| Soybean oils are rich in nutrients and low price with high production and consumption in the world.Government regulation（GB/T1535-2017）of soybean oils specifies that the color of refined,bleached and deodorized（RBD）soybean oils should be visually clear and bright yellow to light yellow.The issue that oil factories often encountered during oil processing is that soybean oils from quality-inferior raw materials cannot reach qualified color after the traditional refining process.Heat damage is one of the main factors affecting the color of RBD soybean oils.Therefore,soybeans with different degrees of heat damage were selected as raw materials in the work to analyze the quality and composition of corresponding oils and fats as well as to study the composition of pigments.The stability of four kinds of pigments（β-carotene,lutein,chlorophyll,and oxidized pigments of tocopherols）under different conditions（refer to oil processing conditions）was investigated,followed by comparative effects of different types and sources of adsorbents on removal of those pigments,and the most effective adsorbents were screened out.First,12 kinds of soybeans with different degrees of heat damage（normal,general,and severe heat damage）were selected as raw materials to analyze the quality and composition（triglycerides,fatty acids,tocopherols,phospholipids,and major pigments）of corresponding oils and fats.Results showed that heat damage accelerated hydrolysis and oxidation of oils and fats.Compared with oils and fats from normal soybeans,oils and fats form heat-damaged soybeans had significant increases in acid value,peroxide value,anisidine value,total oxidation value,conjugated dienes,and conjugated triene values.Moreover,the increasing rates of values of the above-mentioned indicators followed:severe heat-damaged soybeans>general heat-damaged soybean.The increasing rates of acid value and total oxidation value of oils and fats from severe heat-damaged soybeans were as high as 642 and148%,respectively.The hydrolysis reaction was dominated by sn-1,3 hydrolysis,resulting in the decrease of triglyceride content and the increase of diglyceride and free fatty acid contents.The oxidation reaction led to a significant decrease in the content of polyunsaturated fatty acids,and the severer the heat damage was,the more obvious the change was.Heat damage reduced the content of total phospholipids in oils and fats,of which the relative content of phosphatidic acid significantly increased up to 68.97%,the content of phosphatidic acid in soybean oils with severe heat damege was 1.6～2.2 times that in normal soybean oils.Oxidative loss of tocopherols was prone to occur during heat damage,and the loss rate of total tocopherols in oils and fats from severe heat-damaged soybeans was as high as 27.33%.The content of chlorophyll in oils and fats from severe heat-damaged soybeans was 28～39 times higher than that from normal soybeans.Combined with the analysis of phosphatic acid content,it was concluded that immature soybeans were more prone to heat damage.Theβ-carotene content showed an upward trend but lutein content decreased significantly.Tocopherols generated redγ-tocopherol-5,6-quinone during oxidation loss.Second,with the loss rate of pigments as the evaluation index,the stability and color changes ofβ-carotene,lutein,chlorophyll,and oxidation pigments of tocopherols in n-hexane or RBD soybean oil systems were investigated under different conditions such as light（indoor natural light,outdoor sunlight,and ultraviolet light）,metal ions(Fe2+,Fe3+,and Cu2+),acid（phosphoric acid and citric acid）,alkali（Na OH）,hydrogen peroxide,and heat（solvent system:60°C;oil system:90,120,180,and 240°C）.Results showed that in the n-hexane system,outdoor sunlight,ultraviolet light,and Fe3+could easily disrupt the conjugated structure ofβ-carotene and lutein,and the color of solutions changed from yellow to light yellow.Phosphoric acid,Na OH,outdoor sunlight,and ultraviolet light had strong destructive effect on chlorophyll,and the color of chlorophyll solution changed significantly.The solution of oxidized pigments of tocopherols changed from red to yellow after exposure to outdoor sunlight and ultraviolet light.In soybean oil system,the four pigments showed different degrees of fading after reaction due to being sensitive to heat,outdoor sunlight,and ultraviolet light.Thermal stability of the four pigments followed the order:lutein<β-carotene<oxidized pigments of tocopherols<chlorophyll.Under vacuum conditions,the loss rates ofβ-carotene,lutein,and oxidized pigments of tocopherols was up to more than 80%at 240°C,thus the color of corresponding solutions was close to colorless under naked-eye observation.Light stability of the four pigments followed the order:oxidized pigments of tocopherols<lutein≈β-carotene<chlorophyll.Due to the high viscosity and complicated composition of oils and fats,the variation trends of partial stability of pigments in the two systems were the same,but the variation amplitude was different.Comprehensive analysis showed that chlorophyll was relatively stable and difficult to fade,it was crucial for oils and fats from heat-damaged soybeans with high chlorophyll content to select appropriate adsorbents to remove chlorophyll in the bleaching process.Finally,the bleaching conditions commonly used in oil factories were taken to evaluate the effects of 18 kinds of adsorbents（active clay,attapulgite,and activated carbon）on pigment（lutein,chlorophyll,and oxidized pigments of tocopherols）removal in RBD soybean oil system and deacidified oils（obtained from the factory）with the removal rates of pigments and color as evaluation index.Results showed that in the RBD oil system with individual pigment,the effect of active clay and attapulgite on pigment removal followed the order:lutein>chlorophyll>oxidized pigments of tocopherols;the effect of activated carbon on pigment removal followed the order:chlorophyll>lutein>oxidized pigments of tocopherols;the effect of active clay on the removal of lutein and oxidized pigments of tocopherols was better than that of attapulgite and activated carbon;and the effect of activated carbon on the removal of chlorophyll was better than that of active clay and attapulgite.In deacidified oil system,the effect of different adsorbents on the color of deacidified oil and the removal of chlorophyll in deacidified oil followed the order:activated clay>attapulgite>activated carbon.Among the above-tested 18 adsorbents,#1-3,#2-4,and#3-5 adsorbents had good performance on pigment removal.When the mass ratio of mixed adsorbents（active clay and attapulgite of 3:1,m/m）to activated carbon#3-5 was 20:1,the best bleaching performance was achieved with the best cost-efficiency.Therefore,for soybean oils and fats with similar composition,reasonable approach for pigment removal was that chlorophyll with better thermal stability is removed by bleaching using suitable adsorbents,andβ-carotene,lutein,and oxidized pigments of tocopherols are removed by high-temperature deodorization.The purpose of this study was to provide data and theoretical support for reducing adsorbent consumption in the bleaching process,reducing production costs,and improving the refining rate and oil quality. |
