Expel and Extract
Corn oil processing starts with corn germ, which is the embryo of a corn plant, distinct from the starchy nutrients in the endosperm. The oil content of the germ is about 85 percent of the total oil in the seed. The first step in corn oil processing is mechanical: Corn kernels are dehulled and then crushed with a grooved roller to break down the cell walls. The resultant cake is then “wet milled,” steeped in water acidified with sulphur dioxide to separate the components of the seed.
Oil is expelled from the germ using a heated screw press, which can yield as much as 50 percent of the germ oil. And the remaining oil is stripped from the “press cake” with the solvent hexane, a volatile by-product of gasoline production. (Hexane is introduced to the cake bed with an exposure time as brief as possible so that hexane residual in the oil will be limited.) The corn oil in the hexane solution is heated to vaporize the volatile solvent, which is captured as a condensate and used again. Then the heat expelled and hexane extracted oils are combined as crude corn oil; the remaining cake is processed for livestock feed.
Crude corn oil is filtered and then degummed. In steam degumming, steam is introduced at a volume of not more than 3 percent of the oil and is absorbed by lecithin or other gums, making them heavy enough to be removed by centrifugation. In alkali degumming, a heated sodium hydroxide solution is used to absorb the gums and also neutralize any free fatty acids, followed by “washing” the oil with hot water to remove any soap formed in the process. By-products of the alkali process are sold as soap stock.
Bleach and Deodorize
Corn oil is “bleached” with clay that has been “activated” with acid wash to remove any metals native to the clay. As a result, the clay absorbs color pigments, residual soap products and metal ions from the corn oil. The clay is then removed from the oil by filtration.
If a partially hydrogenated product, such as margarine or shortening, is being produced at this point in the process, the heated oil is exposed to hydrogen gas under pressure. The oil is refrigerated to crystallize any waxes, which are then removed by filtration. A continuous stream steam treatment deodorizes the oil in a distillation tower, carrying any remaining impurities out the top of the tower with the rising steam as the finished oil is removed from the bottom.
Although this outline of the process is an industry standard, some processors use as many as 40 steps to complete their corn oil products.