Advanced Glycation End Products In Tea And The Factors Affecting Their Formation

Advanced glycation end products(AGEs)are a group of Maillard reaction products produced by non-enzymatic reactions between an amino compound(such as protein or amino acids)and a carbonyl compound(mainly reducing sugar).AGEs are associated with the changes in human aging and the development of many diseases,such as diabetes,chronic kidney disease,cardiovascular diseases,aging and neurodegenerative diseases.Due to the potential health risks of AGEs,there is increasing interest in AGEs from dietary sources.N~?-(carboxymethyl)lysine(CML)and N~?-(carboxyethyl)lysine(CEL)are important AGEs and are widely used as markers for AGEs levels in foods.The contents of CML and CEL in foods are directly related to the composition and processing of foods.Many hot-processed foods,including baking,fried,roasted,and hot-dried foods,have been found containing AGEs.Previous studies in our laboratory found that high levels of AGEs were in various types of teas including green tea,black tea and oolong tea.Tea is rich in protein,amino acids and sugars,and it needs to be thermal treated during processing.Therefore,it is not unexpected that teas contain AGEs.However,tea is also rich in polyphenols,mainly catechins.Catechins have been shown to have the inhibitory effects on AGEs in many model systems due to their ability of antioxidation and trapping active carbonyl compounds.It is puzzling that tea with such high levels of catechins still have high levels of AGEs.However,it has rarely been reported that the formation of AGEs and the factors affecting their formation in tea,and the relationship between polyphenols and AGEs in tea.Therefore,this research will study the content distribution and formation of CML and CEL in tea,and the role of polyphenols in their formation.Firstly,this study investigated the levels of CML,CEL and polyphenols in tea,and then discussed the effects of tea processing on CML and CEL levels,and the correlation between CML,CEL and polyphenols.It was found that commercial tea products contained a lot of CML and CEL.In general,except for some oolong tea samples with high CEL content,the contents of CML and CEL present an increasing trend in the order of green tea,oolong tea,black tea,and dark tea.This trend indicated that the levels of CML and CEL were mainly affected by the tea processing.In addition to drying,withering,fermentation(oxidation)and pile fermentation might promote the formation of CML and CEL.There were significantly negative correlations between catechins and CML/CEL,while gallic acid(GA)was significantly positively correlated with CML and CEL.Although the formation of CML and CEL was affected by tea processing,polyphenols in tea might also have effects on it.In order to investigate the effects of tea processing on the formation of CML and CEL,the changes of CML,CEL and polyphenols during the processing of green tea and black tea were studied,as well as the effects of the different parameters of fermented black tea processing on the levels of CML,CEL and polyphenols in the final black tea products.The results showed that during green and black tea processing,CML and CEL were mainly formed during drying which had the most thermal intensity(maintaining at a higher temperature for a longer time),while the other steps also affected the amount of CML and CEL by changing the cellular structure of tea leaves and the compounds in tea.Among the different processing parameters of fermented black tea,natural withering,long-term rolling and fermentation,and higher drying temperature significantly enhanced the formation of CML;natural withering and higher drying temperature also significantly promoted the formation of CEL,while the effects of rolling and fermentation were not significant.Higher temperatures had the greatest impact on the formation of CML and CEL than the other factors.The correlation between polyphenols and CML/CEL indicated that polyphenols had effects on the formation of CML and CEL.During storage,CML and CEL levels in black tea and CML level in green tea increased gradually with the increase of storage time.Due to the differences in composition and tissue state between the two teas,the rate of accumulation of CML and CEL in black tea was faster than that in green tea.Although the above results indicated that there were significant correlations between the contents of polyphenols and CML/CEL in tea,in order to explore how polyphenols affect the formation of CML and CEL,this study established green and black tea model systems.In the green tea model system,epicatechin gallate(ECG),epigallocatechin(EGC),and epigallocatechin gallate(EGCG)inhibited the formation of CML or CEL by scavenging free radicals,trapping dicarbonyl compounds,and reacting with lysine and sugar,but the content of fructoselysine(FL,an Amadori product,another intermediate of CML)increased.According to the changes in the content of intermediates,although the content of CML was the result of the combined effects of the two pathways(from dicarbonyl compounds or FL),the main pathway for the formation of CML might through dicarbonyl compounds;the formation pathway of CEL might also be mainly through dicarbonyl compounds,but be more affected than CML by other pathways.In the black tea model system,ECG,EGC and EGCG all facilitated the formation of CML and CEL.Since the pH of the black tea model system was lower than that of the green tea model system,the above-mentioned inhibitory effects of catechins were weakened by the influence of pH.According to the changes in the content of intermediates,although the contents of CML and CEL were the result of the combined effects of the two pathways,the main pathway of CML formation might be the oxidation of FL.The formation pathway of CEL might also be mainly directly through an analogous Amadori product.Although GA has antioxidant capacity,it promoted the formation of CML and CEL in the two model systems,thus its antioxidant capacity did not play a leading role in the formation of CML and CEL in the tea model system.Metal elements have important effects on the growth of tea tree and the formation of tea quality,as well as Maillard reaction.In order to study the effects of metal ions on CML and CEL formation,and the combined effects of metal ions and polyphenols on CML and CEL generation,this study established green and black tea model systems containing metal ions.The results showed that copper and iron ions significantly promoted the formation of CML or CEL through their oxidizing ability,and in the black tea model system,the promoting effect was greater than that in the green tea model system.This might be related to the higher levels of CML and CEL in black tea than in green tea.In the model system simultaneously containing catechins and copper or iron ions,the promoting effect of copper and iron ions was suppressed to some extent by the chelation ability of catechins.Aluminum and calcium ions inhibited the formation of CML and CEL in most cases in both model systems,and their inhibitory may be mainly related to binding to lysine.In the model system simultaneously containing catechins and aluminum or calcium ions,due to the metal chelation of catechins,the inhibitory effects of metal ions and catechins in the system were weakened,resulting in a higher CML or CEL content in system than that in system without catechins.In summary,tea contains a lot of CML and CEL,which is one of the sources of dietary AGEs.Their formation is closely related to the tea processing,while fermented tea contain more CML and CEL.The effects of polyphenols on the formation of CML and CEL in tea are complex,not simply promoting or inhibiting.Whether polyphenols can be used as the inhibitors of AGEs in the future requires more researches.