Oil separation by extraction
Nowadays, oil separation by extraction is the commonest process used in vegetable oil production. Nearly all vegetable oils, apart from olive oil, are produced in this way, ultimately appearing on the market in the form of refined vegetable oils.
During extraction, a solvent – normally food grade n-hexane – is used to separate the vegetable oil from cellular material. In order to ensure that the solvent enjoys unobstructed access to the interior of the seeds, the cell walls need to be broken down and diffusion pathways need, at the same time, to be kept short. In order to achieve this, seed is heated, where necessary, then pulverised mechanically before being pressed into fine flakes on cylinder rollers. Protein coagulation and steam generation during this process help to break down cell walls.
In practice, two extraction processes – percolation and immersion – are used, both of which share a common feature, in that the solvent (extraction agent) flows in the opposite direction to the seed (extraction material). Fresh solvent is being brought continuously into contact with seed from which most of the oil has been extracted, whilst fresh seed is being brought into contact with heavily oil-saturated solvent, referred to as miscella. After the extraction process, solvent is separated by distillation from oil and is then re-used.
The percolation process, also known as the continuous extraction process, is based upon the principle of uninterrupted wetting of extraction material. Here, solvent streams past extraction material, allowing interrupted exchanges between this and the free-flowing solvent that extracts it. Maintenance of a constant solvent flow ensures that locally saturated solvent flows away and is replaced by noon-saturated solvent. This process requires free extraction agent flow within the extraction material.
The advantage of this procedure is that extraction material is not exposed to mechanical stress. This process is used mainly where "free" oil has to be extracted. Any fine particles, such as are inevitably formed during extraction, are filtered out by seed particles and are thereby prevented from entering the miscella.
The immersion process is used where oil has to be obtained from a matrix that is not easy to extract, as is the case where there are large amounts of unrefined fibre in the extraction material.
In the immersion process, the entire quantity of seed from which material is to be extracted is immersed in solvent. The design of the system means that there are no forced movements, ensuring that oil-saturated solvent (miscella) is constantly being exchanged with fresh solvent. The static system therefore needs to be stirred, in order to balance out any differences in concentration. Stirring inevitably causes abrasion of the extraction material, so the miscella has subsequently to be filtered out.
Factors influencing the choice of extraction processes
The two extraction processes are influenced by a wide range of factors. However, as a rule, there are only five factors that are of any real significance.
1. Seed water content
Being a polar substance, water interferes with wetting of the seed surface and solvent penetration into the seed. In addition, it reduces diffusion. What is needed, however, is a certain degree of residual moisture to maintain seed flake elasticity and to prevent them from crumbling, which would make it difficult for solvent to penetrate the seed.
2. Particle size and shape
First of all, the shape of particles in the extraction material must be such as to allow solvent to flow freely, without any great resistance. Secondly, the particle size must allow the best possible extraction from each individual particle, by minimising diffusion pathways. Seed must not be presented in the form of meal, as this would make solvent percolation impossible.
3. Amount of solvent
The quantitative ratio of solvent to extraction material will depend on seed composition. Generally, the quantity of extraction solvent increases in proportion to the raw fibre content in the seed. The miscella concentration also plays a further role. As a general rule, the higher the latter, the less energy is needed to remove solvent at the end of the process.
4. Extraction temperature
High temperatures reduce solvent viscosity and increase extract solubility in the solvent. Reduced solvent viscosity and enhanced solvent function at high temperatures make for improved extraction. Whilst there may well be no major differences, it is worth using heated extraction agents. The increase in oil yield outweighs the cost of heating the solvent.
5. Extraction time
The extraction time has some bearing upon the degree of extraction and will depend on the nature and structure of the extraction material.