The objective of this research was to compare the content of soluble oxalates (SO) in water infusions of green and herbal teas made using four different extraction procedures, i.e. a traditional extraction procedure with boiling water of 100°C, a microwave radiation supported extraction at 80 °C (EWPM), and an ultrasound supported extraction at 40°C (EWU-40) and at 60 °C. The average SO content in the water infusions analyzed, determined by a manganometric method, was the highest after the microwave EWPM procedure of extraction was applied; it ranged from 7.73 to 14.89 mg/g (dry mass) as for the green teas and from 3.53 to 18.11 mg/g as for the herbal teas. The lowest values were obtained after the use of ultrasound EWU-40 procedure; those values ranged from 5.06 to 10.88 mg/g as for green teas, and from 1.5 to 11.03 mg/g as for herbal teas. In the case of herbal teas delivered for the analysis in tea bags for instant brewing, a significant dependency was found between the SO amounts determined and the kind of anatomical parts of the initial raw material. Similarly to the green teas, the water infusions prepared from whole herbal tea leaves with the use of EWPM procedure were characterized by the highest SO content, for example: peppermint tea: 18.11 mg/g; sage tea: 12.23 mg/g; and nettle tea: 11.76 mg/g. In the case of fennel fruit (seed) tea, the content of soluble oxalate determined was at a level of 3.53 mg/g of dry matter of the product, and in the case of linden inflorescence tea: at a level of 4.01 mg/g of dry matter. The exception in this group was the camomile inflorescence tea with a 16.05 mg/g of SO content determined. The results obtained can be applied to standardize analytical procedures used to determine SO contents in imported green teas and local herbal teas for such purposes as: identification of the country and region of origin of a given tea; fixing the harvesting period and determining the type of production process; finding admixtures and stating/excluding falsification; verifying the declared expiration date; and confirming the healthful properties (quality) of those products.
oxalic acid, oxalates, green tea, herbal tea, extraction, microwave radiation, ultrasounds