Method for reducing lignin content of tobacco stems, tobacco stem product with low lignin content and application of tobacco stem product

Abstract

The invention provides a method for reducing the lignin content of tobacco stems, a low-lignin-content tobacco stem product and application of the low-lignin-content tobacco stem product, and belongs to the technical field of tobacco processing. The method for reducing the lignin content of the tobacco stems comprises the following steps: in a normal-pressure environment, mixing a tobacco stem raw material with water, and carrying out hydrothermal pretreatment at 70-120 DEG C to obtain hydrothermal pretreated tobacco stems; the deep-eutectic solvent and the hydrothermal pretreatment tobacco stems are mixed and subjected to a heating reaction, and the deep-eutectic solvent is formed by mixing a hydrogen bond donor and a hydrogen bond acceptor.

Description

Technical Field

[0001] The present invention belongs to the technical field of tobacco processing, and in particular relates to a method for reducing the lignin content of tobacco stems, a tobacco stem product with low lignin content and applications thereof. Background Art

[0002] Tar reduction and harm reduction have become essential development priorities for the tobacco industry. Tobacco stems, a key byproduct of tobacco processing, account for approximately 25% of tobacco leaves and are a key raw material for cutting stems and tobacco flakes. However, the rich lignin content in tobacco stems has become a key obstacle to their efficient utilization. Tobacco stem lignin is the primary aromatic component in tobacco leaves and a significant source of harmful tar compounds. Pyrolysis produces harmful substances such as catechol, nicotine, and carbon monoxide (CO). These substances not only mask the aroma of the tobacco itself, diminishing the flavor of cigarettes, but also pose a potential threat to smokers' health. Therefore, developing methods to reduce tobacco stem lignin content that combine efficient removal, structural protection, and green economics is of great significance for increasing the value of tobacco stems, improving cigarette quality, and promoting energy conservation and emission reduction in the tobacco industry. Summary of the Invention

[0003] To address the above technical issues, the present invention provides a method for reducing the lignin content of tobacco stems, a tobacco stem product with low lignin content, and its application, in order to at least partially resolve the above technical issues. The technical solutions provided by the present invention are as follows.

[0004] As a first aspect of the present invention, a method for reducing the lignin content of tobacco stems is provided, comprising: mixing tobacco stem raw materials with water under normal pressure and performing hydrothermal pretreatment at 70-100°C to obtain hydrothermally pretreated tobacco stems; mixing a low eutectic solvent and the hydrothermally pretreated tobacco stems and performing a heating reaction, wherein the low eutectic solvent is a mixture of a hydrogen bond donor and a hydrogen bond acceptor.

[0005] As a second aspect of the present invention, a tobacco stem product with low lignin content prepared by the above method is provided.

[0006] As a third aspect of the present invention, there is provided a use of a tobacco stem product with a low lignin content in a cigarette product.

[0007] Based on the above technical solution, the present invention provides a method for reducing the lignin content of tobacco stems, a tobacco stem product with low lignin content, and its application, which have at least one of the following beneficial effects:

[0008] (1) In the technical solution of the present invention, the soluble substances in the tobacco stem raw material are first dissolved by low-temperature (70-100°C) hydrothermal pretreatment, thereby destroying the tight network structure of the lignin-carbohydrate complex and fully exposing the lignin. Then, the low eutectic solvent and the hydrothermally pretreated tobacco stems are mixed and heated. The strong hydrogen bond network structure formed by the hydrogen bond acceptor (HBA) and the hydrogen bond donor (HBD) in the low eutectic solvent (DES) is utilized to destroy the ether bonds and hydrogen bonds of the tobacco stem lignin itself, thereby effectively improving the lignin removal rate and reducing the lignin content in the tobacco stems. Compared with single hydrothermal pretreatment or single deep eutectic solvent (DES) treatment, the lignin removal rate in the tobacco stems can be significantly improved. Even without pulverization treatment, the tobacco stem lignin removal rate can be guaranteed to exceed 35%, solving the dependence of the existing technology on pulverization degree while ensuring the filling capacity of subsequent processing.

[0009] (2) In the technical solution of the present invention, low-temperature hydrothermal pretreatment is used to avoid irreversible damage to plant cell walls caused by high temperatures (120-200°C). The use of a normal pressure reaction system can effectively reduce equipment investment costs and eliminate the need for high-pressure or special reaction vessels. By optimizing reaction parameters, the treatment can be completed under mild conditions, significantly reducing energy consumption and safety risks.

[0010] (3) In the technical solution of the present invention, the tobacco stem product with low lignin content prepared by the above method is applied to the field of cigarettes, which can effectively reduce the manufacturing cost of cigarettes, as well as reduce the content of tumorigenic active substances and tar in mainstream smoke condensates, and improve the smoke index and sensory quality of tobacco sheet cigarette products after delignining. It also maintains high stability after multiple cycle tests, indicating that it has high accuracy and reproducibility in actual pretreatment experiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 X-ray diffraction (XRD) patterns of tobacco stem particles obtained in Example 1 and Comparative Examples 1 to 3 of the present invention;

[0012] Figure 2 Fourier transform infrared spectroscopy (FT-IR) diagrams of the structures of tobacco stem lignin extracted from Examples 1, 2, 6 of the present invention and Comparative Examples 3 and 5;

[0013] Figure 3 This is a graph showing the test results of recycling the deep eutectic solvent in Example 1 of the present invention. DETAILED DESCRIPTION

[0014] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present invention. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of embodiments of the present invention. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of the concept of the present invention.

[0015] Before implementing the technical solution of the present invention, it was found that the current pretreatment methods for tobacco stems include: acid, alkali, organic solvents, ionic liquids and biological enzymatic hydrolysis, but there are problems such as poor treatment effect, high energy consumption, high cost, easy pollution of the environment and equipment corrosion. Its feasibility is limited and is not conducive to the development of the green economy.

[0016] For example, related technologies use microwave ovens to heat cut tobacco stems and then treat them with complex enzymes to reduce the lignin content in the stems. The resulting low-lignin tobacco products have improved odor, strength, pungency, and aftertaste. However, this method is complex and costly, and the enzymatic hydrolysis reaction time is long. Furthermore, natural enzymes are easily inactivated during microwave treatment, making large-scale production difficult.

[0017] Another example: Deep eutectic solvents (DES), which are composed of a mixture of natural chemicals such as amino acids, alcohols, sugars and quaternary ammonium salts, are a new type of solvent system. They have the characteristics of good solubility, high stability and adjustable strength. They also have the advantages of low price, simple operation, non-toxicity and recyclability. They are widely used in metal processing, lignocellulosic biomass pretreatment, chemical reaction synthesis and other fields. However, single deep eutectic solvents have poor applicability to dense tobacco stems, and there are problems such as low lignin removal efficiency and long processing time. In addition, the penetration resistance caused by the lignin-carbohydrate complex (LCC) barrier is not resolved.

[0018] Another example: Tobacco stems are mixed with glycerol and potassium hydroxide, heated for reaction, and then cooled. The resulting cooled product is stirred with a glycerol-water solution, and the insoluble matter in the stirred product is filtered and collected. The insoluble matter is then washed to neutrality and air-dried to obtain a low-lignin tobacco stem product. However, this method heats the tobacco stems at excessively high temperatures (e.g., above 120°C), which can damage the stem cell walls to a certain extent. This prevents the stems from forming a porous structure during subsequent expansion, resulting in reduced filling power and lower tobacco processing efficiency. Furthermore, the smoke produced by burning cigarettes can have a strong alkaline flavor, significantly impacting the sensory quality of the cigarettes.

[0019] Another example is a method for synergistically reducing the lignin content in tobacco stems using a microwave-assisted deep eutectic solvent. This involves mixing a hydrogen bond donor and a hydrogen bond acceptor in a suitable proportion and stirring in a water bath to produce a green deep eutectic solvent. Subsequently, the tobacco stems are shredded, mixed with the deep eutectic solvent, and then subjected to microwave-assisted oil bath heating to produce treated tobacco stems. However, this method requires the purchase of a microwave reactor, which is relatively expensive and only suitable for laboratory-scale operation, which to some extent restricts its application in large-scale industrial production. During the hydrothermal pretreatment process, a large number of heat-soluble non-lignin components are preferentially dissolved, significantly increasing the relative content of lignin in the remaining solids. However, the current high temperature of hydrothermal pretreatment alone (160-180°C) may cause the dissolved lignin to undergo structural recombination in the high-temperature water environment, failing to effectively remove lignin from the tobacco stems. As a result, the physical properties and sensory quality of the hydrothermally treated tobacco stems not only fail to improve, but may even deteriorate, failing to meet the quality requirements for use as a raw material for tobacco sheet or reconstituted tobacco leaf.

[0020] In response to the problems of poor treatment effect, high energy consumption, high cost, easy environmental pollution and equipment corrosion in the existing tobacco stem pretreatment technology, the present invention provides a step-by-step low-temperature hydrothermal pretreatment-low eutectic solvent coordinated treatment method. The tobacco stems are first pretreated by low-temperature hydrothermal treatment to gently destroy the tight network structure of lignocellulose in the tobacco stems, so that the lignin is partially exposed and loosened, while keeping the cellulose skeleton intact, and at the same time reducing the content of phenolic substances (substances produced when the tobacco stems are burned) in the tobacco stems; then, the ether bonds and hydrogen bonds in the exposed lignin molecules are selectively destroyed and dissolved by a low eutectic solvent, further removing the lignin in the tobacco stems, thereby improving the sensory quality of the tobacco sheet cigarette products after the lignin is removed.

[0021] As a first aspect of the present invention, a method for reducing the lignin content in tobacco stems is provided, comprising: steps A1 and A2.

[0022] Step A1: Under normal pressure, the tobacco stem raw material is mixed with water and subjected to hydrothermal pretreatment at 70-100° C. to obtain hydrothermally pretreated tobacco stems.

[0023] Step A2: mixing a low eutectic solvent and hydrothermally pretreated tobacco stems and performing a heating reaction, wherein the low eutectic solvent is formed by mixing a hydrogen bond donor and a hydrogen bond acceptor.

[0024] In an embodiment of the present invention, low-temperature (70-100°C) hydrothermal pretreatment is first performed to dissolve soluble substances (such as phenols) in the tobacco stem raw material, disrupting the tight network structure of the lignin-carbohydrate complex and fully exposing the lignin. A deep eutectic solvent and the hydrothermally pretreated tobacco stems are then mixed and heated. The strong hydrogen bond network formed by the hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) in the deep eutectic solvent (DES) disrupts the ether bonds and hydrogen bonds in the tobacco stem lignin, effectively increasing the lignin removal rate and reducing the lignin content in the tobacco stems. Compared to hydrothermal pretreatment alone or deep eutectic solvent (DES) treatment alone, this method significantly improves the lignin removal rate in tobacco stems. Even without pulverization, the tobacco stem lignin removal rate can exceed 35%, eliminating the reliance on pulverization degree in existing technologies while ensuring filling capacity for subsequent processing. Furthermore, the present method for reducing the lignin content in tobacco stems is highly efficient, inexpensive, environmentally friendly, non-toxic, and low-cost.

[0025] According to an embodiment of the present invention, in step A1, the tobacco stem raw material is a tobacco stem raw material that has been dried and crushed into 3-5cm strips and passed through a 5-60 mesh sieve. For example: the mesh size that can be sieved is 5-10 mesh, 10-20 mesh, 20-40 mesh, and 40-60 mesh, and the raw material source of tobacco stems is wide and easy to obtain, and the waste biomass resource of tobacco stems can be efficiently converted and reused, which is not only economical and efficient, but also has significant environmental benefits, which is in line with the sustainable development concept advocated by green chemistry. In addition, the technical solution of the present invention can also be adopted without grinding or sieving the tobacco stem raw material to achieve the reduction of the lignin content in the tobacco stems, but it is preferred to crush the tobacco stems to improve the lignin removal effect. In the present invention, the tobacco stem raw material and water are mixed in a mass ratio of 1:10-20, and a hydrothermal reaction is carried out at 70-100°C, wherein the hydrothermal pretreatment time is 0.5-1h. For example, at atmospheric pressure, washed and dried tobacco stems are mixed with distilled water in a reactor (such as a round-bottom flask or pressure flask, with the reaction performed at atmospheric pressure) at a mass ratio of 1:10. The mixture is then hydrothermally reacted at 70°C for 1 hour. After cooling to room temperature, the stems are rinsed with distilled water and then dried in an oven for later use. Low-temperature hydrothermal pretreatment gently removes water-soluble substances from the stems and gently disrupts the tight network of lignocellulose within them, exposing and loosening the lignin while preserving the integrity of the cellulose skeleton.

[0026] According to an embodiment of the present invention, in step A2, the hydrogen bond donors of the present invention are lactic acid (LA) and ethylene glycol (EG); the hydrogen bond acceptor is choline chloride (ChCl), and lactic acid, ethylene glycol, and choline chloride are mixed in a molar ratio of 1:5:0.5-1 (preferably 1:5:0.6) to form a deep eutectic solvent (DES). The prepared deep eutectic solvent (DES) can be expressed as [Ch][LA]5[EG] 0.6 Specifically, the preparation of the deep eutectic solvent includes: mixing lactic acid, ethylene glycol, and choline chloride in a molar ratio of 1:5:0.6, heating and stirring in a magnetic stirring oil bath at 80°C and 300 rpm until a homogeneous and transparent solution is formed, and then storing the mixture in a silica desiccator after cooling.

[0027] According to an embodiment of the present invention, in step A2, hydrothermally pretreated tobacco stems are mixed with a deep eutectic solvent at a mass ratio of 1:10-20; after uniform mixing, the mixture is heated at 90-120°C for a reaction of 2-6 hours, so as to utilize the strong hydrogen bond network structure formed by the hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) in the deep eutectic solvent to selectively destroy and dissolve the ether bonds and hydrogen bonds in the exposed lignin molecules. The low-temperature hydrothermal pretreatment and the synergistic dissolution of the deep eutectic solvent form a step-by-step synergistic mechanism of "physical loosening-chemical dissolution", thereby achieving efficient lignin removal while reducing damage to the main structure of the tobacco stems. Specifically, the low-temperature hydrothermal pretreatment-deep eutectic solvent synergistic pretreatment of tobacco stems includes: placing the hydrothermally pretreated dried tobacco stems and the deep eutectic solvent at a mass ratio of 1:20 in a stoppered round-bottom flask, mixing them uniformly, and then heating them at different temperatures (e.g., 90-120°C) and times (e.g., 2-6 hours).

[0028] According to an embodiment of the present invention, after step A2, the method for reducing tobacco stem lignin content further includes adding a quencher and stirring to terminate the reaction, followed by vacuum filtration to obtain a solid residue and a filtrate. The quencher is anhydrous ethanol, and the amount of quencher added is the same as the amount of the deep eutectic solvent, i.e., anhydrous ethanol:DES (v:m) = 1:1. The quencher is added and stirred to terminate the reaction for 2 hours. Specifically, after the heating reaction in step A2 is completed, the mixture is cooled to room temperature, and anhydrous ethanol equal to the amount of the deep eutectic solvent is added and stirred for another 2 hours to terminate the reaction. After stirring is completed, the mixture is vacuum filtered through a G3 sand core funnel to obtain a solid residue and a filtrate. Using anhydrous ethanol as a quencher facilitates terminating the reaction and reduces the solvent viscosity during rotary evaporation, allowing for better fusion of DES and anhydrous ethanol and better separation of tobacco stem residue from DES. In addition, during the rotary evaporation process, since anhydrous ethanol has a lower boiling point, the rotary evaporation process will evaporate all the anhydrous ethanol and retain the DES solvent, which has little impact on the recovery of the low eutectic solvent, and the recovered low eutectic solvent can be recycled.

[0029] According to an embodiment of the present invention, the solid residue is rinsed with a mixed solvent of ethanol and water until neutralized and the rinse liquid is retained; the volume ratio of ethanol to water is 1:9. Subsequently, the washed tobacco stems are placed in an oven for drying, thereby obtaining a tobacco stem product that has a low lignin content through a step-by-step low-temperature hydrothermal pretreatment and deep eutectic solvent co-treatment.

[0030] According to an embodiment of the present invention, the rinse liquid and the filtrate are mixed, excess water is added, stirred, allowed to stand, and separated to obtain a lignin precipitate and a supernatant; wherein the amount of excess water added is 10-20 times the volume of the deep eutectic solvent; the standing time is 24-48 hours; the separation speed is 9000-10000 rpm, and the running time is 8-12 minutes.

[0031] According to an embodiment of the present invention, the obtained supernatant is concentrated by rotary evaporation, centrifugal washing, filtration, and freeze-drying to obtain an absolutely dry regenerated lignin, wherein the temperature of the rotary evaporation is 40-50°C; the concentrate of the rotary evaporation is a recovered low eutectic solvent, and the recovered low eutectic solvent is recycled; the freeze-drying time is 24h-48h. Specifically, the supernatant is used to evaporate the ethanol and water in the solvent using a rotary evaporator to obtain a concentrate, the precipitate is washed by centrifugation, and then vacuum filtered using a 0.45μm organic membrane to obtain a clean lignin sample, which is placed in a refrigerator and frozen, and then dried in a freeze dryer to obtain an absolutely dry regenerated lignin sample.

[0032] In summary, the method of the present invention of step-by-step low-temperature hydrothermal pretreatment-deep eutectic solvent synergistically reducing the lignin content in tobacco stems has high stability, strong adjustability, simple operation, and the low eutectic solvent can be recycled, and can process tobacco stems of different particle sizes.

[0033] As a second aspect of the present invention, a tobacco stem product with low lignin content prepared by the above method is provided.

[0034] As a third aspect of the present invention, provided is the use of the tobacco stem product with low lignin content as described above in cigarette products.

[0035] In an embodiment of the present invention, a method for synergistically reducing the lignin content of tobacco stems by using a step-by-step low-temperature hydrothermal pretreatment and a low-eutectic solvent is provided. The method is applied to tobacco processing technology, has good tobacco stem lignin removal efficiency, can reduce cigarette manufacturing costs, and reduce the content of tumorigenic active substances and tar in mainstream smoke condensates, improve the smoke indicators and sensory quality of tobacco sheet cigarette products after lignin removal, maintain high stability after multiple cycle tests, and have high accuracy and reproducibility in actual pretreatment experiments.

[0036] The present invention is further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments. Obviously, the embodiments described below are only a part of the embodiments, and all other embodiments obtained by professionals in this field without creative work are still within the scope of protection of the present invention.

[0037] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0038] The method of the present invention for synergistically reducing the lignin content of tobacco stems by using a step-by-step low-temperature hydrothermal pretreatment and a deep eutectic solvent comprises two stages: low-temperature hydrothermal pretreatment of tobacco stems and deep eutectic solvent treatment.

[0039] The present invention determines the moisture content of tobacco stems with reference to the tobacco industry standard of the People's Republic of China (YC / T 31-1996 oven method) and the standard lignocellulose composition analysis method provided by the National Renewable Energy Laboratory (NREL) of the United States. The cellulose and hemicellulose contents in the tobacco stems are determined by a two-step acid hydrolysis method and high-performance liquid chromatography analysis. The lignin content in the tobacco stems is determined with reference to the agricultural industry standard NY / T3494-2019. The acid-soluble lignin and acid-insoluble lignin contents are determined by ultraviolet absorbance and gravimetry, respectively, and the total lignin content is then calculated. The tobacco stem recovery rate and lignin removal rate after pretreatment are calculated according to the following formulas (1) and (2), respectively, where the lignin content in the raw material is 13.31% as referenced in Control Example 1.

[0040] (1)

[0041] (2)

[0042] The following examples further illustrate the method of synergistically reducing the lignin content of tobacco stems by step-by-step low-temperature hydrothermal pretreatment-deep eutectic solvent treatment.

[0043] Example 1

[0044] A method for synergistically reducing tobacco stem lignin content by step-by-step low-temperature hydrothermal pretreatment and deep eutectic solvent comprises the following steps ① to ④.

[0045] Step 1: Pre-dry the tobacco stems, crush them, and pass them through a 40-60 mesh sieve. The moisture content of the tobacco stems is 13%. Add 20 g of the washed and dried tobacco stem granules to 200 mL of distilled water and stir at 70°C for 30 minutes. After cooling, filter with filter paper and wash with water until the filtrate is colorless. Dry the washed tobacco stems in a 60°C oven for 12 hours to obtain hydrothermally pretreated tobacco stems.

[0046] Step 2: 2 g of the dried hydrothermally pretreated tobacco stems obtained in step 1 were added to 30 g of a deep eutectic solvent consisting of ChCl, LA, and EG (molar ratio of 1:5:0.6), mixed evenly in a 100 mL stoppered round-bottom flask, and heated and stirred at 100°C and 300 rpm for 4 h.

[0047] Step 3: After the reaction is complete and the mixture is cooled to room temperature, 30 mL of anhydrous ethanol is added to the round-bottom flask and stirring is continued for 2 hours. After stirring, the mixture is vacuum filtered through a G3 fritted funnel to obtain a solid residue and a filtrate. The solid residue is washed with an ethanol-water mixture (v:v = 1:9). The filtrate is pH-tested with a pH test paper until it shows neutrality, indicating washing is complete. The rinse solution is retained. The washed tobacco stems are dried in a 60°C oven to complete the step-by-step low-temperature hydrothermal-deep eutectic solvent synergistic pretreatment of the tobacco stems.

[0048] Step 4: After mixing the filtrate and rinse liquid obtained in step 3, add excess distilled water to a total volume of 600 mL. Stir magnetically for 30 minutes and let stand to obtain a lignin precipitate. Use a 40°C rotary evaporator to evaporate the ethanol and water in the solvent from the supernatant to obtain a concentrate. The precipitate is centrifuged at 9500 rpm for 10 minutes and then vacuum filtered using a 0.45 μm organic membrane to obtain a clean lignin sample. After freezing in a refrigerator, dry it in a freeze dryer for 24 hours to obtain a tobacco stem product with low lignin content.

[0049] The low lignin content tobacco stem product prepared in Example 1 was tested. The test results showed that the tobacco stem particles (40-60 mesh) were subjected to the low-temperature hydrothermal (70°C) pretreatment of the present invention with a deep eutectic solvent (HTP) 70 -[Ch][LA]5[EG] 0.6 ) After treatment, the lignin removal rate was 70.16±1.16%.

[0050] Example 2

[0051] Example 2 uses the same method as in Example 1 to prepare a tobacco stem product with a low lignin content, except that: in step ①, the particle size of the tobacco stem after crushing is 20~40 mesh, and the other steps and parameters are the same as in Example 1.

[0052] The low lignin content tobacco stem product prepared in Example 2 was tested. The test results showed that the tobacco stem particles (20-40 mesh) were subjected to the hydrothermal (70°C) pretreatment of the present invention with a deep eutectic solvent (HTP) 70 -[Ch][LA]5[EG] 0.6 ) After treatment, the lignin removal rate was 66.75±0.44%.

[0053] Example 3

[0054] Example 3 uses the same method as in Example 1 to prepare a tobacco stem product with a low lignin content, except that: in step ①, the particle size of the tobacco stem after crushing is 10~20 mesh, and the other steps and parameters are the same as in Example 1.

[0055] The low lignin content tobacco stem product prepared in Example 3 was tested. The test results showed that the tobacco stem particles (10-20 mesh) were subjected to the low-temperature hydrothermal (70°C) pretreatment of the present invention with a deep eutectic solvent (HTP) 70 -[Ch][LA]5[EG] 0.6 ) After treatment, the lignin removal rate was 52.65±1.00%.

[0056] Example 4

[0057] Example 4 uses the same method as in Example 1 to prepare a tobacco stem product with a low lignin content, except that: in step ①, the tobacco stem particles are 5 to 10 mesh in size after crushing, and the other steps and parameters are the same as in Example 1.

[0058] The low lignin content tobacco stem product prepared in Example 4 was tested. The test results showed that the tobacco stem particles (5-10 mesh) were pretreated with low-temperature hydrothermal (70°C) pretreatment-deep eutectic solvent (HTP) of the present invention. 70 -[Ch][LA]5[EG] 0.6) After treatment, the lignin removal rate was 42.22±1.28%.

[0059] Example 5

[0060] Example 5 uses the same method as in Example 1 to prepare a tobacco stem product with a low lignin content, except that: in step ①, the tobacco stems are not crushed, but cut into lengths of 3 to 5 cm. The other steps and parameters are the same as in Example 1.

[0061] The low lignin content tobacco stem product prepared in Example 5 was tested. The test results showed that the tobacco stem (3-5 cm strips) was subjected to the low-temperature hydrothermal (70°C) pretreatment of the present invention with a deep eutectic solvent (HTP) 70 -[Ch][LA]5[EG] 0.6 ) After treatment, the lignin removal rate was 36.75±1.65%.

[0062] Example 6

[0063] Example 6 uses the same method as in Example 1 to prepare a tobacco stem product with a low lignin content, with the following differences: in step ①, 20 g of the washed and dried tobacco stems are added to 200 mL of distilled water, and the reaction is carried out at 100°C for 1 h (the reactor is a round-bottom flask, the reaction is a normal pressure reaction, and the heating is carried out in an oil bath). After cooling to room temperature, the product is filtered with filter paper and washed with water until the filtrate is colorless. The washed tobacco stems are placed in an oven at 60°C and dried for 12 h to obtain hydrothermally pretreated tobacco stems. The other steps and parameters are the same as in Example 1.

[0064] The low lignin content tobacco stem product prepared in Example 6 was tested. The test results showed that the tobacco stem particles (40-60 mesh) were subjected to the low-temperature hydrothermal (100°C) pretreatment of the present invention - deep eutectic solvent (HTP) 100 -[Ch][LA]5EG] 0.6 ) After treatment, the lignin removal rate was 76.27±0.73%.

[0065] Comparative Example 1

[0066] The method for reducing the lignin content of tobacco stems in this comparative example 1 includes the following steps ① to ③.

[0067] Step ①: Pre-dry the tobacco stems, crush them, and pass them through a 40-60 mesh sieve, wherein the moisture content of the tobacco stems is 13%.

[0068] Step ②: Take 10g of washed and dried tobacco stems and place them in a round-bottom flask. Stir with 100mL of stoppered distilled water at room temperature for 30min. After stirring, filter the washed tobacco stem particles with filter paper and rinse the tobacco stems with distilled water until the filtrate is colorless.

[0069] Step ③: Dry the washed tobacco stem particles in an oven at 60°C for 12 hours to obtain washed and dried tobacco stems.

[0070] Comparative Example 2

[0071] The method for reducing the lignin content of tobacco stems in this comparative example 2 includes the following steps ① to ②.

[0072] Step ①: Pre-dry the tobacco stems, crush them, and pass them through a 40-60 mesh sieve, wherein the moisture content of the tobacco stems is 13%.

[0073] Step ②: Take 20 g of washed and dried tobacco stem particles and add them to 200 mL of distilled water. React in a round-bottom flask at 100°C (normal pressure) for 1 hour. After cooling, filter with filter paper and wash with water until the filtrate is colorless. Place the washed tobacco stem residue in a 60°C oven and dry it for 12 hours to obtain dry hydrothermally pretreated tobacco stems.

[0074] The hydrothermal pre-treated tobacco stems prepared in control example 2 were tested. The test results showed that tobacco stem particles (40-60 mesh) were hydrothermally pre-treated (HTP) 100 ) After pretreatment, the lignin removal rate was 16.46±0.74%.

[0075] Comparative Example 3

[0076] The method for reducing the lignin content of tobacco stems in this comparative example 3 comprises the following steps ① to ④:

[0077] Step 1: Pre-dry the tobacco stems, crush them, and pass them through a 40-60 mesh sieve, wherein the moisture content of the tobacco stems is 13%.

[0078] Step 2: Add 2 g of washed and dried tobacco stem particles to 30 g of a deep eutectic solvent consisting of ChCl and LA and EG (molar ratio of 1:5:0.6) in a 100 mL stoppered round-bottom flask, mix thoroughly, and heat and stir at 100°C and 300 rpm for 4 h.

[0079] Step 3: After the reaction is complete and the mixture is cooled to room temperature, 30 mL of anhydrous ethanol is added to the round-bottom flask and stirring is continued for 2 hours. After stirring, the mixture is vacuum filtered through a G3 fritted funnel to obtain a solid residue and a filtrate. The solid residue is washed with an ethanol-water mixture (v:v = 1:9). The filtrate is pH-tested with a pH test paper until it shows neutrality, indicating washing is complete. The rinse solution is retained. The washed tobacco stems are dried in a 60°C oven to obtain single DES-pretreated tobacco stems.

[0080] Step 4: After mixing the filtrate and rinse liquid obtained in step 3, add excess distilled water to a total volume of 600 mL. After magnetic stirring for 30 minutes, let the mixture stand to obtain a lignin precipitate. The ethanol and water in the solvent were evaporated from the supernatant using a 40°C rotary evaporator to obtain a concentrate. The precipitate was washed by centrifugation at 9500 rpm for 10 minutes and then vacuum filtered using a 0.45 μm organic membrane to obtain a clean lignin sample. After freezing in a refrigerator, it was dried in a freeze dryer for 24 hours to obtain tobacco stem lignin.

[0081] The tobacco stems obtained by treating with the deep eutectic solvent in control example 3 were tested. The test results showed that the tobacco stem particles (40-60 mesh) were treated with a single DES ([Ch][LA]5EG] 0.6 ) After treatment, the lignin removal rate was 57.71±1.8%.

[0082] Comparative Example 4

[0083] The method for reducing the lignin content of tobacco stems in this comparative example 4 comprises the following steps ① to ②:

[0084] Step 1: Pre-dry the tobacco stems, crush them, and pass them through a 20-40 mesh sieve, wherein the moisture content of the tobacco stems is 13%.

[0085] Step ②: Take 20 g of washed and dried tobacco stem particles and add them to 200 mL of distilled water. React in a high-pressure sterilizer at 120°C for 1 hour. After cooling, filter with filter paper and wash with water until the filtrate is colorless. Place the washed tobacco stem residue in a 60°C oven and dry it for 12 hours to obtain dry hydrothermally pretreated tobacco stems.

[0086] The hydrothermal pre-treated tobacco stems prepared in control example 4 were tested. The test results showed that tobacco stem particles (20-40 mesh) were hydrothermally pre-treated (HTP) 120 ) After pretreatment, the lignin content was 17.37±0.17%, and the lignin content increased instead.

[0087] Comparative Example 5

[0088] Control Example 5 uses the same method as in Example 2 to prepare a tobacco stem product with a low lignin content, with the following differences: in step ①, the particle size of the tobacco stem after crushing is 20~40 mesh, 20 g of the washed and dried tobacco stems are added to 200 mL of distilled water, and reacted at 120°C for 1 hour (the reactor is a high-pressure sterilizer, and the reaction is a normal pressure reaction), cooled to room temperature, filtered with filter paper, washed with water until the filtrate is colorless, and the washed tobacco stems are placed in a 60°C oven and dried for 12 hours to obtain hydrothermally pretreated tobacco stems. The other steps and parameters are the same as in Example 2.

[0089] The low lignin content tobacco stem product prepared in control example 5 was tested. The test results showed that the tobacco stem particles (20-40 mesh) were subjected to the low temperature hydrothermal (120°C) pretreatment of the present invention with a deep eutectic solvent (HTP) 120 -[Ch][LA]5EG] 0.6 ) After treatment, the lignin removal rate was 56.61±0.56%.

[0090] The lignin content, tobacco stem recovery rate and lignin removal rate of the tobacco stems in the above examples and control examples are shown in Table 1 below.

[0091] Table 1

[0092]

[0093] As shown in Table 1, the problem with using a single hydrothermal pretreatment is that it cannot effectively remove lignin from tobacco stems (for example, the lignin content obtained by single hydrothermal treatment at 100°C is 12.1±0.2%, the lignin removal rate is 16.46±0.74%, and the relative lignin removal rate is 1.21%), and only a portion of water-soluble substances can be removed. Compared with the single DES method (Control Example 3, the lignin content is 8.67±0.23%, the lignin removal rate is 57.71%, and the relative lignin removal rate is 4.64%). The present invention significantly reduces the lignin content in tobacco stems after the step-by-step low-temperature hydrothermal pretreatment and deep eutectic solvent co-treatment, with the highest lignin removal rate reaching 76.27% and the relative lignin removal rate reaching 7.66%. This shows that the method for reducing the lignin content in tobacco stems provided by the present invention can improve the efficiency of tobacco stem lignin removal by 10-20% (an increase of more than 15%). Compared to the hydrothermal method alone (lignin removal rate of 16.46%), this represents a nearly 60% increase. Even without pulverization, the tobacco stem lignin removal rate can exceed 35%, eliminating the existing technology's reliance on pulverization while ensuring sufficient filling capacity for subsequent processing. Furthermore, tobacco stems, as a unique biomass raw material, are rich in hot-water-soluble substances such as pectin and starch. Experiments have found that the hydrothermal treatment conditions that are effective for lignin removal efficiency are in the temperature range of 70~100℃. When the hydrothermal temperature is ≥120℃, on the one hand, the hydrothermal conditions will cause a large amount of hot water-soluble substances in the tobacco stems to dissolve out, resulting in the relative content of tobacco stem lignin increasing instead of decreasing (such as in Control Example 4, the lignin content increased from 13.31% to 17.37% after hydrothermal treatment at 120℃; or Control Example 5); on the other hand, the DES treatment at 120℃ is close to the boiling point of DES (the boiling point of lactic acid is 122℃), which will inhibit the removal of lignin. The degradation of lignin and the recombination of lignin degradation products (intermediates) are a set of competing reactions. In an acidic environment and high temperature (≥120℃) reaction conditions, too long a reaction time may promote the condensation and re-precipitation of the hydrolyzed lignin, and the lignin content will increase (such as in Control Example 5). Therefore, high-temperature (≥120°C) hydrothermal pretreatment technology alone is not only unable to effectively remove lignin, but the removal of water-soluble substances leads to the concentration of lignin, which increases the difficulty of subsequent treatment.

[0094] Furthermore, the tobacco stem particles obtained in Example 1 and Comparative Examples 1 to 3 were subjected to X-ray diffraction (XRD) analysis, and the crystallinity was calculated. The calculation results are as follows: Figure 1 shown.

[0095] Figure 1 1 and 2 are X-ray diffraction (XRD) patterns of tobacco stem particles obtained in Example 1 of the present invention and Comparative Examples 1 to 3.

[0096] like Figure 1The XRD pattern shown can reflect the crystallinity index (CrI) of the tobacco stem sample to a certain extent. The higher the crystallinity index, the more regular the structure of the cellulose and other components in the sample is, and the more complete the intermolecular hydrogen bond network is, which may lead to improved thermal stability of the sample. Figure 1 It can be seen that the diffraction peaks of Example 1 are significantly stronger than those of the control at the characteristic peak of native cellulose type I (2θ≈22°) and the 101 crystal plane (2θ≈16°), and the position of the diffraction peaks at these locations of the pretreated tobacco stem particles does not change, indicating that the low-temperature hydrothermal pretreatment-deep eutectic solvent synergistic treatment process does not change the crystal morphology of cellulose. Figure 1 As can be seen in the figure, the crystallinity of the tobacco stems increased significantly from 30.8% to 53.7%. This is because hemicellulose and lignin are amorphous components, and the removal of amorphous components will increase the CrI of cellulose. This indicates that hemicellulose and lignin are removed during the low-temperature hydrothermal pretreatment-deep eutectic solvent co-treatment process.

[0097] The structure of tobacco stem lignin extracted from Examples 1, 2, 6 and Comparative Examples 3 and 5 was analyzed by Fourier transform infrared spectroscopy (FT-IR). Figure 2 shown.

[0098] Figure 2 Fourier transform infrared spectroscopy (FT-IR) diagrams are provided for the structures of tobacco stem lignin extracted from Examples 1, 2, 6 of the present invention and Comparative Examples 3 and 5.

[0099] from Figure 2 It can be seen that the overall structure of the characteristic peaks of different lignin samples remains consistent, and the characteristic peaks of lignin can be clearly observed. -1 、1456cm -1 The absorption peak at 1377cm is the stretching vibration of the aromatic benzene ring skeleton of lignin, which is clearly visible in all spectra, indicating that the basic structure of lignin has not been significantly damaged; -1 and 1213cm -1 The characteristic signal peak of CO of lignin syringyl S unit is Figure 2 It can be seen that the tobacco stem lignin extracted in Example 1 shows a stronger signal in the spectrum; 1317 cm -1 The characteristic signal of the aromatic C—C bond of the S unit is at 1266 cm -1 and 1030cm -1 The C-O bond vibration of the guaiacyl G unit and the -OH stretching vibration of the aliphatic and ether bonds are at 1160 cm -1The absorption peak at represents the carbonyl CO absorption peak of the G unit. The existence of these characteristic peaks confirms that the step-by-step low-temperature hydrothermal pretreatment-low eutectic solvent co-treatment method of the present invention can effectively extract tobacco stem lignin, and its structural integrity is well maintained without significant denaturation.

[0100] The deep eutectic solvent [Ch][LA]5[EG] prepared in Example 1 0.6 After rotary evaporation recovery, the lignin removal rate and content of tobacco stems were obtained after 5 hydrothermal deep eutectic solvent circulation experiments. Figure 3 As shown in the figure, the five cycle experiments were completed under the same conditions (reaction temperature 100℃, time 4h, solid-liquid ratio 1:15).

[0101] Figure 3 This is a graph showing the test results of recycling the deep eutectic solvent in Example 1 of the present invention.

[0102] from Figure 3 It can be seen from the results that the deep eutectic solvent prepared by the present invention has good recyclability and stability: the lignin removal rate reaches 70.79% in the first cycle and can still be maintained at above 60% after 5 cycles; at the same time, the lignin content in the tobacco stems decreases from the initial 13.31% to 6.56% (in the first cycle) and only slightly recovers to 7.33% after the 5th cycle. This shows that [Ch][LA]5[EG] 0.6 No significant structural decomposition or loss of active sites occurred during repeated use, and its hydrogen bond donor / acceptor ability and selective solubility for lignin remained stable, indicating that the low eutectic solvent designed in the present invention has good reusability.

[0103] Comparative Example 6

[0104] In Control Example 6, the tobacco stems were processed using the same method as in Example 1, except that the eutectic solvent consisted of choline chloride and lactic acid, wherein the molar ratio of choline chloride to lactic acid was 1:5.

[0105] The tobacco stem product prepared in Control Example 6 was tested, and the test results showed that after the tobacco stem particles were hydrothermally pretreated at 70°C and then treated with a low eutectic solvent consisting of choline chloride and lactic acid, the lignin removal rate was 58.25±1.12%.

[0106] Comparative Example 7

[0107] In Control Example 7, the tobacco stems were processed using the same method as in Example 1, except that the deep eutectic solvent consisted of choline chloride and ethylene glycol, wherein the molar ratio of choline chloride to ethylene glycol was 1:2.

[0108] The tobacco stem product prepared in Control Example 7 was tested, and the test results showed that after the tobacco stem particles were hydrothermally pretreated at 70°C and then treated with a low eutectic solvent consisting of choline chloride and ethylene glycol, the lignin removal rate was 45.3±0.98%.

[0109] It can be seen from the above examples and comparative examples that the composition and specific ratio of the deep eutectic solvent in the present invention, as well as the conditions of the hydrothermal treatment are crucial to the removal of lignin from tobacco stems.

[0110] Comparative Example 8

[0111] The same method for reducing the lignin content of tobacco stems as in Control Example 3 was used, with the only difference being that in step ②, the molar ratio of choline chloride:lactic acid:ethylene glycol was 1:5:1.

[0112] The tobacco stems obtained by treating with a single deep eutectic solvent in Control Example 8 were tested. The test results showed that after the tobacco stem particles (40-60 mesh) were treated with a single DES ([Ch][LA]5EG]1), the lignin content was 8.146%, the recovery rate was 65.95%, and the lignin removal rate was 59.64%.

[0113] In summary, the method of reducing the lignin content in tobacco stems of the present invention is characterized by high efficiency, low cost, and green environmental protection. Compared with the single DES method, the tobacco stem lignin removal rate is significantly improved, and low-temperature hydrothermal pretreatment can avoid irreversible damage to plant cell walls caused by high temperature, reduce cigarette manufacturing costs and harmful components in smoke, and improve product sensory quality. The DES solvent can be recycled, is simple to operate, and has high stability.

[0114] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for reducing the lignin content of tobacco stems, characterized in that: include: Under normal pressure, the tobacco stem raw material is mixed with water and subjected to hydrothermal pretreatment at 70-100° C. to obtain hydrothermally pretreated tobacco stems; A low eutectic solvent and the hydrothermally pretreated tobacco stems are mixed and heated for reaction, wherein the low eutectic solvent is formed by mixing a hydrogen bond donor and a hydrogen bond acceptor.

2. The method according to claim 1, characterized in that The tobacco stem raw material is dried and crushed into 3-5 cm strips, and passed through a 5-60 mesh sieve; The tobacco stem raw material and water are mixed in a mass ratio of 1:10-20, and the hydrothermal pretreatment time is 0.5-1h.

3. The method according to claim 1, characterized in that The hydrogen bond donors are lactic acid and ethylene glycol; the hydrogen bond acceptor is choline chloride; The lactic acid, ethylene glycol, and choline chloride are mixed in a molar ratio of 1:5:0.5-1 to form the deep eutectic solvent.

4. The method according to claim 1 or 3, characterized in that The hydrothermally pretreated tobacco stems are mixed with the deep eutectic solvent in a mass ratio of 1:10-20; The heating reaction temperature is 90-120° C. and the time is 2-6 hours.

5. The method according to claim 4, characterized in that After the heating reaction is completed, the method further comprises: A quenching agent was added and stirred to terminate the reaction, and a solid residue and a filtrate were obtained after vacuum filtration.

6. The method according to claim 5, characterized in that The solid residue is washed with a mixed solvent of ethanol and water until it becomes neutral and the washing liquid is retained; Wherein, the volume ratio of the ethanol to water is 1:

9.

7. The method according to claim 6, characterized in that The washing liquid and the filtrate are mixed, excess water is added, stirred, allowed to stand, and separated to obtain a lignin precipitate and a supernatant; Wherein, the amount of excess water added is 10-20 times the volume of the deep eutectic solvent; The standing time is 24-48h; The separation speed is 9000-10000 rpm and the separation time is 8-12 minutes.

8. The method according to claim 6, characterized in that The supernatant is concentrated by rotary evaporation, centrifuged, filtered, and freeze-dried to obtain absolute dry regenerated lignin; The rotary evaporation temperature is 40-50° C., the concentrate obtained by rotary evaporation is a recovered low eutectic solvent, and the recovered low eutectic solvent is recycled.

9. A tobacco stem product with low lignin content prepared by the method according to any one of claims 1 to 8.

10. Use of the tobacco stem product with low lignin content according to claim 9 in cigarette products.

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