Microbial Community Analysis in Sichuan South-road Dark Tea Piled Center at Pile-Fermentation Metaphase and Insight Into Organoleptic Quality Development Mediated by Aspergillus niger M10

Abstract

Microbes are critical in the Sichuan South-road Dark Tea (SSDT) organoleptic quality development during pile-fermentation. Piled tea center at fermenting metaphase is crucial for the conversion of its quality components. In this study, we investigated the microbial community of piled SSDT center below the stacked tea surface of 15 cm (SSDTB), 50 cm (SSDTX), and 85 cm (SSDTH) on the second turning time of pile-fermentation, respectively. Results showed that SSDTH and SSDTB had a higher similarity in the microbial community. Pantoea (36.8%), Klebsiella (67.7%), and Aspergillus (35.3%) were the most abundant in SSDTH, SSDTB, and SSDTX, respectively. We found 895 species were common among all samples, but 86, 293, and 36 species were unique to SSDTB, SSDTX, and SSDTH, respectively. Aspergillus niger showed high co-occurrence and was positively correlated with numerous microbes in SSDT samples, and Aspergillus niger M10 isolated from SSDTX was excellent at enhancing soluble sugar (SS), amino acids (AAs), theaflavin (TF), and thearubigins (TR) contents, while decreasing catechin (Cat), tea polyphenols (TPs)/AA, Caf/SS, Cat/SS, TPs/SS, and (TPs + Caf)/SS levels in AM10 post-fermentation, as compared with the control. Moreover, it also produced a noticeable difference in the CIELab parameters in dried, liquor, and infused tea colors between AM10 and control during fermentation. When it was further inoculated on differential mediums, we detected glycoside hydrolases, namely, β-glucosidase, mannosidase, pectinase, cellulase, amylase, and α-galactosidase being secreted by Aspergillus niger M10. Taken together, SSDXT presented a more unique microbial community. Aspergillus niger M10 probably improved the sweet and mellow taste, and the yellow brightness and red color of SSDT during fermentation. It also provided new insights into the microbial profile and organoleptic quality development mechanism of SSDT during pile-fermentation.

The glycoside hydrolase gene family profile and microbial function of Debaryomyces hansenii Y4 during South-road dark tea fermentation

“…Additionally, most of the GHs detected were located in the extracellular space, that means they can act directly on the tea leaves to stimulate chemical reactions. Regrettably, the GH genes detected in D. hansenii Y4 were not as abundant as those found in Aspergillus fungi (Ma et al, 2021;Zou et al, 2022Zou et al, , 2023.…”

“…In general, the transformation of the chemical component in SSDT during pile-fermentation always results from a synergistic effect of moist heat and microbial activity ( Zou et al, 2022 ), whereas, after excluding the effect of hygrothermal action used the control treatment, apparently, D. hansenii Y4 seriously reduced the contents of TPs and Caf, and altered the levels of catechin monomers in tea. Previous literature suggested that GHs, tannase, glycosyltransferases and so on could catalyze the hydrolysis of phenolic glycosides and phenolic esters, oxidation or polymerization of phenolic compounds, and destruction of the aromatic rings in phenolic compounds, thus significantly reducing the TPs levels in tea during fermentation ( Ma et al, 2021 ).…”

“…In this work some functions of D. hansenii Y4 during SSDT fermentation were verified, but compared to the functional research of filamentous fungi during dark tea fermentation, which is obviously insufficient ( Ma et al, 2021 ; Zou et al, 2022 , 2023 ; Liao et al, 2023 ). We expect to explore more functional microbes during SSDT production, and we will continue to investigate the activities of unique GHs in D. hansenii Y4 for elucidating its action mechanism during SSDT pile-fermentation and facilitating its application in our subsequent work.…”

“…Recently, some of the functional microorganisms involved in the distinctive flavor formation of dark tea have been identified, for example, Aspergillus , Debaryomyces , and Lichtheimia were confirmed to be the primary beneficial agents of Pu-erh tea during fermentation ( Li et al, 2018 ; Ma et al, 2021 ), while Aspergillus and Debaryomyces also play an important role in the volatile metabolism of Fuzhuan tea during production ( Li M. Y. et al, 2020 ). Furthermore, Aspergillus niger M10 could significantly influence the transformation of key quality components in SSDT during fermentation via the expression of GH genes ( Zou et al, 2022 , 2023 ). It seems that the functional microbes are very important in the production of dark tea.…”

“…South-road Dark Tea (SSDT) is a well-known health beverage in China, it is produced in Ya' an City, Sichuan Province (Figure 1), and characterized by the sensory qualities of brick shape, reddish-brown appearance, mellow mouthfeel, and aged and pure aroma (Zou et al, 2022). Pile-fermentation, a spontaneous fermentation stimulated by environmental Zou et al 10.3389/fmicb.2023.1229251 Frontiers in Microbiology 02 frontiersin.org microorganisms in workshop, is the key procedure responsible for SSDT quality formation, during which microbial metabolism, extracellular enzyme activities and natural oxidation, etc.…”