Comparison of solid-state fermentation with different Bacillus species on the volatile organic compounds and non-volatile metabolites of dark teas.

In this study, four Bacillus spp. (i.e., Bacillus oleronius YC-1, Bacillus velezensis YC-2, Bacillus subtilis YC-3, and Bacillus siamensis YC-4) isolated from the processing of Fu brick tea (FBT), were used to make solid-state fermentation (SSF) of dark teas. The effects of these Bacillus spp. on the volatile organic compounds (VOCs) and non-volatile metabolites of dark teas were explored. Both Gas Chromatography-Mass Spectrometry (GC-MS) and Electronic nose (E-nose) revealed that there were great differences in aroma profiles of dark teas fermented by four Bacillus spp. A total of 58 VOCs were identified by GC-MS in dark teas during SSF, mainly consisting of ketones, alcohols, hydrocarbons and aldehydes. There were 13, 9, 12 and 10 key differential VOCs in dark teas fermented by B. oleronius YC-1, B. velezensis YC-2, B. subtilis YC-3 and B. siamensis YC-4 using orthogonal partial least squares discriminant analysis and relative odor activity value analysis. Specifically, 2-heptanone, benzaldehyde and phenylacetaldehyde were identified as common key markers in four Bacillus spp. fermented dark teas. Furthermore, α-cyclocitral and β-homocyclocitral were the specific markers of B. oleronius YC-1. Hexanal, safranal, 2,2,6-trimethylcyclohexanone and hexanol were the specific markers in B. velezensis YC-2, and (Z)-furan linalool oxide was specific marker in B. subtilis YC-3. Linalool, β-ionone, 1-octen-3-ol and β-cyclocitral having strong floral and woody fragrance were the shared key markers of B. oleronius YC-1 and B. subtilis YC-3. Additionally, total phenols, catechins and free amino acids of dark tea decreased by Bacillus spp. fermentations, whereas soluble sugar increased and alkaloids remained stable. Electronic tongue (E-tongue) revealed that astringency, aftertaste-astringency, bitterness and umami decreased and aftertaste-bitterness increased. Overall, SSF with Bacillus spp. had remarkable effect on dark tea metabolites, revealing that Bacillus spp. play an important role in the formation of VOCs and non-volatile metabolites of FBT.