Multi-stage picking and baking method for improving tobacco leaf oil content

Abstract

The invention discloses a multi-stage picking and baking method for improving tobacco leaf oil content, which comprises the following steps of: performing anti-oxidation pretreatment before baking, adopting damp heat-low oxygen coordinated regulation and a baking room top and bottom alternate air supply mode in a baking stage, and adopting sandwich type airflow distribution (top air supply, bottom air return and middle air supplement) to realize hot air homogenization; by means of humidity attenuation and oxygen concentration control in stages, humidity heat-low oxygen coordinated regulation and control remarkably improves the tobacco leaf oil content and stability, and the method is suitable for flue-cured tobacco, air-cured tobacco and other types and has the advantages of being efficient, low in consumption and uniform in quality.

Description

Technical Field

[0001] The present invention relates to a multi-stage harvesting and baking method for improving the oil content of tobacco leaves, belonging to the technical field of tobacco leaf baking. Background Art

[0002] According to the description of the national standard (GB / T2635—1992), oil content is a soft semi-liquid or liquid substance contained in the tissue cells of tobacco leaves. Under the condition that the water content of tobacco leaves reaches 16% - 18%, the degree of oiliness, plumpness or dryness shown in its appearance reflects the full or poor state of tobacco leaves. According to the oil content state of tobacco leaves and people's sensory perception, the oil content of the tobacco leaf appearance quality index is divided into 4 grades: much, some, slightly, and little. Generally, it is considered that the material basis of tobacco leaf oil content with characteristic components such as sugars and their derivatives, lipids, terpenoids, etc. is related to petroleum ether extracts, mainly including volatile oils, fats, resins, fatty acids, waxes, lipids, sterols, and pigments, etc. (Shi Hongzhi and Liu Guoshun, 1998). Oil content is one of the core indicators for measuring the quality of tobacco leaves, and its abundance directly affects the physical properties, chemical composition, and sensory quality of tobacco leaves, as specifically shown below:

[0003] 1. Physical properties: Tobacco leaves with sufficient oil content have a dense tissue structure, and the cell gaps are filled with oils, showing the characteristics of softness and strong elasticity, and are not easily broken during the processing; tobacco leaves with insufficient oil content are hard and brittle in texture, have weak hygroscopicity, and are prone to breakage and debris.

[0004] 2. Chemical composition: Tobacco leaves with more oil content have a high sugar content, a large proportion of carbohydrates, relatively low total nitrogen and nicotine contents, and the chemical composition is more coordinated. Moreover, the oil content is rich in lipids (such as solanesol and neophytadiene) and aromatic substances, which are the direct sources of the aroma of high-quality tobacco leaves.

[0005] 3. Sensory quality: When the oil content is sufficient, the aroma quality is pure, the aroma quantity is sufficient, the miscellaneous gas and irritation are small, and the aftertaste is comfortable. Tobacco leaves with insufficient oil content are prone to produce green miscellaneous gas or woody gas, and the taste is rough.

[0006] Therefore, in order to obtain tobacco leaves with a high oil content, it is necessary to optimize production management, implement mature harvesting and precise baking processes, and ultimately improve the industrial value and economic benefits of tobacco leaves.

[0007] Starch degradation products such as sugars, such as glucose and maltose, are closely related to the oil content of tobacco leaves. The existing literature (authorized announcement number: CN105192874B) discloses a baking process for reducing starch and enhancing fragrance in flue-cured tobacco. By implementing three baking stages: the high-temperature sweating and low-temperature yellowing stage, the early moisture removal and slow color fixation stage, and the stage of increasing fragrance and drying the veins by raising the temperature, the starch content of the flue-cured tobacco leaves after baking can be reduced and the oil content can be increased; the literature (authorized announcement number: CN104839884B) discloses a process method for efficiently degrading starch in tobacco leaves. By subjecting fresh tobacco leaves to high-temperature induction treatment to activate enzyme activity, and passing through the early yellowing stage, the middle yellowing stage, the late yellowing stage, the color fixation stage, and the vein drying stage, the purpose of degrading starch in tobacco leaves is achieved; the literature (authorized announcement number: CN112998303B) discloses a "half-leaf" baking method for reducing the starch content of flue-cured tobacco. Using the half-leaf harvesting method, the two "half-tobacco leaves" without tobacco stalks are strung on a pole and baked in a pit. By implementing three stages: the leaf yellowing stage, the leaf wilting and midrib yellowing stage, and the leaf delayed color fixation stage, the starch content of the flue-cured tobacco leaves after baking can be significantly reduced, and the smoking quality of the tobacco leaves can be improved. The literature (authorized announcement number: CN107411161B) discloses a method for reducing the nicotine and starch content of upper tobacco leaves. When the upper tobacco leaves are ready to be picked, the tobacco plants are transferred to a greenhouse or a house that shields sunlight together with their roots and placed together for 2-8 days, effectively promoting the degradation and transformation of substances such as starch in the tobacco leaves and improving the maturity and quality of the tobacco leaves.

[0008] Maturity is an important factor affecting the oil content of flue-cured tobacco leaves after baking. Shen Yanjin (2015) believes that delaying the harvest by 7 days can significantly increase the oil content of flue-cured tobacco leaves after baking; Yang Mingkun et al. (2020) believe that the upper six leaves harvested after delaying the harvest by 10-11 days have sufficient oil content. In addition, Meng Zhiyong et al. (2012) pointed out that the oil content of the middle tobacco leaves after conventional harvesting is good, and the oil content of the upper tobacco leaves harvested after delaying maturity by 9 days is sufficient.

[0009] In terms of the baking process, extending the yellowing stage at 38°C for 12 hours can improve the appearance oil content of the upper tobacco leaves after baking (Meng Zhiyong et al., 2023). Using a slow-accelerating temperature increase during the color-fixing stage results in better oil content of the tobacco leaves after baking (Meng Zhiyong et al., 2012). For the upper tobacco leaves, extending the late yellowing and late color-fixing periods leads to a higher oil content in the tobacco leaves after baking (Xie Pengfei, 2010). Research by Zhan Jun et al. (2012) shows that increasing the temperature-holding time at 35°C and 60°C and drying the stems at a low temperature of 65°C can significantly improve the oil content of the tobacco leaves by yellowing and drying the stems at a lower temperature. In addition, appropriately increasing the wet-bulb temperature during the baking process can improve the oil content of the tobacco leaves after baking, with a more obvious effect on the middle and upper leaves (Meng Zhiyong et al., 2011; Zhang Baozhan et al., 2012; Yang Bo and Zhang Xifeng, 2021; He Xiaolei et al., 2022). When the dry-bulb temperature is 47°C during the color-fixing period of baking and the wet-bulb temperature is 39°C for the middle tobacco leaves and 38°C for the upper tobacco leaves, it is beneficial to improve the oil content state of the tobacco leaves after baking (Xu Wei et al., 2017). When the dry-bulb temperature is 65 - 67°C and the wet-bulb temperature is 41 - 42°C during the stem-drying period, the oil content of the tobacco leaves after baking is relatively better (Shen Shaojun et al., 2014).

[0010] Gong Changrong et al. (2007) believe that using a low-temperature and medium-humidity yellowing and medium-humidity color-fixing baking process results in sufficient oil content in the tobacco leaves after baking. Huang Ke et al. (2017) believe that slow baking at a low temperature has a good effect on improving the oil content of the tobacco leaves after baking, while Meng Zhiyong et al. (2018) believe that the medium-temperature and medium-humidity yellowing and medium-humidity color-fixing process treatment results in a high appearance oil content in the tobacco leaves after baking. Existing literature (authorized publication number: CN101416767B) discloses a flue-cured tobacco baking process that can improve the quality and aroma of tobacco leaves. By using low-temperature humidity adjustment for yellowing, temperature-holding moisture removal for wilting, ventilation dehydration for leaf drying, and temperature and humidity control for stem drying, it can increase the aroma amount and oil content of the tobacco leaves after baking. Another piece of literature (application publication number: CN116746701A) discloses a method for color-fixing flue-cured tobacco to improve the oil content of tobacco leaves, including measuring the thickness and moisture content of fresh tobacco leaves, calculating their average values, and substituting them into the calculation formulas for the initial wet-bulb temperature, final wet-bulb temperature, and total color-fixing time during color-fixing, which can accurately determine the wet-bulb temperature and color-fixing time in the tobacco barn and significantly improve the quality of tobacco leaf baking and the oil content of the tobacco leaves after baking.

[0011] The rotational speed of the circulation fan is also an important aspect affecting the oil content of flue-cured tobacco leaves. Fan Junhui et al. (2010) believed that when the rotational speed of the circulation fan was 960 r / min in the late color-fixing stage and 720 r / min in the stem-drying stage, it could significantly improve the oil content of tobacco leaves; while Xu Qi et al. (2013) believed that from the start of baking to 52 °C, the fan speed was 1450 r / min, from 52 to 60 °C it was 960 r / min, and from 60 to 65 °C to the end of baking it was 720 r / min, which was conducive to the accumulation of petroleum ether extract content. In addition, Zhan Jun et al. (2011) believed that when the dry-bulb temperature was 42 - 47 °C, the fan speed was 1440 r / min, from 47 to 60 °C it was 960 r / min, and from 60 to 68 °C it was 720 r / min, the oil content of the flue-cured tobacco leaves after baking was relatively high.

[0012] Regarding the precise baking process for improving the oil content of tobacco leaves, many existing literatures have pointed out many feasible solutions in the research field, but the above-mentioned literatures are still in the laboratory stage or for a certain stage, and do not provide the process parameters for industrial application.

[0013] At the same time, analyzing the existing baking process in terms of the theory of improving the oil content of tobacco leaves, the existing baking process divides the baking of tobacco leaves into three stages: yellowing stage, color-fixing stage and stem-drying stage based on the morphological changes of tobacco leaves. There are the following defects in the baking process for improving the oil content of tobacco leaves in the above three stages:

[0014] 1. The synthesis and accumulation of the oil precursor substances in fresh tobacco leaves are insufficient, resulting in a reduction in the oil substances of the flue-cured tobacco leaves after baking; the humidity control in the early and middle stages of the yellowing stage is improper, manifested as too low humidity from ignition to 38 °C, which will inhibit the activities of amylase and lipoxygenase and hinder the conversion of oil precursor substances.

[0015] 2. The temperature in the color-fixing stage is too high and the oxygen concentration is relatively large, resulting in intensified lipid oxidation, blocked lipid synthesis, and a relatively high degradation rate of oil substances.

[0016] 3. The hot air circulation is uneven and the wind speed in the stem-drying stage is too large, resulting in uneven distribution of the oil on the surface of the tobacco leaves and relatively large losses. Summary of the Invention

[0017] Based on the above, the present invention provides a multi-stage harvesting and baking method for improving the oil content of tobacco leaves, which can significantly improve the oil retention rate of tobacco leaves and the appearance sensory smoothness, and can overcome the deficiencies of the prior art.

[0018] A multi-stage harvesting and baking method for improving the oil content of tobacco leaves includes the following steps:

[0019] a. Pretreatment process before harvesting: Spraying exogenous regulators 5 - 7 days before harvesting fresh tobacco leaves to inhibit the lipid oxidation of cell membranes and promote the synthesis and accumulation of starch and lipids.

[0020] Objective: To increase the synthesis and accumulation of starch and lipid substances in fresh tobacco leaves.

[0021] b Harvesting process: Harvest the fresh tobacco leaves when they reach the stage of being still mature, i.e., the physiological maturity state.

[0022] c Stage-by-stage baking process: Load the fresh tobacco leaves into a dense baking house with a dual-mode of internal dehumidification and external moisture exhaust and capable of switching between top air supply and bottom air supply, and conduct coordinated temperature and humidity control in the following five stages:

[0023] i. Enzyme activation promotion stage: After ignition, adopt the external moisture exhaust mode, raise the dry-bulb temperature to 33 - 34 °C at a speed of 1 °C / h, control the wet-bulb temperature at 32.5 - 33.5 °C, and keep the temperature stable for baking for 6 - 8 h to start the sweating and yellowing of the tobacco leaves and promote the starch and lipid metabolism enzymes to enter the activation stage.

[0024] Objective: Raise the dry-bulb temperature to 33 - 34 °C, and appropriately extend the low-temperature and humidity-keeping yellowing time by 4 - 5 h to activate the activities of amylase and lipoxygenase and start the metabolism and transformation of starch and lipid substances in the tobacco leaves.

[0025] ii. Lipid synthesis substrate accumulation stage: Adopt the external moisture exhaust mode and raise the dry-bulb temperature to 37 - 38 °C at a speed of 1 °C / 2 h, with the wet-bulb temperature at 36.5 - 37.5 °C, and keep the temperature stable for baking for 20 - 35 h; then, raise the dry-bulb temperature to 39 - 40 °C at a speed of 1 °C / 2 h, control the wet-bulb temperature at 38 °C, and reduce the wet-bulb temperature by 0.5 °C every 3 - 5 h, and finally control the wet-bulb temperature between 35.5 - 36.5 °C, and keep the temperature stable for baking until the tobacco leaves turn yellow by 8 - 9 tenths and the leaves are fully softened.

[0026] Objective: Appropriately extend the low-temperature and humidity-keeping yellowing at 37 - 38 °C in the early stage of the lipid synthesis substrate accumulation stage by 5 - 10 h to prevent the rapid destruction of the cell membrane structure and lead to the premature termination of starch and lipid metabolism; in the later stage of the lipid synthesis substrate accumulation stage, the dry-bulb temperature is 39 - 40 °C, control the wet-bulb temperature at 38 °C, and reduce the wet-bulb temperature by 0.5 °C every 3 - 5 h to increase the metabolism rate of starch and lipid substances and further promote the accumulation of lipid synthesis substrates.

[0027] iii. Lipid synthesis stage: Adopt the internal dehumidification mode and construct a low-oxygen environment, and raise the dry-bulb temperature to 42 - 43 °C at a gradient of 1 °C / 3 - 4 h, control the wet-bulb temperature at 35 - 36.5 °C, and keep the temperature stable for baking for 6 - 8 h; then, raise the dry-bulb temperature to 44 - 45 °C at a speed of 1 °C / 3 - 4 h, control the wet-bulb temperature at 35.5 - 37 °C, and keep the temperature stable for baking until the tobacco leaves form small rolls.

[0028] Objective: Appropriately reduce the wet-bulb temperature by 0.5 - 1°C, combined with an intermittent hypoxic environment during the lipid synthesis stage, inhibit the oxidation reaction, reduce the oxidation ratio of unsaturated fatty acids, and promote the synthesis of lipid substances from oil precursor substances such as carbohydrate substances, fatty acids, and terpenoid substances, thereby enhancing the accumulation of oil substances in tobacco leaves during the baking process.

[0029] iv. Lipid stable conversion stage: Adopt the external exhaust humidity mode and alternate air supply at the top and bottom of the baking room. Raise the dry-bulb temperature to between 48 - 49°C at a speed of 1°C every 2 - 3 hours, control the wet-bulb temperature at 38 - 39°C, and bake at a stable temperature for 10 - 15 hours until more than 2 / 3 of the leaves are dry; then, raise the dry-bulb temperature to between 51 - 52°C at a speed of 1°C every 2 - 3 hours, control the wet-bulb temperature at 38.5 - 39.5°C, and bake at a stable temperature for 6 - 10 hours until more than 3 / 4 of the leaves are dry; then, raise the dry-bulb temperature to 55 - 56°C at a speed of 1°C every 2 - 3 hours, control the wet-bulb temperature at 39 - 40°C, and bake at a stable temperature for 10 - 18 hours until all the tobacco leaves are dry and more than 1 / 4 of the main veins are dry.

[0030] Objective: During the stable conversion stage, adopt alternate air supply at the top and bottom of the baking room to reduce the volatilization of tobacco leaf lipids and oil; extend the stable baking time by 6 - 10 hours at 48 - 49°C, 51 - 52°C, and 55 - 56°C respectively, increase the wet-bulb temperature by 0.5 - 1°C, and further promote the synthesis of lipid substances from oil precursor substances such as carbohydrate substances and terpenoid substances.

[0031] v. Safe dry vein stage: Adopt the external exhaust humidity mode. Raise the dry-bulb temperature to between 58 - 59°C at a speed of 1°C every 1 - 2 hours, control the wet-bulb temperature at 41 - 42°C, and bake at a stable temperature for 6 - 8 hours; then, raise the dry-bulb temperature to between 63 - 64°C at a speed of 1°C every 1 - 2 hours, control the wet-bulb temperature at 41 - 42°C, reduce the leaf surface wind speed, and bake at a stable temperature until all the tobacco leaves in the whole baking room are completely dry.

[0032] Objective: Appropriately reduce the dry-bulb temperature by 2 - 4°C, increase the wet-bulb temperature by 1 - 2°C, reduce the leaf surface wind speed, reduce the volatilization of tobacco leaf lipids and oil, and promote the accumulation of oil substances in tobacco leaves.

[0033] The multi-stage harvesting and baking method for improving the oil content of tobacco leaves described above is that the exogenous regulator capable of enhancing the synthesis and accumulation of starch and lipid substances in fresh tobacco leaves is a mixed solution containing 50 - 100 mg / L of ascorbic acid and 50 - 80 mg / L of salicylic acid, and the dosage is 4 - 6 L per mu.

[0034] The multi-stage baking method for improving the oil content of tobacco leaves described above is that the internal dehumidification mode is to set a return air duct connecting the external dehumidification window and the heating chamber intake window, and a condensation device is set on the return air duct to collect the water vapor in the return air duct. The internal dehumidification mode runs intermittently, and the intermittent operation mode is to turn on the external dehumidification mode for 10 minutes every 4 - 5 hours.

[0035] The multi-stage baking method for improving the oil content of tobacco leaves described above is that top air supply, bottom air supply and intermediate air make-up are simultaneously set in the baking house. The top and bottom of the baking house supply air alternately, that is, the top air supply and the bottom air supply alternate every 1 hour, and the air speed is 0.5 - 1.0 m / s, which is used to reduce lipid volatilization.

[0036] The multi-stage baking method for improving the oil content of tobacco leaves described above is that an infrared auxiliary heating module for promoting the uniform migration of lipids inside the tobacco leaves is equipped on the roof and the ground against the wall in the baking house, and the wavelength of the infrared auxiliary heating module is 3 - 5 μm.

[0037] The multi-stage baking method for improving the oil content of tobacco leaves described above is that in the safe dry leaf stage, the circulating fan adopts a low-speed gear air speed, and the air speed in the loading chamber of the bulk curing barn is ≤ 0.3 m / s to ensure the stable solidification of oil substances.

[0038] The multi-stage baking method for improving the oil content of tobacco leaves described above, the low-oxygen environment is formed by the self-metabolism of tobacco leaves consuming oxygen during the internal dehumidification stage to form a low-oxygen environment.

[0039] The multi-stage baking method for improving the oil content of tobacco leaves described above: the low-oxygen environment is to introduce carbon dioxide gas into the baking house during the lipid synthesis stage, so that the O2 concentration in the baking house is between 10% and 18%.

[0040] The multi-stage baking method for improving the oil content of tobacco leaves described above is that the bulk curing barn with the internal dehumidification and external dehumidification dual-use mode and equipped with top air supply, bottom air supply and intermediate air make-up is an air source heat pump heating type tobacco baking house.

[0041] Compared with the prior art, the beneficial effects of the present invention are:

[0042] The present invention analyzes the existing technical solutions for improving the oil content of tobacco leaves, combines them with the existing tobacco leaf baking process, and obtains a baking process suitable for large-scale use that can improve the oil content of tobacco leaves. First, exogenous regulators are sprayed on the tobacco leaves 5 to 7 days before harvesting, that is, a composite solution of ascorbic acid + salicylic acid is used to inhibit the generation of free radicals, which can promote the synthesis and accumulation of starch and lipids; the fresh tobacco leaves are harvested when they reach the physiological maturity state to increase the synthesis and accumulation amount of starch and lipid substances in the fresh tobacco leaves and reduce the consumption of oil precursor substances in the fresh tobacco leaves; then, based on the coordinated regulation of the baking temperature, humidity, and wind speed in five stages, the metabolism of substances related to the oil content of tobacco leaves is regulated. In the enzyme activation promotion stage, the dry bulb temperature is raised to 33-34°C, and the low-temperature humidity-keeping yellowing time is appropriately extended by 4-5 hours to activate the activities of amylase and lipoxygenase, etc., and initiate the metabolism and transformation of starch and lipid substances in the tobacco leaves; in the lipid synthesis substrate accumulation stage, the low-temperature humidity-keeping yellowing at 37-38°C in the early stage of the lipid synthesis substrate accumulation stage is appropriately extended by 5-10 hours to prevent the sudden destruction of the cell membrane lipid structure, resulting in the premature termination of starch and lipid metabolism; in the later stage of the lipid synthesis substrate accumulation stage, the dry bulb temperature is 39-40°C, the wet bulb temperature is controlled at 38°C, and the wet bulb temperature is reduced by 0.5°C every 3-5 hours to increase the metabolism rate of starch and lipid substances and further promote the accumulation of lipid synthesis substrates; in the lipid synthesis stage, the wet bulb temperature is appropriately reduced by 0.5-1°C, and in combination with a low-oxygen environment in the lipid synthesis stage, the oxidation reaction is inhibited, the oxidation ratio of unsaturated fatty acids is reduced, and oil precursor substances such as carbohydrate substances, fatty acids, and terpenoid substances are promoted to synthesize lipid substances, thereby enhancing the accumulation of oil substances in the tobacco leaves during the baking process; in the lipid stable transformation stage, alternating air supply at the top and bottom of the baking room is adopted to reduce the volatilization of tobacco leaf lipids and oil content, and at the same time, setting air supply in the middle can further improve the air flow uniformity in the baking room; at 48-49°C, 51-52°C, and 55-56°C, the constant temperature baking time is extended, and the wet bulb temperature is increased by 0.5-1°C to further promote the synthesis of lipid substances from oil precursor substances such as carbohydrate substances and terpenoid substances; in the safe stem drying stage, the dry bulb temperature is appropriately reduced by 2-4°C, the wet bulb temperature is increased by 1-2°C, and the leaf boundary wind speed is reduced to reduce the volatilization of tobacco leaf lipids and oil content and promote the accumulation of oil substances in the tobacco leaves.

[0043] The hypoxic environment in the lipid synthesis stage can be achieved by supplementing carbon dioxide during the internal dehumidification stage, or by the tobacco leaves' own metabolism consuming oxygen during the internal dehumidification stage to form a hypoxic environment. Specifically, during the internal moisture removal stage, the tobacco leaves are equivalent to being placed in a sealed environment. During the tobacco leaf baking process, the tobacco leaves will consume oxygen through internal physiological and biochemical reactions and gradually form hypoxia. The specific manifestations are as follows: The baking of tobacco leaves is accompanied by complex physiological and biochemical changes. Among them, oxidoreductases will catalyze the decomposition and transformation of organic substances. This process requires the participation of oxygen and produces carbon dioxide and heat. This reaction is called the oxidation reaction during the baking stage. Oxygen consumption under the action of enzymes: During the baking process, the activities of enzymes such as respiratory enzymes and hydrolases in the tobacco leaves increase, resulting in the gradual decomposition of macromolecular organic substances (such as starch and protein) into small molecule substances. This process involves an oxidation reaction, and oxygen is consumed as a reactant.

[0044] By comparing with the existing tobacco leaf baking technology, after baking with the technical solution of the present invention, the appearance quality of the tobacco leaves has significantly increased in oiliness, the starch content has been significantly reduced, the content of oil substances such as fatty acids and petroleum ether extracts in the tobacco leaves has increased, and the sensory quality of the baked tobacco leaves has been significantly improved.

[0045] At the same time, in order to meet the baking process proposed by the present invention, the hot air circulation of the existing baking house has been optimized. It is proposed to adopt a "sandwich-style" air distribution (top air supply + bottom air return + middle air make-up) to achieve the air flow uniformity in the baking house. In this way, the temperature difference can be ≤1.0 °C and the humidity difference can be ≤3%; at the same time, an infrared auxiliary heating module (wavelength 3 - 5 μm) is equipped in the baking house to promote the uniform migration of lipids inside the tobacco leaves. Description of the Drawings

[0046] Figure 1 is the temperature and humidity control curve graph of the five-stage baking;

[0047] Figure 2 is a schematic diagram of a bulk curing barn with both internal dehumidification and external moisture removal dual modes and with the top air supply and bottom air supply being switchable. Detailed Embodiments

[0048] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention is provided. Many specific details are set forth in the following description to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein. Those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0049] Example 1, taking the upper tobacco leaves of the flue-cured tobacco variety Yunyan 87 as an example:

[0050] a Pretreatment process before harvesting: Spraying exogenous regulators 5 - 7 days before harvesting fresh tobacco leaves; the exogenous regulator is a mixed solution containing 50 - 100 mg / L of ascorbic acid and 50 - 80 mg / L of salicylic acid, with a dosage of 4 - 6 L / acre.

[0051] b Harvesting process: Harvesting when fresh tobacco leaves reach the physiological maturity state;

[0052] Control Example 1 is conventional production, without spraying exogenous regulators, and harvested when mature.

[0053] Control Example 2 is conventional production, without spraying exogenous regulators, and harvested at physiological maturity.

[0054] Taking the tobacco leaves at the 16th leaf position as samples, detecting the lipoxygenase activity, malondialdehyde (MDA) and starch content of the harvested fresh tobacco leaves, and the comparison results are as follows:

[0055] Table 1 Effects of different treatments on the enzyme activity physiological indexes of fresh tobacco leaves

[0056]

[0057] Note: Different lowercase letters in the same column indicate significant differences at the 0.05 level, and the same below.

[0058] It can be seen from Table 1 that compared with Control 1 and Control 2, for the physiologically mature tobacco leaves harvested after spraying exogenous regulators, the lipoxygenase activity and MDA content are significantly reduced, and the starch content is significantly higher, which is beneficial to the accumulation of the precursor substances of tobacco leaf oil.

[0059] Example 2, Selection and preparation of the baking house, preferably a bulk curing barn with dual modes of internal dehumidification and external moisture exhaust and with the switchable top air supply and bottom air supply. Among them, the structure of the bulk curing barn with switchable top air supply and bottom air supply can be achieved by the patent No. 202122318239.1, and the patent name is a baking house with mutually switchable upward and downward airflows. And the dual mode of internal dehumidification and external moisture exhaust can be improved on the basis of the baking house with mutually switchable upward and downward airflows. By adding a return air duct to connect the external moisture exhaust window and the heating chamber air inlet window, and setting a condensation device on the return air duct to collect the water vapor in the return air duct, internal moisture exhaust can be achieved. As Figure 2 shown, the baking house includes a heating chamber 1 and a baking chamber 2 separated by a partition wall 3. An upper air inlet 4 and a lower air inlet 5 are respectively provided at the upper and lower sections of the partition wall 3. Controllable sluices capable of opening and closing the air inlets are provided on both the air inlet 4 and the lower air inlet 5. An air inlet window 6 is provided on the heating chamber, and a moisture exhaust port 7 is provided on the baking chamber. Three-way valves with valves are respectively provided on the air inlet window 6 and the moisture exhaust port 7, and then the air inlet window 6 and the moisture exhaust port 7 are connected by a return air duct 8, and a condensation device 9 is provided on the return air duct to collect the water vapor in the return air duct, so as to achieve internal moisture exhaust.

[0060] Taking the middle leaves of the Yunyan 87 flue-cured tobacco variety as an example:

[0061] a Pretreatment process before harvesting: Spray exogenous regulators 5 - 7 days before fresh tobacco leaves are harvested; the exogenous regulator is a mixed solution containing 50 - 100 mg / L ascorbic acid and 50 - 80 mg / L salicylic acid, with a dosage of 4 - 6 L / acre.

[0062] b Harvesting process: Harvest when the fresh tobacco leaves reach the physiological maturity state;

[0063] c Stage - type baking process: Load the fresh tobacco leaves into a dense baking house with a dual - mode of internal dehumidification and external moisture exhaust, and conduct coordinated temperature and humidity control in the following five stages:

[0064] i. Enzyme activation promotion stage: After ignition, adopt the external moisture exhaust mode, raise the dry - bulb temperature to 33 - 34 °C at a speed of 1 °C / h, control the wet - bulb temperature at 32.5 - 33.5 °C, and keep the temperature stable for baking for 6 - 8 h to start the sweating and yellowing of the tobacco leaves, and promote the starch and lipid metabolism enzymes to enter the activation stage; at the same time, an infrared auxiliary heating module for promoting the uniform migration of lipids inside the tobacco leaves is equipped in the baking house, and the wavelength of the infrared auxiliary heating module is 3 - 5 μm;

[0065] ii. Lipid synthesis substrate accumulation stage: Adopt the external moisture exhaust mode, raise the dry - bulb temperature to 37 - 38 °C at a speed of 1 °C / 2 h, with the wet - bulb temperature at 36.5 - 37.5 °C, and keep the temperature stable for baking for 20 - 35 h; then, raise the dry - bulb temperature to 39 - 40 °C at a speed of 1 °C / 2 h, control the wet - bulb temperature at 38 °C, and reduce the wet - bulb temperature by 0.5 °C every 3 - 5 h, and finally control the wet - bulb temperature between 35.5 - 36.5 °C, and keep the temperature stable for baking until the tobacco leaves turn yellow by 8 - 9 tenths and the leaves are fully softened; at the same time, an infrared auxiliary heating module for promoting the uniform migration of lipids inside the tobacco leaves is equipped in the baking house, and the wavelength of the infrared auxiliary heating module is 3 - 5 μm;

[0066] iii. Lipid synthesis stage: Adopt the internal dehumidification mode and construct a low - oxygen environment, raise the dry - bulb temperature in a gradient manner to 42 - 43 °C at a speed of 1 °C / 3 - 4 h, control the wet - bulb temperature at 35 - 36.5 °C, and keep the temperature stable for baking for 6 - 8 h; then, raise the dry - bulb temperature to 44 - 45 °C at a speed of 1 °C / 3 - 4 h, control the wet - bulb temperature at 35.5 - 37 °C, and keep the temperature stable for baking until the tobacco leaves are slightly rolled; in the lipid synthesis stage, adopt the internal dehumidification mode, and turn on the external moisture exhaust mode for 10 min every 4 - 5 h. At this time, during the internal moisture exhaust process, the self - metabolism of the tobacco leaves consumes oxygen to form a low - oxygen environment, and then the tobacco leaves can inhibit the oxidation reaction in the low - oxygen environment;

[0067] iv. Lipid stabilization and transformation stage: Adopt the external moisture exhaust mode, raise the dry bulb temperature to between 48 and 49 °C at a rate of 1 °C every 2 - 3 hours, control the wet bulb temperature at 38 - 39 °C, and bake at a stable temperature for 10 - 15 hours until more than 2 / 3 of the leaves are dry; then, raise the dry bulb temperature to between 51 and 52 °C at a rate of 1 °C every 2 - 3 hours, control the wet bulb temperature at 38.5 - 39.5 °C, and bake at a stable temperature for 6 - 10 hours until more than 3 / 4 of the leaves are dry; then, raise the dry bulb temperature to 55 - 56 °C at a rate of 1 °C every 2 - 3 hours, control the wet bulb temperature at 39 - 40 °C, and bake at a stable temperature for 10 - 18 hours until all the tobacco leaves are dry and more than 1 / 4 of the main veins are dry. During the stable transformation stage, adopt alternating air supply at the top and bottom of the curing barn. The alternating air supply at the top and bottom of the curing barn means that the top air supply and the bottom air supply alternate every 1 hour, and the wind speed is 0.5 - 1.0 m / s, which is used to reduce lipid volatilization.

[0068] v. Safe stem drying stage: Adopt the external moisture exhaust mode, raise the dry bulb temperature to between 58 and 59 °C at a rate of 1 °C every 1 - 2 hours, control the wet bulb temperature at 41 - 42 °C, and bake at a stable temperature for 6 - 8 hours; then, raise the dry bulb temperature to between 63 and 64 °C at a rate of 1 °C every 1 - 2 hours, control the wet bulb temperature at 41 - 42 °C, reduce the leaf boundary wind speed, and bake at a stable temperature until all the tobacco leaves in the whole barn are completely dry. Maintain weak ventilation during the safe stem drying stage to ensure the stable solidification of oil substances. The weak ventilation refers to that the wind speed in the curing barn is ≤ 0.3 m / s.

[0069] The baking operation of Comparative Example 3 was carried out according to the three - stage baking process (GB / T 23219—2008), and other treatments (spraying exogenous regulators, physiological maturity harvesting, and intensive curing barn transformation) were the same as those in Example 2. The specific operation of the three - stage baking process is as follows:

[0070] Yellowing period: After loading the tobacco, burn a small fire to raise the curing barn temperature to 35 °C, keep the wet bulb temperature at 34 °C. After the leaf tips turn yellow, raise the dry bulb temperature to 38 °C at a rate of 1 °C every 2 hours, extend the stable temperature time, control the wet bulb temperature at 35.5 °C, so that about 80% of the tobacco leaves at the thermometer hanging positions in the curing barn reach 70 - 80% yellow, and at the same time the leaves become soft (the water loss is about 30%). Then raise the temperature to 42 °C, keep the wet bulb temperature at 36.5 °C, extend the time, so that the tobacco leaves reach yellow leaves with green veins and slightly green, withered and collapsed, and the main veins are soft.

[0071] Color - fixing period: First, raise the dry bulb temperature to 45 °C at an average rate of 1 °C every 3 hours and keep it stable, and bake at a stable temperature until the tips are hooked and the edges are curled; then, raise the temperature to 48 °C at a rate of 1 °C every 2 hours, extend the time, so that the tobacco veins turn yellow and the water loss reaches small - roll; then raise the temperature to 54 °C at a rate of 1 °C every 2 hours, and bake at a stable temperature for 10 hours to achieve leaf drying. As the dry bulb temperature rises, the wet bulb temperature gradually rises and stabilizes at 39 °C.

[0072] Drying period: Raise the temperature from 54℃ to 65℃ at a rate of 1℃ per hour and keep it stable until the tobacco leaves are completely dry. Before the dry-bulb temperature reaches 60℃, keep the wet-bulb temperature at 40℃; after 60℃, gradually close the moisture inlet and outlet, reduce the ventilation volume, and keep the wet-bulb temperature at 41℃.

[0073] The 10th leaf tobacco was used as a sample to measure the activities of lipoxygenase and amylase in the tobacco leaves during the baking process, evaluate the appearance quality of the tobacco leaves after baking, and detect the contents of petroleum ether extracts, fatty acids, starch and other substances. The comparison results are as follows:

[0074] Table 2 Changes in enzyme activity of tobacco leaves during baking under different treatments

[0075]

[0076] As can be seen from Table 2, compared with the conventional baking process, during the baking process of the method of the present invention, the activities of tobacco leaf amylase and lipoxygenase at 34°C are relatively low, which is in the enzyme activation stage; at 38-40°C, the activities of amylase and lipoxygenase are relatively high, which is the lipid synthesis substrate accumulation stage; at 38-40°C, the activities of amylase and lipoxygenase are also higher than those in the conventional baking process, but the enzyme activity values ​​have been significantly reduced, which is the lipid synthesis stage; at 38-40°C, the activities of amylase and lipoxygenase are slightly different from those in the conventional baking process, which is the lipid stabilization conversion stage.

[0077] Table 3 Effects of different treatments on the appearance quality of flue-cured tobacco leaves

[0078]

[0079] As shown in Table 3, compared with the conventional baking process, the appearance quality of tobacco leaves baked by the method of the present invention is significantly improved, especially the oil content and chroma are significantly improved.

[0080] Table 4 Effects of different treatments on petroleum ether extract and starch content in flue-cured tobacco leaves (%)

[0081]

[0082] Note: Different lowercase letters in the same column indicate significant differences among different treatments.

[0083] As shown in Table 4, compared with the conventional baking process, the petroleum ether extract content of tobacco leaves baked by the method of the present invention is significantly increased, and the starch content is significantly reduced.

[0084] Table 5 Effects of different treatments on the fatty acid content of flue-cured tobacco leaves (μg / g)

[0085]

[0086]

[0087] As can be seen from Table 5, compared with the conventional baking process, the total amount of fatty acids and the content of its components in the tobacco leaves after baking by the method of the present invention have increased significantly.

[0088] Table 6 Effects of Different Treatments on the Sensory Quality of Tobacco Leaves after Baking

[0089]

[0090] Note: The sensory quality evaluation is the sum of the scores of 6 items: aroma quality, aroma quantity, taste, off-flavor, irritation, and strength. Each item is 9 points. The same below

[0091] As can be seen from Table 6, 7 smoking evaluation experts were organized to evaluate the sensory quality of the tobacco leaves after baking. Compared with the conventional baking process, the average total score of the sensory quality of the tobacco leaves after baking by the method of the present invention is 0.6 points higher, indicating that the method of the present invention significantly improves the sensory quality of the tobacco leaves after baking.

[0092] Example 3, to optimize the implementation of the baking scheme proposed by the present invention, it is necessary to first improve the existing tobacco barn. This type of tobacco barn preferably uses a new generation of air source heat pump tobacco barn, and its core system includes an air source heat pump (evaporator, compressor, condenser, expansion valve and control system), which is combined with a tobacco leaf drying room (tobacco barn, tobacco leaf hanging rack and observation window) and a temperature and humidity control system. There are already many related public technologies, such as patent application number 202010203717.1, patent name an automatic control instrument and energy analysis method for an intelligent heat pump tobacco barn; patent number 202222774396.8, patent name an air source heat pump system and a tobacco barn for a tobacco barn, both of which use air source heat pump technology and are equipped with internal and external dehumidification control systems, and use electric heating when the heat exchange efficiency is low, so the specific principle will not be described in detail.

[0093] At the same time, top air supply, bottom air supply and intermediate air supply are simultaneously set in the new generation of air source heat pump tobacco barn. This "sandwich type" air flow distribution (top air supply + bottom air return + intermediate air supply) is used to achieve the air flow uniformity in the tobacco barn, so that the temperature difference ≤ 1.0 °C and the humidity difference ≤ 3%; at the same time, an infrared auxiliary heating module (wavelength 3 - 5 μm) is equipped in the tobacco barn to promote the uniform migration of lipids inside the tobacco leaves, and an infrared auxiliary heating module is added in the tobacco barn to ensure the temperature control accuracy.

[0094] Taking the upper tobacco leaves of the K326 variety as an example:

[0095] a Pretreatment process before harvesting: Spray exogenous regulators 5 - 7 days before harvesting fresh tobacco leaves; the exogenous regulator is a mixed solution containing 50 - 100 mg / L ascorbic acid and 50 - 80 mg / L salicylic acid, and the dosage is 4 - 6 L / mu.

[0096] b Harvesting process: Harvest when the fresh tobacco leaves reach the physiological maturity state;

[0097] c - stage baking process: Load fresh tobacco leaves into a bulk curing barn with both internal dehumidification and external moisture - exhaust functions, and conduct coordinated temperature and humidity control in the following five stages:

[0098] i. Enzyme activation promotion stage: After ignition, adopt the external moisture - exhaust mode, raise the dry - bulb temperature to 33 - 34 °C at a rate of 1 °C / h, control the wet - bulb temperature at 32.5 - 33.5 °C, and bake at a constant temperature for 6 - 8 h to initiate the sweating and yellowing of tobacco leaves, and promote the starch and lipid - metabolizing enzymes to enter the activation stage; at the same time, an infrared auxiliary heating module for promoting the uniform migration of lipids inside the tobacco leaves is equipped in the curing barn, and the wavelength of the infrared auxiliary heating module is 3 - 5 μm;

[0099] ii. Lipid synthesis substrate accumulation stage: Adopt the external moisture - exhaust mode, raise the dry - bulb temperature to 37 - 38 °C at a rate of 1 °C / 2 h, with the wet - bulb temperature at 36.5 - 37.5 °C, and bake at a constant temperature for 20 - 35 h; then, raise the dry - bulb temperature to 39 - 40 °C at a rate of 1 °C / 2 h, control the wet - bulb temperature at 38 °C, and reduce the wet - bulb temperature by 0.5 °C every 3 - 5 h, and finally control the wet - bulb temperature between 35.5 - 36.5 °C, and bake at a constant temperature until the tobacco leaves turn yellow by 8 - 9 tenths and the leaves are fully softened; at the same time, an infrared auxiliary heating module for promoting the uniform migration of lipids inside the tobacco leaves is equipped in the curing barn, and the wavelength of the infrared auxiliary heating module is 3 - 5 μm;

[0100] iii. Lipid synthesis stage: Adopt the internal dehumidification mode and construct a low - oxygen environment, raise the dry - bulb temperature to 42 - 43 °C at a gradient rate of 1 °C / 3 - 4 h, control the wet - bulb temperature at 35 - 36.5 °C, and bake at a constant temperature for 6 - 8 h; then, raise the dry - bulb temperature to 44 - 45 °C at a rate of 1 °C / 3 - 4 h, control the wet - bulb temperature at 35.5 - 37 °C, and bake at a constant temperature until the tobacco leaves form small rolls; in the lipid synthesis stage, adopt the internal dehumidification mode, and turn on the external moisture - exhaust mode for 10 min every 4 - 5 h to inhibit the oxidation reaction with a low - oxygen environment; at this time, during the internal moisture - exhaust process, the self - metabolism of the tobacco leaves consumes oxygen to form a low - oxygen environment, and then the tobacco leaves can inhibit the oxidation reaction in the low - oxygen environment. If the oxygen concentration is still relatively high, carbon dioxide can be introduced to make the O2 concentration in the curing barn between 10% and 18%.

[0101] iv. Lipid stable transformation stage: Adopt external moisture removal and alternating air supply at the top and bottom of the curing barn. Raise the dry-bulb temperature to between 48 - 49°C at a rate of 1°C every 2 - 3 hours, control the wet-bulb temperature at 38 - 39°C, and bake at a stable temperature for 10 - 15 hours until more than 2 / 3 of the leaf is dry; then, raise the dry-bulb temperature to between 51 - 52°C at a rate of 1°C every 2 - 3 hours, control the wet-bulb temperature at 38.5 - 39.5°C, and bake at a stable temperature for 6 - 10 hours until more than 3 / 4 of the leaf is dry; then, raise the dry-bulb temperature to 55 - 56°C at a rate of 1°C every 2 - 3 hours, control the wet-bulb temperature at 39 - 40°C, and bake at a stable temperature for 10 - 18 hours until the entire tobacco leaf is dry and more than 1 / 4 of the main vein is dry; during the stable transformation stage, adopt alternating air supply at the top and bottom of the curing barn. There is top air supply, bottom air return, and intermediate air make-up in the curing barn at the same time. The alternating air supply at the top and bottom of the curing barn rotates forward and backward every 1 hour, and the wind speed is 0.5 - 1.0 m / s, which is used to reduce lipid volatilization.

[0102] v. Safe stem-drying stage: Adopt the external moisture removal mode. Raise the dry-bulb temperature to between 58 - 59°C at a rate of 1°C every 1 - 2 hours, control the wet-bulb temperature at 41 - 42°C, and bake at a stable temperature for 6 - 8 hours; then, raise the dry-bulb temperature to between 63 - 64°C at a rate of 1°C every 1 - 2 hours, control the wet-bulb temperature at 41 - 42°C, reduce the leaf boundary wind speed, and bake at a stable temperature until the entire barn of tobacco leaves is completely dry. Maintain weak ventilation during the safe stem-drying stage to ensure the stable solidification of oil substances. The weak ventilation means that the wind speed in the curing barn ≤ 0.3 m / s.

[0103] The bulk curing barn of Comparative Example 4 uses a standard bulk curing barn without modification of the alternating air supply at the top and bottom of the curing barn and the infrared auxiliary heating module. Other treatments (spraying exogenous regulators, harvesting at physiological maturity, and supporting baking technology) are the same as in Example 2. Taking the tobacco leaves at the 16th leaf position as samples, evaluate the appearance quality of the cured tobacco leaves, and detect the contents of petroleum ether extracts, fatty acids, starch, etc. The comparison results are as follows:

[0104] Table 7 Effects of different treatments on the appearance quality of cured tobacco leaves

[0105]

[0106] As can be seen from Table 7, compared with the standard bulk curing barn, the appearance quality of the upper tobacco leaves cured by the method of the present invention (using pulsating hot air and infrared auxiliary heating module) is significantly improved, especially the oil content and chroma are significantly improved.

[0107] Table 8 Effects of different treatments on the contents of petroleum ether extracts and starch in cured tobacco leaves (%)

[0108]

[0109] As can be seen from Table 8, compared with the standard bulk curing barn, the content of petroleum ether extract in the tobacco leaves cured by the method of the present invention (using "sandwich type" air supply and infrared auxiliary heating module) increased significantly, while the starch content decreased significantly.

[0110] Table 9 Effects of Different Treatments on Fatty Acid Content in Cured Tobacco Leaves (μg / g)

[0111]

[0112] As can be seen from Table 9, compared with the standard bulk curing barn, the total fatty acid content and its components (except methyl palmitate and methyl linolenate) in the tobacco leaves cured by the method of the present invention (using "sandwich type" air supply and infrared auxiliary heating module) increased significantly.

[0113] Table 10 Effects of Different Treatments on Sensory Quality of Cured Tobacco Leaves

[0114]

[0115] As can be seen from Table 10, 7 panelists were organized to evaluate the sensory quality of the cured tobacco leaves. Compared with the standard bulk curing barn, the average total score of the sensory quality of the tobacco leaves cured by the method of the present invention (using "sandwich type" air supply and infrared auxiliary heating module) was 0.5 points higher, indicating that the method of the present invention significantly improved the sensory quality of the cured tobacco leaves.

[0116] The above embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention should be subject to the appended claims.

Claims

1. A multi-stage harvesting and curing method for improving the oil content of tobacco leaves, characterized in that, It includes the following steps: a Pre-harvest pretreatment process: Spraying exogenous regulators 5 - 7 days before fresh tobacco leaves are harvested to increase the synthesis and accumulation of starch and lipid substances in fresh tobacco leaves; b Harvesting process: Harvesting when fresh tobacco leaves reach the physiological maturity state; c Stage-by-stage baking process: Loading fresh tobacco leaves into a dense baking house with a dual mode of internal dehumidification and external moisture discharge and capable of switching between top air supply and bottom air supply, and carrying out coordinated temperature and humidity control in the following five stages: i. Enzyme activation promotion stage: After ignition, adopt the external moisture discharge mode, raise the dry bulb temperature to 33 - 34 °C at a speed of 1 °C / h, control the wet bulb temperature at 32.5 - 33.5 °C, and bake at a constant temperature for 6 - 8 h to initiate sweating and yellowing of the tobacco leaves, activate the activities of amylase and lipoxygenase, and initiate the metabolism and transformation of starch and lipid substances in the tobacco leaves; ii. Lipid synthesis substrate accumulation stage: Adopt the external moisture discharge mode, raise the dry bulb temperature to 37 - 38 °C at a speed of 1 °C / 2 h, with the wet bulb temperature of 36.5 - 37.5 °C, and bake at a constant temperature for 20 - 35 h. Extend the baking time in the early stage of the lipid substrate accumulation stage to prevent the rapid destruction of the cell membrane lipid structure, resulting in the premature termination of starch and lipid metabolism; then, raise the dry bulb temperature to 39 - 40 °C at a speed of 1 °C / 2 h, control the wet bulb temperature at 38 °C, and reduce the wet bulb temperature by 0.5 °C every 3 - 5 h. Finally, control the wet bulb temperature between 35.5 - 36.5 °C and bake at a constant temperature until the tobacco leaves turn yellow by 8 - 90% and the leaves are fully softened to increase the metabolism rate of starch and lipid substances and further promote the accumulation of lipid synthesis substrates; iii. Lipid synthesis stage: Adopt the internal dehumidification mode and construct a low-oxygen environment, raise the dry bulb temperature to 42 - 43 °C at a gradient of 1 °C / 3 - 4 h, control the wet bulb temperature at 35 - 36.5 °C, and bake at a constant temperature for 6 - 8 h; then, raise the dry bulb temperature to 44 - 45 °C at a speed of 1 °C / 3 - 4 h, control the wet bulb temperature at 35.5 - 37 °C, and bake at a constant temperature until the tobacco leaves form small rolls; by reducing the wet bulb temperature in combination with the low-oxygen environment, inhibit the oxidation reaction, reduce the oxidation ratio of unsaturated fatty acids, and promote the synthesis of lipid substances from oil precursor substances such as sugar substances, fatty acids, and terpenoid substances, thereby enhancing the accumulation of oil substances in tobacco leaves during the baking process; iv. Lipid stabilization transformation stage: Adopt external moisture removal and alternate air supply at the top and bottom of the baking chamber. Raise the dry-bulb temperature to between 48 and 49 °C at a rate of 1 °C every 2 - 3 h, control the wet-bulb temperature at 38 - 39 °C, and bake at a stable temperature for 10 - 15 h until more than 2 / 3 of the leaves are dry; then, raise the dry-bulb temperature to between 51 and 52 °C at a rate of 1 °C every 2 - 3 h, control the wet-bulb temperature at 38.5 - 39.5 °C, and bake at a stable temperature for 6 - 10 h until more than 3 / 4 of the leaves are dry; then, raise the dry-bulb temperature to 55 - 56 °C at a rate of 1 °C every 2 - 3 h, control the wet-bulb temperature at 39 - 40 °C, and bake at a stable temperature for 10 - 18 h until all the tobacco leaves are dry and more than 1 / 4 of the main veins are dry; by adopting the alternate air supply mode of the top and bottom of the baking chamber, reduce the volatilization of lipids and oil in the tobacco leaves, and promote the synthesis of lipid substances from oil precursor substances such as saccharide substances and terpenoid substances; v. Safe stem drying stage: Adopt the external moisture removal mode. Raise the dry-bulb temperature to between 58 and 59 °C at a rate of 1 °C every 1 - 2 h, control the wet-bulb temperature at 41 - 42 °C, and bake at a stable temperature for 6 - 8 h; then, raise the dry-bulb temperature to between 63 and 64 °C at a rate of 1 °C every 1 - 2 h, control the wet-bulb temperature at 41 - 42 °C, reduce the inter-leaf air velocity, and bake at a stable temperature until all the tobacco leaves in the whole baking chamber are completely dry; by reducing the inter-leaf air velocity, reduce the volatilization of lipids and oil in the tobacco leaves, and promote the accumulation of oil substances in the tobacco leaves.

2. The multi-stage harvesting and curing method for improving the oil content of tobacco leaves according to claim 1, characterized in that: The exogenous regulator that can improve the synthesis and accumulation of starch and lipid substances in fresh tobacco leaves is a mixed solution containing 50 - 100 mg / L of ascorbic acid and 50 - 80 mg / L of salicylic acid, with a dosage of 4 - 6 L per mu.

3. The multi-stage harvesting and curing method for improving the oil content of tobacco leaves according to claim 2, wherein: The internal dehumidification mode is to set a return air duct connecting the external moisture removal window and the heating chamber intake window, and set a condensation device on the return air duct to collect the water vapor in the return air duct. The internal dehumidification mode operates intermittently, and the intermittent operation method is to turn on the external moisture removal mode for 10 minutes every 4 - 5 h.

4. The multi-stage harvesting and curing method for improving the oil content of tobacco leaves according to claim 3, wherein: Both top air supply, bottom air supply and intermediate air supply are set in the baking chamber. The alternate air supply at the top and bottom of the baking chamber is that the top air supply and the bottom air supply alternate every 1 h, and the air velocity is 0.5 - 1.0 m / s, which is used to reduce lipid volatilization.

5. The multi-stage harvesting and curing method for improving the oil content of tobacco leaves according to claim 4, characterized in that: Infrared auxiliary heating modules for promoting the uniform migration of internal lipids in tobacco leaves are equipped on the roof and the ground against the wall in the baking chamber. The wavelength of the infrared auxiliary heating module is 3 - 5 μm.

6. The multi-stage harvesting and curing method for improving the oil content of tobacco leaves according to claim 5, characterized in that: During the safe stem drying stage, the circulation fan adopts a low-speed air velocity, and the air velocity in the loading chamber of the bulk curing barn is ≤ 0.3 m / s to ensure the stable solidification of oil substances.

7. The multi-stage harvesting and curing method for improving the oil content of tobacco leaves according to claim 3, wherein: The low-oxygen environment is formed by the self-metabolism of tobacco leaves consuming oxygen during the internal dehumidification stage.

8. The multi-stage harvesting and curing method for improving the oil content of tobacco leaves according to claim 3, wherein: The low-oxygen environment is to introduce carbon dioxide gas into the baking chamber during the lipid synthesis stage, so that the O2 concentration in the baking chamber is between 10% and 18%.

9. The multi-stage harvesting and curing method for improving the oil content of tobacco leaves according to claim 7 or 8, characterized in that: The bulk curing barn with the dual functions of internal dehumidification and external moisture removal and equipped with top air supply, bottom air supply and intermediate air supply is an air-source heat pump heating type tobacco baking barn.

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