Effects of solid-state fermentation with Bacillus subtilis LK-1 on the volatile profile, catechins composition and antioxidant activity of dark teas.

In this study, the solid-state fermentation (SSF) of dark tea was carried out using Bacillus subtilis LK-1, which was isolated from Fu brick tea (FBT). The effects of SSF with B. subtilis on volatile organic compounds (VOCs), non-volatile metabolites, and antioxidant activities of dark tea was investigated. A total of 45 VOCs were identified, primarily consisting of ketones (18), hydrocarbons (8), aldehydes (7), and alcohols (6). Following fermentation, the content of key odor active substances such as linalool, ß-ionone, and 3,5-octadiene-2-one significantly increased, resulting in an enhanced floral and fruity aroma of dark tea. Furthermore, new flavor substances like geranyl isovalerate and decanal were produced during SSF, enriching the aroma profile of dark tea. Non-ester catechins demonstrated a drastic increase, while ester catechins remarkably decreased after SSF. Furthermore, SSF led to a slight decrease in the total polyphenols content and antioxidant activity of dark tea. There is a close relationship between VOCs and the main non-volatile metabolites during SSF. Overall, this study highlighted the great impact of SSF with B. subtilis on the metabolites of dark tea and provided valuable insights into the role of bacteria in shaping the metabolite profile of FBT.

Discrimination and characterization of the volatile profiles of five Fu brick teas from different manufacturing regions by using HS-SPME/GC-MS and HS-GC-IMS.

Although aroma is one of the most essential factors determining the quality of Fu brick tea (FBT), the aroma profiles of FBTs from different manufacturing areas are rarely investigated. The aroma profiles of FBTs manufactured in five typical provinces of China were comprehensively analyzed on the basis of headspace gas chromatography-ion mobility spectrometry (HS-GC-IMS), headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME-GC-MS), sensory evaluation, odor activity value (OAV), and relative odor activity value (ROAV). HS-GC-IMS and HS-SPME-GC-MS identified 63 and 93 volatile organic compounds (VOCs), respectively. Multivariate statistical analysis indicated that the FBTs from different production regions had remarkably varied aromas. HS-SPME-GC-MS revealed that 27 VOCs (OAV >1) contributed to the overall aroma of the samples, of which 15 key differential compounds can effectively distinguish the aroma profiles of different FBTs. FBT from Shaanxi manifested a strong floral and fruity aroma; that from Hunan had a floral, grassy, and pine-woody aroma; that from Guizhou presented a grassy and herbal aroma; that from Guangxi exhibited a sweet, floral, and minty aroma; and that from Zhejiang possessed various fruit flavors and floral fragrance. OAV analysis identified the biomarkers responsible for the variation in the aroma characteristics of diverse FBTs. These biomarkers included linalool, 6-methyl-5-hepten-2-one, α-ionone, hexanal, and ethyl hexanoate. Sensory evaluation demonstrated that the infusion color and aroma of FBT samples from different provinces also greatly varied. Network correlation analysis revealed that Aspergillus and Eurotium were the crucial microorganisms for the metabolism and formation of VOCs. These findings provide new insight into the VOCs and fragrance features of FBTs produced in different regions of China.