Dark tea fermentation process capable of controlling temperature and humidity in stages

Abstract

The invention relates to a black tea fermentation process capable of controlling temperature and humidity in stages, and belongs to the technical field of fermented food. According to the dark green tea fermentation process, the fermentation process is divided into five stages, and compared with a traditional process, the content of thearubigins and theaflavins in dark green tea is remarkably increased by precisely regulating and controlling the temperature, the humidity and the oxygen content in stages and adjusting the pH value of the fermentation environment through a linkage buffer solution, and meanwhile the residual amount of tea polyphenol is reduced. Therefore, the taste, aroma and color of the finished dark tea are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of fermented foods, and particularly relates to a black tea fermentation process with staged temperature and humidity control. Background Art

[0002] As a uniquely post-fermented tea in China, the core quality formation of black tea depends on the dynamic transformation of microbial metabolism and tea internal substances during the pile fermentation process. In traditional black tea fermentation processes, such as pile fermentation, the temperature and humidity conditions are mainly controlled empirically, and the fermentation uniformity is achieved by combining regular turning of the pile.

[0003] However, there are some technical defects in the existing processes: (1) The temperature and humidity control is rough, and the fermentation consistency is poor. The traditional process uses static temperature and humidity parameters, such as a single temperature range of 40 - 50°C, without considering the dynamic changes in microbial activity at different fermentation stages. Due to the lack of staged control in the existing process, problems such as insufficient early fermentation, inhibited microbial activity in the middle stage, and loss of aroma substances in the later stage are likely to occur.

[0004] (2) It relies on empirical judgment, and the quality stability is insufficient. Traditional turning of the pile relies on empirical judgment, such as turning the pile once every 24 - 48 hours. Improper turning timing may disrupt the microbial growth cycle. For different batches of tea, fluctuations in the color of the tea soup, differences in aroma components, and large fluctuations in the content of active ingredients (such as tea polysaccharides and thearubigins) may occur, affecting the functionality of the product.

[0005] The patent with the publication number CN102265945A discloses a three-stage fermentation process, which divides the pile fermentation into low temperature (28°C - 32°C), high temperature (40°C - 50°C), and medium temperature (35°C - 38°C) stages, and promotes the step-by-step action of yeast, biological enzymes, and lactic acid bacteria by controlling the temperature and humidity. The patent with the publication number CN103564074A further proposes to implement three-stage fermentation on an automatic control bed, extend the low temperature stage to 25°C - 35°C, raise the upper limit of the high temperature stage to 55°C, and extend the duration of each stage.

[0006] Although such processes have improved compared to the traditional one-step fermentation method, they still have core defects.

[0007] (1) The stage division is rough. The existing segmented processes only divide the stages based on temperature ranges (such as low temperature, high temperature, medium temperature). Failure to synchronously control the oxygen content in the high temperature stage (36°C - 55°C) may lead to over-oxidation.

[0008] (2) The temperature and humidity control are independently adjusted, and the parameter linkage is insufficient. For example, when the humidity control in the high temperature stage is 25% - 30%, if only relying on a fixed humidification amount and ignoring the dynamic changes in the moisture content of the tea leaves, it may lead to locally high humidity and cause spoilage.

[0009] Therefore, there is an urgent need to develop a more scientific and reasonable dark tea fermentation process to improve the fermentation quality of dark tea. Summary of the Invention

[0010] Starting from two core technologies of controlling temperature, humidity, and oxygen in stages and regulating pH with buffer solutions, the present invention specifically addresses the pain points of traditional processes. Through precise stage division, parameter coordination, stable pH, and dual action mechanisms, the fermentation is ensured to be stable and efficient, significantly improving the quality of dark tea.

[0011] Specifically, the technical solutions are as follows: The dark tea fermentation process of the present invention that controls temperature and humidity in stages includes the following steps: (1) Initial treatment of dark tea: Fixing the green color and rolling of dark tea; (2) Fermentation process: Stage A: Temperature 25°C - 28°C, humidity 65% - 70%, oxygen content 18% - 20%, for 8 - 12 hours, without turning the pile; Stage B: Turn the pile, evenly spray the buffer solution, let it stand, gradually increase the temperature to 30°C - 33°C, humidity 70% - 75%, oxygen content 15% - 18%, turn the pile once every 6 - 8 hours, for a total of 12 - 24 hours; Stage C: Temperature 35°C - 38°C, humidity 75% - 80%, oxygen content 10% - 15%, turn the pile once every 4 - 6 hours, for a total of 24 - 48 hours; Stage D: Slowly decrease the temperature to 32°C - 35°C, humidity 70% - 75%, oxygen content 8% - 10%, turn the pile once every 8 - 10 hours, for a total of 12 - 24 hours.

[0012] Stage E: Temperature 25°C - 30°C, humidity 60% - 65%, oxygen content 5% - 8%, turn the pile once every 8 - 12 hours, for a total of 24 - 36 hours.

[0013] (3) Drying: Low-temperature segmented drying until the water content of the dark tea is lower than 10% to obtain the finished dark tea.

[0014] Wherein, the buffer solution is a citric acid - sodium citrate buffer solution containing 0.02 - 0.05 M sodium chloride; the pH of the buffer solution is 4.5 - 5.0.

[0015] Furthermore, the gradual increase in temperature in Stage B means a gradual increase at a rate of 1 - 1.5°C / h. The slow decrease in temperature in Stage D means a slow decrease at a rate of 1 - 1.5°C / h. The low-temperature segmented drying method is to dry at a temperature of 35 - 45°C for 10 - 15 hours and then dry at a temperature of 50 - 60°C for 6 - 10 hours.

[0016] Compared with the prior art, the technical effects of the present invention are as follows: (1) Temperature, humidity, and oxygen control in stages: The fermentation process of the present invention is divided into five stages, namely A - E. The temperature, humidity, and oxygen parameters change synergistically in each stage, precisely matching the needs of microbial growth and enzymatic reactions. The triple linkage of temperature, temperature - humidity, and oxygen content guides the directional conversion of tea polyphenols into thearubigins and theaflavins, avoiding over - oxidation or blocked synthesis.

[0017] (2) pH regulation with buffer solution: In stage B of the present invention, a specific buffer solution is sprayed to stabilize the pH at 4.5 - 5.0, avoiding situations such as the reduction of microbial activity due to pH fluctuations, and accelerating the conversion of tea polyphenols. The citric acid - sodium citrate system regulates the pH, and sodium chloride maintains the osmotic pressure. The two work together to create good growth and metabolic conditions for microorganisms, significantly improving the material conversion efficiency compared with single components. Description of the Drawings

[0018] Figure 1 : Contents of tea polyphenols and thearubigins in dark tea of Examples 1 - 3 and Comparative Examples 1 - 9.

[0019] Figure 2 : Contents of theaflavins in dark tea of Examples 1 - 3 and Comparative Examples 1 - 9. Detailed Embodiments

[0020] In order to make the objectives and technical solutions of the present invention clearer, the following further describes the present invention with reference to embodiments. However, the protection scope of the present invention is not limited to these embodiments, and the embodiments are only used to explain the present invention. Those skilled in the art should understand that any changes or equivalent substitutions that do not deviate from the concept of the present invention are included in the protection scope of the present invention.

[0021] Example 1 Fermentation Process of Dark Tea (1) Pretreatment of dark tea: The freshly picked dark tea raw materials are subjected to a fixation treatment. The fixation temperature is controlled at 210°C, and the fixation time is 4 minutes; after fixation, it is quickly cooled to room temperature (22°C), and then a rolling operation is carried out. The rolling speed is set at 30 r / minute, and the rolling time is 15 minutes; after rolling, the raw materials are evenly stacked in the fermentation equipment to prepare for the subsequent fermentation process.

[0022] (2) Fermentation process: Stage A: The fermentation environment temperature is controlled at 26.5°C, the relative humidity is maintained at 68%, and the oxygen content is maintained at 19%. The fermentation duration in this stage is 10 hours, and no turning operation is required in this stage.

[0023] Phase B: First, turn the fermentation raw materials and evenly spray a citric acid - sodium citrate buffer solution with pH = 4.8 containing 0.03 M sodium chloride (90 ml / kg dark tea raw materials). After spraying, let it stand for 0.8 hours. Then, gradually increase the temperature to 32°C at a rate of 1°C / h, while adjusting the relative humidity to 72% and controlling the oxygen content at 17%. During this phase, turn the pile every 7 hours, and the fermentation duration is 21 hours.

[0024] Phase C: Keep the fermentation temperature stable at 37°C, the relative humidity at 78%, and reduce the oxygen content to 12%. Turn the pile every 5 hours, and the fermentation duration is 30 hours.

[0025] Phase D: Slowly decrease the temperature to 33°C at a rate of 1.2°C / h, adjust the relative humidity to 72%, and maintain the oxygen content at 9%. Turn the pile every 9 hours, and the fermentation duration is 18 hours.

[0026] Phase E: Keep the temperature at 27°C, set the relative humidity at 62%, and control the oxygen content at 6%. Turn the pile every 10 hours, and the fermentation duration is 30 hours to complete the entire fermentation process.

[0027] (3)Drying: After fermentation, adopt a low - temperature segmented drying method. First, dry at 40°C for 12 hours, then dry at 55°C for 8 hours until the water content of the dark tea is less than 10% to obtain the finished dark tea.

[0028] Example 2 Dark Tea Fermentation Process (1)Preliminary treatment of dark tea: Carry out the fixation treatment on the freshly picked dark tea raw materials. The fixation temperature is controlled at 180°C, and the fixation time is 3 minutes. After fixation, quickly cool to room temperature (20°C), and then carry out the rolling operation. The rolling speed is set at 25 r / min, and the rolling time is 10 minutes. After rolling, evenly stack the raw materials in the fermentation equipment to prepare for the subsequent fermentation process.

[0029] (2)Fermentation process: Phase A: Control the fermentation environment temperature at 25°C, the relative humidity at 65%, and the oxygen content at 18%. The fermentation duration of this phase is 8 hours, and no pile - turning operation is required in this phase.

[0030] Phase B: First, turn the fermentation raw materials and evenly spray a citric acid - sodium citrate buffer solution with pH = 4.5 containing 0.02 M sodium chloride (80 ml / kg dark tea raw materials). After spraying, let it stand for 0.5 hours. Then, gradually increase the temperature to 30°C at a rate of 1°C / h, while adjusting the relative humidity to 70% and controlling the oxygen content at 15%. During this phase, turn the pile every 6 hours, and the fermentation duration is 12 hours.

[0031] Phase C: Stabilize the fermentation temperature at 35°C, maintain the relative humidity at 75%, reduce the oxygen content to 10%, turn the pile every 4 hours, and the fermentation duration is 24 hours.

[0032] Phase D: Slowly decrease the temperature at a rate of 1°C / h to 32°C, adjust the relative humidity to 70%, maintain the oxygen content at 8%, turn the pile every 8 hours, and the fermentation duration is 16 hours.

[0033] Phase E: Keep the temperature at 25°C, set the relative humidity at 60%, control the oxygen content at 5%, turn the pile every 8 hours, and the fermentation duration is 24 hours to complete the entire fermentation process.

[0034] (3) Drying: After fermentation, adopt a low-temperature segmented drying method. First, dry at 35°C for 10 hours, then dry at 50°C for 6 hours until the moisture content of the dark tea is lower than 10% to obtain the finished dark tea.

[0035] Example 3 Dark Tea Fermentation Process (1) Initial treatment of dark tea: Subject the freshly picked dark tea raw materials to a fixation treatment. Control the fixation temperature at 250°C and the fixation time at 5 minutes. After fixation, quickly cool to room temperature (25°C), and then perform a rolling operation. Set the rolling speed at 35 r / min and the rolling time at 20 minutes. After rolling, evenly stack the raw materials in the fermentation equipment to prepare for the subsequent fermentation process.

[0036] (2) Fermentation process: Phase A: Control the fermentation environment temperature at 28°C, maintain the relative humidity at 70%, and maintain the oxygen content at 20%. The fermentation duration in this phase is 12 hours, and no pile turning operation is required in this phase.

[0037] Phase B: First, turn the fermentation raw materials and evenly spray a citric acid-sodium citrate buffer solution with pH = 5.0 containing 0.05 M sodium chloride (100 ml / kg dark tea raw materials). After spraying, let it stand for 1 hour. Then gradually increase the temperature to 33°C at a rate of 1.5°C / h, while adjusting the relative humidity to 75% and controlling the oxygen content at 18%. In this phase, turn the pile every 8 hours, and the fermentation duration is 24 hours.

[0038] Phase C: Stabilize the fermentation temperature at 38°C, maintain the relative humidity at 80%, reduce the oxygen content to 15%, turn the pile every 6 hours, and the fermentation duration is 48 hours.

[0039] Phase D: Slowly decrease the temperature at a rate of 1.5°C / h to 35°C, adjust the relative humidity to 75%, maintain the oxygen content at 10%, turn the pile every 10 hours, and the fermentation duration is 30 hours.

[0040] Stage E: Maintain the temperature at 30°C, set the relative humidity to 65%, control the oxygen content at 8%, turn the pile every 12 hours, and the fermentation duration is 36 hours to complete the entire fermentation process.

[0041] (3)Drying: After fermentation, adopt a low-temperature segmented drying method. First, dry at 45°C for 15 hours, and then dry at 60°C for 10 hours until the moisture content of the dark tea is lower than 10% to obtain the finished dark tea.

[0042] Influence of the segmented temperature, humidity, and oxygen control process on physical and chemical indexes The content of tea polyphenols was detected by the Folin-Ciocalteu colorimetric method according to the national standard method (GB / T 8313-2018). The contents of thearubigins and theaflavins were determined by high-performance liquid chromatography (HPLC).

[0043] Table 1 Fermentation process parameters of dark tea in Comparative Example 1 The process is the same as that of Example 1, but the oxygen content is not controlled, and it is in an open environment with a natural oxygen content of about 21% in the environment.

[0044] Table 2 Fermentation process parameters of dark tea in Comparative Example 2 The process is the same as that of Example 1, but the fermentation process is divided into three stages.

[0045] Table 3 Fermentation process parameters of dark tea in Comparative Example 3 The process is the same as that of Example 1, but the fermentation process is divided into four stages.

[0046] Table 4 Fermentation process parameters of dark tea in Comparative Example 4 The process is the same as that of Example 1, but the oxygen content is constant at 15%.

[0047] Table 5 Fermentation process parameters of dark tea in Comparative Example 5 The process is the same as that of Example 1, but the fermentation process is divided into four stages.

[0048] Table 6 Fermentation process parameters of dark tea in Comparative Example 6 The process is the same as that of Example 1, but the oxygen content in Stage A is maintained at 15%.

[0049] Table 7 Fermentation process parameters of dark tea in Comparative Example 7 The other processes are the same as those in Example 1, but the temperature in the B stage is controlled at 36°C.

[0050] Table 8 Fermentation process parameters of black tea in Comparative Example 8 The other processes are the same as those in Example 1, but the oxygen content in the C stage is controlled at 8%.

[0051] Table 9 Fermentation process parameters of black tea in Comparative Example 9 The other processes are the same as those in Example 1, but the turning of the pile is carried out for 2 h each time in the A stage.

[0052] Table 10 Contents of tea polyphenols, thearubigins, and theaflavins in the finished black tea Table 10 elaborates on the effects of fermentation process differences on the contents of tea polyphenols, thearubigins, and theaflavins. Examples 1-3 adopt a method of controlling temperature, humidity, and oxygen in stages to promote the synergistic effect of microorganisms and enzymes, appropriately accelerate the degradation of tea polyphenols, promote the oxidative polymerization of tea polyphenols to form thearubigins, and optimize the conversion of theaflavins. In Comparative Example 1, the lack of control of the oxygen content led to excessive oxidation of tea polyphenols, disordered consumption of thearubigin precursor substances, and blocked synthesis, and theaflavins were degraded due to intensified high-temperature oxidation. In Comparative Example 2, the traditional three-stage fermentation was adopted, and the stage division was rough, resulting in an early decline in the activity of microorganisms, incomplete degradation of tea polyphenols, insufficient accumulation of thearubigins, and damage to theaflavins due to high-temperature oxidation. In Comparative Example 3, the lack of the D stage led to insufficient material conversion in the later stage, high residual tea polyphenols, stagnant synthesis of thearubigins, and degradation of theaflavins. In Comparative Example 4, the oxygen was not regulated in stages, resulting in blocked synthesis of thearubigins. In Comparative Example 5, the lack of the B stage led to insufficient fermentation in the early stage, incomplete degradation of tea polyphenols, low conversion efficiency of thearubigin precursor substances, and low synthesis amount of theaflavins. In Comparative Example 6, the lack of oxygen in the A stage of fermentation led to insufficient conversion of the initial substances, increased residual tea polyphenols, weak basis for the synthesis of thearubigins, and insufficient accumulation of theaflavin precursors. In Comparative Example 7, the temperature in the B stage of fermentation was relatively high, resulting in premature inactivation of the microbial enzymes, inhibition of the synthesis of thearubigin precursors, and degradation of theaflavins. In Comparative Example 8, the too low oxygen content in the C stage of fermentation led to incomplete polymerization reaction of thearubigins, and the synthesis amount of thearubigins decreased significantly. In Comparative Example 9, frequent turning of the pile in the A stage of fermentation damaged the initial colonization of microorganisms, reduced the material conversion efficiency, resulting in blocked degradation of tea polyphenols, decreased synthesis amount of thearubigins, and insufficient accumulation of theaflavin precursors. It shows that the triple-stage regulation of temperature-humidity-oxygen in the present invention promotes the directional conversion of tea polyphenols into thearubigins and theaflavins, and the lack of any stage or parameter imbalance will lead to deterioration of the physical and chemical indexes.

[0053] Effect of buffer solution on physical and chemical indexes in the B stage of fermentation Table 11 Fermentation processes of Comparative Examples 10-15 The contents of tea polyphenols, thearubigins, and theaflavins in the finished dark teas of Comparative Examples 10 to 14 were detected according to the above method.

[0054] Table 12 Real-time monitoring data of the pH value Table 12 shows that after spraying the buffer solution in Phase B of Examples 1 to 3, the pH was stabilized at 4.5 - 5.0, which was positively correlated with the thearubigin content in Table 13, proving that a stable acidic environment promotes the activity of polyphenol oxidase. It shows that the precise control of the pH value in stages during the fermentation process is one of the core advantages of this process. Through the synergistic effect of the citric acid - sodium citrate buffer system and microbial metabolism, the directional conversion of tea polyphenols into thearubigins and theaflavins was achieved.

[0055] Table 13 Contents of tea polyphenols, thearubigins, and theaflavins in the finished dark tea As can be seen from Table 12, the buffer solution can maintain the pH stability of the fermentation environment. Without spraying the buffer solution, the pH will fluctuate greatly during the fermentation process. The instability of the pH will affect the activity of microorganisms and the catalytic action of enzymes, hinder the degradation and conversion process of tea polyphenols, and thus lead to an increase in the residual amount of tea polyphenols. At the same time, the synthesis of thearubigins and theaflavins will also be inhibited, and the content will decrease. Water cannot maintain the pH stability like the buffer solution, and the pH of the fermentation environment will change with the accumulation of metabolites. Although water can increase the humidity, it cannot adjust the pH, which will affect the growth of microorganisms and the activity of enzymes, and further affect the conversion of tea polyphenols and the synthesis of thearubigins and theaflavins. Sodium chloride can maintain the osmotic pressure balance of microbial cells and promote the metabolic activities of microorganisms, and also has a certain influence on the conversion of tea polyphenols and the synthesis of thearubigins and theaflavins. Both Table 12 and Table 13 prove that the precise regulation of the pH value during the fermentation process is the core factor affecting the quality of dark tea. By maintaining the pH stable between 4.5 - 5.0, the buffer solution significantly promotes the conversion of tea polyphenols into thearubigins and theaflavins, while the pH fluctuation or deviation from the optimal range will lead to a decrease in the material conversion efficiency. The two sets of data corroborate each other and jointly support the scientificity and superiority of the process of "controlling temperature, humidity, and oxygen in stages + regulating pH with buffer solution".

[0056] 5 grams of the dark teas of Examples 1 to 3 and Comparative Examples 1 to 9 were respectively taken, brewed with boiling water above 95 °C, quickly rinsed the tea once, and then slowly poured boiling water along the cup wall at a fixed point. The brewing time was 30 seconds. 10 volunteers tasted them respectively and all said that the finished dark teas of Examples 1 to 3 were better in terms of taste, aroma, and color.

Claims

1. A black tea fermentation process with staged temperature and humidity control, characterized in that, The dark tea fermentation process includes: (1) Initial treatment of dark tea: Fixing and rolling the dark tea; (2) Fermentation process: Stage A: Temperature 25°C - 28°C, humidity 65% - 70%, oxygen content 18% - 20%, for 8 - 12 hours, no need to turn the pile; Stage B: Turn the pile, evenly spray the buffer solution, let it stand, gradually increase the temperature to 30°C - 33°C, humidity 70% - 75%, oxygen content 15% - 18%, turn the pile every 6 - 8 hours, for a total of 12 - 24 hours; Stage C: Temperature 35°C - 38°C, humidity 75% - 80%, oxygen content 10% - 15%, turn the pile every 4 - 6 hours, for a total of 24 - 48 hours; Stage D: Slowly decrease the temperature to 32°C - 35°C, humidity 70% - 75%, oxygen content 8% - 10%, turn the pile every 8 - 10 hours, for a total of 12 - 24 hours; Stage E: Temperature 25°C - 30°C, humidity 60% - 65%, oxygen content 5% - 8%, turn the pile every 8 - 12 hours, for a total of 24 - 36 hours; (3) Drying: Low - temperature segmented drying until the water content of the dark tea is less than 10% to obtain the finished dark tea.

2. The dark tea fermentation process according to claim 1, characterized in that, The buffer solution is a citric acid - sodium citrate buffer solution containing 0.02 - 0.05M sodium chloride.

3. The dark tea fermentation process according to claim 1, characterized in that, The buffer solution is a citric acid - sodium citrate buffer solution containing 0.03M sodium chloride.

4. The dark tea fermentation process according to claim 1, wherein The pH of the buffer solution is 4.5 - 5.

0.

5. The dark tea fermentation process according to claim 1, characterized in that, The pH of the buffer solution is 4.

8.

6. The dark tea fermentation process according to claim 1, wherein The gradual increase in temperature in Stage B means increasing at a rate of 1 - 1.5°C / h.

7. The dark tea fermentation process according to claim 1, characterized in that, The slow decrease in temperature in Stage D means decreasing at a rate of 1 - 1.5°C / h.

8. The dark tea fermentation process according to claim 1, characterized in that, The low - temperature segmented drying method is to dry at 35 - 45°C for 10 - 15 hours and then dry at 50 - 60°C for 6 - 10 hours.

9. The dark tea fermentation process according to claim 1, characterized in that, The dark tea fermentation process includes: (1) Initial treatment of dark tea: Fixing fresh dark tea raw materials at 180 - 250°C for 3 - 5 minutes, cooling to room temperature, and rolling at 25 - 35 r / min for 10 - 20 minutes; (2) Fermentation process: Stage A: Temperature 25°C - 28°C, relative humidity 65% - 70%, oxygen content 18% - 20%, fermentation duration is 8 - 12 hours, no need to turn the pile; Stage B: Turn the pile, evenly spray a citric acid - sodium citrate buffer solution with pH = 4.5 - 5.0 containing 0.05 - 0.1M sodium chloride, let it stand for 0.5 - 1 hour, gradually increase the temperature to 30°C - 33°C at a rate of 1 - 1.5°C / h, adjust the relative humidity to 70% - 75%, control the oxygen content at 15% - 18%, turn the pile every 6 - 8 hours, fermentation duration is 12 - 24 hours; Stage C: Temperature 35°C - 38°C, relative humidity 75% - 80%, oxygen content reduced to 10% - 15%, turn the pile every 4 - 6 hours, fermentation duration is 24 - 48 hours; Stage D: Slowly decrease the temperature to 32°C - 35°C at a rate of 1 - 1.5°C / h, adjust the relative humidity to 70% - 75%, oxygen content 8% - 10%, turn the pile every 8 - 10 hours, fermentation duration is 16 - 30 hours; Stage E: Maintain the temperature at 25°C - 30°C, set the relative humidity at 60% - 65%, control the oxygen content at 5% - 8%, turn the pile every 8 - 12 hours, and the fermentation duration is 24 - 36 hours; (3) Drying: Adopt the low-temperature segmented drying method. First, dry at a temperature of 35 - 45°C for 10 - 15 hours, and then dry at a temperature of 50 - 60°C for 6 - 10 hours until the water content of the dark tea is lower than 10% to obtain the finished dark tea.

10. The dark tea fermentation process according to claim 9, wherein, The dark tea fermentation process includes: (1) Primary treatment of dark tea: Fresh dark tea raw materials are de-enzymed at 210°C for 4 minutes, cooled to room temperature, and then kneaded at 30 r / min for 15 minutes; (2) Fermentation process: Stage A: The temperature is 26.5°C, the relative humidity is 68%, the oxygen content is 19%, the fermentation duration is 10 hours, and there is no need to turn the pile; Stage B: Evenly spray the citric acid - sodium citrate buffer solution with pH = 4.8 containing 0.08 M sodium chloride during turning the pile, let it stand for 0.8 hours, gradually raise the temperature to 32°C at a rate of 1°C / h, adjust the relative humidity to 72%, control the oxygen content at 17%, turn the pile every 7 hours, and the fermentation duration is 21 hours; Stage C: The temperature is 37°C, the relative humidity is 78%, the oxygen content is reduced to 12%, turn the pile every 5 hours, and the fermentation duration is 35 hours; Stage D: Slowly lower the temperature to 33°C at a rate of 1.2°C / h, adjust the relative humidity to 72%, the oxygen content is 9%, turn the pile every 9 hours, and the fermentation duration is 18 hours; Stage E: Maintain the temperature at 27°C, set the relative humidity at 62%, control the oxygen content at 6%, turn the pile every 10 hours, and the fermentation duration is 30 hours; (3) Drying: Adopt the low-temperature segmented drying method. First, dry at a temperature of 40°C for 12 hours, and then dry at a temperature of 55°C for 8 hours until the water content of the dark tea is lower than 10% to obtain the finished dark tea.

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