| Photodynamic technology(PDT)has many advantages such as safe and non-toxic,maintaining the original flavor,color and freshness of food and low drug resistance,and is widely used in fruit and vegetable preservation.Curcumin has antibacterial,antioxidant and other biological activities.It is also a natural photosensitizer that can be used for PDT technology and has reliable sources,low toxic and side effects,and strong photosensitive effects.However,its poor stability and low water solubility hinder its development and application in the food field.Embedding curcumin with nanoemulsion can improve its stability and water solubility,thereby improving its bioavailability.In this study,curcumin was extracted from different varieties of Curcuma plants by selecting the best sterilization effect of Curcuma varieties.Secondly,curcumin nanoemulsion was constructed with whey protein-dextran sulfate conjugate(Wpi-Ds)as emulsifier.Finally,curcumin nanoemulsion-mediated PDT was applied to the preservation of fresh-cut mango and the inactivation of Escherichia coli and Listeria monocytogenes.The main results are as follows:1.Study on the bactericidal properties of characteristic compounds in different varieties of Curcuma plants:Curcumin was extracted from eight different varieties of turmeric by ultrasonic assisted extraction.The content of curcuminoids in different varieties of turmeric was explored,and the content of total phenols and total flavonoids and antioxidant activity were further determined.The results showed that the contents of curcumin,demethoxycurcumin and bisdemethoxycurcumin in Curcuma aromatica were the highest,which were 531.80±36.64μg/g,281.39±13.36μg/g and 162.63±21.17μg/g,respectively.The contents of total phenols,total flavonoids and antioxidant capacity were second only to Curcuma longa and Curcuma wenyujin.The flavor quality of different varieties of Curcuma plants was explored by GC-MS and electronic nose.The results showed that terpenes were the main volatile components,and the relative content of terpenes in Curcuma wenyujin and Curcuma aromatica was higher.In addition,the bactericidal effects of different varieties of Curcuma plants and curcumin standards on Escherichia coli and Listeria monocytogenes were compared.The results showed that Curcuma aromatica had the highest bactericidal rate against Listeria monocytogenes and Escherichia coli,which were 82.21±1.60%and 85.12±1.36%,respectively.The results of correlation analysis showed that the contents of curcuminoids and terpenes were significantly correlated with the sterilization rate.Therefore,Curcuma aromatica was selected for subsequent embedding and photodynamic experiments.2.Preparation and properties of curcumin nanoemulsion:Wpi-Ds complex coacervation system was prepared,and the interaction force of Wpi-Ds conjugate was investigated.The results showed that Wpi-Ds conjugate was formed by electrostatic interaction between Wpi and Ds,and the secondary and tertiary structures of Wpi were affected after reaction.Then the Wpi-Ds curcumin nanoemulsion system was constructed,and the morphology of the nanoemulsion was investigated by transmission electron microscopy.It was found that the emulsion system was uniform and stable.The stability of curcumin nanoemulsion stored at 4°C was investigated by particle size,PDI,Zeta potential,embedding rate,turbidity and K value.The results showed that the particle size and PDI value of nanoemulsion with SOR of 7:3 were the smallest and the absolute value of potential was the highest.The minimum turbidity;k value is the largest;the embedding rate was the highest;and the change range during storage is small,indicating that the emulsion has the highest stability,so it is used for subsequent sterilization and preservation experiments.3.The preservation and sterilization mechanism of curcumin nanoemulsion photodynamic sterilization on fresh-cut mango:The curcumin nanoemulsion mediated PDT technology was applied to the preservation of fresh-cut mango,and the effect of PDT treatment on the sterilization and enzyme inactivation effect and quality of fresh-cut mango was evaluated after 5 days of storage at 4°C.The bactericidal effect and bactericidal mechanism of different concentrations of curcumin nanoemulsion on Listeria monocytogenes and Escherichia coli were investigated.The results showed that PDT treatment could better maintain the color of fresh-cut mango on the 5th day,and the weight loss rate was only 1.69±0.01%.The contents of titratable acid,soluble solids and total phenols were 0.08%,1.89%and 0.06 mg/g higher than those of the control group,respectively,and the relative conductivity was 9.21%lower than that of the control group.DPPH and superoxide anion radical scavenging rates were 37.80%and 13.36%higher than those of the control group.The total number of colonies,the total number of molds and yeasts were 3.13×10~4CFU/g,1.64×10~3CFU/g lower than the control group,and the inhibition rate of PPO activity was 44.4%.The sterilization rate of Escherichia coli and Listeria monocytogenes increased with the increase of curcumin nanoemulsion concentration.When the concentration was 25μmol/L,the sterilization rates were 98.83±0.08%and 98.94±0.04%,respectively,and both reached a stable level.After PDT treatment,the cell wall of the bacteria was destroyed,the permeability of the cell membrane was changed,and the DNA damage and protein degradation of the bacteria resulted in bacterial death. |
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