Effects Of Metal Ions And Enzymes On The Thermal Stability Of EGCG Under Anaerobic Conditions

Catechins,the major group of polyphenols in tea leaves,are responsible for various physiological activities,such as antioxidation and scavenging free radicals et.al,and has a close relationship with the quality of tea.Among them,EGCG is the most abundant and active catechins in tea.The nature of catechins is not stable.Degradation,epimerization and oxidation reactions occur during the heat treatment of tea and tea-related products,causing a decline in sensory quality of tea products.The epimerization of catechins is considered to be one of the most important reactions occurring during the heat treatment process.Studies have shown that the stereoisomerism of catechins influences their activities of scavenging free radicals and antioxidants.The isomerization reaction of the tea hormone has gradually attracted the attention of many researchers.At present,the influencing factors of catechin isomerization include temperature,pH,catechin concentration,citric acid and ascorbic acid.Temperature and pH are two key factors.There are few studies on the effect of metal ions on the isomerization of catechins,but studies have also found that catechins are more likely to undergo isomerization reactions in tap water than in pure water.The reason may be that tap water contains metal ions.Polyphenol oxidase（PPO）and peroxidase（POD）catalyze the oxidation of theaflavins and thearubigins in black tea production.No studies have shown that PPO and POD have an effect on the epimerization of catechins.In this paper,the effect of Ca²⁺,Mg²⁺,Al³⁺,Co²⁺and Fe²⁺on the thermal stability of EGCG was investigated.The mechanism of Ca²⁺and Mg²⁺in the isomerization reaction of EGCG was preliminarily explored by theoretical chemical calculations.In addition,the effects of PPO and POD on the isomerization and degradation of EGCG at 30,60 and90°C were studied.The main results are as follows:Under anaerobic conditions and heating at 90°C,Ca²⁺,Mg²⁺,Al³⁺,Co²⁺and Fe²⁺all have different effects on the thermal stability of EGCG.Among them,Ca²⁺,Mg²⁺,and Al³⁺showed a promoting effect on the isomerization reaction of EGCG,and Co²⁺and Fe²⁺had an inhibitory effect on the isomerization reaction of EGCG;The five kinds of metal ions all showed a promoting effect on the degallation reaction of EGCG.At the same time,the pH of the EGCG solution is also changed.The change of the pH will,to a certain extent,interfere with the influence of metal ions on the isomerization reaction of EGCG.The effect of PPO and POD on the thermal stability of EGCG at 30,60 and 90°C was studied under anaerobic conditions.The results showed that the addition of PPO to EGCG solution heated at 60 and 90°C showed an inhibitory effect on the isomerization of EGCG,and at the same time it promoted the hydrolysis of EGCG at 90°C POD showed inhibitory effects on the isomerization reactions of EGCG at 60°C,and with the increase of POD content,the inhibitory effects were stronger.No GA was detected by UPLC at 30 and60°C when POD added.At 90°C POD promoted on the isomerization reaction and the hydrolysis reaction of EGCG.