The influence of pH on auto-oxidative browning of flavan-3-ols and gallic acid as a promising browning inhibitor

The effect of pH and of flavan-3-ols' structural features on undesired auto-oxidative browning of ready-to-drink green tea is still largely unknown. Thus, we investigated auto-oxidative browning of epicatechin (EC), epicatechin gallate (ECg), epigallocatechin (EGC), epigallocatechin gallate (EGCg) in pH-regulated aqueous model systems. In the typical pH range of green tea infusions (pH 5-7), the brown color intensity of flavan-3-ols after auto-oxidation was found to be EC > ECg > EGCg > EGC. Characterization of brown auto-oxidation products by RP-UHPLC-ESI-HRMSⁿ revealed that most of these can only be formed through multi-step reactions. Therefore, oxidation potentials of the starting flavan-3-ols are unsuitable for predicting browning. Browning inhibition in real green tea can be achieved by maintaining a pH below 4 with either conventional additives (i.e. ascorbic or citric acid) or with gallic acid, a compound naturally abundant in tea. In conclusion, gallic acid is a promising candidate for clean-label auto-oxidative browning inhibition.