

Filed under  Plants and preparation