Mechanism Of The Effect Of Ascorbic Acid On Catechins Stability And Dimeric Oxidation Pathway

Ascorbic acid(AA)is a rich organic acid in tea and is often used to protect catechins in green tea beverage,which also contributes to the sensory quality and healthy function.Catechins,theaflavins(TFs)and theasinesins(TSs)are widely found in tea and tea beverages,and paly a very important role in the sensory quality and healthy function.The effect of AA on the stability of catechins has not been clearly documented.The effect of AA on the catechin conversion pathway and the color and taste characteristics of TFs and TSs monomers have not been reported.Therefore,the study of the mechanism of the effect of AA on the stability and conversion pathway of catechins and the influence of AA on the sensory properties of TFs and TSs is of great significance for the production,quality control and development of health functions of tea and tea beverages.In this paper,the effects of AA on the stability and conversion pathway of catechins and their mechanism was studied through treatments such as high temperature heating,room temperature storage,enzymatic oxidation.And the color and taste characteristics of TFs and TSs and the effects of AA on them were analyzed by machine vision,electronic tongue system and sensory evaluation.The research results as follows:1.A series of incubation systems of pure(-)-Epigallocatechin gallate(EGCG),ascorbic acid(AA)and dehydroascorbic acid(DHAA)at 80? and room temperature were performed to investigated the effect and mechanism of AA on the stability of EGCG.Results showed the dual function of AA,protecting action and promoting degradation.AA protected EGCG by inhibiting the oxidation of EGCG.DHAA promoted the consumption of EGCG by generating DHAA-EGCG adducts.AA and DHAA could inhibit the hydrolysis pathway of EGCG.DHAA reacted with EGCG in two steps,and AA inhibited the first step.The mechanism of the effect of AA on the stability of EGCG can macroscopically lead to the phenomenon that low concentrations of AA protected EGCG and high concentrations of AA promoted the consumption of EGCG,with the critical concentration being about 5-10 mmol/L.Or it can lead to the phenomenon that AA protected EGCG in the early stage and then promoted the consumption of EGCG.2.AA oxidase was used to catalyze the mixed reaction system of four catechin monomers and AA,and product identification and catechin consumption rate analysis were used to elucidate the mechanism of the effect of AA on catechin stability and the binding capacity of DHAA to catechins.DHAA could react with EGCG,ECG,EC,and EGC to form ascorbyl catechins.AA has dual function on the stability of catechins.AA protected the stability of catechins by inhibiting the oxidation pathway.DHAA increased the oxidation pathway by generating ascorbyl catechins.AA and DHAA both inhibit the hydrolyzation of catechins.The depletion rates of EGCG,ECG,EC and EGC were 30.08% and 22.78%.21.45% and 13.55%.Thus,the binding capacity of DHAA and catechins from high to low was EGCG,ECG,EC and EGC.The binding capacity with ester catechins was significantly stronger than that with non-esterified catechins.3.Enzymatic oxidation of catechins was used to investigate the mechanism of the effect of AA on the catechin oxidation pathway.The results showed that the binding capacity between different catechins is,EGCG+ECG>EGCG+EGCG,EGCG+EC> EGCG+EGCG,EGC+EGC>EGC+EC.AA reduced the consumption rate of EC,EGC,ECG and EGCG by 24.29%.15.20%,17.71% and-3.00%.Thus,the protective effect of AA on catechins was EC>ECG>EGC> EGCG.The differential protective capacity of AA against catechins and the differential ability of DHAA to bind catechins lead the shift in the oxidative conversion pathway of catechins.4.Machine vision system,electronic tongue,colorimeter and sensory evaluation were used to analysis the color and taste characteristics of TFs,TSs and the effect of AA on them.The results showed that:(1)the color characteristic of TFMG and TFDG is urantius,and with increasing concentration the color changes from orange-yellow to orange red.The values of “L” of TFMG and TFDG gradually decrease and the values of “a” and “b” gradually increase with increasing concentration and “L,a,b” values were linearly correlated with changes in concentration,with a correlation coefficient of 0.99.(2)AA could increase the brightness and reddening of TFMG and TFDG,and the effect was more pronounced for TFDG.(3)The taste characteristics of TFMG and TFDG are bitterness and freshness,with bitterness dominating.AA has a masking effect on the bitterness and freshness of TFMG and TFDG,and the higher the concentration,the better the masking effect.AA masks the bitterness of TFMG and TFDG better than it does the freshness;(4)TSA taste characteristics are bitter and astringent,with astringency stronger than bitterness,and there is a synergy of bitterness and astringency between AA and TSA.