Oil Extraction and Refining

Oil extraction involves pressing the oil bearing material to separate crude oil from the solids high in protein or washing flaked or modestly pressed material with solvent, almost always hexane. The defatted solids after pressing are known as cake and after solvent extraction as meal. The oil, crude oil because it contains undesirable components, such as pigments, phosphatides, free fatty acids, and off-flavors and off odors, must be refined to remove these contaminants and produce high quality edible oils (Johnson, 2002).

Most oilseeds require some degree of cleaning and preparation before the oil is separated from the solid portion of the seed. Foreign matter reduces oil and protein yields, adversely affects oil quality, and increases wear and damage to the processing equipment. Stems, pods, leaves, broken grain, dirt, small stones, and extraneous seeds are the typical components of the foreign material found in some seeds. After cleaning, most of the seeds are dehulled. The hulls have low oil content (usually less than 1%), will absorb and retain oil in the press cake, and reduce the capacity of the extraction equipment (O’Brien, 2009).

Three general types of processes are used to crush oilseeds: hard pressing, prepress solvent extraction, and direct solvent extraction. The extraction process of choice depends primarily upon the oil content of the source material, the amount of residual oil in the meal allowed, the amount of protein denaturation allowed, the amount of investment capital available, and local environmental laws concerning emissions of volatile organic compounds (VOCs) (Johnson, 2002).

All crude fats and oils obtained after rendering, crushing, or solvent extraction inevitably contain variable amounts of nontriglyceride components, such as fatty acids, mono- and diglycerides, phosphatides, sterols, tocopherols, hydrocarbons, pigments (gossypol, chlorophyll), vitamins (carotene), sterol glucosides, glycolipids, protein fragments, traces of pesticides, and trace metals, as well as resinous and mucilaginous materials. The quantities of the nontriglycerides vary with the oil source, extraction process, season, and geographical source. Removal of the objectionable nontriglyceride constituents in the fat or oil with the least possible damage to the triglycerides and minimal loss of desirable constituents is the objective of the refining process. The objectionable components interfere with further processing and cause the oil to darken, foam, smoke, precipitate, and develop off-flavors (Weindermann, 1981 in O’Brien, 2009).

The two major purification-processing methods are chemical refining and physical refining. In chemical refining, FFA, most of the phosphatides, and other impurities are neutralized with an alkaline solution, usually NaOH (sodium hydroxide). In physical refining, the FFA is removed by distillation during deodorization, and the phosphatides and other impurities must be removed prior to steam distillation (O’Brien, 2009).

Physical refining is still some distance away from being applicable to all fats and oils. Currently, the refining method of choice is determined by the characteristics of the individual crude fats and oils: (1) fats and oils that are normally physically refined, (2) fats and oils that can be physically or chemically refined, and (3) fats and oils that can only be chemically refined. Low-phosphatide crude oils, such as palm, palm kernel, and coconut, are almost always physically refined. Tallow and lard processing can also be identified as a form of physical refining (O’Brien, 2009).

Seed oils, such as canola, sunflower, and corn, fall into the either/or category, the choice being determined by process economics and local environmental requirements for handling soapstock and wastewater generated by chemical refining. Soybean oil can also be physically refined depending on the treatment of the bean before and during extraction. To obtain good-quality fats and oils with physical refining, it is essential to have a phosphorus content lower than 5 ppm before steam stripping. Some oils, such as cottonseed, cannot be physically refined; an alkali treatment is required to remove the gossypol pigment (O’Brien, 2009).

Degumming is the first stage in refining, and it is used to precipitate metal salts and other hydratable phosphatides and mucilaginous materials likely to cause the oil to develop flavors and odors, and induces neutralization. Degumming is a process that separates resin or gum in oil which consists of phosphatide, protein, carbohydrate, residue, resin, without decreasing free fatty acid in oil (Ketaren, 1986). Commonly, the process is conducted by adding water, therefore gum and oil can be separated more easily. There are seven methods of gum removal in oil: (a) hydration degumming, (b) acid degumming, (c) hot water degumming, (d) adsorbent degumming, (e) physical degumming, (f) special reagent degumming, and (g) alkali degumming (Fennema, 1985).

According to Syah (2006), degumming is a process that carried out for removal all undesirable material, like phosphatide, free fatty acid, wax, tocopherol, or pigment which can retard reaction. Phosphatide causes turbidity in oil during storage. Water soluble phosphatide can be thrown away by adding water to the oil in temperature 60 – 90oC and followed by centrifugation separation (water degumming). According to Weiss (1983), gum separation will be conducted easily in temperature 60 – 70oC. In batch condition, stirring and mixing 30 – 60 minutes result a perfect hydration process, therefore gum separation can be well conducted. According to Sukarjo (1997), temperature is linier with dissolving velocity, however not in gas dissolved power condition. Some water must be added in a certain number, too little water results oil which still contain phosphatide, however too much water results 3 phases that are oil, gum, and water.