Seed oil extractionThere is a variety of manufacturing methods of manufacture associated with the large range of oil sources. However a number of key steps are typically always undertaken. Coarse extraneous matter (wood, stones, metal) are first removed with sieves, cyclones, magnets, etc. Fine impurities (e.g. plant residues, dust, sand) are then removed by vibrating sieves, wind sifters, etc. The husks and hulls of some seeds (e.g. soya, sunflower) are removed, and the seeds are then cracked, followed by air classification, to expose the “meats” of the raw material. The meats are conditioned or dried to a required moisture level (typically 9-10%) and rolled into flakes which releases oil and improves subsequent extraction.
The extraction may be achieved about by either pressing (for seeds and nuts) or boiling (for fruit), by solvent extraction, or by a combination of the two. The oil is skimmed after boiling and filtered after pressing.
Solvent extraction generally consists of a sequence of four operations:
- physical removal of oil from the feedstock (seed, husk, etc.) in the extractor
- desolventising-toasting of the de-oiled meal, often combined with drying and cooling of the meal
- distillation, to remove the solvent of the extracted oil
- recovery of the solvent for re-use in the reactor.
Hexane is typically used for this purpose. The recovered solvent/oil mixture is called “miscella” and the extracted flakes “spent flake” or “cake”.
In the production of olive oil, there are three systems currently in use for the extraction of the oil: traditionally by pressure, and using three-phase or two-phase decanter centrifuges. In Spain, most plants are of the two-phase type, while in most other Mediterrean countries the bigger plants use the three-phase technique, and smaller plants typically still use traditional pressing.
While the two-phases generate a paste-like waste, both the traditional and the three-phases systems produce a liquid phase (olive mill waste water, `alpechin') and a press cake (pomace, husk, `orujo'). This latter may be further treated (`husk' or `pomace' oil). The remaining solid (husk) is dried (to 3-6% of humidity) and used as fuel.