Last reviewed 1 Oct 2026 · 8 min read
Oilseeds and their oil content
| Oilseed | Oil content (approx., %) |
|---|---|
| Groundnut (kernel) | 45–50 |
| Sesame | 45–50 |
| Mustard / rapeseed | 35–42 |
| Sunflower (seed) | 38–45 (kernel 50–55) |
| Safflower | 30–35 |
| Soybean | 18–21 |
| Copra (dried coconut) | 62–68 |
| Cottonseed | 15–20 |
| Linseed | 35–42 |
| Castor | 45–55 |
| Palm kernel / palm fruit (mesocarp) | 45–50 / 20–25 (fruit) |
| Rice bran | 15–20 |
| Maize germ | 30–40 |
- Edible oils: groundnut, mustard, sunflower, soybean, sesame, coconut, palm, rice bran, safflower, cottonseed. Non-edible oils (industrial): castor, linseed, neem, karanja (pongamia), jatropha — used for soaps, paints, lubricants and biodiesel.
- Oil content is determined in the laboratory by Soxhlet extraction with hexane or petroleum ether.
Pre-treatment of oilseeds
The seed must be prepared to release the oil efficiently.
- Cleaning — remove stones, dust, metal, and other seeds (sieves, aspirators, destoners, magnets).
- Dehulling (decorticating) — removes the hard outer hull (sunflower, cottonseed, groundnut shells) to increase the oil and protein quality of the cake and reduce wear of equipment; by hullers, decorticators or disc shellers; followed by air separation of the hulls. (Mustard and soybean usually go with the hull or have a thin one.)
- Size reduction (cracking, grinding, flaking) — cracking rolls or hammer mills break the seed into fragments to rupture the oil cells; flaking rolls produce thin (0.2–0.4 mm) flakes — a large surface area is vital for solvent extraction.
- Conditioning (cooking, heating) — heating with steam to 70–110 °C, with moisture controlled (about 4–8 %), for 20–60 minutes. Purposes: rupture the cell walls; coagulate proteins; lower the oil's viscosity so it flows easily; inactivate enzymes (lipase, myrosinase); sterilise; improve flaking and pressing characteristics.
Methods of oil extraction
1. Mechanical expression (pressing)
| Machine | Features |
|---|---|
| Ghani (kolhu) — traditional | A wooden or iron mortar and a rotating pestle driven by bullocks or an electric motor; the seed is crushed and pressed; oil recovery low (about 80–85 % of the oil present); the cake retains 10–15 % oil; oil retains flavour (cold pressed, "kachi ghani"); small scale |
| Hydraulic press | The seed (in cloth bags or cages) is pressed between plates at high pressure (150–400 kg/cm²); batch operation; low capacity; good oil quality; used for cocoa, groundnut, mustard (batch), neem |
| Screw press (expeller) | A continuous machine: a worm (screw) rotating inside a barrel with narrow slots (cage); the seed is conveyed, compressed and squeezed as the volume decreases along the screw; oil flows through the slots, and the cake exits through an adjustable choke (cone) at the end. Pressure up to 1000–1500 kg/cm² at the end; capacities from 0.5 to 25 t/day (small) up to 100+ t/day (large) |
- Expeller: residual oil in cake 6–10 % (from a full press); pre-press operation (leaving 15–20 % oil in the cake) is used before solvent extraction.
- Parameters: screw speed, choke setting, seed temperature and moisture, pre-treatment; temperature rise (friction) up to 100 °C affects oil colour and quality.
2. Solvent extraction
The oil is dissolved out by a solvent (food-grade hexane) — a leaching (solid–liquid extraction) operation, used for low-oil seeds (soybean, rice bran, cottonseed) and for the pre-pressed cake of high-oil seeds.
- Process: prepared flakes or pre-press cake are brought into contact with hexane in countercurrent extractors (basket, belt (de-Smet), rotary (Rotocel), or percolation type) at about 50–60 °C; the miscella (oil + solvent) is drained; the solvent is recovered from the miscella by distillation (evaporators and strippers) and from the marc (spent meal) by desolventiser-toaster (DT) with steam.
- Hexane losses are small (about 1 kg or less per tonne of seed).
- Residual oil in meal: about 0.5–1.5 % — very high recovery.
- Advantages: maximum oil recovery, good-quality meal (de-oiled), large capacity, low residual oil. Disadvantages: explosion and fire hazard (hexane), high capital cost; needs large throughput to be economic.
3. Supercritical CO₂ and aqueous methods
Supercritical CO₂ extraction (specialty oils, spice extracts) and aqueous/enzymatic extraction are emerging or niche (sensitive oils; solvent-free); cold-press for high-value oils.
4. Combined process
Pre-press followed by solvent extraction is used for high-oil seeds (sunflower, rapeseed, cottonseed): the press removes about 60–70 % of the oil cheaply, and the solvent extracts the rest from the cake.
Refining of crude oil
Crude oil contains free fatty acids (FFA), gums (phospholipids), colour pigments, wax, and odorous compounds, which must be removed to obtain edible oil.
FormulaSteps in refining
- Degumming — treat with hot water or phosphoric acid at about 70–80 °C; the phospholipids (gums) become insoluble and are centrifuged out (the source of lecithin).
- Neutralisation (de-acidification) — remove free fatty acids by alkali (caustic soda) washing; soap stock is separated (chemical refining), or by steam distillation at high temperature and vacuum (physical refining, for palm and rice bran oil).
- Bleaching — adsorption of colour pigments by bleaching earth (activated clay) or activated carbon at 90–110 °C under vacuum; filter the earth.
- Dewaxing / winterisation — cool and filter to remove waxes (sunflower, rice bran oil), preventing cloudiness in the cold.
- Deodorisation — steam distillation under high vacuum at 220–260 °C removes volatile odour and flavour compounds and the remaining FFA; the finished oil is cooled and antioxidants (TBHQ, citric acid) added.
- Fractionation / hydrogenation / interesterification (optional) — to make vanaspati, shortenings, margarine.
Refining loss — total loss in refining is about 2–10 % depending on the FFA and gums.