Sequential determination of tannin and total amino acid contents in tea for taste assessment by a fluorescent flow-injection analytical system

A novel fluorometric flow-injection analytical system is utilised to determine tannin and total amino acid levels in tea for taste assessment of astringency and umami, respectively. The quenching effect on fluorescence of 3-aminophthalate was employed to determine tannin levels, and the fluorogenic reaction of o-phthalaldehyde (OPA) was applied to quantify amino acid content. Both reactions were measured by a single fluorescence sensing system with same excitation and emission wavelengths (340/425 nm). A three-way valve was introduced to switch reagents for sequential determination of tannin and amino acids levels. Parameters of operational conditions were optimised, and the effect of tannic acid on OPA-based amino acid determination was also investigated. The linear dynamic ranges for tannic acid and theanine are 50–250 and 0.1–1.0 μg mL−1 (CV < 5%, n = 3), respectively. Each sample throughout was 20 samples h−1. This system was also applied to quantitatively assess the quality of partially fermented teas.