Mechanical Pressing

Oil in a seed is not sitting in a reservoir waiting to be poured out. It is held in cells distributed through the kernel tissue, and it has to be both freed from those cells and given a path out. Mechanical pressing supplies the second half of that: pressure that drives free oil through a restricted opening while the solids are held back. It cannot supply the first half, which is why preparation — rupturing the oil cells and shortening the distance oil must travel — happens before the press, not in it.

A screw press works by continuously reducing the volume available to the material as it advances along the screw. The solids compact; the liquid, being incompressible and mobile, is squeezed out through gaps sized to retain the solids. The cake emerges compressed and depleted of the oil that would flow, and the expressed oil leaves carrying suspended fines that a later clarification step removes.

Pressing splits one commodity into two co-products, and the word matters. Neither the oil nor the cake is the by-product of making the other. A crusher sells both, prices both, and would not run the plant on either alone. Describing the meal as what is left over after the oil is taken misrepresents both the material flow and the economics.

Expressed oil

The liquid fraction, leaving the press as crude oil carrying suspended solids. It goes forward to clarification before any further refining.

Press cake or meal

The compacted solid fraction: protein, fibre, and the oil that pressure could not free. A co-product with an established feed market, sold on protein.

What happens next to the cake depends on the plant. Where pressing is the only recovery step — the configuration behind oils marketed as expeller-pressed or cold-pressed — the cake leaves as a finished co-product with its residual oil intact. Where a plant is configured to recover more, the cake goes forward to solvent extraction, and pressing is then a prepress stage rather than the whole crushing operation. High-oil seeds such as sunflower and rapeseed are commonly prepressed this way; the choice is a plant-configuration and market decision, not a quality ranking.

Pressing does not create oil quality; it inherits it. Oil that arrives in sound, properly stored seed presses out sound. Seed that has been damaged, heated, or allowed to develop free fatty acids yields oil carrying those faults, and no press setting reverses them. What pressing does influence is how much heat the material sees — pressure against a compacting solid generates it — and heat has consequences for both streams, affecting colour and minor components in the oil and protein quality in the cake.

The dominant loss point is the one built into the method: oil retained in the cake. Whether that counts as a loss at all depends on the plant. In an expeller-only operation it is not lost — it is sold, as part of a higher-oil feed cake. In a prepress operation it is not lost either, because extraction recovers it. It becomes a genuine loss only where a plant intends to recover it and does not.

• Oil retained in the cake — recovered downstream in prepress plants, sold in the cake in expeller-only plants
• Fines carried into the crude oil, to be removed at clarification
• Heat generated by compaction, affecting oil colour and cake protein quality
• Spillage and residual stock at transfers

Pressing is mechanically demanding: forcing oil out of a compacting solid takes substantial electrical energy, and press wear parts are consumables. Against that, it is a solvent-free route, and both of its streams are utilised. Comparing its overall environmental burden with that of solvent extraction is not a simple matter of counting inputs — the two recover different amounts of oil from the same seed, so any comparison has to be drawn on a comparable basis, and none is asserted here.