Effect of extraction conditions on the content of soluble oxalate in aqueous infusions of green and herbal teas

Green (GTs) and herbal (HTs) teas are recommended as the alternative hot beverages for subjects who tend to form calcium oxalate stones [1]. The aim of study was to compare the effect of extraction procedures [2] on the content of soluble oxalate (SOX) in drinkable infusions of six GTs and seven HTs sold in Poland. The traditional method of extraction using boiling water at 100°C was compared with the microwave extraction at 80°C (EWPM), the ultrasound extraction at 40°C (EWU-40) and the ultrasound extraction at 60°C. The highest amount of SOX, as determined by oxidimetric and HPLC methods, was obtained in the infusions made by the EWPM procedure, i.e. 7.73–14.89mg/g (dry mass) for studied GTs, and in range of 3.53–18.11mg/g for studied HTs. The lowest values were obtained after the EWU-40 procedure (5.06–10.88mg/g for GTs, and 1.50–11.03mg/g for HTs). Highest average content of SOX were observed in brews obtained from green whole leaf teas by the EWPM method (14.89mg/g). The reduced content of SOX was observed in brews obtained by EWPM method from the highly crushed, ready-to-use, express green teas (7.73mg/g). Similarly, the highest content of SOX after the EWPM extraction was obtained for herbal whole leaf teas as in the case of peppermint (18.11mg/g), salvia (12.23mg/g), and nettle (11.76mg/g). These results could be used for standardization of analytical methods applied to determination of SOX in imported green teas and domestic herbal teas.

Acknowledgements: Nicolaus Copernicus University, Torun, Poland (Internal Grant No. 407/2010).

References: 1. Massey, L.K. (2007)J. Am. Diet. Assoc. 107(7): 1191–1194.

2. Honow, R., Hesse, A. (2002) Food Chem. 78(4): 511–521.