A method for preparing low-factor coffee beans rich in natural thiol aroma by low-temperature countercurrent extraction coupled with enzymatic directed conversion

Abstract

The application discloses a method for preparing low-cafeine coffee beans rich in natural thiol aroma by low-temperature countercurrent extraction coupled with enzyme-directed conversion. The method adopts low-temperature countercurrent gradient extraction technology to realize efficient and mild removal of caffeine in coffee beans, and avoids the damage of high temperature to coffee precursor aroma substances; through the use of specific composite enzyme preparation, precise temperature and pH control enzyme-directed conversion reaction, part of the glycosylation precursor substances and sulfur-containing amino acid precursors in the coffee beans are directed to convert into natural thiol characteristic aroma substances such as methyl mercaptan and ethyl mercaptan, so as to make up for the aroma loss in the decaffeination process. The method can stably prepare high-quality coffee beans with low caffeine and high natural thiol aroma, the content of caffeine in the finished product is less than or equal to 0.08%, the total content of thiol aroma substances is increased by 40%-65% compared with traditional water washing decaffeination coffee, the original rich flavor of coffee is maximally retained, the process is mild, green and environmentally friendly, and the method can be used for large-scale production, and is suitable for industrialized production of high-quality low-caffeine coffee.

Description

Technical Field

[0001] This invention belongs to the field of coffee deep processing and food biomodification technology, specifically relating to a method for preparing decaffeinated coffee beans by removing caffeine through low-temperature countercurrent extraction and combining it with enzymatic directional conversion to enrich natural thiol aromas. Background Technology

[0002] Decaffeinated coffee beans have a low caffeine content, do not cause insomnia or palpitations, and are suitable for pregnant women, the elderly, caffeine-sensitive individuals, and nighttime drinking occasions. This represents an important development direction for the specialty coffee market. Currently, the mainstream decaffeination processes for coffee beans are mainly divided into three categories: washed decaffeination, organic solvent extraction decaffeination, and supercritical CO2 decaffeination. Traditional washed decaffeination requires high-temperature soaking and long-term boiling, which results in the loss of a large amount of sulfur-containing amino acids and glycosides, aroma precursors, from the coffee beans, leading to a thin aroma and bland flavor in the finished coffee. Organic solvent extraction decaffeination carries the risk of solvent residue, which does not meet the green and safe requirements of high-end food. Supercritical CO2 decaffeination equipment is expensive and energy-intensive, making it difficult to scale up and popularize, and it still suffers from the loss of some heat-sensitive aroma substances.

[0003] Existing decaffeinated coffee processing technologies generally suffer from the following problems: During the decaffeination process, a large amount of water-soluble aroma precursors are lost with the extract, and the high-temperature process destroys the activity of endogenous enzymes, making it impossible to achieve the endogenous transformation of aroma substances. The final decaffeinated coffee generally suffers from weak aroma, poor layering, and incomplete flavor. Among the existing technologies, a few studies have attempted to compensate for aroma defects by adding flavorings later, but artificial flavorings have a harsh and layered aroma and do not conform to the trend of natural food consumption. Some other technologies use a single enzyme preparation to process coffee, but they do not combine it with a gentle decaffeination process. After high-temperature decaffeination, a large amount of aroma precursors are lost, and the enzymatic aroma enhancement effect is extremely poor, which cannot fundamentally solve the contradiction between decaffeination and aroma preservation and enhancement in decaffeinated coffee.

[0004] This invention aims to overcome the technical problems of high-temperature aroma loss, incomplete decaffeination, severe loss of natural thiol aroma, and poor flavor in existing decaffeinated coffee bean preparation processes. It provides a method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzymatic directional conversion to solve the above problems. Summary of the Invention

[0005] To address the shortcomings and defects of existing technologies, this invention proposes a method for preparing decaffeinated coffee beans rich in natural thiol aromas through low-temperature countercurrent extraction coupled with enzymatic directional conversion. This method ensures efficient decaffeination while maximizing the retention of coffee aroma precursors. Furthermore, it utilizes bio-enzymatic reactions to directionally enrich the characteristic aromas of natural thiols, thereby producing high-quality natural decaffeinated coffee beans that are low in caffeine, highly aromatic, and have a rich flavor.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzymatic directional conversion includes the following steps: Step S1: Raw material pretreatment: Select green coffee beans with uniform maturity, no mold, and no insect infestation. After washing and air drying, perform low-temperature light cell wall breaking treatment, controlling the cell wall breaking rate to 15%-25%, to obtain pretreated coffee beans. Step S2, Low-temperature countercurrent gradient dedecanoation: Place the pretreated coffee beans in a low-temperature countercurrent extraction device, use food-grade pure water as the extraction solvent, set the extraction temperature to 30-45℃, the countercurrent flow rate to 8-15L / h, and the solid-liquid ratio to 1:4-1:6, and perform three-stage gradient countercurrent extraction. The extraction time for each extraction is 20-30 minutes. After extraction, drain the surface water to obtain decanoated pretreated coffee beans. Step S3, Enzymatic Directed Flavor Enhancement Conversion: Place the decaffeinated pretreated coffee beans in a constant temperature sealed reactor, spray with a compound enzyme preparation buffer solution to evenly wet the surface of the coffee beans and break the cell walls, adjust the system temperature to 35-42℃ and pH value to 5.0-6.5, and keep the system at a constant temperature for 40-90 minutes to achieve the directional conversion of sulfur-containing aroma precursor substances and obtain flavor-enhanced modified coffee beans. Step S4, passivation and drying treatment: After the enzymatic reaction, the coffee beans are heated to 55-65℃ and kept at a constant temperature for 10-15 minutes to passivate the enzymes. Then, they are subjected to segmented low-temperature drying treatment: the first stage is pre-drying at 38-46℃ for 25-40 minutes, and the second stage is deep drying at 45-60℃ until the moisture content of the coffee beans is ≤13%. Step S5, Low-temperature roasting and ripening: The dried coffee beans are roasted and ripened using a gradient low-temperature roasting process: the first stage is preheating at 120-140℃ for 8-12 minutes, the second stage is medium roasting at 160-175℃ for 10-15 minutes, and the third stage is shaping roasting at 180-190℃ for 3-5 minutes; after ripening and shaping, the beans are cooled and screened to obtain decaffeinated coffee bean products rich in natural thiol aromas.

[0007] Preferably, in step S1, the temperature of the low-temperature mild cell wall breaking treatment is 20-28℃, the cell wall breaking method is low-temperature airflow micro-cell wall breaking, the cell wall breaking pore size is controlled at 0.1-0.3mm, only the coffee bean seed coat and surface cell wall are damaged, while the internal endosperm structure is preserved.

[0008] Preferably, in step S2, the gradient temperatures of the three-stage gradient countercurrent extraction are 30-33℃, 38-40℃, and 43-45℃ respectively, and the extraction pressure of each stage is 0.08-0.12MPa. There is no high temperature or high heat throughout the process, thus preserving the coffee's intrinsic aroma precursor substances.

[0009] Preferably, in step S3, the buffer solution of the compound enzyme preparation is prepared as an acetate-sodium acetate buffer solution with a pH of 5.5. The compound enzyme preparation contains β-glucosidase, cysteine ​​desulfurase, and flavor protease, and the mass ratio of the three is (2-3):(1-2):(0.5-1). During the enzymatic reaction, a low-oxygen closed environment is maintained throughout, with an oxygen content of ≤3%, to avoid the oxidation and loss of thiol aroma substances.

[0010] Preferably, in step S3, the total amount of the compound enzyme preparation added is 0.15%-0.35% of the dry weight of coffee beans, and the enzyme activities are: β-glucosidase ≥10000U / g, cysteine ​​desulfurase ≥8000U / g, and flavor protease ≥5000U / g.

[0011] Preferably, in step S4, the enzyme inactivation is performed using a low-temperature, humid heat inactivation method, with the relative humidity controlled at 45%-55%, to prevent the coffee beans from drying out and cracking, and the aroma from evaporating.

[0012] Preferably, in step S5, the baking temperature does not exceed 190°C throughout the entire baking process.

[0013] A type of low-caffeine coffee bean rich in natural thiol aromas, with a finished coffee bean caffeine content ≤0.08%, a moisture content of 10%-12%, and a total content of thiol characteristic aroma substances ≥350μg / kg.

[0014] Compared with the prior art, the beneficial effects of this invention are as follows: 1. The present invention uses a decaking temperature of ≤45℃ throughout the process, which is much lower than that of traditional processes. This completely avoids the high-temperature damage to heat-sensitive aroma precursors. The three-stage low-temperature countercurrent extraction can stably reduce the caffeine content of coffee beans to below 0.08%, with a decaking rate of ≥92%. The decaking process is gentle and efficient, with very low aroma loss. The decaking effect is better than that of traditional water washing processes.

[0015] 2. Through the synergistic catalysis of specific compound enzyme preparations, a large number of natural thiol characteristic aroma substances are generated in a targeted manner. No artificial flavorings are needed. The thiol aroma content of the finished product is 40%-65% higher than that of traditional decaffeinated coffee, which perfectly makes up for the lack of flavor in decaffeinated coffee. The aroma is rich, mellow and naturally strong.

[0016] 3. This invention uses pure water extraction and bio-enzyme modification throughout the entire process, leaving no organic solvent residue. The low-temperature process has low energy consumption and minimal pollution, meeting green food production standards. The process parameters are highly controllable, making it suitable for large-scale industrial mass production as well as small-batch customized production of high-quality products, thus having wide applicability.

[0017] 4. The decaffeinated coffee beans prepared by this invention have moderate moisture content, uniform color, and no off-flavors. They are characterized by low caffeine content, high natural roasted aroma, nutty aroma, and caramel aroma. They have a mellow and smooth taste, excellent finished product quality, and are suitable for various brewing scenarios, which can meet the market demand for high-end specialty decaffeinated coffee. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the process for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzyme-catalyzed directional conversion, as proposed in this invention. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0020] The coffee beans used in Examples 1-3 and Comparative Examples 1-2 of this invention are Example

[0021] A method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzymatic directional conversion includes the following steps: Step S1: Raw material pretreatment: Select green coffee beans with uniform maturity, free from mold and insect infestation, remove broken particles, rinse surface impurities with clean water, and air dry at room temperature until there is no moisture on the surface; perform low-temperature airflow micro-wall breaking treatment at 25℃, control the wall breaking rate at 20%, and control the wall breaking pore size at 0.2mm to obtain pretreated coffee beans.

[0022] Step S2, Low-Temperature Countercurrent Gradient Dedecanoation: The pretreated coffee beans are put into a low-temperature countercurrent extraction device, using food-grade pure water as the solvent, with a solid-liquid ratio of 1:5 and a countercurrent flow rate of 12L / h. Three-stage gradient countercurrent extraction is performed, with the gradient temperatures of 32℃, 39℃, and 44℃ respectively, each extraction time being 25min, and the extraction pressure being 0.1MPa. After extraction, the surface moisture of the coffee beans is drained, and the caffeine content of the coffee beans is measured to be reduced to 0.075%, resulting in decanoic pretreated coffee beans.

[0023] Step S3, Enzymatic Directed Flavor Enhancement Conversion: Prepare an acetate-sodium acetate buffer solution with a pH of 5.5, and add a compound enzyme preparation with a total mass fraction of 0.25%. The mass ratio of β-glucosidase, cysteine ​​desulfurase, and flavor protease in the compound enzyme is 2.5:1.5:0.8, and the enzyme activities are β-glucosidase 12000 U / g, cysteine ​​desulfurase 10000 U / g, and flavor protease 8000 U / g, respectively. Place the decaffeinated pretreated coffee beans in a constant-temperature sealed reactor, control the oxygen content to ≤3%, and then spray the compound enzyme preparation buffer solution to evenly wet the surface of the coffee beans and the cell wall pores. Adjust the system temperature to 38℃ and keep the enzymatic reaction at a constant temperature for 65 min to achieve the directional conversion of sulfur-containing aroma precursors and obtain flavor-enhanced modified coffee beans.

[0024] Step S4, passivation and drying treatment: The coffee beans after the enzymatic reaction are heated to 60°C and the relative humidity is controlled at 50%. Enzyme inactivation is carried out for 12 minutes under low temperature and humid heat conditions. Then, the beans are dried in stages at low temperature: the first stage is pre-drying at 42°C for 30 minutes, and the second stage is deep drying at 52°C until the moisture content of the coffee beans is 11%.

[0025] Step S5, Low-temperature roasting and ripening: The dried coffee beans are roasted and ripened using a gradient low-temperature roasting process: the first stage is preheated at 130℃ for 10 minutes, the second stage is medium roasted at 170℃ for 12 minutes, and the third stage is shaped and roasted at 185℃ for 4 minutes; after ripening and shaping, the beans are cooled and screened to obtain decaffeinated coffee bean products rich in natural thiol aromas.

[0026] According to the test results, the caffeine content of the finished coffee beans in this embodiment is 0.075%, the total content of thiol aroma substances is 385μg / kg, there is no artificial flavor, and the roasted and nutty aromas are rich and the flavor is layered. Example

[0027] A method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzymatic directional conversion includes the following steps: Step S1: Raw material pretreatment: Select green coffee beans with uniform maturity, free from mold and insect infestation, remove broken particles, rinse the surface impurities with clean water, and air dry at room temperature until there is no moisture on the surface; perform low-temperature airflow micro-wall breaking treatment at 22℃, control the wall breaking rate at 18%, and control the wall breaking pore size at 0.15mm to obtain pretreated coffee beans.

[0028] Step S2, Low-Temperature Countercurrent Gradient Dedecanoation: Pretreated coffee beans are placed in a low-temperature countercurrent extraction device, using food-grade pure water as the solvent, with a solid-liquid ratio of 1:4.5 and a countercurrent flow rate of 10L / h. Three-stage gradient countercurrent extraction is performed, with gradient temperatures of 31℃, 38℃, and 43℃, and an extraction time of 22min for each stage. The extraction pressure is 0.09MPa. After extraction, the surface moisture of the coffee beans is drained, and the caffeine content of the coffee beans is measured to be reduced to 0.078%, resulting in decanoic pretreated coffee beans.

[0029] Step S3, Enzymatic Directed Flavor Enhancement Conversion: Prepare an acetate-sodium acetate buffer solution with a pH of 5.2, and add a compound enzyme preparation with a total mass fraction of 0.20%. The mass ratio of β-glucosidase, cysteine ​​desulfurase, and flavor protease in the compound enzyme is 2.0:1.0:0.5, and the enzyme activities are β-glucosidase 11000 U / g, cysteine ​​desulfurase 9000 U / g, and flavor protease 6000 U / g, respectively. Place the decaffeinated pretreated coffee beans in a constant-temperature, sealed reaction vessel, control the oxygen content to ≤3%, and then spray the compound enzyme preparation buffer solution to evenly wet the surface of the coffee beans and the cell wall pores. Adjust the system temperature to 36℃ and keep the enzymatic reaction at a constant temperature for 70 min to achieve the directional conversion of sulfur-containing aroma precursors and obtain flavor-enhanced modified coffee beans.

[0030] Step S4, passivation and drying treatment: After the enzymatic reaction, the coffee beans are heated to 58°C and the relative humidity is controlled at 46%. Enzyme inactivation is carried out for 15 minutes under low temperature and humid heat conditions. Then, the beans are dried in stages at low temperature: the first stage is pre-drying at 40°C for 30 minutes, and the second stage is deep drying at 50°C until the moisture content of the coffee beans is 10.5%.

[0031] Step S5, Low-temperature roasting and ripening: The dried coffee beans are roasted and ripened using a gradient low-temperature roasting process: the first stage is preheated at 125℃ for 12 minutes, the second stage is medium roasted at 165℃ for 14 minutes, and the third stage is shaped and roasted at 182℃ for 5 minutes; after ripening and shaping, the beans are cooled and screened to obtain decaffeinated coffee bean products rich in natural thiol aromas.

[0032] Testing revealed that the finished coffee beans in this embodiment contained 0.078% caffeine and 362 μg / kg of thiol aroma compounds, exhibiting a rich flavor and long-lasting aroma. Example

[0033] A method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzymatic directional conversion includes the following steps: Step S1: Raw material pretreatment: Select green coffee beans with uniform maturity, free from mold and insect infestation, remove broken particles, rinse the surface impurities with clean water, and air dry at room temperature until there is no moisture on the surface; perform low-temperature airflow micro-wall breaking treatment at 28℃, control the wall breaking rate at 22%, and control the wall breaking pore size at 0.25mm to obtain pretreated coffee beans.

[0034] Step S2, Low-Temperature Countercurrent Gradient Dedecanoation: Pretreated coffee beans are placed in a low-temperature countercurrent extraction device, using food-grade pure water as the solvent, with a solid-liquid ratio of 1:5.5 and a countercurrent flow rate of 14 L / h. Three-stage gradient countercurrent extraction is performed, with gradient temperatures of 33℃, 40℃, and 45℃, and an extraction time of 28 min for each stage. The extraction pressure is 0.11 MPa. After extraction, the surface moisture of the coffee beans is drained, and the caffeine content of the coffee beans is measured to be reduced to 0.072%, resulting in decaffeinated pretreated coffee beans.

[0035] Step S3, Enzymatic Directed Flavor Enhancement Conversion: Prepare an acetate-sodium acetate buffer solution with a pH of 6.0, and add a compound enzyme preparation with a total mass fraction of 0.32%. The mass ratio of β-glucosidase, cysteine ​​desulfurase, and flavor protease in the compound enzyme is 3.0:2.0:1.0, and the enzyme activities are β-glucosidase 10000 U / g, cysteine ​​desulfurase 8000 U / g, and flavor protease 5000 U / g, respectively. Place the decaffeinated pretreated coffee beans in a constant-temperature, sealed reaction vessel, control the oxygen content to ≤3%, and then spray the compound enzyme preparation buffer solution to evenly wet the surface of the coffee beans and the cell wall pores. Adjust the system temperature to 34℃ and keep the enzymatic reaction at a constant temperature for 75 minutes to achieve the directional conversion of sulfur-containing aroma precursors and obtain flavor-enhanced modified coffee beans.

[0036] Step S4, passivation and drying treatment: After enzymatic reaction, the coffee beans are heated to 62°C and the relative humidity is controlled at 45%. Enzyme inactivation is carried out for 10 minutes under low temperature and humid heat conditions. Then, the beans are dried in stages at low temperature: the first stage is pre-drying at 45°C for 30 minutes, and the second stage is deep drying at 55°C until the moisture content of the coffee beans is 11.5%.

[0037] Step S5, Low-temperature roasting and ripening: The dried coffee beans are roasted and ripened using a gradient low-temperature roasting process: the first stage is preheated at 135℃ for 8 minutes, the second stage is medium roasted at 175℃ for 10 minutes, and the third stage is shaped and roasted at 188℃ for 3 minutes; after ripening and shaping, the beans are cooled and screened to obtain decaffeinated coffee bean products rich in natural thiol aromas.

[0038] Tests showed that the finished product contained 0.072% caffeine and 410 μg / kg of thiol aroma compounds. It had a full aroma with prominent caramel and roasted notes.

[0039] Comparative Example 1 (Traditional High-Temperature Water Washing Degenerative Process): Coffee beans from the same batch were selected and dedecenolated using traditional 90℃ high-temperature water immersion for 2 hours, followed by conventional drying and roasting. The finished product has a caffeine content of 0.082%, a total thiol aroma content of 242μg / kg, a thin aroma, a bland flavor, no obvious roasted nutty aroma, and a weak taste.

[0040] Comparative Example 2 (Unenzyme-free flavor enhancement and low-temperature degenerative process): Using the low-temperature countercurrent decaine removal process of the present invention, the enzymatic directional conversion step is omitted, and the remaining parameters are the same as in Example 1. The caffeine content of the finished product is 0.076%, and the total content of thiol aroma substances is 278 μg / kg. The aroma is significantly weaker than that of Example 1, and the layering is lacking.

[0041] Performance comparison of Examples 1-3 and Comparative Examples 1-2: Cause-free mode Low temperature countercurrent gradient Low temperature countercurrent gradient Low temperature countercurrent gradient High temperature boiling (90℃) Low temperature countercurrent gradient Does it have enzymatic flavor enhancement? yes yes yes no no Caffeine content of finished product 0.075% 0.078% 0.072% 0.082% 0.076% Total thiol aroma content (μg / kg) 385 362 410 242 278 Aroma quality evaluation Rich in roasted and nutty aromas, with a strong sense of layering. Rich flavor and long-lasting aroma Full-bodied aroma with prominent caramel notes The aroma is weak and the flavor is bland. The aroma is weak and lacks complexity. Performance comparison analysis: 1. Total amount of thiol aroma (enzymatic aroma enhancement is a key variable that can effectively generate key aroma substances derived from Maillard reaction and lipid oxidation): The total amount of thiol aroma in Examples 1-3 was 362-410 μg / kg, which was much higher than the total amount of thiol aroma in Comparative Example 1 (242 μg / kg) and Comparative Example 2 (278 μg / kg), showing a significant increase in the total amount of thiol aroma.

[0042] 2. The caffeine content of Examples 1-3 is 0.072%-0.078%, which meets the conventional standards for decaffeinated coffee. The low-temperature countercurrent gradient decaffeination method is superior to the traditional high-temperature boiling method in terms of decaffeination efficiency, and the low-temperature condition avoids the destruction of flavor precursors by high temperature.

[0043] 3. At similar caffeine content, the total aroma content of the low-temperature process is about 50% higher, while high-temperature washing will seriously lose volatile aroma. Under the same dedecanoation method, enzymatic treatment increases the total aroma content by about 30%-47%, indicating that enzymatic hydrolysis can release more bound aroma precursors.

[0044] In summary, compared with traditional processes, the finished coffee beans prepared by Examples 1-3 of this invention have a higher decaffeination qualification rate and a significantly increased content of thiol core aroma substances. This solves the problem of aroma loss and flavor defects after decaffeination of traditional decaffeinated coffee. Moreover, the entire process is a low-temperature green process with no safety hazards, and the product quality is far superior to products produced by existing processes.

[0045] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzyme-catalyzed directional conversion, characterized in that, Includes the following steps: Step S1: Raw material pretreatment: Select green coffee beans with uniform maturity, no mold, and no insect infestation. After washing and air drying, perform low-temperature light cell wall breaking treatment, controlling the cell wall breaking rate to 15%-25%, to obtain pretreated coffee beans. Step S2, Low-temperature countercurrent gradient dedecanoation: Place the pretreated coffee beans in a low-temperature countercurrent extraction device, use food-grade pure water as the extraction solvent, set the extraction temperature to 30-45℃, the countercurrent flow rate to 8-15L / h, and the solid-liquid ratio to 1:4-1:6, and perform three-stage gradient countercurrent extraction. The extraction time for each extraction is 20-30 minutes. After extraction, drain the surface water to obtain decanoated pretreated coffee beans. Step S3, Enzymatic Directed Flavor Enhancement Conversion: Place the decaffeinated pretreated coffee beans in a constant temperature sealed reactor, spray with a compound enzyme preparation buffer solution to evenly wet the surface of the coffee beans and break the cell walls, adjust the system temperature to 35-42℃ and pH value to 5.0-6.5, and keep the system at a constant temperature for 40-90 minutes to achieve the directional conversion of sulfur-containing aroma precursor substances and obtain flavor-enhanced modified coffee beans. Step S4, passivation and drying treatment: After the enzymatic reaction, the coffee beans are heated to 55-65℃ and kept at a constant temperature for 10-15 minutes to passivate the enzymes. Then, they are subjected to segmented low-temperature drying treatment: the first stage is pre-drying at 38-46℃ for 25-40 minutes, and the second stage is deep drying at 45-60℃ until the moisture content of the coffee beans is ≤13%. Step S5, Low-temperature roasting and ripening: The dried coffee beans are roasted and ripened using a gradient low-temperature roasting process: the first stage is preheating at 120-140℃ for 8-12 minutes, the second stage is medium roasting at 160-175℃ for 10-15 minutes, and the third stage is shaping roasting at 180-190℃ for 3-5 minutes; after ripening and shaping, the beans are cooled and screened to obtain decaffeinated coffee bean products rich in natural thiol aromas.

2. The method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzyme-catalyzed directional conversion according to claim 1, characterized in that, In step S1, the temperature of the low-temperature mild cell wall breaking treatment is 20-28℃, the cell wall breaking method is low-temperature airflow micro-cell wall breaking, the cell wall breaking pore size is controlled at 0.1-0.3mm, only the coffee bean seed coat and surface cell wall are damaged, while the internal endosperm structure is preserved.

3. The method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzyme-catalyzed directional conversion according to claim 1, characterized in that, In step S2, the gradient temperatures of the three-stage gradient countercurrent extraction are 30-33℃, 38-40℃, and 43-45℃ respectively, and the extraction pressure of each stage is 0.08-0.12MPa. There is no high temperature or high heat throughout the process, which preserves the coffee's endogenous aroma precursor substances.

4. The method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzyme-catalyzed directional conversion according to claim 1, characterized in that, In step S3, the buffer solution of the compound enzyme preparation is prepared with an acetate-sodium acetate buffer solution at pH 5.

5. The compound enzyme preparation contains β-glucosidase, cysteine ​​desulfurase, and flavor protease, with a mass ratio of (2-3):(1-2):(0.5-1). During the enzymatic reaction, a low-oxygen, closed environment is maintained throughout, with an oxygen content ≤3%, to prevent the oxidation and loss of thiol aroma substances.

5. The method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzyme-catalyzed directional conversion according to claim 1, characterized in that, In step S3, the total amount of the compound enzyme preparation added is 0.15%-0.35% of the dry weight of coffee beans, and the enzyme activities are: β-glucosidase ≥10000U / g, cysteine ​​desulfurase ≥8000U / g, and flavor protease ≥5000U / g.

6. The method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzyme-catalyzed directional conversion according to claim 1, characterized in that, In step S4, enzyme inactivation is performed using a low-temperature, humid heat inactivation method, with relative humidity controlled at 45%-55% to prevent coffee beans from drying out and cracking, and aroma loss.

7. The method for preparing decaffeinated coffee beans rich in natural thiol aroma through low-temperature countercurrent extraction coupled with enzyme-catalyzed directional conversion according to claim 1, characterized in that, In step S5, the baking temperature shall not exceed 190°C throughout the entire baking process.

8. The decaffeinated coffee beans rich in natural thiol aroma prepared by the method according to any one of claims 1-7, characterized in that, The finished coffee beans have a caffeine content of ≤0.08%, a moisture content of 10%-12%, and a total content of thiol-like characteristic aroma substances of ≥350μg / kg.

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