Steps in Oil and Meal Processing

Pre-press solvent extraction

This is the most common means of processing canola. After the seed is crushed, oil is separated from meal using solvent.

The process usually includes:

During processing, meal quality can be affected by temperature and other factors.

Double-pressing is a less common processing method. Instead of extracting oil with solvent, the seed can be run through the expeller a second time. The resulting meal has higher oil content and therefore higher metabolizable, digestible and net energy content. The meal is not desolvented/toasted so there is less potential for temperature to affect meal quality.

Steps in the process

Seed cleaning

Dockage materials are removed at the plant before processing.

Seed pre-conditioning and flaking

In colder climates, many crushing plants use grain dryers to preheat the seed and prevent shattering as seed enters the flaking unit. The cleaned seed is first flaked by roller mills set for a narrow clearance to physically rupture the seed coat. The objective is to rupture as many cell walls as possible without damaging the quality of the oil. Optimum flake thickness is 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.

Seed cooking

Flakes are passed through steam-heated drum or stack-type cookers to:

  • Thermally rupture oil cells that have survived flaking
  • Reduce oil viscosity, which promotes oil collection
  • Increase the diffusion rate of prepared oil cake
  • Denature hydrolytic enzymes
  • Adjust the moisture of the flakes to prepare for prepressing.

Temperature is rapidly increased to 80-90°C to inactivate the myrosinase enzyme. These enzymes can hydrolyze the small amounts of glucosinolates in canola and produce undesirable breakdown products affecting oil and meal quality.

The cooking cycle usually lasts 15-20 minutes. Temperatures range from 80-105°C, with an optimum of about 88°C.

Temperatures up to 120°C have been traditionally used in China and some other countries to process high-glucosinolate rapeseed. However, these high temperatures can negatively affect meal protein.

Pressing

The cooked canola seed flakes are then pressed in a series of screw presses or expellers. The process removes 50-60% of the oil while avoiding excessive pressure and temperature.

A cylindrical barrel contains a rotating screw shaft and flat set steel bars set along the edge. Oil flows between the bars while the cake stays in the barrel. The rotating shaft presses the cake against an adjustable choke, which controls discharge of cake from the barrel.

The process produces a presscake that is ideal for solvent extraction.

Solvent extraction

To remove the remaining oil, the cake is treated with hexane that is specially refined for the vegetable oil industry. The cake is flooded with solvent or miscella. The extractor then moves the cake and the miscella (solvent plus oil) in opposite directions to create a continuous counter-current extraction.

A series of pumps spray a progressively "leaner" miscella over the cake, gradually increasing the ratio of solvent to oil. The solvent percolates by gravity through the cake bed, diffusing into, and saturating, the cake fragments. After a fresh solvent wash, the marc (hexane-saturated meal) leaving the solvent extractor contains less than 1% oil.

Various types of extractors can be used. The basket and continuous loop type designs are the most common.

Meal Desolventizing and toasting

The solvent is removed from the marc in a desolventizer-toaster. Most of the solvent is flashed from the meal by heating it on a series of steam-heated plates in compartments or kettles. The final stripping of the solvent is completed by injecting live steam through the meal - a process called toasting.

During the desolventization-toasting process, the meal is heated to 95-115°C and moisture increases to 12-18%. The meal is in the desolventizer-toaster for about 30 minutes. Then air is blown through it to cool and dry the meal to about 12% moisture. Finally, the meal is granulated to a uniform consistency using a hammer mill, and then either pelleted or sent directly to storage as a mash.

Oil refining

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 moved through a natural clay. Although this process is called bleaching, it 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.

At this point, the canola oil is ready to be packaged and sold as a cooking oil, or further processed into other products.

Further processing

Canola oil can be made into products like margarine and shortening through a process called 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 hydrogenated.

Cold pressing

Canola oil can also be extracted from the seed through cold-pressing, which follows the same general steps but does not involve hexane extraction. The oil is removed primarily through mechanical pressing, which is a less efficient way of collecting the oil.