As a first step the seed/bean is cleaned and dried. Foreign material (like stones, glass and metal) is taken out by sieving and magnets and disposed of outside the feed chain.
Drying is performed by avoiding contact with combustion gasses unless natural gas is used.
The preparation of the seeds before an extraction step depends on the kind of seed/bean and the required quality of the meal.
Some oilseeds, like soybeans and sunflower seed, may be dehulled after the cleaning step.
After dehulling the meal will have a lower crude fibre content, and hence a higher protein content.
The hulls can also be used for feeding purposes, as such or in pelletized form.
In the past, crushing was done between mill stones that later became steel rolls. Hence the factories are known as oil mills and the process as oil milling or oilseed crushing.
Seeds with a high oil content, like rape seed and sunflower seed, are usually mechanically pressed in expellers after a preheating step in indirectly heated conditioners. The expeller cake (or pressed cake) will then be further treated in the extractor, since it might still content up to 18 per cent of oil. In some cases the expeller cake is not further extracted but after deep expelling sold as such for feed purposes.
The expeller works as follows: the oil bearing material is fed into one end of a cylinder within which a power-driven worm conveyor forces the material to the other end of the cylinder and out against resistance. The pressure exerted in the process squeezes out the oil.
Soybeans, with a relatively low oil content, are thermally treated, mechanically cracked and flaked for further extraction.
Sometimes the raw material is pressed without heating; such oils are known as cold-pressed oils. Since cold pressing does not extract all the oil, it is practiced only in the production of a few special edible oils, i.e. olive oil.
Crude oils having relatively high levels of phosphatides (e.g., soybean oil) may be degummed prior to refining to remove the majority of those phospholipid compounds.
During the degumming process the crude oil is treated with a limited amount of water in order to hydrate the phosphatides and then separate them by centrifugation. After the degumming process, the crude oil is dried.
Soybean oil is the most common oil to be degummed; the phospholipids are often recovered and further processed to yield a variety of lecithin products.
The resulting pressed crude oil can be consumed as such or be further refined.
Solvent extraction is used to separate oil from seeds/beans where the principle is to employ a volatile liquid in which the oil is freely soluble. The common solvent used by crushers is hexane*. The pre-processed seeds/beans are treated in a multistage counter-current process with solvent until the remaining oil content is reduced to the lowest possible level.
* ‘Hexane’ in accordance with Council Directive 88/344/EEC on extraction solvents used in the production of foodstuffs and food ingredients.
The miscella, a mixture of oil and solvent, is separated by distillation into two components, oil and solvent. The solvent is recycled into the extraction process.
Crude oils obtained by pressing and/or extraction are sometimes used directly for food and feed purposes. In most cases, however, the crude oils are refined in a multistage process.
Crude oils might contain substances and trace components, which are undesirable for taste, stability, appearance, and odour or may interfere with further processing. These substances and trace components include seed particles, impurities, phosphatides, carbohydrates, proteins, and traces of metals, pigments, waxes, oxidation products of fatty acids, polycyclic aromatic hydrocarbons and pesticide residues.
Internal specifications developed by the oils and fats sector stipulate that crude oils should meet certain quality requirements. In fact, this is a key step in ensuring that when refining is applied to this raw material, the fully refined oil is suitable for human consumption.
The hexane-containing meal is treated in the desolventising toaster with the help of indirect heating and steam. The desolventising toasting process serves three purposes. Firstly, to win back the solvent from the meal, secondly to increase the nutritional value of the meal e.g. by reducing the content of glucosinolates or trypsin inhibitors, and thirdly to minimise the risk of biological contamination.
To obtain a stable and transportable feed material that is fit for storage, the meal is subsequently dried and cooled. During this process, the water content of the meal is reduced from 18 – 20% to below 14%. In general, oil meals are stored in silos. At present, the packing in bags is limited to exceptional cases. In order to avoid the sticking of the oil meals to the wall of the silo, it is common practice that an anti caking agent (amongst others mineral clays like bleaching clay) is added. This is particularly necessary when the silos reach considerable heights. The anti caking agents used are those permitted by feedstuff legislation.