Study On Heat-and Photo-induced Chemical Conversions Of Catechins And Relevant Mechanisns

Catechins are the most important quality-related and bioactive compounds in tea leaves,which are acknowledged with many functionalities such as anti-bacterial and anti-radiation properties.However,catechins are unstable during processing and storage,which have been associated with the browning of tea and tea extract-containing products.Different substitution group and configuration are the important reasons for the differential biological activities and chemical properties of catechins,potentially leading to the different contribution of individual catechin to the browning of tea and tea extract-containing products.So far,the information on the differential chemical behaviors of catechins is scarce.Temperature and illumination are the most important environmental factors during the processing and storage of tea and tea extract-containing products.In the present study,the heat-and photo-induced chemical conversions of catechins have been investigated by UPLC-MS/MS,UV absorption spectroscopy and NMR,the chemical conversions and related products were separated and identified,the relevant reaction mechanisms were interpreted,and the influence of substitution group and configuration on the chemical properties were discussed.The major obtained results were shown as follows:(1)Heat-induced chemical conversions of 8 catechins.Isomerization,hydrolysis and oxidation are the main reactions to catechins under heat treatment,which are impacted by substitution groups.Non-enzymatic oligomerization occurred to epi type catechins(EC?EGC?EGCg?ECg)and catechin under heat treatment,and dehydrodicatechins of A and B types were generated from EC and C samples.UV absorption spectrum showed that the generation and accumulation of dehydrodicatechins A were responsible for the browning of epicatechin and catechin solutions.(2)Catechins were stable under low temperature and dark condition,EC and C turned yellow under either room temperature or light conditions.The influence of UVB,fluorescent lamp and solar radiation on the stabilities of EC and C were studied.Results showed that the maintaining rates of EC and C were highest under fluorescent lamp radiation,followed by solar radiation,while EC and C were greatly lost under UVB radiation.(3)UVB induced chemical conversions of 8 catechins.EC and C are UVB sensitive with low maintaining rates,While other 6 catechins were UVB insensitive with the maintaining%being above 90.7%.UVB induced chemical conversions of EC and C were influenced by radiation time,substrate concentration and solvent.Photoisomerization and photolysis were two important UVB-induced reactions to EC whereas photolysis was the dominant reaction for C.8 reaction products were generated from EC and C under UVB radiation,including 1 photoisomer and 7 photolysis products.The photolysis reaction of EC and C involved two pathways,including radical reaction and photo-induced electron transfer reaction.