ABSTRACT
Auto-oxidation of flavan-3-ols leads to browning and consequently loss of product quality during storage of ready-to-drink (RTD) green tea. The mechanisms and products of auto-oxidation of galloylated catechins, the major flavan-3-ols in green tea, are still largely unknown. Therefore, we investigated auto-oxidation of epicatechin gallate (ECg) in aqueous model systems. Oxidation products tentatively identified based on MS included δ- or γ-type dehydrodicatechins (DhC2s) as the main contributors to browning. Additionally, various colourless products were detected, including epicatechin (EC) and gallic acid (GA) from degalloylation, ether-linked ε-type DhC2s, and 6 new coupling products of ECg and GA possessing a lactone interflavanic linkage. Supported by density function theory (DFT) calculations, we provide a mechanistic explanation on how presence of gallate moieties (D-ring) and GA affect the reaction pathway. Overall, presence of gallate moieties and GA resulted in a different product profile and less intense auto-oxidative browning of ECg compared to EC.
Subject(s)
Catechin , Catechin/analysis , Gallic Acid , Tea/metabolism , Oxidative StressABSTRACT
Green tea catechins are well known for their health benefits. However, these compounds can easily be oxidized, resulting in brown color formation, even in the absence of active oxidative enzymes. Browning of catechin-rich beverages, such as green tea, during their shelf life is undesired. The mechanisms of auto-oxidation of catechins and the brown products formed are still largely unknown. Therefore, we studied auto-oxidative browning of epicatechin (EC) and epigallocatechin (EGC) in model systems. Products of EC and EGC auto-oxidation were analyzed by reversed-phase ultra-high-performance liquid chromatography with photodiode array detection coupled to mass spectrometry (RP-UHPLC-PDA-MS). In the EC model system, 11 δ-type dehydrodicatechins (DhC2s) and 18 δ-type dehydrotricatechins (DhC3s) that were related to browning could be tentatively identified by their MS2 signature fragments. In the EGC model system, auto-oxidation led to the formation of 13 dihydro-indene-carboxylic acid derivatives and 2 theaflagallins that were related to browning. Based on the products formed, we propose mechanisms for the auto-oxidative browning of EC and EGC. Furthermore, our results indicate that dimers and oligomers that possess a combination of an extended conjugated system, fused rings, and carbonyl groups are responsible for the brown color formation in the absence of oxidative enzymes.