Vegetable oil obtained by pressing or extracting method without refining is commonly called crude oil. The main components of the oil is fatty acid mixture and triglycerides, commonly known as neutral oil. In addition, it also contains various non-triglycerides of different quantities, collectively called the oil impurities.These impurities are different in the quantity and composition, depending entirely on oil varieties, quality and the way of oil production and processing. To meet the requirements for food or industrial purposes, necessary technical means must be used to remove the impurities in accordance with a certain standard, this is called the “oil refining”.
Except few oils like sesame oil, superfine fragrant peanut oil and soyabean oil that do not need or only need simple physical refining before consumption, most oils must be “refined” to meet the cooking standards. With the continuous development of edible oil refining technology, the “five de” continuous oil refining process is widely used around the world, that is “hydration degumming, alkali refining and deacidification, adsorption decolorizing, winterization dewaxing, high-temperature deodorization”. The following is the introduction of the “five de” technology.
1, Hydration degumming
So-called hydration degumming makes use of the hydrophilicity of glue soluble impurities like phospholipids by adding a certain amount of water or electrolyte to dilute solution and mixed with the oil. Then, the colloid swells and condenses to the “hydrate” with higher relative density than oil. Gravity settling or centrifugation is used then to achieve the purpose of the separation and purification. In hydration degumming process, phospholipid is the main material that can be removed in settlement separation, and other substances such as protein, phlegmatic temperament and trace metals combined with phospholipids can be removed as well.
2, Alkali refining and deacidification
Alkali refining and deacidification refers to the refining method that uses alkali lye (NaOH solution) to neutralize the free fatty acids in the oil, then precipitation separation is made after the soap is produced. The soap produced has strong adsorption ability, it can absorb quite a number of other impurities like solid grain, protein, pectin and pigment into soapstock and separate them.
3, Adsorption and decolorization
Pure triglyceride is colorless in liquid state and white in solid. However, all sorts of common oils have different colors due to the different numbers and varieties of pigments contained in the oil. Most of the pigments are non-toxic, but they will affect the appearance of the oil. Therefore, decoloring treatment must be made in the production of high-grade oil products. Adsorption and decolorization of oil utilizes materials that have strong selective adsorption on some of pigment (e.g. activated clay, activated carbon, etc.) to absorb pigments and other impurities under certain conditions so as to achieve the aim of decolorization. The color of oil can be improved and pectin in the oil can be removed through adsorbent processing. Besides, some of trace metal ions can also be effectively removed that provides a good condition for further refining of oil (deodorization).
4, Winterization and dewaxing There are a small amount of wax contained in the oil, which reduces product transparency and digestion and absorption, the flavor and palatability of oil become worse, thus lowers the oil edible quality and nutritional value. Wax is an important industrial raw material, on the other hand, it can be used for making stencil, waterproofing agent, luster agent, etc. Therefore, removal and extraction of the wax from the oil can improve the nutritional value and quality of the edible oil and the industrial utilization value of oil and the comprehensive utilization of vegetable oil wax source. Winterization and dewaxing process makes use of the different freezing points of wax, saturated fat and liquid oil to seed out saturated fat and wax by reducing the oil temperature, and then use mechanical or solvent method to separate out solid lipid (wax) and liquid oil to make the refined oil meet the standard of freezing test.
5, High-temperature deodorization Pure triglyceride has no smell, but oils obtained by different extraction processes have varying degrees of odor, some of which are loved by people, such as the fragrance of sesame oil and peanut oil, some are not popular like the smell of vegetable oil and rice bran oil. Usually, all kinds of smells in the oil are collectively called odor. Some of the smells are natural, while some of them are produced in the oil extraction and processing. Oil deodorization can not only remove the odor material, improve the oil smoke point and the flavor of edible oil, but also improve the stability, color and quality of the oil. Because in the process of deodorization, the free fatty acid, peroxide and some heat-sensitive pigment can also be removed, volatile decomposition in the protein of mildew oil seeds can be eliminated as well as the PAH with small relative molecular weight and residual pesticide to be within the safety degree. Therefore, importance is attached to deodorization in the production of high-grade oil products.