Decolorization is a washing process, so what are the impurities it hopes to remove and to what extent? It is described as follows:

Oxidation products of oil and grease

Adsorption decolorization process hopes to completely remove the oxidation products of fats and oils, including automatic oxidation of fats and oils, enzyme oxidation of primary oxidation products (hydroperoxides) and their decomposition of the formation of secondary oxidation products, as well as tocopherols, sterols, pigments and other lipid oxidation products. These substances exist in the oil to be decolorized, and if they cannot be removed, the stability of the oil will be seriously affected.

Oxidation products usually have a certain polarity, can be effectively adsorbed by the white clay and removed, at the same time, the activation of the white clay can be hydroperoxides and epoxy compounds, such as degradation into volatile aldehydes and ketones, which ensures that these compounds in the next step of the deodorization process to remove efficiently. This is the first task of decolorization.

Trace metals

Trace metals such as iron and copper are naturally present in oil seeds, either from the plantation or from the oil processing facility; these trace metals must be removed to the maximum extent possible by adsorption prior to deodorization in order to ensure the stability and quality of the deodorized oils. Especially for post decolorization of hydrogenated oils, the main purpose is to remove the trace metal catalysts.

Soap

Adsorption decolorization is the best way to remove trace soap. Although most of the water-soluble sodium soap can be removed by water washing of alkaline refining oil, there still exists a small amount of water-insoluble metal soap, such as calcium/magnesium soap, etc. in the oil. Phospholipids can also be partially hydrolyzed in the alkali refining process to make the alkali refining oil calcium/magnesium soap residue increases. These substances must be decolorized to maximize their removal.

Residual phospholipids and glycolipids

Soybean oil, rapeseed oil, sunflower oil, peanut oil and cottonseed oil have high content of phospholipids, palm oil, palm kernel oil and coconut oil have low content of phospholipids but contain a certain amount of glycolipids, and most of the oils and fats will still leave a small amount of phospholipids or glycolipids after degumming.

Recently, the research of rice bran oil found that there is a special kind of glycolipids in the oil – phosphorus-containing glycolipids, which are almost impossible to be removed by traditional degumming methods, but they can be maximally removed by suitable adsorbents.

Polycyclic aromatic hydrocarbons and pesticide residues

The environmental pollutants of oil mainly include pesticide residues and PAHs, and adsorption and decoloration can also remove them substantially.

Pesticide residues mainly come from pesticides applied during the growth of oil crops and contaminated soil and water, pesticides containing phosphorus and nitrogen are absorbed by the oil after biodegradation, and enter into the oil by oil production, and they can be removed by adsorption of white clay in the process of decolorization; organochlorine pesticides are difficult to biodegrade, and have a long period of pollution of the soil and water, and although the use of such pesticides has been banned in many countries, the environment is still contaminated, and it will be difficult to use adsorption to remove them once they enter into the oil. Once in the oil is difficult to remove by adsorption, can only be removed in the deodorization.

Polycyclic aromatic hydrocarbons mainly from the combustion process of pollutants, such as flue gas drying oil will increase the content of polycyclic aromatic hydrocarbons in the oil, coconut oil in particular. Generally, white clay cannot adsorb non-polar PAHs, but activated carbon can adsorb heavy PAHs with more than 4 rings, and the remaining light PAHs can be removed during deodorization.

Pigmentation

Partial removal of color (decolorization) is of course one of the purposes of decolorization of fats and oils.

The pigments in oils and fats can be divided into primary pigments and newborn pigments.

The former are carotenoids and chlorophyll in their natural state, as well as cotton phenol in cottonseed oil. Chlorophylls are mainly removed by the decolorization process, and chlorophylls are a type of natural pigment that we hope to remove as much as possible during the decolorization process. Carotenoids are heat sensitive and can be effectively removed by thermal cracking during deodorization. White clay will not adsorb cotton-phenol, but cotton-phenol is acidic, can be removed by neutralization in alkaline refining.

The latter is the oxidative decomposition products of primary pigments or their reaction products with other lipids, which are newborn pigments in the process. Newborn pigment is in the process of decolorization hope completely removed, but more difficult.

The removal of pigment in oil does not rely entirely on the decolorization process, the removal of pigment in oil, throughout the refining process, degumming, alkali refining, acid refining, deodorization process have the effect of removing pigment, are accompanied by a reduction in the color of the oil and grease, which relatively reduces the need to remove a large number of pigments from the decolorization section. In fact, for palm oil and soybean oil, decolorization process does not play a major role in the improvement of oil color.

In short, the purpose of decolorization, is to make the decolorized oil in the color improvement, not theoretically remove all the pigments in the oil, and sometimes even the main purpose is not to remove the pigment, but to reduce the oxidation products in the oil, residual phosphorus, residual soap and trace metals, agricultural residues and other impurities, thus improving the flavor and oxidation stability of the oil to improve the role of the oil for the next step of the process of deodorization to provide qualified raw material oil. Therefore, the decolorization process can not be omitted, even if the gross oil color is very light. Otherwise, the flavor of fully refined oil is easy to deteriorate.

In terms of the safety of edible oil, the color is small, the flavor (stability) is big, so the task of the decolorization process is: to remove miscellaneous affairs, and the removal of pigment can be moderate.