Effect Of Cooking Methods On Microcystins Residues And Nutritional Quality In Lettuce

Eutrophication cause the increasing cyanobacteria bloom problem.Microcystins（MCs）produced by cyanobacteria bloom entered the farmland ecosystem with agricultural irrigation and other methods,and were concentrated in the edible parts of crops,threatening the food safety of crops.The existing health risk assessment of MCs in edible parts of crops is based on the MCs concentration in harvested samples in the field to calculate the possible daily intake（EDI）per person,and then compared with the maximum allowable daily intake（TDI）.However,most crops are usually eaten after cooking,and the cooking processing and human digestion process can change the contaminant residues and toxicity in food,thus affecting the health risk assessment results.To study the effect of cooking methods on the consumption risk of MCs in lettuce,Lactuca sativa L.was used to simulate the common concentrations of MCs(5,20,50 and 100μg·L^-1)in the 35-day-old lettuce for 7 days.Effect of cooking methods（microwave,boiling and frying）on the MCs residues and the changes of nutrients（soluble sugar,soluble protein and vitamin C）in lettuce were studied by enzymelinked immunoassay.Effect of cooking methods on the bioavailability of MCs and nutrients in lettuce were studied by static digestion model in vitro.Finally,the cytotoxic changes of MCs in lettuce after cooking methods were investigated by the combination of human hepatocellular toxicity assessment and multiple linear regression.This not only provided a theoretical basis for accurately assessing the health risks of MCs residues in crops,but also provided a new thinking direction for reducing MCs residues in food.Main results were as follows:（1）Effect of cooking methods on microcystins residues and nutritional quality in lettuce indicated:The concentration of MCs in lettuce treated with simulated MCs(5～100μg·L^-1)was 65.3～215.4μg·kg^-1,and its EDI was 1.08～3.59 times that of TDI.After microwave（low,medium,and high power）,boiling（100 ^oC）,frying（100 ^oC,140 ^oC,and 180 ^oC）treatment,the residues of MCs in lettuce were 54.8～185.9μg·kg^-1,40.5～121.2μg·kg^-1,and39.7～176.7μg·kg^-1,respectively.EDI was 0.91～3.09 times,0.67～2.3 times,and 0.66～2.9 times of TDI,respectively.The results showed that microwave,boiling and frying methods all reduced the MCs residues in lettuce,and the residues decreased with the increase of microwave and frying temperature.Among these three cooking methods,boiling was the most effective in reducing MCs residue in lettuce.However,it is worth noting that the health risks of MCs(5μg·L^-1)treated lettuce changed from a safety risk to a low safety risk after cooking,while the health risks of MCs(20～100μg·L^-1)treated lettuce remained after cooking.Further research on the reasons for the reduction of MCs residue in lettuce by cooking methods showed that MCs were detected in both boiled water and fried oil,indicated that the reduction of MCs residue in lettuce by cooking treatment might be related to the dissolution of MCs in lettuce.The concentration of MCs in boiled water(1.48μg·L^-1)was higher than that in fried oil(1.02μg·L^-1),which was related to the hydrophilicity of MCs.In addition,microwaving,boiling,and frying reduced the content of nutrients（vitamin C,soluble protein,and soluble sugar）in lettuce containing MCs residues to a greater extent than that in non-MCs lettuce（excess content:5.9～14.2%）,indicated that MCs in lettuce could aggravate the decrease of nutrients in lettuce during cooking,which was related to the decrease of total antioxidant capacity of lettuce aggravated by MCs.（2）Effect of cooking methods on microcystins residues and nutrient bioavailability in lettuceindicated:The bioavailability of MCs residues in lettuce treated with MCs(20～100μg·L^-1)ranged from 16.77～19.49%,and the decrease of MCs bioavailability was mainly in the intestinal digestion stage（9.11～19.49%）,followed by the oral stage（12.28～21.94%）,and finally in the gastric digestion stage（23.25～37.38%,）.The bioavailability of MCs residues in lettuce after microwave,boiling and frying were 9.11～13.27%,10.45～13.51%and 12.42～13.59%,respectively,which were all lower than those in uncooked lettuce,indicated that microwave,boiling and frying could reduce the bioavailability of MCs in lettuce（the part that could be absorbed by human body）,and microwave cooking had the greatest reduction.Considering the effect of MCs bioavailability,EDI was 0.11～0.316 times of TDI,which was about 10%of the MCs health risk in cooked lettuce,indicated that the health risk of MCs contaminated lettuce was within the safe range.The bioavailability of vitamin C and total phenol in lettuce treated with MCs(20～100μg·L^-1)was 15.06～16.8%and 26.17～40.39%,respectively.The bioavailability of vitamin C and total phenol in lettuce containing MCs was 17.89～22.46%and 50.1～85.46%after three cooking methods was higher than that in uncooked lettuce.In conclusion,microwave,boiling and frying could improve the bioavailability of nutrients in MCs-contaminated lettuce,indicated that cooking could reduce the health risk of MCs in lettuce and improve the edible value of nutrients in lettuce.（3）Hepatocellular toxicity effect of cooking methods on microcystins in lettuce and risk assessment results indicated:The acute health risk of MCs in lettuce treated with MCs(5～100μg·L^-1)was within the acceptable range in all populations,pencentage of reference dose（POR）<100,while chronic health risks of MCs were present in all populations（POR>100）.Microwaving,boiling,and frying could reduce the chronic health risks of MCs.Adult males showed no health risks of MCs after 5μg·L^-1MCs-treated lettuce was cooked,but children and adult females were still at higher risk,suggested that more attention should be paid to the health risk assessment of children and women under exposure to MCs.Quantitative analysis of cytotoxicity of MCs in lettuce showed that the inhibition degree of MCs residues in uncooked lettuce treated with MCs(20～100μg·L^-1)on the activity of human hepatocytes was 80.3～88.8%.The inhibition degree of MCs residue on hepatocyte activity（7.78～13.5%）of lettuce after microwave,boiling and frying was lower than that of uncooked lettuce,indicated that cooking could reduce the biotoxicity of MCs to human hepatocytes.There was no significant difference in MCs residue between uncooked and uncooked lettuce treated with MCs(20μg·L^-1),but the inhibition of MCs residues in cooked lettuce on hepatocyte activity（12.9%）was greater than that in uncooked lettuce（11.2%）,which might be related to the stress of degradation products or conjugates of MCs on hepatocytes,indicated that the degradation products or conjugates of MCs were still biotoxic after cooking treatment.In addition,the prediction equation of MCs in food on hepatocyte activity was established:Y=10.598-0.081X₁-4.404X₂,where Y is hepatocyte activity,X₁is MCs concentration in food,and X₂is bioavailability of MCs in food.Therefore,the effect of digested MCs on the activity of human hepatocytes could be predicted by the concentration of MCs in food and the results of bioavailability,which provided a scientific basis for the rapid evaluation of the toxicity of MCs in food to human beings.In conclusion,cooking methods（microwave,boiling and frying）could reduce the MCs residues and bioavailability of MCs in lettuce,so as to reduce the biotoxicity of MCs to human hepatocytes,suggested that cooking could reduce the health risk of MCs residues in leafy vegetables,and at the same time,MCs residues could aggravate the loss of nutrients in lettuce during cooking.By comparing the effects of three cooking methods on the health risks and nutritional quality of MCs residues in lettuce,it was found that boiling method was superior to microwave and frying under the experimental conditions of this study.