Characteristics of Organic Selenium Contents in Natural Tea of China and Discussion on the Quality Standard

Objective] In order to discuss the classification of quality standard of selenium (Se) in tea naturally produced in China, contents of the total Se, the organic Se and its percents were analyzed based on the bibliographic database. [Method] This study totally selected 65/15 pieces of organic Se data records for tea/tea soup from the database, analyzed contents and percents of organic Se in natural tea grouped by types, areas and the Se extracted and measured methods, and their relationships to the total Se, and primarily put forward the quality standards of the natural tea and Se-enriched tea. [Result] More studies had been conducted upon the organic Se in tea especially the green tea since 2013. In general, content of the organic Se in tea was 0.74 ± 0.94 mg kg−1 (0.03~5.54 mg kg−1, n = 65), accounting for 87.6% of the total Se (0.85 ± 1.07 mg kg−1, n = 65). Similarly, the average of the organic Se in tea soup was 0.43 ± 0.27 μg/100ml (0.15~0.99 μg/100ml, n = 15), which approximately accounted for 91.2% of the total amount (0.40 ± 0.28 μg/100ml, n = 15). Though the organic Se content was relatively higher in green tea, its percent was not so, and no significant differences were found between different types of tea (P > 0.05). The contents of organic Se and their percents in tea produced in Enshi, Hubei were much higher, but no significant differences were observed among other areas (P > 0.05). The Se extracted and measured methods, direct or indirect, would probably underestimate the contents of organic Se in tea to a certain extent. A positive correlation existed between the organic Se and total Se contents in tea or tea soup (P < 0.05), nevertheless, the organic Se percents firstly increased with the total Se and then tended to be stable gradually. Five grades, namely, V (<0.20 mg kg−1), IV (0.20~0.40 mg kg−1), III (0.40~0.60 mg kg−1), II (0.60~1.00 mg kg−1) and I (≥1.00 mg kg−1), with a percent of ≥70% for organic Se in tea or soup, were validated according to the total Se contents in natural tea based on our results and some issued quality standards of tea in China. As the contents of total Se, organic Se and its percent in the natural Se-enriched samples were higher than those in the ordinary samples (P < 0.05), this study further divided the natural Se-enriched tea into 3 grades, namely III (0.40~0.60 mg kg−1), II (0.60~1.00 mg kg−1) and I (1.00~4.00 mg kg−1), with a percent of not less than 80% for organic Se. [Conclusion] The established grading method based on the total Se contents combined with the organic percents in tea or tea soup is easily operated and the results are 通讯作者。