Reasons Of Refined Soybean Oil Color Reversion And Process Of Retarding Color Reversion

The color of refined soybean oil is faint yellow, but it may deepen in the storage,transportation and consumption process, this is the so-called the phenomenon of colorreversion. Internal components of soybean oil may cause the color reversion of soybean oil,such as phospholipids, tocopherols, metal ions and external factors （temperature, light, air）.This research mainly inspected the above-mentioned factors and identified the main reasonsof color reversion in refining processes, and then achieved the purpose of retarding oil colorreversion according to adjustment of refining processes or improvement of processconditions.The standard detection method of oil color is Lovibond colorimetric method, but theresult is easily influenced by subjective factors and the reproducibility of result is poor.Studies have shown that spectral scanning method could eliminate subjective factors on thedetermination of the color of soybean oil and the measurement results were more accurateand precise. The method was suitable for analysis of oil color reversion. The added value ofthe absorbance area between400nm-700nm (ΔS_A400-A700) and the added value of Lovibondred（ΔR） had linear relation, and the linear regression equation was ΔR=0.0574ΔS_A400-A700,r²=0.9388, R rose by0.10when S_A400-A700increased every1.74. For comparison study, theoil was thought to occur color reversion when ΔR≥0.20.The examine factors were added to the oil system, and their effects on color reversionof the oil were studied by oven heating of oil. The results showed that: the most influentialfactors on the color reversion of soybean oil were phospholipid, tocopherol and externalfactors （temperature, light, air）. After being added lecithin powder5mg/kg （measured inphosphorus）, soybean oil was heated at60℃, the oil color red value rose by0.20after240hours. Orthogonal Experimental results showed that the greatest impact on color reversionwas phospholipid, followed by tocopherol, and both of them showed a significantcorrelation with color reversion. Higher the content of tocopherol, more clearly the colorreversion. The lard color no longer rose when the content of tocopherol was less than26.1mg/kg under100℃. The possible reason of the color reversion caused by tocopherolwas that lipid oxidation provided free radicals to tocopherol, and the degradation productsof tocopherol caused the color rose in lard. The temperature, light and air had apparent impacts on soybean oil color reversion. Metal ions, free fatty acids and different initial acidvalues had no significant effect on soybean oil color reversion.The Soybean oil degumming process was studied in the solvent system （hexane） andsolvent-free system. In the solvent-free system, the phosphorus content of the degummed oilcould be reduced to below4.2mg/kg after adding the acid degumming agent. In the solventsystem, the phosphorus content of soybean oil could be effectively removed to5.2mg/kgwithout adding acid degumming agent with a suitable ratio of oil/solvent（4.0:1, m/m）. Afterdecolorization by adsorption and vacuum deodorized, phosphorus content of soybean oilcould be as low as0.84mg/kg. As a result, the measurements could effectively prevent lipidoxidation and retard the oil color reversion by effectively dephosphorization of the refiningprocess, adding antioxidants, low temperature and sealed storage in the dark place.