Extraneous material should first be removed from the harvested crop seed by screening. This is particularly important to ensure that materials such as small stones are removed prior to crushing and extraction. The seed should also be passed over a magnet to remove metal pieces.
Processors sometimes heat the seeds prior to processing to increase the oil yield. When pressing for fuel production, it is possible to use engine exhaust to warm the oilseeds. Some systems use an integrated steam kettle to heat the oilseeds.
The seeds are sometimes crushed and extruded before the final pressing to further enhance oil extraction. The material left after extracting the oil is referred to as “cake” or "meal." "Cake" is the official term when the oil is extracted mechanically; "meal" is the term used when oil is extracted using a solvent.
Some seeds, such as soybeans, must be cooked before being pressed to destroy anti-nutritional factors in the cake. Cooking makes the cake suitable for animal feed.
Mechanical extraction of oil produces a crude oil and a cake that contains approximately 5 to 10 percent or more of the original oil content of the seeds. The oil in the cake may enhance its value as a livestock feed, depending on the needs of the animals being fed.
Oilseed meal is often used as a protein supplement for cattle. Therefore, cake with a higher protein content often commands a higher price. But protein is not the only thing that matters. Canola meal (which has a lower percentage of protein than soybean meal) has been found to be good for milk production in cows.
Some cake is not suitable for livestock feed. Mustard seed meal, for example, has high concentrations of glucosinolates, which prevent absorption of some nutrients. However, this meal has been shown to have excellent herbicidal and pesticidal properties when used as a soil amendment and has the potential to be a high-value by-product.
Crude oil contains solid material and gums that should be removed from the oil before processing it for biodiesel. The University of Idaho biodiesel laboratory removes impurities by allowing the oil to settle for at least two weeks. This process removes some gums and virtually all of the solids (such as seed coats and dirt). The gums and solids can be drained off the bottom of the tank, or the oil can be decanted off the top, leaving gums and solids behind as a sludge-type material at the bottom.
However, small-scale biodiesel producers may benefit from additional degumming processes because this can enhance the quality and yield of the final biodiesel product. Some seeds (such as soybeans) contain more gums than other seeds. The gums can make it more difficult to separate the glycerin at the end of the transesterification process. In addition, because gums contain phosphorous, too much left in the biodiesel can cause it to fail the ASTM fuel standard specification.
Degumming is accomplished by mixing small quantities of water (2 to 3 percent) with the heated oil. The water combines with the gums and precipitates out after the mixture settles for approximately one hour. The water can then be drained off the bottom of the oil.
Gums can also be removed from oil using an acid degumming process. This is similar to the water degumming described above but includes a small amount of citric or phosphoric acid to dissolve the non-water soluble gums.
Removing all the water from the oil before further processing is important because the presence of water will hydrolize some of the oil to fatty acids, which will form soap rather than biodiesel.
Removing the water can be accomplished by heating the oil to 120° Celsius (248° Fahrenheit) for approximately two hours to boil off the water. Use of a vacuum, if available, permits faster removal of water and allows for gentler heating for prolonged shelf life of the biodiesel.