Fermentation method for improving the aged and woody flavor of fuzhuan tea

Abstract

This invention provides a fermentation method to enhance the aged and woody aroma of Fu brick tea, belonging to the field of Fu brick tea processing technology. Based on *Aspergillus chevaleri*, a strain deposited at the China General Microbiological Culture Collection Center (CGMCC No. 42256), the method involves adding plant-derived methyl selenide, such as selenium-enriched freeze-dried rapeseed powder, to black tea to adjust the total selenium content. After inoculation, fermentation is carried out, followed by drying. The resulting Fu brick tea exhibits significantly increased levels of characteristic aged and woody aroma substances, such as 4-ethylphenol and 1,2,3-methoxybenzene, exhibiting a mellow and robust aged and woody aroma. This invention uses plant-derived methyl selenide instead of chemical selenium additives, eliminating the need to change the fermentation strain and process parameters. The intensity of the aged and woody aroma can be precisely controlled simply by adjusting the amount of naturally selenium-enriched raw materials. The process is simple, controllable, and the product is natural and safe, fully meeting the food safety requirements for tea, making it highly suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention belongs to the field of Fu brick tea processing technology, specifically relating to a fermentation method for improving the aged aroma and woody aroma of Fu brick tea. Background Technology

[0002] Aspergillus chevaleri ( Aspergillus chevalieri *Aspergillus chevalieri* is a filamentous fungus belonging to the genus *Aspergillus*, a non-type strain, native to China, and commonly found in fermentation substrates such as high-temperature daqu (a type of starter culture) and Fu brick tea. *Aspergillus chevalieri* imparts a unique "floral aroma" to tea, a complex fragrance that blends floral, woody, and minty notes, creating its signature aroma characteristics. *Aspergillus chevalieri* is one of the core fungal species responsible for the "golden flower" aroma in Fu brick tea. Woody aromas typically manifest as a calm, restrained, and dry fragrance. Unlike floral aromas, they are not overpowering but rather serve as the "skeleton" or "base" of the dark tea's aroma, giving the tea soup a rich and substantial texture. High-quality woody aromas smell clean and pure, without any musty or off-flavors, and upon tasting, one can experience a lingering sweetness similar to chewing dry twigs.

[0003] Aged aroma is a complex and layered fragrance. It may manifest as medicinal, jujube, ginseng, honey, or even the scent of old paper reminiscent of a vintage book. The core characteristics of aged aroma are "mellowness" and "purity." It smells pleasant and natural, without any harshness. In terms of taste, tea with aged aroma is typically smoother, more mellow, and has a long-lasting aftertaste, leaving the drinker feeling refreshed and relaxed.

[0004] The aged aroma is one of the most captivating characteristics of Fu brick tea or black tea. It is primarily formed during long-term storage (post-fermentation) through the combined action of microorganisms and natural oxidation. This is mainly due to the "golden flowers" (Eurotium cristatum and Aspergillus chevaleri) in Fu brick tea, along with other beneficial microorganisms (such as yeast and Aspergillus niger), which continuously undergo slow metabolic activity under suitable storage conditions. These microorganisms continuously break down large molecules such as proteins and polysaccharides in the tea leaves, transforming them into smaller aromatic molecules. During aging, over time, catechins and theaflavins in the tea leaves undergo auto-oxidation and polymerization, producing darker substances such as theabrownins. In this process, substances with grassy or bitter notes gradually decrease, while compounds with an aged aroma (such as some alkenes and ketones) gradually accumulate. Freshly made Fu brick tea may have a slightly "fired" or slightly musty smell. During aging, these unpleasant odors gradually dissipate, making the original pure woody aroma and the aged flavor developed by microorganisms more prominent.

[0005] Aspergillus chewasserle is a key microorganism in the formation of the "floral and floral aroma" of Fu brick tea by secreting highly active esterases and ferulic acid esterases, which directionally convert precursor substances in tea into core floral and fruity aroma components such as linalool and 1-octen-3-ol. These key floral and fruity aroma substances significantly enhance the complex layers of sweet floral, fruity, and floral aromas in the tea soup.

[0006] The market demands for fermented Fu brick tea with different flavors. Generally, this is achieved by inoculating different *Aspergillus cristatus*, *Aspergillus niger*, or *Aspergillus schaferi* to adjust the flavor. This invention discovered that *Aspergillus schaferi* isolated and preserved under different selenium concentrations exhibits significant differences in aroma during fermentation. Appropriate selenium concentrations can promote the growth and metabolic activity of *Aspergillus schaferi*, leading to different results in the expression of aroma-related enzymes and the synthesis of aroma substances. Therefore, this characteristic can be utilized to ferment Fu brick tea products with different flavors. Summary of the Invention

[0007] Research revealed that the aroma of Aspergillus chevaleri with accession number CGMCC No. 42256 varied under different selenium concentrations. When the total selenium content of the raw material was adjusted to 1.0±0.1 mg / kg, the resulting selenium-enriched Fu brick tea was dominated by floral and fruity aromas. When adjusted to 2.0±0.1 mg / kg, the resulting selenium-enriched Fu brick tea had both aged and woody aromas, as well as floral and fruity aromas. When adjusted to 4.0±0.1 mg / kg, the resulting selenium-enriched Fu brick tea was dominated by aged and woody aromas.

[0008] The purpose of this invention is to provide a fermentation method for enhancing the aged and woody aroma of Fu brick tea, thereby solving the technical problems in existing Fu brick tea production, which rely on lengthy storage and aging or frequent adjustments to the microbial community to obtain a rich aged and woody aroma, resulting in complex processes, long cycles, and batch instability. Furthermore, this invention uses plant-derived methyl selenide as the selenium source, replacing chemical selenium additives, ensuring that the product meets food safety requirements, and the process is green and clean, suitable for large-scale industrial production.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: a fermentation method for improving the aged aroma and woody aroma of Fu brick tea, comprising the following steps: (1) Mix black tea with raw materials containing plant-derived methyl selenium, adjust the total selenium content of the raw materials, and then sterilize them; (2) Inoculate Aspergillus schwanniferus onto the mixed raw materials for fermentation; (3) After fermentation, the tea is dried to obtain selenium-rich Fu tea with enhanced aged and woody aroma.

[0010] Furthermore, in step (1), the raw material containing plant-derived methyl selenide is selenium-enriched rapeseed freeze-dried powder.

[0011] Further, in step (2), the *Aspergillus chevaleri* strain is a specific strain that has been isolated, screened, and identified, with the preservation number CGMCC No. 42256. This strain was isolated from a high-quality fermented Fu brick tea sample from Ziyang County, Ankang City, Shaanxi Province. It has the ability to tolerate 80 μg / mL sodium selenite, and the pure fermentation product exhibits a woody, camphor-like, and light floral aroma, which blends with the woody aroma to form a mellow aged aroma base. During inoculation, the strain is prepared as a seed liquid in advance and sprayed evenly onto the mixed raw materials at a volume-to-mass ratio of 5%.

[0012] Furthermore, in step (1), the black tea is preferably made from Ziyang selenium-rich black tea produced in Ziyang County, Shaanxi Province. This raw material itself is produced in a high-selenium soil area and has selenium-rich properties. It forms a "natural selenium-rich superposition" effect with the raw material containing plant-derived methyl selenium added later, which is more conducive to achieving the target selenium concentration under the overall natural conditions of the raw material, enhancing the flavor control effect, and at the same time giving the product clear origin and quality characteristics.

[0013] Further, in step (1), the sterilization process is as follows: after adjusting the moisture content of the mixed materials to 25%–28%, the materials are placed in breathable packaging and sterilized by high-pressure steam at 121°C for 15–20 minutes. This condition can effectively kill miscellaneous bacteria in the raw materials, providing a clean substrate environment for subsequent pure culture fermentation of Aspergillus sieboldii, ensuring the stability of fermentation and the consistency of product flavor.

[0014] Further, in step (2), the fermentation conditions are as follows: *Aspergillus chevaleri* seed liquid is inoculated at a volume-to-mass ratio of 5%, and fermentation is carried out at a temperature of 28±2℃ and a relative humidity of 70%–75%. The fermentation time is 5–20 days, preferably 12 days, with the pile turned over every 1–3 days, preferably every 3 days, to ensure uniform fermentation of all parts of the tea substrate and prevent local overheating or lack of oxygen from affecting the growth of the microorganisms and the accumulation of aroma metabolites.

[0015] Further, in step (3), the drying is carried out at 60°C with forced air drying until the moisture content is less than 12%, to obtain the finished selenium-enriched Fu tea. Beneficial effects

[0016] (1) In this embodiment, plant-derived methyl selenide such as selenium-enriched rapeseed freeze-dried powder is used to replace chemically synthesized sodium selenite as the selenium source. All components are derived from natural edible plant materials, which meet the national food safety standards and relevant regulations for tea. There is no risk of chemical additive residues or exceeding the standard. The resulting product can be directly marketed as edible-grade selenium-enriched Fu brick tea, which has high consumer acceptance and strong regulatory compliance.

[0017] (2) By directly adding natural selenium-enriched plant powder to adjust the total selenium level of the fermentation raw materials, rather than adding chemical selenium preparations during the fermentation process, the entire flavor control process is based on the compounding of natural raw materials, ensuring the green attributes of the process from the source. The raw material combination of "natural selenium-enriched black tea + natural selenium-enriched plant powder" is in line with the current industry development trend of clean labels and natural ingredients in the food industry, making it easier for the product to obtain organic certification and health food market access.

[0018] (3) The experimental results of Example 4 show that adding only 2% of selenium-enriched freeze-dried rapeseed powder can significantly increase the relative content of woody and aged aroma characteristic substances such as 4-ethylphenol and 1,2,3-trimethoxybenzene, while the proportion of floral and fruity aroma substances such as linalool oxide decreases, and the overall aroma presents a mellow and rich aged woody aroma style. Furthermore, the amount of selenium-enriched freeze-dried rapeseed powder added can be flexibly adjusted between 0.1% and 5%, and the intensity of aged and woody aroma of the Fu tea produced shows a gradient change with the amount added. Production enterprises do not need to change the strain or modify the fermentation process. They can achieve fine-grained control of the intensity of aged and woody aroma flavors by simply changing the ratio of natural selenium-enriched raw materials, which meets the multi-level product development needs from light aged aroma to rich aged woody aroma.

[0019] (4) Rapeseed itself is rich in protein, amino acids, polyphenols and trace elements. The introduction of freeze-dried powder not only serves as a selenium source, but may also provide additional nitrogen source and flavor precursors for the metabolism of Aspergillus chevaleri. The combined effect of selenium stress and natural rapeseed components may further activate or inhibit related enzyme systems, promote protease secretion and biotransformation of lignin and phenolic acid precursors, and make the accumulation of aged / woody aroma characteristic components more efficient and complete. This dual effect of "selenium + natural matrix" is expected to produce flavor enhancement results that surpass those of simple chemical selenium addition, making the product's aged aroma fuller and more layered.

[0020] (5) As a solid powdered raw material, selenium-enriched rapeseed freeze-dried powder can be directly mixed with black tea during the raw material pretreatment stage. No special dissolving, metering, or slow-release equipment is required, and the operation steps are highly consistent with the original process. No adjustments are made to the remaining sterilization, inoculation, fermentation, and drying processes. The same production line can be compatible with this technical solution without any hardware modifications. After the raw materials are mixed evenly, the selenium is stably distributed in the tea leaves, with small batch-to-batch flavor fluctuations and controllable product quality, making it very suitable for standardized and large-scale industrial production.

[0021] (6) Examples 2 and 3 accurately established the dose-response relationship of "selenium concentration - aroma type" using sodium selenite, laying a theoretical and data foundation for flavor regulation; Example 4, based on this, provides a practical implementation plan for natural selenium sources, transforming the chemical selenium model in the research stage into a mass-producible natural raw material formulation. The two are consistent, connecting the path from mechanism verification to product realization, making the patented technology system both scientifically profound and industrially feasible, which is conducive to accelerating the transformation of technology from the laboratory to the factory.

[0022] (7) Although this invention uses selenium-enriched rapeseed freeze-dried powder as a specific implementation example, based on its working principle, methyl selenide powder from any plant source can achieve similar effects. Rapeseed is easy to grow, has a strong selenium conversion capacity, and its freeze-drying powder processing technology is mature and cost-controllable, making it the preferred selenium source for industrial production. However, enterprises can also select other selenium-enriched plant raw materials such as milkvetch, tea tree pruning leaves, and leguminous green manure according to local resource endowments. The raw materials can be adapted to local conditions and sourced locally, further reducing costs and forming local characteristics, thereby enhancing the universality and promotion value of the technical solution.

[0023] (8) Selenium-enriched Fu brick tea, with flavor controlled by naturally selenium-rich raw materials, combines the nutritional and health attributes of "naturally selenium-rich" with the high-end flavor experience of "aged woody aroma." This dual selling point significantly enhances the product's market positioning. Compared to traditional Fu brick tea products that rely on long-term storage and aging to achieve their aged aroma, this solution has a short fermentation cycle, controllable flavor, and is naturally safe. It can help Fu brick tea companies launch high-value-added differentiated products in a shorter period of time, occupy the mid-to-high-end market of functional fermented tea, and improve product profit margins and brand competitiveness.

[0024] (9) Examples 2-4 collectively demonstrate that the differences in aroma production during fermentation of *Aspergillus chevaleri* CGMCC No. 42256 under different selenium concentrations are stable and reproducible. The regulatory effects of selenium on the secondary metabolism of the strain—whether it's inhibiting the synthesis pathway of floral and fruity aromas, activating the lignin metabolic pathway, or increasing the secretion of proteases and precursors of aged aromas through selenium stress—all indicate a regular correlation between "trace element concentration—microbial metabolism—flavor output." This discovery and regulatory method are not only applicable to Fu brick tea, but may also extend to other post-fermented teas such as Pu-erh tea and green brick tea, and even fermented soy products and fermented beverages. Further verification is needed to confirm this. It provides a controllable and efficient new engineering strategy for the flavor design of fermented foods, demonstrating significant methodological originality and broad application prospects. Attached Figure Description

[0025] Figure 1 Colony morphology of Aspergillus schwannii CGMCC No. 42256 at different stages on PDA medium.

[0026] Figure 2Hyphae and conidial structures of Aspergillus chevaleri CGMCC No. 42256 under an optical microscope.

[0027] Figure 3 Metabolite diagram of Aspergillus chevaleri CGMCC No. 42256.

[0028] Figure 4 Growth curves of Aspergillus schwanniferus CGMCC No. 42256 at different selenium concentrations.

[0029] Figure 5 Aroma data of black tea fermented with Aspergillus schwanniferus CGMCC No. 42256 and containing 1.0 mg / kg selenium.

[0030] Figure 6 Extraction ion chromatogram and total ion chromatogram of black tea fermented with Aspergillus schwannii CGMCC No. 42256 and containing 1.0 mg / kg selenium.

[0031] Figure 7 Aroma data of black tea fermented with Aspergillus schwanniferus CGMCC No. 42256 with a selenium content of 2.0 mg / kg.

[0032] Figure 8 Extraction ion chromatogram and total ion chromatogram of black tea fermented with Aspergillus chevalericus CGMCC No. 42256 and containing 2.0 mg / kg selenium.

[0033] Figure 9 Aroma data of black tea fermented with Aspergillus schwanniferus CGMCC No. 42256 with a selenium content of 4.0 mg / kg.

[0034] Figure 10 Extraction ion chromatogram and total ion chromatogram of black tea fermented with Aspergillus schwannii CGMCC No. 42256 with a selenium content of 4.0 mg / kg.

[0035] Figure 11 Aroma data of black tea from Example 4.

[0036] Figure 12 Extraction ion chromatogram and total ion chromatogram of black tea in Example 4.

[0037] Figure 13 Aroma data of black tea in the control group of Example 4.

[0038] Figure 14 Example 4: Extraction ion chromatogram and total ion chromatogram of black tea from the control group. Detailed Implementation

[0039] The present invention will be further described below with reference to specific embodiments.

[0040] In the embodiments, the fermentation substrate for Examples 1-3 was Ziyang selenium-rich black tea, specifically a semi-finished product made from fresh leaves harvested in late spring and early summer in Ziyang County through processes such as fixation, rolling, piling, and sun-drying. The soil in Ziyang County has a high selenium content, with some soils exceeding 10 mg / kg. The selenium-rich black tea used in this study contained between 0.631 and 4.906 mg / kg of selenium, exceeding the national standard of 0.22 mg / kg for selenium enrichment.

[0041] The fermentation substrate of Example 4 was tea grown in ordinary soil. The black tea obtained by fermentation using the same method was tested and found to have a selenium content of 0.15 mg / kg, which is lower than the national standard of 0.22 mg / kg for selenium enrichment, and therefore does not belong to selenium-enriched tea.

[0042] In this embodiment, the selenium-enriched rapeseed freeze-dried powder was prepared by Wuhan Kangmao Biotechnology Co., Ltd., and its selenium content was 500 mg / kg. The preparation method of the selenium-enriched rapeseed freeze-dried powder is as follows: 1. Rapeseed grading: Rapeseed plants are 50-60 cm tall. Leave 10-20 cm for secondary bud growth and harvest about 40 cm of bud for processing. First, remove the leaves from the bottom 35-50 cm of stem, which is classified as Grade 1. Second, remove the leaves from 20-35 cm of stem, which is classified as Grade 1. Third, remove the leaves from 10-20 cm of stem, which is classified as Grade 1. The remaining stem is put into a segmenting machine and divided into 4 segments, which are collected separately from top to bottom: 10 cm bud, 10-20 cm stem, 20-30 cm stem, and 30-40 cm stem.

[0043] 2. Selection: Lay the rapeseed stalks flat on the work surface and remove any rotten, yellow, or scorched parts. Place the selected rapeseed stalks into a storage basket.

[0044] 3. Cleaning: After connecting the bubble cleaner to the power supply, turn on the tap in the first pool to add water. Add water until it is close to the overflow point and then turn off the tap. Add hydrogen peroxide cleaning agent, stir well, and then put the rapeseed into the cleaning pool. Set the cleaning program. This equipment uses a combination of circulating water (inside the pool) and clean water spraying to disinfect and rinse simultaneously. Then, the rapeseed enters the second clean water cleaning machine to continue cleaning and reduce disinfectant residue.

[0045] 4. Draining: After washing, the raw materials directly enter the draining equipment. The excess water from the washing is removed by setting the amplitude, time, and wind speed.

[0046] 5. Chopping: Turn on the power switch of the vegetable cutter, set the blade speed to 50 and the conveyor belt speed to 23, start the equipment, and then evenly put the cleaned rapeseed stalks into the feed port of the vegetable cutter.

[0047] 6. Pre-freezing: Spread the chopped rapeseed stalks evenly in the freeze-drying tray, ensuring the thickness does not exceed the height of the tray, and push them into the pre-freezing chamber of the freeze dryer through the inlet for 1 hour. Transfer the pre-frozen material cart from the pre-freezing chamber to the freeze-drying chamber, select the freeze-drying tray with the thickest rapeseed pile, place the temperature probe inside, and close the door after loading is complete. 7. Freeze-drying: A stepped heating drying process is adopted, in which the temperature inside the chamber is gradually increased from -45℃ to 30℃ to complete the dehydration and drying process.

[0048] 8. Ultrafine grinding: Using ultrafine grinding technology, the material is ground to a fineness of 600-1000 mesh, and finally high-quality selenium-enriched rapeseed freeze-dried powder is obtained. Example 1

[0049] This embodiment provides a method for the isolation, identification, and preservation of the strain *Aspergillus schwanniferus* of the present invention, as detailed below.

[0050] Sample collection: Fermented Fu brick tea samples were collected from Ziyang County, Ankang City, Shaanxi Province.

[0051] Separation and purification: Take 10 g of a sample of fermented Fu brick tea from Ziyang County with good aroma, add 90 mL of sterile water, and shake to mix. After serial dilution of the supernatant, spread it onto Martin's agar plates containing 2% glucose (with 0.5% K₂Cr₂O₇ added to inhibit bacteria), and incubate at 28℃ for 5–7 days. Look for well-grown, yellowish-brown typical mold colonies on the plates. Pick the target colonies with an inoculation needle and perform repeated streak plating on CYA agar plates until multiple morphologically identical pure colonies are obtained.

[0052] The fermented *Aspergillus chevaleri* exhibits woody, camphorate, and light floral aromas, along with coumarin-like notes and a slight musky undertone, blending with the woody notes to create a mellow, aged aroma base. Its colony morphology at different stages on PDA medium is shown in [Figure / image / description]. Figure 1 The hyphae and conidia structures under an optical microscope are shown below. Figure 2 .

[0053] Depend on Figure 1 and Figure 2 It can be seen that the strain initially forms white, fluffy colonies on PDA medium. As the culture time increases, it gradually produces a large number of brown to dark brown conidia. The overall colony morphology is full and the mycelium grows densely. Under the microscope, septate hyphae, smooth conidiophores, and typical hemispherical vesicles are visible. The sporulation structure is double-layered, which is consistent with the morphological characteristics of Aspergillus chevaleri.

[0054] Molecular biological identification: Genomic DNA was extracted, and the ITS region and β-tubulin gene were amplified.

[0055] Sequencing yields the sequence (as shown in the provided sequence fragments; the full-length sequence is assembled).

[0056] BLAST comparison in the NCBI database showed that it had more than 99% homology with Aspergillus chevalieri.

[0057] The genome sequence of *Aspergillus schwanniferus* was _TSS20251021-022-01559-BSND . Preservation: The purified strain was inoculated onto a PDA slant and sent to the China General Microbiological Culture Collection Center (CGMCC). The preservation date was October 27, 2025, the preservation number was CGMCC No. 42256, and the name was AKFX-003. Example 2

[0058] This embodiment uses black tea culture medium as a basis to provide growth curves of Aspergillus chevaleri at different selenium concentrations. The specific method is as follows.

[0059] The preserved *Aspergillus chevaleri* was inoculated onto fresh PDA plates and incubated at 28°C for 7 days until colonies matured and spores formed in large quantities. The colony surface was rinsed with sterile water, and the spore suspension was collected. Mycelial fragments were removed by filtration through gauze, and the spore concentration was adjusted to 1 × 10⁻⁶ using a hemocytometer. 6 The concentration of Aspergillus schwanniferus spores was obtained at 1 / mL.

[0060] Black tea was steeped in 95℃ hot water for 1 hour, and 3% glucose was added. The final concentration of sodium selenite in the culture medium was adjusted to 0, 10, 20, 40, and 80 μg / mL according to different gradients. Then, the medium was sterilized at 121℃ to obtain black tea culture medium with different selenium concentrations. Aspergillus spores were inoculated into the medium at a rate of 5% and cultured on a shaker.

[0061] Set up 3 parallel culture flasks, and terminate the culture and take samples at each time point (0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7 days).

[0062] The culture medium was filtered through a glass fiber filter of known constant weight. The mycelium was washed 2-3 times with a small amount of sterile water or physiological saline to remove residual culture medium components. The filter paper containing the mycelium was placed in an oven and dried at 80-105℃ until constant weight. After cooling, it was weighed, and the dry weight of the mycelium was calculated. Mycelial dry weight (g / 100mL) = Mycelium / Culture medium volume (100mL) A growth curve was plotted with culture time on the x-axis and mycelial dry weight on the y-axis, as shown in the figure. Figure 4 .

[0063] Depend on Figure 4 It can be seen that within the sodium selenite concentration range of 0~20μg / mL, the addition of sodium selenite has little effect on the growth rate of Aspergillus chevaleris. Overall, the mycelial dry weight during the stable period of the selenium-containing fermentation broth is lower than that of the unselenium-added fermentation broth. When the sodium selenite concentration increases to 40μg / mL, the growth rate of the strain slows down, but can still maintain a relatively normal level. When the selenium content is 80μg / mL, the growth of the strain is significantly inhibited, and the mycelial dry weight is only about 30% of that of the control group. This indicates that this strain can tolerate a sodium selenite concentration of no more than 80μg / mL. Within this concentration range, the strain can still survive and grow, meeting the requirements of selenium-enriched Fu brick tea fermentation. Example 3

[0064] This embodiment provides a method for fermenting Ziyang selenium-rich black tea using Aspergillus schwanniferus, as detailed below.

[0065] Raw material pretreatment: Take Ziyang selenium-rich black tea, adjust the moisture content to 25%~28%, calculate the amount of each type of black tea according to the measured selenium content in each batch of black tea, mix the black teas in proportion, and finally make the total selenium content of the black teas 1.0±0.1mg / kg, 2.0±0.1mg / kg and 4.0±0.1mg / kg respectively. After mixing, pack the tea into a tea box with breathable cotton paper packaging, sterilize with high pressure steam at 121℃ for 20 minutes, and cool to room temperature for later use.

[0066] Inoculation and fermentation: The activated Aspergillus schwanniferus CGMCC No.42256 was inoculated into PDA liquid seed medium and cultured in shake flasks at 28℃ and 180rpm for 48 hours to obtain seed liquid; the seed liquid was evenly sprayed onto the sterilized black tea at an inoculation rate of 5% and placed in a constant temperature and humidity incubation room at 28±2℃ and 70%~75% relative humidity for fermentation. The fermentation time was 12 days, and the pile was turned over every 3 days to ensure uniform fermentation.

[0067] Drying the finished product: After the fermentation process is completed, place the Fu tea in a 60℃ forced-air drying oven and dry it until the moisture content is below 12% to obtain the finished selenium-enriched fermented Fu tea.

[0068] The selenium-enriched fermented Fu brick tea was brewed with 95℃ hot water and steeped for 10 minutes. The aroma components in the tea soup were then measured. The specific method is as follows: The aroma components in Fu brick tea liquor were determined using a headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS). The specific method is as follows.

[0069] ①Headspace Extraction Fiber selection: 50 / 30 µm DVB / CAR / PDMS or 65 µm PDMS / DVB fibers are recommended (they have good adsorption properties for phenols, alcohols, and esters). Insertion: Insert the fiber needle into the rubber stopper of the bottle cap. Extraction conditions: Extract at 50°C-70°C for 30-60 minutes.

[0070] ②GC-MS Analysis (Instrumental Analysis) Thermal desorption: Immediately insert the fiber into the GC inlet (250°C) and desorb for 5-10 minutes.

[0071] Chromatographic column: Commonly used columns are DB-5ms (30m × 0.25mm × 0.25µm) or DB-Wax (60m × 0.25mm × 0.25µm). Polar / non-polar columns are selected according to the target components.

[0072] Temperature program: Example (DB-5ms): 35°C (keep warm for 5 minutes) Increase temperature by 3°C / min to 150°C (hold for 5 min). Increase temperature at 10°C / min to 250°C (hold for 10 min). ③Mass spectrometry conditions: Electron impact (EI) 70 eV, scan range 33-475 m / z.

[0073] Injection mode: Splitless Injection time: 2 min ⑤ Carrier gas: Pressure: 112.4 MPa, Total flow rate: 30.3 ml / min, Column flow rate: 1.40 ml / min, Purge flow rate: 3.0 ml / min, Split ratio: -1.0.

[0074] The data results for a total selenium content of 1.0 mg / kg are as follows: Figure 5 As shown, the extracted ion chromatogram and total ion chromatogram are shown below. Figure 6 As shown.

[0075] The data results for a total selenium content of 2.0 mg / kg are as follows: Figure 7 As shown, the extracted ion chromatogram and total ion chromatogram are shown below. Figure 8 As shown.

[0076] The data result for a total selenium content of 4.0 mg / kg is as follows: Figure 9 As shown, the extracted ion chromatogram and total ion chromatogram are shown below. Figure 10 As shown.

[0077] The aroma composition of fermented Fu brick tea with different selenium contents clearly shows that when the total selenium content is 1.0 mg / kg, the content of floral / fruity aroma substances (linalool oxide, benzaldehyde, benzyl alcohol, etc.) is relatively high, while the proportion of woody / aged aroma substances (4-ethylphenol, 1,2,3-trimethoxybenzene, etc.) is relatively low, and the overall aroma is dominated by floral and fruity flavors. When the total selenium content increases to 2.0 mg / kg, the proportion of woody / aged aroma substances increases, while the proportion of floral / fruity aroma substances decreases slightly. When the total selenium content reaches 4.0 mg / kg, the content of floral aroma substances decreases significantly, while the content of aged and woody aroma substances increases significantly. This indicates that the aroma components obtained by this strain in the fermentation of black tea with different selenium concentrations are different. Fu brick tea with floral and fruity flavors is obtained at lower selenium addition levels, while Fu brick tea with more woody / aged aroma substances is obtained under higher selenium addition levels.

[0078] In other words, the preparation of Fu brick tea can achieve targeted production of selenium-enriched Fu brick tea with different flavors by adjusting the selenium content of the raw materials. When a woody / aged aroma Fu brick tea is needed, only black tea with a high selenium content needs to be added to participate in fermentation to increase the aged and woody aromas. Unlike traditional Fu brick tea, there is no need to constantly adjust the microorganisms to achieve the purpose of flavor adjustment. After this adjustment, the overall flavor of the fermented Fu brick tea is relatively stable. The flavor can be targeted and controlled simply by adjusting the selenium content of the raw materials. The process is simple and controllable, can adapt to the flavor needs of different consumers, reduces the difficulty of flavor control, and is more suitable for large-scale industrial production. Example 4

[0079] This embodiment provides a fermentation method to enhance the aged aroma and woody flavor of Fu brick tea, as detailed below.

[0080] Raw material pretreatment: Accurately weigh the black tea leaves, add 2% selenium-enriched rapeseed freeze-dried powder, adjust the moisture content to 25%~28%, pack it into a tea box with breathable cotton paper packaging, sterilize it with high pressure steam at 121℃ for 20 minutes, and cool it to room temperature for later use.

[0081] Inoculation and fermentation: The activated Aspergillus schwanniferus CGMCC No.42256 was inoculated into PDA liquid seed medium and cultured in shake flasks at 28℃ and 180rpm for 48 hours to obtain seed liquid; the seed liquid was evenly sprayed onto the sterilized black tea at an inoculation rate of 5% and placed in a constant temperature and humidity incubation room at 28±2℃ and 70%~75% relative humidity for fermentation. The fermentation time was 12 days, and the pile was turned over every 3 days to ensure uniform fermentation.

[0082] Drying the finished product: After the fermentation process is completed, place the Fu tea in a 60℃ forced-air drying oven and dry it until the moisture content is below 12% to obtain the finished selenium-enriched fermented Fu tea.

[0083] The control group consisted of rapeseed freeze-dried powder without selenium enrichment, but all other steps were the same.

[0084] Both the selenium-enriched fermented Fu tea product obtained in the example and the control group were brewed with hot water at 95°C. After soaking for 10 minutes, the aroma components in the tea soup were measured using the same method as in Example 3.

[0085] The aroma component data results of this embodiment with the addition of 2% selenium-enriched freeze-dried rapeseed powder are as follows: Figure 11 As shown, the extracted ion chromatogram and total ion chromatogram are shown below. Figure 12 .

[0086] The aroma component data of the control group are as follows: Figure 13 As shown, the extracted ion chromatogram and total ion chromatogram are shown below. Figure 14 .

[0087] Aroma data show that adding selenium-enriched freeze-dried rapeseed powder significantly increases the relative content of woody and aged aroma characteristic substances such as 4-ethylphenol and 1,2,3-trimethoxybenzene in Fu brick tea, while decreasing the proportion of floral and fruity aroma substances such as linalool oxide. The overall aroma is more mellow and rich, with more prominent aged and woody aroma characteristics. This method does not require adjustment of fermentation strains or other process parameters; it can target and enhance the aged and woody aroma of Fu brick tea simply by adding selenium-enriched raw materials. It exhibits good fermentation stability, simple operation, and meets the needs of industrial production.

[0088] Similarly, adding 0.1% to 5% selenium-enriched freeze-dried rapeseed powder to black tea can achieve the present invention. The amount added can be flexibly adjusted according to the desired intensity of aged aroma and woody aroma, ultimately resulting in finished Fu brick tea with different flavor intensities to suit different production and consumption needs. The longer the fermentation time, the better the aged aroma effect. According to experimental results, the reasonable range for aged aroma is 5 to 20 days of fermentation. Turning the pile every 1 to 3 days can ensure uniform fermentation.

[0089] Examples 2, 3, and 4 all demonstrate that the *Aspergillus serrata* preserved in this invention produces different aromas during fermentation under different selenium concentrations. It is speculated that selenium may inhibit or activate the activity of certain enzymes, leading to a decrease in the yield of floral and fruity aroma compounds, or that selenium stress increases protease secretion, resulting in the generation of more precursors related to aged and woody aromas through various biotransformations, ultimately enhancing the aged and woody aroma flavor of Fu brick tea. This flavor regulation method does not require changing the core fermentation strain or adjusting other process parameters such as fermentation temperature and time; it can be achieved simply by adjusting the selenium content in the raw materials. The operation is simple and controllable, with good flavor stability, effectively reducing the difficulty of flavor regulation in industrial production of Fu brick tea.

[0090] Sodium selenite is a toxic additive, and its addition in food or tea is strictly limited. Therefore, this invention uses natural selenium-rich black tea or raw materials containing plant-derived methyl selenide to adjust the total selenium content of the fermentation raw materials. Based on this principle, it can be inferred that adding any extract containing plant-derived methyl selenide can achieve this invention. In other words, plant-derived high-organic selenium food raw materials that meet the group standard T / CHC 1001-2019 can achieve this invention. In summary, this invention can be achieved at any concentration where this strain can grow normally. This method meets the relevant requirements for tea food safety and is more suitable for producing edible-grade selenium-rich Fu brick tea products.

Claims

1. A fermentation method for enhancing the aged aroma and woody flavor of Fu brick tea, characterized in that, Includes the following steps: (1) Mix black tea with raw materials containing plant-derived methyl selenium, adjust the total selenium content of the raw materials, and then sterilize them; (2) Inoculate Aspergillus schwanniferus onto the mixed raw materials for fermentation; (3) After fermentation, the tea is dried to obtain selenium-rich Fu tea with enhanced aged and woody aroma.

2. The fermentation method for enhancing the aged aroma and woody aroma of Fu brick tea according to claim 1, characterized in that, In step (1), the raw material containing plant-derived methyl selenide is selenium-enriched rapeseed freeze-dried powder.

3. The fermentation method for enhancing the aged aroma and woody aroma of Fu brick tea according to claim 2, characterized in that, The amount of selenium-enriched rapeseed freeze-dried powder added is 0.1% to 5% of the weight of black tea.

4. The fermentation method for enhancing the aged aroma and woody aroma of Fu brick tea according to claim 1, characterized in that, In step (2), the preservation number of Aspergillus chevaleri is CGMCC No. 42256.

5. The fermentation method for enhancing the aged aroma and woody aroma of Fu brick tea according to claim 1, characterized in that, In step (1), the black tea is Ziyang selenium-rich black tea.

6. The fermentation method for enhancing the aged aroma and woody aroma of Fu brick tea according to claim 1, characterized in that, In step (1), the sterilization is performed by high-pressure steam sterilization at 121°C for 15-20 minutes.

7. The fermentation method for enhancing the aged aroma and woody aroma of Fu brick tea according to claim 1, characterized in that, In step (2), the fermentation conditions are as follows: the Aspergillus serrata seed liquid is inoculated at a volume mass ratio of 5%, and fermentation is carried out under the conditions of temperature 28±2℃ and relative humidity 70%~75%.

8. The fermentation method for enhancing the aged aroma and woody aroma of Fu brick tea according to claim 1, characterized in that, In step (2), the fermentation time is 5 to 20 days, during which the pile is turned over once every 1 to 3 days.

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