Determination of oxalate content in tea samples by high performance liquid chromatography

Abstract

This study was conducted to determine oxalate content in tea samples and study the effect of steeping duration on the oxalate content. One-hundred and twenty samples of both leaf and bag forms of tea-white, green, oolong, and black teas from 40 brands-were purchased from markets in Thailand from February to June 2011. The oxalate content was measured by the high performance liquid chromatography (HPLC) method. The results showed that in leaf form, the highest oxalate content was found in black tea (2.32 ± 1.10 mg/g tea), then in oolong tea (1.62 ± 0.79 mg/g tea), green tea (1.41 ± 0.56 mg/g tea) and lowest in white tea (1.29 ± 0.33 mg/g tea). In bag form, the oxalate content in black tea (2.37 ± 0.80 mg/g tea) was higher than that of oolong tea (1.88 ± 1.04 mg/g tea). The amount of oxalate in green tea (1.24 ± 0.48 mg/g tea) was comparable with that of white tea (1.25 ± 0.45 mg/g tea). The forms (leaf or bag) of tea did not have significant influence on the amount of oxalate. Steeping the tea for a longer duration provided more oxalate content. People with high risk of kidney stones should avoid drinking high amounts of black tea. Steeping tea for a short duration is also recommended for limiting oxalate intake.