| During the transportation,storage and consumption of vegetable oil,the color of oil is significantly deepened with the extension of time.This phenomenon is called oil color reversion.Many factors such as raw material quality,processing technology and storage conditions can affect oil color reversion.With the in-depth study of oil color reversion,it has been confirmed that oxidation is the inducement of oil color reversion.The γ-tocopherol oxidation derivative is endogenous substance that causes the oil color reversion.Therefore,controlling oil color reversion can be achieved by slowing down oil oxidation.Adding antioxidants is a convenient and efficient method to slow down the oxidation of oil.Through the screening of commercial antioxidants,it was found that tert-butyl hydroquinone(TBHQ)and ascorbyl palmitate(AP)can effectively slow down the oil oxidation and oil color reversion.Due to the potential risks of TBHQ,as well as consumers’ emphasis on health,green antioxidant AP is gradually gaining importance.However,AP has disadvantages such as difficulty dissolving,which limits its application in oils and fats.Based on this,the solubility of ascorbic acid fatty acid ester in oil is improved by increasing the fatty acid chain length and unsaturation of ascorbic acid fatty acid ester.The preparation of ascorbic acid fatty acid ester was carried out by using biological enzyme as catalyst.In order to reduce the cost of immobilized enzymes and improve the catalytic activity of liquid lipase,a simple and lowcost immobilization method was developed.The fat solubility of ascorbic acid stearate(AS)and ascorbic acid oleate(AO)in oil and their effect on oil color reversion were studied.The specific results are as follows:(1)Firstly,the experimental conditions of accelerated color reversion in oven were determined.It was found that there was a correlation between the significant deepening stage of corn oil color reversion and oil peroxide value,and significant color reversion occurred in the primary stage of oxidation;In the process of oil color reversion,the oxidation index of oil increased significantly,and the AV didn’t change significantly.Through the screening of commercial antioxidants,it was found that Polyhydroxyphenol antioxidants(TBHQ)and olefin glycol antioxidants(AP)have better effects on slowing down the color reversion and antioxidation of corn oil.When the amount of TBHQ and AP is 200 ppm,compared with blank,the difference in red value(ΔR6.0)down to ΔR3.3 and ΔR2.5.However,AP is difficult to dissolve in oils and fats.Under normal temperature,when the addition amount of AP is 200 ppm,the solubility is about 20%.The solubilization temperature has a significant impact on its solubility.When the solubilization temperature is 65℃,it has better solubility,and the solubility can reach 92.21±2.93%.(2)In order to increase the solubility of ascorbic acid in oil,ascorbic acid stearate(AS)and ascorbic acid oleate(AO)were synthesized by increasing the chain length and unsaturation of fatty acid.In order to reduce the catalytic cost and improve the catalytic ability of liquid lipase,liquid lipase CALB was immobilized with low-cost domestic resin.Using adsorption resin,when enzyme solution:buffer for 2:1(v/v),immobilized for 24 h,the immobilized loading rate can reach 72.17%,the enzyme activity is 383.44 U/g.The immobilized lipase CALB was prepared under this immobilized conditions.The optimum reaction conditions for the synthesis of as were obtained by single factor and response surface optimization: the reaction temperature was 72°C,the substrate ratio was 1:12,the enzyme addition was 22.49%,the substrate concentration was 0.12 mol/L,the molecular sieve addition was 100 g/L,and the yield was 41.33±1.81% after 24 h.The effects of reaction variables on the preparation of AS catalyzed by immobilized lipase CALB were as follows: temperature >substrate ratio > enzyme addition.The optimum reaction conditions for the synthesis of AO were obtained by single factor and response surface optimization: the reaction temperature was 70°C,the substrate ratio of ascorbic acid to oleic acid was 1:11,the addition of enzyme was 28.02%,the concentration of ascorbic acid was 0.12 mol/L,the addition of molecular sieve was 100 g/L,the reaction time was 17.61 h,and the yield was 39.26±1.43%.The effects of reaction variables on the preparation of as catalyzed by immobilized lipase CALB were as follows: reaction time > reaction temperature > enzyme addition > substrate ratio.(3)Finally,the solubility of AS and AO and the effect on oil color reversion in purified corn triglyceride system and corn oil system were studied.Increasing the length of fatty acid chain can’t improve the solubility of ascorbic acid ester.Increasing the unsaturation of fatty acid chain can significantly improve the solubility of ascorbic acid ester.Through DSC and solubility measurement,AO has lower melting temperature(41.22℃)and higher solubility.In the purified corn triglyceride system,only α-Tocopherol,AS and AO can’t cause oil color reversion;The higher the total tocopherol content,the more serious the oil color reversion;Adding 200 ppm AS and AO to the total tocopherol oil sample and recoloring for 10 h under the same conditions can reduce the R by about 0.5.In corn oil system,AS and AO have better effects of slowing down oil oxidation and color reversion.When 200 ppm AS and AO were added respectively and the color return was accelerated for 10 h,the R of corn oil could be reduced by 1.6;In the process of oil oxidation and color reversion,AS and AO have better ability to retain tocopherol in oil.The oxidation induction time of corn oil was determined by Rancimat experiment.The oxidation induction time of adding 100 ppm AS or AO was similar to that of adding 50 ppm TBHQ.Under the same addition amount,there was no significant difference between AS and AO on the oxidation induction time of corn oil.Finally,the study found that AS and AO can also slow down the color reversion of soybean oil. |
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