Expeller pressing (also called oil pressing) is a mechanical method for extracting oil from raw materials. The raw materials are squeezed under high pressure in a single step. When used for the extraction of food oils, typical raw materials are nuts, seeds and algae, which are supplied to the press in a continuous feed. As the raw material is pressed, friction causes it to heat. In the case of harder nuts, which require higher pressure, the material temperature can exceed 120 °F (49 °C). "Expeller" is a trademarked term of Anderson International Corporation since 1900, although it has become genericized, is often confused with screw press equipment in general, and does not indicate whether oil extraction is done hot or cold.
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In this Dossier
Expeller pressing in the context of Grain
A grain is a small, hard, dry fruit (caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legumes.
After being harvested, dry grains are more durable than other staple foods, such as starchy fruits (plantains, breadfruit, etc.) and tubers (sweet potatoes, cassava, and more). This durability has made grains well suited to industrial agriculture, since they can be mechanically harvested, transported by rail or ship, stored for long periods in silos, and milled for flour or pressed for oil. Thus, the grain market is a major global agricultural market that includes crops such as maize, rice, soybeans, wheat and other grains.
Expeller pressing in the context of Linseed oil
Linseed oil, also known as flaxseed oil or flax oil (in its edible form), is a colorless to yellowish oil obtained from the dried, ripened seeds of the flax plant (Linum usitatissimum). The oil is obtained by pressing, sometimes followed by solvent extraction.
Owing to its polymer-forming properties, linseed oil is often blended with combinations of other oils, resins, or solvents as an impregnator, drying oil finish, or varnish in wood finishing; as a pigment binder in oil paints; as a plasticizer and hardener in putty; and in the manufacture of linoleum. Linseed oil use has declined since the 1950s with increased availability of synthetic alkyd resins, which function similarly, are petroleum-based, and resist yellowing.
Expeller pressing in the context of Press cake
A press cake or oil cake is the solid matter remaining after pressing something to extract the liquids. Their most common use is in animal feed.
Some foods whose processing creates press cakes are olives for olive oil (pomace), peanuts for peanut oil, coconut flesh for coconut cream and milk (sapal), grapes for wine (pomace), apples for cider (pomace), mustard cake, and soybeans for soy milk (used to make tofu) (this is called soy pulp) or oil. Other common press cakes come from flax seed (linseed), cottonseed, and sunflower seeds. However, some specific kinds may be toxic, and are rather used as fertilizer; for example, cottonseed contains a toxic pigment, gossypol, that must be removed before processing.