9,10-Anthraquinone contamination in tea processing using coal as heat source

Abstract

9,10-Anthraquinone (AQ) is a contaminant with a potential carcinogenic risk and occurs in tea worldwide. The maximum residue limit (MRL) of AQ in tea set by the European Union (EU) is 0.02 mg/kg. The possible sources of AQ in tea processing and the main stages of its occurrence were investigated based on a modified AQ analytical method and gas chromatography-tandem mass spectrometry (GC-MS/MS) analysis. Compared with electricity as the heat source in green tea processing, AQ increased by 4.3 to 23.9 times in tea processing with coal as the heat source, far exceeding 0.02 mg/kg, while the AQ level in the environment tripled. The same trend was observed in oolong tea processing under coal heat. The steps with direct contact between tea leaves and fumes, such as fixation and drying, are considered as the main steps of AQ production in tea processing. The levels of AQ increased with the rising contact time, suggesting that high levels of AQ pollutant in tea may be derived from the fumes caused by coal and combustion. Fourty samples from different workshops with electricity or coal as heat sources were analyzed, ranged from 50.0%−85.0% and 5.0%−35.0% for detection and exceed rates of AQ. In addition, the maximum AQ content of 0.064 mg/kg was observed in the tea product with coal as the heat source, indicating that the high levels of AQ contamination in tea products are likely to be contributed by coal.