Selenium-enriched black tea selenium preservation and aroma preservation fermentation method

Abstract

This invention belongs to the field of tea processing technology, specifically relating to a method for preserving selenium and aroma in fermented selenium-enriched black tea, comprising: S1. Spreading the kneaded selenium-enriched tea leaves onto a stainless steel mesh tray, uniformly spraying a selenium stabilizer solution, the spraying amount being 3-8% of the tea leaf mass, and letting it stand for 15-30 minutes after spraying; S2. Placing the treated tea leaves in a container with a relative humidity of 90-95% and an aeration rate of 0.1-0.3 m³ / h. 3 / min·m 3 S3. After fermentation, the tea leaves are transferred to a vacuum drying oven for drying. This invention utilizes a composite stabilizer of chitosan, L-theanine, and vitamin C, sprayed immediately after kneading. The amino groups of chitosan form electrostatic complexes with the carboxyl groups of selenoamino acids, L-theanine provides sulfhydryl coordination sites to further stabilize organic selenium, and vitamin C maintains a reducing microenvironment to inhibit selenium oxidation and volatilization. Through gradient vacuum drying, the selenium retention rate can reach 82-91%. The theaflavins content reaches 1.2-1.8%, thearubigins content is 5-8%, and theabrownin content is controlled below 3%. The tea liquor is bright red, with a mellow and refreshing taste, free of any astringent or burnt flavor.

Description

Technical Field

[0001] This invention belongs to the field of tea processing technology, specifically relating to a method for preserving selenium and aroma in the fermentation of selenium-enriched black tea. Background Technology

[0002] Selenium is an essential trace element for the human body, possessing various physiological functions such as anti-oxidation, enhanced immunity, and prevention of cardiovascular diseases. Approximately 72% of my country's soil is selenium-deficient, making dietary selenium supplementation through selenium-enriched agricultural products an economical and effective approach. Tea, with its wide consumer base and excellent selenium accumulation capacity, is an ideal carrier of selenium. Selenium-enriched black tea combines the rich flavor of black tea with the nutritional and health benefits of selenium, and market demand for it is increasing year by year.

[0003] Current processing methods for selenium-enriched black tea follow the traditional black tea process: withering, rolling, fermentation, and drying. The fermentation process is typically controlled at 28-32℃, relative humidity above 90%, and time 3-5 hours; the drying process uses two stages: initial drying and final drying. This process has certain shortcomings: high-temperature drying causes a large amount of organic selenium (mainly selenomethionine and selenocysteine) in the tea leaves to decompose and volatilize, with a selenium retention rate of only 50-65%; furthermore, prolonged high-temperature treatment causes excessive oxidation of tea polyphenols and excessive Maillard reactions between amino acids and reducing sugars, destroying the unique floral and fruity aroma components of black tea (linalool, geraniol, and phenylethyl alcohol, etc.), resulting in a decline in aroma quality; additionally, under the traditional constant-temperature fermentation mode, the peak activity period of polyphenol oxidase does not match the temperature curve, making it difficult to precisely control the degree of fermentation and causing large batch-to-batch quality fluctuations. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preserving selenium and aroma in the fermentation of selenium-enriched black tea, so as to solve the problems existing in the prior art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for preserving selenium and aroma in the fermentation of selenium-enriched black tea includes: S1. Spread the kneaded selenium-enriched tea leaves on a stainless steel mesh tray, and spray the selenium stabilizer solution evenly. The amount of spraying is 3-8% of the tea leaf mass. After spraying, let it stand for 15-30 minutes to allow the stabilizer to fully penetrate into the leaf tissue. The selenium stabilizer solution, by mass percentage, comprises the following components: 0.5-1.5% chitosan, 0.3-0.8% L-theanine, 0.1-0.3% vitamin C, and the balance being purified water; the degree of deacetylation of the chitosan is ≥90%. S2. After treatment, the tea leaves are placed in an environment with a relative humidity of 90-95% and an aeration rate of 0.1-0.3 m³ / h. 3 / min·m 3 Lower fermentation; S3. After fermentation, the tea leaves can be transferred to a vacuum drying oven for further drying.

[0006] Furthermore, step S2 fermentation includes the following stages: Low-temperature start-up stage: fermentation temperature 18-22℃, fermentation time 1-2h; Mesophilic conversion stage: The temperature is increased to 26-28℃ at a rate of 0.5-1.0℃ / min, and fermentation is maintained for 3-5 hours; Low-temperature termination phase: The temperature is reduced to 16-20℃ at a rate of 0.3-0.5℃ / min and maintained for 0.5-1h.

[0007] Furthermore, step S3 employs a two-stage drying process: Low-temperature dehydration section: temperature 42-48℃, vacuum degree -0.08 to -0.09MPa, drying time 2-4h, until the leaf moisture content drops to 15-18%; Medium-temperature aroma enhancement stage: temperature 55-62℃, vacuum degree -0.06 to -0.07MPa, drying time 1-2h, until the leaf moisture content drops to 5-7%.

[0008] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention involves spraying a composite stabilizer of chitosan, L-theanine, and vitamin C immediately after kneading. The amino groups of chitosan form electrostatic complexes with the carboxyl groups of selenoamino acids, L-theanine provides thiol coordination sites to further stabilize organic selenium, and vitamin C maintains a reducing microenvironment to inhibit selenium oxidation and volatilization. After gradient vacuum drying, the selenium retention rate can reach 82-91%.

[0009] (2) In the three-stage variable temperature fermentation of this invention, the low-temperature start-up stage retains the endogenous aroma precursors of tea leaves, the medium-temperature conversion stage promotes the full release of the activity of aroma precursor hydrolytic enzymes such as β-glucosidase, and the low-temperature termination stage avoids the volatilization and excessive conversion of aroma substances at high temperatures. The medium-temperature aroma-enhancing stage of gradient vacuum drying moderately stimulates the Maillard reaction and caramelization reaction, generating roasting aroma components such as pyrazines and furans, so that the content of geraniol, a characteristic aroma substance of black tea, reaches 0.25-0.35 mg / g and the content of linalool reaches 0.18-0.28 mg / g.

[0010] (3) The black tea of ​​this invention has a theaflavin content of 1.2-1.8%, a thearubigin content of 5-8%, and a theabrownin content controlled below 3%. The tea soup is bright red, with a mellow and refreshing taste, and no astringent or burnt taste. Detailed Implementation

[0011] The technical solution of this invention patent will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0012] The fresh leaves of the selenium-enriched tea all come from the selenium-enriched tea production area of ​​Enshi, Hubei (one bud and two leaves, with a total selenium content of 0.45 mg / kg dry weight).

[0013] Example 1 A method for preserving selenium and aroma in the fermentation of selenium-enriched black tea includes the following steps: S1. Spread the kneaded selenium-enriched tea leaves on a stainless steel mesh tray, and spray the selenium stabilizer solution evenly. The amount of spraying is 3% of the tea leaf mass. After spraying, let it stand for 15 minutes to allow the stabilizer to fully penetrate into the leaf tissue. The selenium stabilizer solution, by mass percentage, comprises the following components: 0.5% chitosan (dissolved in 1% acetic acid solution), 0.3% L-theanine, 0.1% vitamin C, and the balance being purified water; the degree of deacetylation of the chitosan is ≥90%. S2. The treated tea leaves are placed in an environment with a relative humidity of 90% and an air circulation of 0.1 m³ / h. 3 / min·m 3 Fermentation is divided into three stages: Low-temperature start-up stage: fermentation temperature 18℃, fermentation time 1 hour; Mesothermal conversion stage: The temperature is increased to 26℃ at a rate of 1.0℃ / min and fermentation is maintained for 3 hours; Low-temperature termination phase: The temperature is reduced to 16℃ at a rate of 0.5℃ / min and maintained for 0.5h; S3. After fermentation, the tea leaves are transferred to a vacuum drying oven for two-stage drying: Low-temperature dehydration section: temperature 42℃, vacuum degree -0.08MPa, drying until the leaf moisture content drops to 18%; Medium-temperature aroma enhancement stage: temperature 55℃, vacuum degree -0.06MPa, drying until the leaf moisture content drops to 6.8%.

[0014] Example 2 A method for preserving selenium and aroma in the fermentation of selenium-enriched black tea includes the following steps: S1. Spread the kneaded selenium-enriched tea leaves on a stainless steel mesh tray, and spray the selenium stabilizer solution evenly. The amount of spraying is 8% of the tea leaf mass. After spraying, let it stand for 30 minutes to allow the stabilizer to fully penetrate into the leaf tissue. The selenium stabilizer solution, by mass percentage, comprises the following components: 1.5% chitosan (dissolved in 1% acetic acid solution), 0.8% L-theanine, 0.3% vitamin C, and the balance being purified water; the degree of deacetylation of the chitosan is ≥90%. S2. The treated tea leaves are placed in an environment with a relative humidity of 95% and an aeration rate of 0.3 m³ / h. 3 / min·m 3 Fermentation is divided into three stages: Low-temperature start-up stage: fermentation temperature 22℃, fermentation time 1-2h; Mesophilic conversion stage: The temperature is increased to 28℃ at a rate of 1.0℃ / min and fermentation is maintained for 5 hours; Low-temperature termination phase: The temperature is reduced to 20℃ at a rate of 0.5℃ / min and maintained for 1 hour; S3. After fermentation, the tea leaves are transferred to a vacuum drying oven for two-stage drying: Low-temperature dehydration section: temperature 48℃, vacuum degree -0.09MPa, drying until the leaf moisture content drops to 15%; Medium-temperature aroma enhancement stage: temperature 62℃, vacuum degree -0.07MPa, drying until the leaf moisture content drops to 5.2%.

[0015] Example 3 A method for preserving selenium and aroma in the fermentation of selenium-enriched black tea includes the following steps: S1. Spread the kneaded selenium-enriched tea leaves on a stainless steel mesh tray, and spray the selenium stabilizer solution evenly. The amount of spraying is 5% of the tea leaf mass. After spraying, let it stand for 20 minutes to allow the stabilizer to fully penetrate into the leaf tissue. The selenium stabilizer solution, by mass percentage, comprises the following components: 1.0% chitosan (dissolved in 1% acetic acid solution), 0.5% L-theanine, 0.2% vitamin C, and the balance being purified water; the degree of deacetylation of the chitosan is ≥90%. S2. The treated tea leaves are placed in an environment with a relative humidity of 92% and an aeration rate of 0.2 m³ / h. 3 / min·m 3 Fermentation is divided into three stages: Low-temperature start-up stage: fermentation temperature 20℃, fermentation time 1.5h; Mesophilic conversion stage: The temperature is increased to 27℃ at a rate of 1.0℃ / min and fermentation is maintained for 4 hours; Low-temperature termination phase: The temperature is reduced to 18℃ at a rate of 0.5℃ / min and maintained for 0.8h; S3. After fermentation, the tea leaves are transferred to a vacuum drying oven for two-stage drying: Low-temperature dehydration section: temperature 45℃, vacuum degree -0.085MPa, drying until the leaf moisture content drops to 17%; Medium-temperature aroma enhancement stage: temperature 60℃, vacuum degree -0.065MPa, drying until the leaf moisture content drops to 6%.

[0016] Example 4 A method for preserving selenium and aroma in the fermentation of selenium-enriched black tea includes the following steps: S1. Spread the kneaded selenium-enriched tea leaves on a stainless steel mesh tray, and spray the selenium stabilizer solution evenly. The amount of spraying is 6% of the tea leaf mass. After spraying, let it stand for 25 minutes to allow the stabilizer to fully penetrate into the leaf tissue. The selenium stabilizer solution, by mass percentage, comprises the following components: 0.8% chitosan (dissolved in 1% acetic acid solution), 0.6% L-theanine, 0.15% vitamin C, and the balance being purified water; the degree of deacetylation of the chitosan is ≥90%. S2. The treated tea leaves are placed in an environment with a relative humidity of 95% and an aeration rate of 0.25m³ / h. 3 / min·m 3 Fermentation is divided into three stages: Low-temperature start-up stage: fermentation temperature 19℃, fermentation time 1.58h; Mesophilic conversion stage: The temperature is increased to 27.5℃ at a rate of 1.0℃ / min and fermentation is maintained for 3.5h; Low-temperature termination phase: The temperature is reduced to 17℃ at a rate of 0.5℃ / min and maintained for 0.6h; S3. After fermentation, the tea leaves are transferred to a vacuum drying oven for two-stage drying: Low-temperature dehydration section: temperature 44℃, vacuum degree -0.08MPa, drying until the leaf moisture content drops to 17%; Medium-temperature aroma enhancement stage: temperature 60℃, vacuum degree -0.065MPa, drying until the leaf moisture content drops to 5.8%.

[0017] Comparative Example 1 The fresh leaf raw materials were the same as in Example 3. After kneading, no selenium stabilization pretreatment was performed. Fermentation was carried out using traditional constant temperature fermentation (28℃, RH 95%, 4h). Drying was carried out using traditional hot air drying (110℃ for 15min for initial drying; 85℃ for 45min for final drying).

[0018] Comparative Example 2 The selenium stabilization pretreatment in step S1 is omitted, and the remaining operations are the same as in Example 3.

[0019] Comparative Example 3 Step S2 uses constant temperature fermentation (27℃, RH 92%, 5h) instead of three-stage variable temperature fermentation, and the remaining operations are the same as in Example 3.

[0020] Comparative Example 4 Step S3 uses vacuum freeze drying (-50℃, 13Pa, 18h) instead of gradient vacuum drying, and the remaining operations are the same as in Example 3.

[0021] Selenium retention rate The total selenium content of the samples was determined according to GB 5009.93-2017, "National Food Safety Standard - Determination of Selenium in Food" (Hydride Atomic Fluorescence Spectrometry). The samples were digested with a mixture of nitric acid and perchloric acid, and the results are shown in Table 1.

[0022] Selenium retention rate (%) = Selenium content of finished tea / Selenium content of rolled leaves × 100%.

[0023] Table 1 Selenium Retention Rate

[0024] As shown in Table 1, the selenium retention rates of Examples 1-4 were all ≥84%, significantly higher than that of Comparative Example 1. The selenium retention rate of Comparative Example 2, which used a selenium-deficient stabilizer, decreased to 68.9%, indicating that selenium stabilization pretreatment is a key step in improving selenium retention. The selenium retention rate of Comparative Example 4, which used freeze-drying, was only 80.0%, lower than the gradient vacuum drying method of this invention, indicating that gradient vacuum drying helps stabilize the selenium-protein complex.

[0025] Determination of main quality components Tea polyphenols were analyzed according to GB / T 8313-2018 (Folin-Ciocalteu method); theaflavins, thearubigins, and theabrownins were analyzed according to systematic method; free amino acids were analyzed according to GB / T 8314-2013 (ninhydrin colorimetric method). The results are shown in Table 2.

[0026] Table 2 Determination of Main Quality Components

[0027] As shown in Table 2, the theaflavins and thearubigins in Examples 1-4 were significantly higher than those in Comparative Example 1, while the theabrownins were lower, indicating that the three-stage variable-temperature fermentation could effectively control the degree of fermentation. The theabrownins in Comparative Example 3, which underwent constant-temperature fermentation, were 3.6%, higher than in all other examples, indicating that the low-temperature termination stage effectively inhibited the excessive oxidation of polyphenols.

[0028] Sensory quality evaluation Referring to GB / T 23776-2018 "Sensory Evaluation Methods for Tea", five trained tea tasters independently scored the tea using a coded evaluation method, with each item having a maximum score of 5 points, and the average of the results was taken.

[0029] Table 3 Sensory Quality Evaluation

[0030] As shown in Table 3, Example 3 had the highest overall sensory score, with an aroma score of 4.5 (rich floral and fruity aroma, no off-odors), a liquor color score of 4.3 (bright red), and a taste score of 4.4 (mellow and refreshing). Comparative Example 1 had the lowest overall score, with a weak aroma and a slightly burnt taste. Comparative Example 4 had poor leaf compaction (3.2 points) and a darker leaf base (3.0 points), indicating that moderate heat treatment is essential for the formation of black tea quality.

[0031] In summary, this invention utilizes a composite stabilizer of chitosan, L-theanine, and vitamin C, sprayed immediately after kneading. The amino groups of chitosan form electrostatic complexes with the carboxyl groups of selenoamino acids, L-theanine provides thiol coordination sites to further stabilize the organic selenium, and vitamin C maintains a reducing microenvironment to inhibit selenium oxidation and volatilization. After gradient vacuum drying, the selenium retention rate can reach 82-91%.

[0032] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for fermenting selenium-enriched black tea to preserve selenium and aroma, characterized in that, include: S1. Spread the kneaded selenium-enriched tea leaves on a stainless steel mesh tray, and spray the selenium stabilizer solution evenly. The amount of spraying is 3-8% of the tea leaf mass. After spraying, let it stand for 15-30 minutes to allow the stabilizer to fully penetrate into the leaf tissue. The selenium stabilizer solution, by mass percentage, comprises the following components: 0.5-1.5% chitosan, 0.3-0.8% L-theanine, 0.1-0.3% vitamin C, and the balance being purified water; the degree of deacetylation of the chitosan is ≥90%. S2. After treatment, the tea leaves are placed in an environment with a relative humidity of 90-95% and an aeration rate of 0.1-0.3 m³ / h. 3 / min·m 3 Lower fermentation; S3. After fermentation, the tea leaves can be transferred to a vacuum drying oven for further drying.

2. The method for preserving selenium and aroma in fermented selenium-enriched black tea according to claim 1, characterized in that, Step S2 fermentation includes the following stages: Low-temperature start-up stage: fermentation temperature 18-22℃, fermentation time 1-2h; Mesophilic conversion stage: The temperature is increased to 26-28℃ at a rate of 0.5-1.0℃ / min, and fermentation is maintained for 3-5 hours; Low-temperature termination phase: The temperature is reduced to 16-20℃ at a rate of 0.3-0.5℃ / min and maintained for 0.5-1h.

3. The method for preserving selenium and aroma in fermented selenium-enriched black tea according to claim 1, characterized in that, Step S3 employs a two-stage drying process: Low-temperature dehydration section: temperature 42-48℃, vacuum degree -0.08 to -0.09MPa, drying time 2-4h, until the leaf moisture content drops to 15-18%; Medium-temperature aroma enhancement stage: temperature 55-62℃, vacuum degree -0.06 to -0.07MPa, drying time 1-2h, until the leaf moisture content drops to 5-7%.

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