Canola seed is traditionally processed using solvent extraction in order to separate the oil from the meal. This process, called pre-press solvent extraction, usually includes:
In Canada canola seed is graded according to strict grading standards established by the Canadian Grain Commission. These include specifications for maximum moisture content, seed damage and chlorophyll level. The seed delivered to the processing plant may contain weed seeds, stems, pods and other materials termed “dockage” which is removed by cleaning operations prior to processing.
Seed pre-conditioning and flaking
Many canola processing plants in colder climates pre-heat the seed with grain dryers to approximately 35°C to prevent shattering which may occur when cold seed from storage enters the flaking unit (Unger, 1990). The cleaned seed is first flaked by roller mills set for a narrow clearance to physically rupture the seed coat. The objective, therefore, is to rupture as many cell walls as possible without damaging the quality of the oil. The thickness of the flake is important, with an optimum of 0.3-0.38 mm. Flakes thinner than 0.2 mm are very fragile while flakes thicker than 0.4 mm result in lower oil yield.
Flakes are cooked/conditioned by passing them through a series of steam-heated drum or stack type cookers. Cooking serves to thermally rupture oil cells which have survived flaking, reduce oil viscosity and thereby promote coalescing of oil droplets, increase the diffusion rate of prepared oil cake, and denature hydrolytic enzymes. Cooking also adjusts the moisture of the flakes, which is important in the success of subsequent prepressing operations.
At the start of cooking, the temperature is rapidly increased to 80-90°C which serves to inactivate the myrosinase enzyme present in canola. This enzyme can hydrolyze the small amounts of glucosinolates in canola and produce undesirable breakdown products which affect both oil and meal quality.
The cooking cycle usually lasts 15-20 minutes and the temperatures normally range between 80° and 105°C, with an optimum of about 88°C. In some countries, cooking temperatures of up to 120°C have been traditionally used when processing high-glucosinolate rapeseed to volatize some of the sulphur compounds which can cause odours in the oil. However, these high temperatures can affect meal protein quality.
The cooked canola seed flakes are then pressed in a series of screw presses or expellers. These units consist of a rotating screw shaft within a cylindrical barrel that contains flat steel bars set edgewise around the periphery and spaced to allow the oil to flow between the bars while the cake is contained within the barrel. The rotating shaft presses the cake against an adjustable choke, which partially constricts the discharge of the cake from the end of the barrel. This action removes part of the oil while avoiding excessive pressure and temperature. The objective of pressing is to remove as much oil as possible, usually 50-60% of the seed oil content, while maximizing the output of the expellers and producing a presscake that is ideal for solvent extraction.
Since pressing alone cannot remove all of the oil from the canola seed, the presscake is usually solvent extracted to remove the remaining oil. The cake from the expellers, containing 18-20% oil, is sometimes broken into uniform pieces prior to solvent extraction in which a solvent (n-hexane) is used that is specially manufactured for the vegetable oil industry. Various mechanical designs of solvent extractors have been developed for moving the cake and the miscella (solvent plus oil) in opposite directions to effect a continuous counter current extraction. Basket and continuous loop type extractors are commonly used for canola. The principles are the same – the cake is deposited in the extractor, which is then flooded with solvent or miscella. A series of pumps spray the miscella over the presscake with each stage using a successively “leaner” miscella, thereby containing a higher ratio of solvent in proportion to the oil. The solvent percolates by gravity through the cake bed, diffusing into, and saturating, the cake fragments. The marc (hexane-saturated meal) that leaves the solvent extractor, after a fresh solvent wash, contains less than 1% oil.
Meal desolventizing and toasting
The solvent is removed and recaptured from the marc in a desolventizer-toaster. In a series of compartments or kettles, the majority of the solvent is flashed from the meal by heating it on a series of steam-heated plates. The final stripping of the solvent is completed by injecting live steam through the meal, a process termed toasting. During the desolventization-toasting process the meal is heated to 95-115°C and moisture increases to 12-18%. The total time spent in the desolventizer-toaster is approximately 50 to 90 minutes. The meal is then cooled and dried to approximately 12% moisture by blowing air through it. The meal is next granulated to a uniform consistency using a hammer mill and is either pelleted or sent directly to storage as a mash.
During processing, meal quality can be affected by temperature and other factors.
To ensure good stability and shelf-life, the crude oil goes through a series of processes involving water precipitation or organic acids in combination with water. These processes remove compounds like phospholipids, mucilaginous gums, free fatty acids, colour pigments and fine meal particles. Once removed, these by-products are added to the canola meal fraction to make it an even more nutritious product for animal feed.
To remove unattractive colour compounds, the oil is then passed through a filter containing natural clay. Although this process is called “bleaching” it is a physical process that doesn't involve any harsh chemicals.
The final step in refining any type of vegetable oil is deodorization. Steam distillation removes any compounds that could give the oil an unpleasant odour or taste.
At this point, the canola oil is ready to be packaged and sold as a cooking oil, or further processed into other products.
Canola oil can be made into products like margarine and shortening through processes called interestification or hydrogenation, which solidifies the oil.
It may also be mixed with other oils that are more solid by nature, including palm kernel oil. This process results in a semi-solid product that does not need to be further processed.
Double pressed canola
A small proportion of Canadian canola seed is processed by a term called double pressing, or expeller processing. The seed is expelled twice to extract oil rather than using solvent extraction in a second step, to extract the residual oil. Up to the point of solvent extraction, the process is similar to the traditional pre-process solvent extraction process. However, it excludes the solvent extraction, desolventization, and drying and cooling stages. The resulting meal has higher oil content which can range from 8-11% and therefore has higher metabolizable, digestible and net energy content than traditional pre-press solvent extracted meal. The meal is not subjected to desolventization/toasting, the primary source of heat that can affect traditional solvent extracted meal, but it is still subject to the potential effects of heat due to the friction generated during the expelling process. The meal temperatures may achieve as much as 160°C but due to the low moisture content and the short duration, protein quality is generally preserved. However, in extreme cases or if the meal is not cooled quickly after extraction, protein quality can be affected.