Reviews of fungi and mycotoxins in Chinese dark tea.

The fermentation is the main process to form the unique flavor and health benefits of dark tea. Numerous studies have indicated that the microorganisms play a significant part in the fermentation process of dark tea. Dark tea has the quality of "The unique flavor grows over time," but unscientific storage of dark tea might cause infestation of harmful microorganisms, thereby resulting in the remaining of fungi toxins. Mycotoxins are regarded as the main contributor to the quality of dark tea, and its potential mycotoxin risk has attracted people's attention. This study reviews common and potential mycotoxins in dark tea and discusses the possible types of masked mycotoxins in dark tea. A summary of the potential risks of mycotoxins and masked mycotoxins in dark tea is presented, intending to provide a reference for the prevention and risk assessment of harmful fungi in dark tea.

Untargeted metabolomics and quantification analysis reveal the shift of chemical constituents between instant dark teas individually liquid-state fermented by Aspergillus cristatus, Aspergillus niger, and Aspergillus tubingensis.

Instant dark teas (IDTs) were individually liquid-state fermented using the fungi Aspergillus cristatus, Aspergillus niger, and Aspergillus tubingensis. To understand how the chemical constituents of IDTs were affected by the fungi, samples were collected and measured by liquid chromatography-tandem mass-tandem mass spectrometry (LC-MS/MS). Untargeted metabolomics analysis revealed that 1,380 chemical constituents were identified in positive and negative ion modes, and 858 kinds of chemical components were differential metabolites. Through cluster analysis, IDTs were different from the blank control, and their chemical constituents mostly included carboxylic acids and their derivatives, flavonoids, organooxygen compounds, and fatty acyls. And the metabolites of IDTs fermented by A. niger and A. tubingensis had a high degree of similarity and were classified into one category, which showed that the fungus used to ferment is critical to the formation of certain qualities of IDTs. The biosynthesis of flavonoids and phenylpropanoid, which involved nine different metabolites such as p-coumarate, p-coumaroyl-CoA, caffeate, ferulate, naringenin, kaempferol, leucocyanidin, cyanidin, and (-)-epicatechin, were significant pathways influencing the quality formation of IDTs. Quantification analysis indicated that the A. tubingensis fermented-IDT had the highest content of theaflavin, theabrownin, and caffeine, while the A. cristatus fermented-IDT had the lowest content of theabrownin, and caffeine. Overall, the results provided new insights into the relationship between the quality formation of IDTs and the microorganisms used in liquid-state fermentation.

Dietary assessment of ochratoxin A in Chinese dark tea and inhibitory effects of tea polyphenols on ochratoxigenic Aspergillus niger.

In recent years, there has been an increasingly heated debate on whether Chinese dark tea is contaminated with mycotoxins and whether it poses health risks to consumers. In this study, a rapid method based on high-performance liquid chromatography was used to detect ochratoxin A (OTA) in Chinese dark tea samples from different regions of China and different years. Of the 228 Chinese dark tea samples tested, 21 were detected for OTA contamination, with a concentration ranging from 2.51 ± 0.16 to 12.62 ± 0.72 µg/kg. Subsequently, a dark tea drinking risk assessment was conducted, and the hazard quotient for each group was far below the acceptable level of 1.0. Of the 12 Aspergillus spp. strains isolated, one strain of Aspergillus niger had the ability to produce OTA. We also found that tea polyphenols and epigallocatechin gallate inhibited the growth of ochratoxin-producing Aspergillus niger and the expression of non-ribosomal peptide synthetase (NRPS), a key gene for ochratoxin synthesis. Thus, OTA contamination of dark tea is at an acceptable risk level, and the inhibition of ochratoxigenic Aspergillus niger by polyphenols provides new insights into the safety of dark tea consumption.

Dynamic changes in the diversity and function of bacterial community during black tea processing.

Among all types of tea, black tea is produced in the largest amount worldwide, and its consumption is still increasing. Enzymatic fermentation is considered majorly contribute to quality formation of black tea, and the information about the roles of bacterial community in black tea processing is scarce. This study aimed to analyze the dynamic changes in composition, structure, and function of microbial communities during black tea processing and reveal the roles of bacterial community in black tea processing. Results showed that the genera Sphingomonas and Variovorax were dominant throughout the processing of black tea. Prediction function analysis of bacterial community showed that the mean proportions of glucuronoarabinoxylan endo - 1,4 - beta - xylanase, aminopeptidase B, phosphoserine phosphatase, homoserine O-acetyltransferase, glycolysis related enzymes, pyruvate dehydrogenase, tricarboxylic acid cycle related enzymes, and glyoxylate bypass were significantly elevated in the rolling and fermentation stages. The contents of amino acids, soluble sugar, theaflavins, thearubigins, and theabrownins increased greatly during the rolling and fermentation processes. Redundancy and Pearson's correlation analyses showed that the relative abundance of bacteria was closely related to the contents of water extract, tea polyphenols, epigallocatechin, epicatechin gallate, catechin gallate, thearubigins, theaflavins, and theabrownins. Overall, the findings provided new insights into the variation of bacterial community during black tea processing and improved our understanding of the core functional bacteria involved in black tea processing.

[Study on Simultaneous Mercury and Lead Detection in Water Samples with PGNAA-XRF].

Prompt gamma neutron activation analysis (PGNAA) technology is used in heavy metals measurement. It is found that the detection accuracy of lead (Pb) is impacted heavily by mercury (Hg), because of thermal neutron cross section of mercury is much bigger than lead. In this paper, a new combined detection method was proposed to improve the lead measurement accuracy in situ environmental water rejects analysis by PGNAA-XRF. Thus, a combined measurement facility was developed to analyze the mercury and lead in water simultaneously. The geometry of set-up is determined by a series of simulations with the MCNP code to improve the detection efficiency of the prompt gamma-ray intensity (Iγ) and characteristic X-ray fluorescence intensity (IX) of element. The ideal sample height and cavity are 33 and 16 cm, respectively. The influence of the relationship between Iγ, IX and different concentration (ｃi) of Hg and Pb was researched by MCNP calculations, respectively. The simulation results showed that there were good linear relationships between Iγ, IX and ｃi, respectively. The empirical formula of combined detection method was proposed based on the above calculations. The limits of detection for Hg and Pb with the combined measurement instrument were 3.89 and 4.80 mg·kg-1, respectively. It is a significant increase in performance of the mercury and lead detection simultaneously.

[New methodology for heavy metals measurement in water samples by PGNAA-XRF].

In the present paper, a new combined detection method was proposed using prompt gamma neutron activation analysis (PGNAA) and characteristic X-ray fluorescence to improve the heavy metals measurement accuracy for in-situ environmental water rejects analysis by PGNAA technology. Especially, the characteristic X-ray fluorescence (XRF) of heavy metals is induced by prompt gamma-ray directly instead of the traditional excitation sources. Thus, a combined measurement facility with an 241 AmBe neutron source, a BGO detector and a NaI-Be detector was developed to analyze the pollutants in water. The two detectors were respectively used to record prompt gamma-ray and characteristic X-ray fluorescence of heavy metals. The prompt gamma-ray intensity (I(γ)) and characteristic X-ray fluorescence intensity (I(x)) was determined by MCNP calculations for different concentration (c(i)) of chromium (Cr), cadmium (Cd), mercury (Hg) and lead (Pb), respectively. The simulation results showed that there was a good linear relationship between I(γ), I(x) and (c(i)), respectively. The empirical formula of combined detection method was given based on the above calculations. It was found that the combined detection method was more sensitive for high atomic number heavy metals like Hg and Pb measurement than low atomic number like Cr and Cd by comparing and analyzing I(γ) and I(x). The limits of detection for Hg and Pb by the combined measurement instrument were 17.4 and 24.2 mg x kg(-1), respectively.