This is the process by which oil is obtained from the source material. Oils are commonly obtained from seeds (such as rapeseed, borage and sesame), but also from kernels (e.g. hazelnut, walnut) and the flesh of oil containing fruit (e.g. avocado, olive). In some cases only part of the seed is used. Wheatgerm oil, for example, is obtained from just the germ of the grain after it is separated during processing of wheat.
After cleaning and drying as required, oil is obtained using two processes:
Crushing or Pressing: These are processes in which pressure is applied to the material to obtain oil, commonly using equipment known as an expeller press. Heat may also be applied to the material in a pre-conditioning step as this can improve the yield. Where heat is not used and the temperature of the process is controlled, it may be described as ‘Cold Pressing’.
Solvent Extraction: In most cases, solvent extraction is used after pressing. Hexane is almost exclusively used for this process, and is mixed with the solid residue from pressing. The oil passes into the solvent, which is subsequently removed by distillation and recycled back into the process. Extracted oil typically contains less than 1 ppm of hexane, in line with EC/US legislation.
In most cases solvent extracted oil is combined with pressed oil before being refined, although in a few cases pressed oil is kept segregated and sold at a premium due to the lower yield.
Pressed Oil v. Solvent Extracted Oil
There is a common perception that pressed oil is better than solvent extracted oil. In fact, the main components (triglycerides and fatty acids) of oil extracted by either method do not differ significantly, although the composition of minor components may differ. However, there is some evidence to suggest the content of some beneficial minor components (for example tocopherols) may actually be higher in solvent extracted oils, so the benefit of one type over another depends upon the intended use and required properties. The content of minor components can be affected during refining, so any differences are in any case often minimised if oil is subsequently refined.
‘Not a lot of people know this’:
Due to the natural variation in raw materials, seasonal variations and crop harvest issues, Cold Pressing only often leads to significant inconsistency and does not guarantee the best quality product for your application, it also does not necessarily guarantee that all of the natural actives are present in the oil. Moreover, current European Food Contaminant regulations render some cold pressed oils non-compliant for use for some applications without further processing.
Crude oils will normally have relatively dark colour and some odour and flavour. They also contain a number of minor components, such as free fatty acids, oxidised products and trace impurities, many of which may not be desirable depending upon the final intended use. The refining process is intended to remove or reduce the presence of any undesirable components and ensure the oil quality is suitable for the intended use. Note that with the exception of Extra Virgin Olive oil, most commodity cooking oils, sold for cooking, are fully refined to enhance product quality and consistency and to ensure compliance to global food regulations
There are a number of different stages that are employed:
Degumming: Many oils, especially those obtained from seeds (for example soyabean and rapeseed oils) contain significant levels of phospholipids, known as gums. These are removed by addition of water (and sometimes a small amount of acid, either citric or phosphoric). The gums precipitate in the presence of the water and are removed from the oil along with the water. Degumming can be performed in conjunction with the extraction process, or may be carried out prior to neutralisation.
Neutralisation: Addition of an alkali solution (caustic soda) enables the removal of free fatty acids, which react with the alkali to form soaps. The soaps are separated from the oil either under gravity or by centrifugation. The oil is then washed with water and dried under vacuum. In addition to the removal of free fatty acids, the process also reduces the content of residual proteins, phosphatides, trace metals and some pigments.
Bleaching: The oil is treated with bleaching earth/clay, usually at around 90°C and under vacuum. Silica may sometimes be used. Pigments (such as chlorophyll and carotenes) are adsorbed onto the bleaching earth, which is subsequently removed by filtration. Activated carbon is an alternative adsorbent that is effective at removing polycyclic aromatic hydrocarbons (PAH), and is employed along with bleaching earth if PAH are suspected to be present. In addition to the reduction in pigments, bleaching is effective at further reducing phosphatides, trace metals, oxidation products and proteins.
Deodorisation: The main purpose of deodorisation is to reduce flavour and odour. Deodorised oils are normally practically bland. The process is effectively steam distillation under vacuum, where the relatively volatile flavour and odour components are distilled off. The process can also reduce the free fatty acid content of the oil, and there can be a further reduction in colour. Temperatures of 180 – 250°C and a vacuum of 0.5 – 8 mbar, together with steam as a stripping gas, are normally used. It has also been demonstrated that the process effectively removes pesticide residues and light PAH if these are present.
Oils refined by these processes possess good organoleptic properties and have improved stability, whilst the key nutritional and physicochemical properties are maintained. In addition, the reduction in protein achieved by the process is considered to be effective at ensuring the absence of allergens.