Rapeseed Oil Production
The main problem in the extraction of rapeseed oil is to achieve a high yield of oil while maintaining high quality of protein. Therefore, operational parameters should be optimized during preliminary operations, processing and isolation of oil. Primary processing includes pre-processing, extraction proper and utilization of by-products.
Pre-treatment-In preparing oilseed for the extraction of its oil, energy must be used to rupture or weaken the walls of the oil-containing cells. This energy is partially mechanical work, which includes; breaking, grinding, rolling, pressing and pelleting. The partially thermal energy to degrade cell walls, reduce oil viscosity and adjust moisture content.
Dehulling-The purpose of dehulling is to remove the major part of the fiber and a group of pigments which, passing into the meal, would lower its feeding value. The overall dehulling process includes: cooking, dehulling itself and separation of the hulls. A cooking period of 10-15 min is usually sufficient, since during this time at 90-97(C the seed reaches moisture content approximately 6%. The dehulling itself takes place in a disc mill, in which the disc spacing and the rotational speed can be varied. Hull separation can be combined with the removal of sulfur compounds.
Rapeseed Oil Extraction – Extraction of oil from flaked rapeseed can proceed by one of the following processes: direct screw pressing, direct solvent extraction, and pre-press solvent extraction. The pre-press solvent extraction process is a classical system of processing rapeseed in which the seed is initially expelled under pressure to release a portion of the available oil; and the residue is than solvent extracted. This method is still used by many oil producing manufacturers, with some modifications, such as pretreatment of rapeseeds described above. Pre-press solvent extraction is probably one of the most economical processes.
A prepress processing steps include:
1. Seed cleaning-Current rapeseed cleaning equipment typically consists of three basic steps: aspiration, screen separation to remove oversized particles, and screen separation to remove undersized particles. Most equipment can provide all three steps in a single unit.
2. Preconditioning-Preheating the whole seed prior to processing (to about 30-40(C) by indirect heating or direct hot air contact. This process improves flaking, screw pressing capacity, cake formation, extractability, and hexane recovery from the extracted canola flakes.
3. Flaking-In order to extract the oil, cell walls must be ruptured to allow the lipid particles to migrate to the outer surface of the flake. The lipid portion is separated from the solid flake. This allows a solvent to penetrate into cellular structure dissolving and diluting the lipid portions. Next, this lipid portion flows out of the cell structure and onto the outer surface of the flake. The preheated rapeseed is flaked between two smooth surface cast-iron rolls.
4. Cooking-Rapeseed flakes are heated in a stacked cooker (at about 75-85(C).
5. Screw pressing-This step is obtained to remove 60% – 70% of the oil from rapeseed flakes, and to compress the small fragile rapeseed flakes into a more dense and durable cake to facilitate good solvent contact and percolation in the extractor.
6. Solvent extraction-Further extraction of oil seeds and press cake with hexane.
7. Desolventizing-Removal of hexane solvent from the extracted cake.
8. Distillation-Hexane recovery from canola oil.
9. Degumming-Removal of the rapeseed gums, and free moisture, cooling of dry oil and then transfer to the refining process or into a storage.
Refining-The refining process involves degumming, neutralization, drying, bleaching, and deodorization. Crude oil from extraction has to be refined to obtain a high quality oil. Natural impurities of crude rapeseed oil include water, dirt, phosphatide gums, free fatty acids, colour matter, odiferous and flavorous substances, natural breakdown and oxidation products of the oil itself. There are two methods for refining edible oils: alkali and physical refining.
1. Degumming-Gums compose about 2% of solvent-extracted rapeseed oil. Degumming treatment commonly uses hot water or steam plus phosphoric acid, citric acid, or other acidic materials. Precipitated gums are removed by continuous centrifugation.
2. Neutralization-Free fatty acids due to the enzymatic breakdown of oil, can be neutralized with alkali solution. After the alkali treatment the oil is washed with hot water to remove traces of soaps that can reduce stability of oil. In addition, pigments of oil, such as chlorophyll, also undergo partial decomposition during neutralization.
3. Drying-The purpose of drying is to remove traces of water from the oil, which improves stability and frying performance.
4. Bleaching-A common method of bleaching is by adsorption of the color producing substances on an adsorbent material. Bentonite (or acid-activated earth clay) and Fuller’s earth have been used most extensively for bleaching rapeseed oils. Rapeseed oil contains large amounts of chlorophylloid pigments that undergo autooxidation, it is more difficult to bleach rapeseed oil than other vegetable oils. However, during refining, degumming removes approximately 38% of chlorophyll b, neutralization removes mainly chlorophyll a, and deodorizing removes carotenes. Overall refining carried out on laboratory scale removes 95.5% of carotene and approximately 85% of chlorophylls.