Study On The Degradation Mechanisms Of The Purple Eggplant Anthocyanins And Changes Of Antioxidant Capacity During Thermal Treatments

Foods with anthocyanins increasingly interest consumers.Fruits and vegetables are the main sources of anthocyanins in daily diets,especially vegetables,which are commonly consumed.Eggplant has been widely cultivated in Asia,Africa,Europe and other parts of the world,especially in China,whose yield ranks the first in the world.Eggplant is rich in various bioactive compounds such as anthocyanins and phenolic acids,and is an important source of nutrients.In particular,the anthocyanin content of eggplant is very high,and mainly delphinidin derivatives,which play an important role in antioxidant,reducing blood lipid and blood glucose,protecting nerves,preventing various chronic and degenerative diseases.The vegetables generally need to be cooked before eating.Anthocyanins are unstable to the temperature.Therefore,Expploring the stability of anthocyanins in thermal treatments is a scientific problem to be solved,and it is of great significance to improve the dietary nutritional value.The previous studies mainly focus on the changes of anthocyanin standards or purified anthocyanins in the aqueous solution,while anthocyanins mainly exist in solid food,and little information is obtained on the conversion rules and changes of anthocyanin antioxidant activity in solid food during thermal treatments.It is due to the complexity of the solid food and more interfering factors.Therefore,the purple eggplant was used as the source of anthocyanins in this study.The simple model food system and complex model food system were designed to explore the anthocyanin conversion rules and changes of anthocyanin antioxidant activity,and then the conversion rules in the purple eggplant were investigated.It was in order to provide references for processing the foods rich in anthocyanins.The main results of this research are as follows:1The dry matter content of the eggplant peel,flesh and the whole eggplant fruit were 9.09±0.04,6.68±0.06 and 6.67±0.06 g/100 g FW,respectively.There was no significant difference in protein content between eggplant peel,flesh and the whole eggplant fruits.The soluble sugar content of the eggplant peel,flesh and the whole eggplant fruits were 24.71±0.59,50.42±1.96 41.64±1.3 g/100 g DW,respectively.The crude fiber content of the eggplant peel,flesh and the whole eggplant fruits were96.30±4.28,9.23±0.01,11.25±1.41 g/100 g DW.The components of eggplant peel and flesh were identified by UPLC-QTOF.Twenty-five compounds were identified,and the compounds only existing in eggplant peel were delphinidin-3-O-rutin-5-O-glucoside,delphinidin-3-O-rutinosylglucoside and delphinidin-3-O-rutinoside.The total anthocyanin content in eggplant peel ranged from 6.24 to 13.42 mg/g DW,the total polyphenols content ranged from 26.10 to 35.62 mg/g DW,and the total flavonoids content ranged from 23.91 to 45.94 mg/g DW.The capacity of the scavenging DPPH and ABTS radicals were 5273.43–8717.78 mg TE/100 g DW and3096.83–5270.49 mg TE/100 g DW,respectively.The reducing capacity?FRAP?was4295.36–8235.42 mg TE/100 g DW.The antioxidant capacity was positively correlated with the total anthocyanin content?r>0.77,p<0.01?.2A simple model food system was designed with dietary fiber as food matrix,and the enriched anthocyanin extract from eggplant peel was designed to explore the conversion rules and changes of antioxidant capacity of anthocyanins during thermal treatments.The results showed that the stability of anthocyanins in solid samples was greater than that in liquid.The degradation of anthocyanins followed the first-order kinetics in steamed,boiled and microwaved solid and liquid samples and fried solid samples.The rate constants of the anthocyanin degradation in the steamed solid and liquid samples were 0.24(t_1/2:2.86 h)and 0.71 h^-1(t_1/2:0.98 h),respectively.The rate constants of anthocyanin degradation in boiled solid and liquid samples were 0.39(t_1/2:1.79 h)and 0.39 h^-1(t_1/2:1.78 h).The rate constants of the anthocyanin degradation in microwaved solid and liquid samples were 0.36(t_1/2:1.90 min)and 0.51 min^-1(t_1/2:1.36 min).The rate constant of the anthocyanin degradation in the fried solid sample was 0.45 min-¹(t_1/2:1.55 min).While during the ultra-high temperature?185±5??heating,the anthocyanin degradation followed the secondary-kinetics,the rate constant of anthocyanin degradation was 3.23?10^-3 min^-1 L mol^-1(t_1/2:1.47 min).The increase of brown index and polymeric color index followed the zero-order kinetics during the thermal treatments.The rate constants of the brown index in steamed solid and liquid samples were 0.06 and 0.10,and the rate constants of polymeric color index were 0.04and 0.06.The rate constants of the brown index in boiled solid and liquid samples were0.10 and 0.10,and the rate constants of polymeric color index were 0.05 and 0.07.The rate constants of brown index in microwaved solid and liquid samples were 0.04 and0.20,and the rate constants of polymeric color index were 0.04 and 0.11.The rate constants of brown index in fried solid and heated liquid samples at the frying temperature were 0.04 and 0.10,and the rate constants of polymeric color index were0.03 and 0.06.The brown index and polymeric index were negatively correlated with the anthocyanin content?r<-0.8?.During the thermal treatments,the antioxidant capacity decreased with time,and there was a positive correlation between the total anthocyanin content and the antioxidant activity?r>0.8?.Our results found that there were two conversion pathways.One was that delphinidin-3-O-rutinoside was hydrolyzed into delphinidin,which was further cleaved into gallic acid and 2,4,6-trihydroxybenzaldehyde,and they might be finally oxidized into pyrogallol and phloroglucinol.The other one was that delphinidin-3-O-rutinoside was first hydrolyzed into form delphinidin-3-O-glucoside,and then it condensed with the p-coumaric acid to form the p-coumaroyl-3-O-glucoside adduct.3The complex model food system was designed with dietary fiber,protein,starch,sugar,and the purple eggplant peel extract,which was actually a fortified food system.Compared with boiling treatment,a higher rate constant of the anthocyanin degradation was observed under steaming.Anthocyanins were subject to much more damage under frying treatment than microwave treatment,and the destruction of frying was the strongest.The anthocyanin degradation in the complex model food system followed the first-order kinetics during steaming and boiling,and the rate constants of anthocyanin degradation were 1.02 h^-1(t_1/2:0.681 h)and 0.59 h^-1(t_1/2:1.174 h)for steaming and boiling,respectively.However,the anthocyanin degradation in the complex model food system followed the second-order kinetics during microwave and frying,and the rate constants of anthocyanin degradation were 0.26(t_1/2:6.26 min)and2.03 mol^-1 L min^-1(t_1/2:1.118 min)for microwave and frying,respectively.The total polyphenolic and flavonoids contents decreased gradually during steaming,boiling and microwave.While the total polyphenolic and flavonoids contents decreased from 0 to1 min,and then increased during frying.There was a significant positive correlation between the total polyphenolic content,total anthocyanin content and the antioxidant activity.The increase of brown index and polymeric index of steamed,boiled,microwaved and fried samples of the complex model system followed the zero-order kinetics.The rate constants of brown index were 0.065,0.048,0.016 and 0.004 during steaming,boiling,microwave and frying,respectively.The rate constants of polymeric color index were 0.059,0.042,0.024 and 0.002 for steamed,boiled,microwaved and fried samples,respectively.Moreover,the brown index/polymeric color index negatively correlated with the anthocyanin content.During the thermal treatments,anthocyanins were mainly decomposed into gallic acid and 2,4,6-trihydroxybenzaldehyde.Gallic acid might be further oxidized,and 2,4,6-trihydroxybenzaldehyde was likely to oxidize and isomerize to dihydroxybenzoic acid.The main phenolic acids?5-O-coffeo-quinic acid and 5-O-feruloquinic acid?might be isomerized and hydrolyzed.4Based on the study of the simple moldel food system and complex moldel food system,the changes of the anthocyanins and the antioxidant activity during thermal treatments were investigated.The results showed that the anthocyanin content decreased obviously during steaming,boiling and frying.After 4 h,the anthocyanin decreased 91.06%and 31.41%for steaming and boiling,respectively.After frying for5 min,the anthocyanin content decreased 60.93%.However,the anthocyanin content of the microwaved purple eggplant peel increased by 20%after 5 min.The destructive effect of steaming on the eggplant peel anthocyanins was stronger than boiling.Frying was the strongest method to reduce the eggplant peel anthocyanins.The increase of brown index and polymeric index of the steamed and boiled eggplant peel also followed the zero-order kinetics.The rate constants of brown index and polymeric color index were 0.166 and 0.084 for the steamed eggplant peel,respectively.The rate constants of brown index and polymeric color index were 0.073 and 0.065 for boiled eggplant peel,respectively.During microwave and frying,the increase of brown index and polymeric color index also followed the zero-order kinetics excluding 0.5 min and 1 min.The rate constants of brown index and polymeric color index were 0.011 and 0.016 for microwave.The rate constants of brown index and polymeric color index were 0.126and 0.084,respectively.The contents of total polyphenolic and total flavonoids contents decreased by 23.81%and 20%after steaming for 4 h.While the total polyphenolic and total flavonoids contents increased by 32.38%and 42.82%after boiling for 4 h,respectively.The increase of the total polyphenolic and flavonoids contents were segmented,they decreased from 4 min,but the total polyphenolic and flavonoids contents were still higher than that of untreated samples after 5 min.The total polyphenolic and total flavonoids contents increased by 20.27%and 36.30%after microwave for 5 min.After frying for 5min,the total polyphenolic and total flavonoids content increased by 36.66%and 3.96%,respectively.During the thermal treatments,the compounds from the eggplant peel might be generated and decomposed,Therefore,the correlation between the antioxidant capacity and the total polyphenolic content was low,especially during microwave and frying.The eggplant peel anthocyanins might be decomposed into gallic acid and 2,4,6-trihydroxybenzaldehyde,and they might also take place chemical reactions,such as hydrolysis reaction,isomerization reaction,Maillard reaction and so on.5.The effect of the extract of the purple eggplant peel,which was processed by different methods,on the antioxidant and liver function in LO2 were investigated.The cell viability tests confirmed that the safe concentration of the processed eggplant peel?uncooking,steaming,boiling,microwave and frying?was 1mg/m L on LO2 cells.When the LO2 cells were treated by 700mmol/L H₂O₂ for 1 h,the cell viability was44.17%.The extract of the processed eggplant peel might significantly improve the activity of SOD,CAT,GSH-PX and GSSH-R enzyme in LO2 cells,and reduced the activity of AST/GOT enzyme and MDA content.The extract of the processed eggplant peel might effectively prevent oxidative damage to LO2 cells.Moreover,the effect of the processed eggplant peel extract was different.