Iron, zinc, copper, manganese and chromium in green teas, their transfer to extracts and correlations between contents of elements and bioactive compounds

Green tea is used worldwide in the preparation of beverages, but also its extracts rich in bioactive compounds, especially flavan-3-ols, are of increasing interest. In addition to bioactive molecules, green tea represents a source of dietary elements. However, knowledge about their content in extracts is limited. The aim of our research was to determine the extent of transfer of selected elements, i.e., iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), and chromium (Cr), from green teas to their extracts and to investigate whether the main bioactive compounds of the extracts affect this transfer. Twelve commercially available green teas were used in the study. The contents of elements in green teas and their extracts obtained with 80% acetone (v/v) were analysed by inductively coupled plasma optical emission spectroscopy (ICP-OES). High performance liquid chromatography in reverse phase (RP-HPLC) was used to determine contents of caffeine, (–)-epigallocatechin (EGC), (–)-epicatechin (EP), (–)-epigallocatechin gallate (EGCG), and (–)-epicatechin gallate (ECG). The element with the highest content in green teas was Mn (711–1402 µg/g), but its transfer to extracts was the lowest (0.269–0.646%). The mean Fe transfer, second abundant element in teas (115–725 µg/g), was 5.52%. The contents of Mn and Fe in extracts were 5.08–30.2 and 10.7–90.1 µg/g, respectively. Zn, Cu, and Cr were transferred with means of 10.4, 20.0, and 26.2%, respectively, which resulted in their contents in the extracts in the ranges of 5.03–12.6, 1.93–13.8, and 0.128–2.03 µg/g, respectively. The significant positive correlations of Zn content in extracts and/or transfer to extracts with EGCG, EGC and total flavan-3-ols as well as between the same Fe variables and EGC were determined, which suggested that these flavan-3-ols may positively affect the transfer of Fe and Zn from green tea to extracts. In turn, significant but negative correlations were found in the case of Mn and Cu. Future research is needed to identify the causes of the various transfer rate of elements from green teas to extracts