Degradation And Transformation Of Organophosphorus Pesticides In Milk By Non-thermal Processing

Milk plays an important role in the human diet and has long been a favorite among consumers for its rich nutrition and immune-boosting benefits.However,due to the widespread use of pesticides,milk may contain pesticide residues,which not only restricts the development of the dairy industry,but also poses a serious threat to human health.Raw milk usually requires proper processing before it can be consumed,but little attention has been paid to the changes in pesticide levels in milk due to processing,and the possible degradation of pesticides resulting in more toxic products.In recent years,the application of non-thermal technology in milk processing has been widely studied,but the effect of non-thermal processing on pesticide residues in milk is still unknown,which will undoubtedly increase the risk of pesticide exposure and even cause major food safety incidents.Therefore,this thesis investigates the degradation of organophosphorus pesticides(OPP)in milk during non-thermal processing(ultrasound,ultraviolet,fermentation)and its impact on milk quality.The focus is on the generation of degradation products and their toxicity evolutions to provide a theoretical basis for a comprehensive and accurate dietary exposure risk assessment.Five parts of the thesis are as follows:1.Degradation of methyl parathion(PM)in milk by high intensity ultrasound.Degradation of PM in milk by ultrasonic treatment was studied in this part.Results showed that the ultrasonic intensity and the initial concentration of PM had a significant effect on the degradation percentage of PM,while temperature had a limited effect on the degradation.The maximum degradation percentage of PM was 97.10%.Three transformation products were identified through UPLC-QTOF/MS,and the oxidation pathway was proposed.Furthermore,according to Quantitative Structure Activity Relationship(QSAR)model prediction,the ecotoxicity of the transformation products may be higher than that of PM.These findings showed that although ultrasonic treatment can effectively degrade pesticide residues in food,it may also generate transformation products with higher ecotoxicity.In addition,the effect of ultrasonic treatment on milk quality was also investigated.Sonication was found to reduce the size of fat globules in milk,resulting in a reduction in average particle size.This is beneficial for improving the processing properties of the milk.However,prolonged sonication significantly reduced the pH and viscosity of milk,thus affecting milk stability.Moreover,ultrasonic treatment changed the original color of the milk,producing more yellow substances.2.Degradation of chlorpyrifos in milk by combined ultrasound(US)and ultraviolet(UV)treatment.This part aimed to compare the degradation kinetics of chlorpyrifos by treatment with US,UV and a combination of both(US/UV),to evaluate the toxicity of the degradation products and the effect of the treatments on milk quality.US/UV markedly accelerated the degradation of chlorpyrifos,the degradation rate constant of chlorpyrifos by US/UV increased by 391% over that of US alone and 184% over UV alone.Five degradation products were identified by GC-MS,and a degradation pathway for chlorpyrifos was proposed,based on density functional theory calculations.According to the luminescent bacteria test and predictions from the QSAR model,the toxicity of the degradation products was lower than that of chlorpyrifos.In addition,US/UV treatment had little effect on the quality of the treated milk.Therefore,US/UV can be used as a potential non-thermal processing method to degrade pesticide residues in milk.3.Screening of lactic acid bacteria(LAB)for degradation of OPP and its degradation mechanism.The ability of 10 strains of LAB to degrade OPP was studied to explore their potential application in the degradation of OPP in fermented foods.The results showed that some LAB were able to survive in the presence of OPP and significantly degraded OPP within a short time.Among them,L.plantarum had the highest degradation capacity.The phosphatase produced by L.plantarum can rapidly degrade OPP in vitro,further confirming that L.plantarum is degrading rather than adsorbing OPP.It is worth noting that the degradation effects of OPPs by L.plantarum(intact cells)varied greatly,the degradation rate constant of phoxim was 1.65-fold higher than that of dichlorvos.However,the phosphatase extracted from L.plantarum had no degradation selectivity for OPP in vitro.It was speculated that the selective uptake of cells determines this degradation selectivity.The results of molecular docking supported this hypothesis because there was no difference in the binding energies between phosphatase and OPP,while the binding energies between phosphate transporter protein and pesticides were different,and they were negatively correlated with the degradation rate constants of the eight OPP by L.plantarum(r =-0.93).In addition,the excellent antioxidant ability of LAB and the tolerance of simulated gastric and intestinal juices showed their potential protective effects against oxidative damage induced by pesticides in vivo.4.Degradation of dimethoate in milk by fermentation with L.plantarum.This work studied the degradation mechanism of dimethoate in milk by L.plantarum,and analyzed the toxicity of degradation products.The results showed that the bacterial inoculum,temperature,initial concentration of dimethoate and exogenous additives were important factors affecting the degradation of dimethoate by L.plantarum.Under optimal conditions,L.plantarum can degrade 81.28% of dimethoate.The pH and titratable acidity of the fermented milk were 4.25 and 85 °T,respectively.Dimethoate had high binding affinities to phosphatase with the free energy of-16.67 kcal/mol,and amino acid residues,Gln375 and SER415 played important roles in the catalysis process.Five degradation products were identified using UPLCQTOF/MS,and their toxicity was estimated using QSAR model and luminescent bacteria test.Some intermediate products were predicted to be toxic,which should not be ignored,but the overall toxicity of milk decreased significantly after fermentation.5.Ultrasonic promotion of L.plantarum to degrade OPP in milk.The effect of ultrasonic treatment on the degradation of OPP by L.plantarum as well as the quality of yogurt were explored.Ultrasonic treatment significantly promoted the growth of L.plantarum and the degradation of pesticides in yogurt.Ultrasonic intensity,pulse duty cycle and duration are key factors affecting microbial growth and pesticide degradation.Under optimal sonication conditions,the degradation rate constants of four pesticides(fenitrothion,chlorpyrifos,profenofos and dimethoate)were at least three times higher than those without sonication.The improvement of ultrasound on CP degradation by L.plantarum arises from two aspects: one is the enhancement of phosphatase activity;the second is the improvement of cell membrane permeability,which accelerates the transport of nutrients and other metabolites through the cell membrane.In addition,ultrasonic treatment significantly shortened the fermentation time of yogurt,increased the water holding capacity,hardness and antioxidant activity of yogurt,and improved the flavor quality of yogurt.These results provide a basis for the application of ultrasound in microbial degradation of pesticide residues and improvement of yogurt quality.In summary,the results of this study show that non-thermal processes such as ultrasound,ultraviolet and fermentation can degrade OPP residues in milk,but the toxicity of some of the degradation products should not be ignored.The combined use of ultrasound-ultraviolet and ultrasound-fermentation technologies can not only improve the efficiency of pesticide degradation and reduce the toxicity of degradation products,but also maintain and even improve the quality of milk and its products.This study provides a theoretical basis and practical foundation for the application of non-thermal technology in milk processing.