Process for preparing green tobacco leaves with low-temperature vacuum freeze-drying

Abstract

The application relates to the technical field of tobacco processing, and discloses a process method for preparing green tobacco leaves with preserved aroma through low-temperature vacuum freeze drying, which comprises the following steps: removing impurities from fresh tobacco leaves and cleaning the fresh tobacco leaves by spraying low-temperature pure water, and controlling the overall water content by centrifugal air drying; uniformly placing the treated fresh tobacco leaves on a tray, and sending the fresh tobacco leaves into a vacuum freeze drying bin for constant-temperature pre-freezing treatment; starting primary sublimation main drying, controlling the vacuum degree in the bin and the material temperature to realize solid-gas sublimation dehydration; then adjusting the vacuum degree and slowly increasing the temperature to realize secondary analysis auxiliary drying; after the drying is completed, the vacuum in the bin is broken, and the material is discharged, sealed and stored in the dark to obtain finished products. Through low-temperature phase change and phased vacuum drying, the application reduces oxidation browning and loss of heat-sensitive aroma components in the processing, maintains the three-dimensional network microskeleton of the tobacco leaves, and obtains finished product tobacco leaves with green color and complete morphology.

Description

Technical Field

[0001] This invention relates to the field of tobacco processing technology, specifically to a process for preparing green and preserved tobacco leaves by low-temperature vacuum freeze drying. Background Technology

[0002] In existing tobacco processing techniques, the dehydration of fresh tobacco leaves is mostly achieved through heat drying methods such as hot air baking. During this drying process, external heat energy acts on the tobacco leaf tissue, causing internal moisture to evaporate. The high-temperature environment intensifies the thermal motion of heat-sensitive aroma components such as terpenes and esters within the tobacco leaf, causing aroma substances to escape with the moisture vapor, thus reducing the intrinsic flavor quality of the finished tobacco leaf. Along with the vaporization and removal of moisture, the capillary surface tension generated by the liquid water within the pores creates stress on the cell walls, causing the shrinkage and collapse of the three-dimensional microstructure of the tobacco leaf, resulting in deformation of the dried tobacco leaf.

[0003] Furthermore, during the temperature and humidity changes of traditional drying processes, polyphenol oxidases and lipoxygenases within the tobacco leaf tissue tend to retain some catalytic activity, causing chlorogenic acid and other phenolic substances to undergo oxidative polymerization reactions, resulting in browning and causing the fresh tobacco leaves to lose their original green appearance. Based on the aforementioned thermal degradation, physical dehydration shrinkage, and oxidative browning reactions, conventional heat-drying processes are insufficient to produce finished tobacco products that possess both the original green appearance and rich natural aroma. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a process for preparing green tobacco leaves by low-temperature vacuum freeze-drying, which solves the problems of aroma component loss, oxidative browning and greening, and dehydration and collapse of tissue structure caused by heating and evaporation during tobacco leaf drying in existing technologies.

[0005] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a process for preparing green tobacco leaves by low-temperature vacuum freeze-drying, comprising the following steps: Remove coarse and hard tobacco stems and damaged or rotten leaves. Use low-temperature pure water spraying at 4℃~10℃ to clean surface impurities. Remove surface free moisture by centrifugal air drying to control the overall moisture content of the tobacco leaves to 70%~80%. The pre-treated fresh tobacco leaves are evenly spread in the freeze-drying tray; The trays with the material laid are sent into a vacuum freeze-drying chamber for constant temperature pre-freezing treatment, with a pre-freezing temperature of -35℃ to -45℃. Start the primary sublimation drying process, control the vacuum degree of the freeze-drying chamber to be 50-80 Pa, and control the material temperature to be maintained at -10℃ to 0℃, and continuously sublimate and dry for 8-12 hours; Adjust the vacuum level of the freeze-drying chamber to 20-50 Pa for secondary desorption and auxiliary drying, and slowly raise the temperature to 15-25℃ for constant temperature desorption and drying for 3-5 hours. After drying, the vacuum inside the silo is broken, the material is discharged, and it is sealed and stored in light-proof packaging to obtain the finished green tobacco leaves.

[0006] By adopting the above technical solution, the use of low-temperature pre-cooling with specific parameters and staged vacuum drying avoids high-temperature thermal degradation, achieving synergistic effects of tobacco leaf dehydration and aroma / color preservation, resulting in a finished product with intact shape, green color, and rich aroma. From a process mechanism perspective, it mainly includes the physicochemical changes corresponding to four stages: washing, pre-freezing, main drying, and auxiliary drying. During the washing stage, low-temperature pure water spraying removes impurities while simultaneously removing field heat from the fresh tobacco leaves. The low temperature reduces the catalytic activity of polyphenol oxidase and lipoxygenase within the tobacco leaf tissue, inhibiting the oxidation reaction that converts phenolic substances such as chlorogenic acid and rutin into ortho-quinone colored polymers, thus cutting off the browning pathway at its source and locking in the green color. Centrifugal dehydration controls the overall moisture content within the target range, limiting the total amount of free water and providing a suitable material basis for the freezing phase transition.

[0007] After entering the constant-temperature pre-freezing treatment stage, as the temperature drops below the eutectic point of the material, the free water within the tissue undergoes a phase transition from liquid to solid, crystallizing. The cryogenic environment promotes a nucleation rate that exceeds the growth rate, forming a fine and uniformly dispersed network of micro-ice crystals inside and outside the cells. The reduced physical expansion stress of the micro-ice crystals prevents mechanical tearing damage to organelles and cell walls, maintaining the original three-dimensional network micro-skeleton structure of the tobacco leaf.

[0008] When the primary sublimation drying process is initiated, the system pressure drops below the triple point pressure of water. Under stable vacuum and temperature gradient, ice crystals absorb heat from the surroundings and undergo a solid-gas phase transition, sublimating and being removed. Water is expelled in gaseous form along the interstitial spaces, and the space originally occupied by ice crystals is transformed into porous capillary channels. Continuous sublimation endothermic processes keep the system at a constant low temperature, inhibiting the molecular thermal motion of volatile aroma components such as terpenes and esters, and reducing their loss.

[0009] Finally, in the secondary desorption and drying stage, for the unfrozen bound water molecules, the desorption driving force for mass transfer is increased by reducing absolute pressure and slowly providing heat energy. The heat energy overcomes the hydrogen bond energy between bound water and the macromolecular skeleton, causing it to desorb into a gaseous state, reducing water activity, and giving the finished product long-term storage stability against microbial contamination.

[0010] Preferably, in the primary sublimation drying, the vacuum degree of the freeze-drying chamber is controlled at 65-70 Pa, the cold trap temperature is -55°C, the material temperature is maintained at -5°C to -2°C, and the sublimation drying is carried out continuously for 10-11 hours. In the secondary desorption drying process, the vacuum degree of the freeze-drying chamber is adjusted to 35-40 Pa, and the temperature is slowly increased to 20-22°C at a rate of 1-3°C / h, and the desorption drying is carried out at a constant temperature for 3.5-4 hours.

[0011] By employing the above technical solution, the defined vacuum level and temperature range constitute the heat and mass transfer boundary conditions adapted to ice crystal sublimation. The -55℃ cold trap provides water vapor capture capability, maintaining the pressure difference at the sublimation interface. The temperature rise rate of 1℃ / h to 3℃ / h matches the internal moisture diffusion rate, preventing excessively rapid external heating from causing the surface tissue to harden and seal the internal airflow channels, thereby improving the final drying quality and morphological uniformity.

[0012] Preferably, pure water is prepared and its temperature is controlled within the range of 4℃ to 10℃; the low-temperature pure water at the set temperature is used for spraying the cleaning area to form a low-temperature pure water spray cleaning work area.

[0013] By adopting the above technical solution, a continuous and stable low-temperature water area is formed into a closed cold environment. This setting reduces the thermal disturbance caused by ambient air to the cleaning area and ensures the consistency of enzyme activity inactivation for different batches of materials.

[0014] Preferably, the vacuum freeze-drying chamber is equipped with a chilled water inlet and waste heat recovery module and a negative pressure sealed direct-connection dehumidification and dust removal component, and operates in a completely sealed and light-proof manner throughout the process.

[0015] By adopting the above technical solution, environmental isolation is achieved through negative pressure airflow control. Simultaneously, light-protected operation is maintained throughout the entire dehydration cycle, avoiding photocatalytic degradation caused by light and blocking the photo-oxidative decomposition chain reaction of chlorophyll in the ultraviolet and visible light bands, thereby maintaining the product's color retention rate.

[0016] Preferably, the chilled water connection and waste heat recovery module is implemented as follows: the inlet of the cascade low-temperature refrigeration unit is directly connected to the existing 7℃ and 12℃ central air conditioning chilled water supply network of the cigarette factory; a waste heat recovery device is set up to recover the low-temperature water vapor waste heat generated during the freeze-drying process and connect it to the fresh air preheating system of the cigarette factory workshop to form a low-temperature water vapor waste heat circulation recovery loop.

[0017] By adopting the above technical solutions, a cross-system energy cascade utilization network was constructed. The air conditioning duct network replaces independent cooling towers, providing a stable source of cold medium and improving the Carnot cycle efficiency on the refrigeration side. On the other hand, the latent heat of low-temperature water vapor is converted into the sensible heat load of the fresh air system, reducing the total energy consumption of the equipment operation.

[0018] Preferably, the specific implementation of the negative pressure sealed direct-connected dehumidification and dust removal component is as follows: a freeze-drying chamber and a Roots vacuum unit are set up using a negative pressure sealed structure; the negative pressure end of the chamber is directly connected to the original dehumidification and dust removal duct of the cigarette factory workshop for airtight connection, so as to ensure that the exhaust process is directly integrated into the workshop exhaust system under negative pressure, so as to achieve zero overflow of dust and odor.

[0019] By adopting the above technical solution, the local exhaust of the equipment is made fluid-mechanically connected to the main exhaust of the workshop. A stable negative pressure buffer zone is established on the exhaust side of the Roots vacuum unit, which reduces the turbulent backflow of dust-laden water vapor at the interface and prevents cross-contamination of the production environment.

[0020] Preferably, mature fresh leaves of the Golden Leaf variety are selected and processed within 1 to 2 hours after harvesting to remove coarse and hard tobacco stems and damaged or rotten leaves; surface impurities are washed by spraying with low-temperature pure water at 5℃ to 7℃, and surface free moisture is removed by centrifugal air drying to control the overall moisture content of the tobacco leaves to 75% to 78%.

[0021] By adopting the above technical solution, the post-harvest settling time is shortened, and processing is carried out before the leaf's vitality and respiratory metabolism enter the decline period, thus preserving physiological metabolic products. In addition, the volume expansion caused by the 75%–78% moisture content during the ice phase transition offsets some of the cold shrinkage deformation, resulting in a smooth shape of the dried tobacco leaves.

[0022] Preferably, the freeze-drying tray is a food-grade stainless steel freeze-drying tray; the pre-treated fresh tobacco leaves are evenly spread in a single layer in the food-grade stainless steel freeze-drying tray, with the thickness of the spread material controlled at 20-30mm, and the tobacco leaves are free from overlap and compression damage.

[0023] By adopting the above technical solution, a short-range mass transfer channel is established in a single-layer, flat material layer with a limited thickness. Radiant heat and conductive heat act simultaneously on the blade surface, causing each piece of material to reach the phase transition endpoint at approximately the same time point, thus reducing the drying blind zone caused by the thickness effect.

[0024] Preferably, the storage chamber is first pre-cooled to -40℃, followed by constant temperature pre-freezing treatment. The pre-freezing temperature is -38℃ to -40℃, and the holding time is 75 to 80 minutes, so that the temperature of the center of the tobacco leaves drops to -31℃ to -32℃.

[0025] By adopting the above technical solution, the pre-cooling of the chamber reduces environmental heat capacity interference and forms a steep cooling curve. The core temperature of -31℃ to -32℃ is close to the complete glass transition temperature of the system, which slows down solute migration at the molecular scale and locks the distribution space of aroma substances within the mesophyll tissue.

[0026] Preferably, after drying, the vacuum inside the chamber is slowly broken by high-purity nitrogen, and the material is discharged quickly at room temperature. It is then sealed and stored in light-proof aluminum foil bags to obtain a finished green tobacco leaf product with a moisture content of 10% to 11%.

[0027] By adopting the above technical solution, high-purity nitrogen is used as an inert protective medium to backfill the porous structure inside the silo, replacing free oxygen and inhibiting the proliferation of aerobic microorganisms and oil oxidation. Simultaneously, the final moisture content of 10%–11% is lower than the critical water activity for mold growth, ensuring the finished product meets long-term storage standards.

[0028] This invention provides a process for preparing green tobacco leaves using low-temperature vacuum freeze-drying. It has the following beneficial effects: 1. This invention inhibits oxidative browning during processing, maintaining the green appearance of tobacco leaves. Low-temperature pure water is used for washing and spraying, which removes impurities while reducing the catalytic activity of polyphenol oxidase and lipoxygenase in the leaves. Combined with the fully enclosed, light-protected operation of the equipment, the oxidative pathway of polyphenols transforming into colored polymers is blocked, reducing the photodecomposition of chlorophyll and thus preserving the original natural color of fresh tobacco leaves.

[0029] 2. This invention reduces the loss of heat-sensitive aroma components and the shrinkage and collapse of cell structures. By combining constant-temperature pre-freezing with staged vacuum drying, the internal moisture of the tobacco leaves undergoes solid-gas sublimation dehydration in the form of tiny ice crystals under low pressure. This low-temperature phase change environment inhibits the thermal movement of volatile aroma components such as terpenes and esters, while avoiding the surface tension pulling of the liquid phase caused by conventional heating evaporation, thus maintaining the inherent three-dimensional network microstructure of the tobacco leaves after drying.

[0030] 3. This invention optimizes the energy utilization rate of the drying equipment and improves the exhaust environment of the workshop. By connecting the chilled water connection and waste heat recovery module to the existing central air conditioning chilled water network and fresh air preheating system of the cigarette factory, a tiered circulation of cooling and heating loads between different systems in the factory is achieved. Simultaneously, a negative pressure sealed structure connects the exhaust side of the vacuum unit to the existing dehumidification and dust removal duct, allowing dust-laden water vapor to converge into the main exhaust pipe under negative pressure buffering, reducing cross-interference of odors and dust with the production area. Attached Figure Description

[0031] Figure 1 The figure shows the test results of the dehydration efficiency and shape retention of the present invention; Figure 2 The following are the results of the chlorophyll and aromatic substance retention test of the present invention, wherein (a) is the result of the test of total chlorophyll and color difference a* value; and (b) is the result of the test of the concentration of key aroma components. Figure 3This is a comparison and evaluation chart of the quality and yield rate of the present invention, wherein (a) is a comparison chart of the appearance defect rate; and (b) is a comparison chart of the retention rate of the internal aroma polyphenols. Figure 4 The diagram shows the microstructure and energy consumption evaluation of the present invention, wherein (a) is a diagram showing the test results of the micropore structure and mechanical tensile properties; and (b) is a diagram showing the energy consumption comparison of the dehydration process system. Figure 5 The diagram shows the rehydration characteristics and sensory evaluation results of the present invention, wherein (a) is a distribution diagram of the difference in the dry sheet rehydration ratio; and (b) is a diagram of the sensory quality evaluation results of a single-material cigarette. Figure 6 The diagram shows the chemical composition and storage stability evaluation of the present invention, wherein (a) is the test result of the retention rate of the core chemical components; and (b) is the test result of the storage decay under simulated high temperature and high humidity environment (180 days). Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for preparing a chilled water docking and waste heat recovery module, including the following steps: The inlet of the cascade low-temperature refrigeration unit is directly connected to the existing 7℃ and 12℃ central air conditioning chilled water supply network of the cigarette factory.

[0034] A waste heat recovery device is installed to recover the low-temperature water vapor waste heat generated during the freeze-drying process and connect it to the fresh air preheating system in the cigarette factory workshop, forming a loop for the recycling of low-temperature water vapor waste heat.

[0035] Preparation Example 2: This preparation example provides a method for preparing a negative pressure closed direct-connection dehumidification and dust removal component, including the following steps: The freeze-drying chamber and Roots vacuum unit are set up using a negative pressure closed structure.

[0036] The negative pressure end of the silo is directly connected to the existing dehumidification and dust removal duct of the cigarette factory workshop in an airtight manner, ensuring that the exhaust process is directly integrated into the workshop exhaust system under negative pressure, achieving zero overflow of dust and odor.

[0037] Preparation Example 3: This preparation example provides a method for preparing a low-temperature constant-temperature cleaning medium and cleaning components, including the following steps: Prepare pure water and control its temperature within a specific setting range of 4℃ to 10℃.

[0038] The cleaning area is sprayed with low-temperature pure water at the specific temperature mentioned above to form a low-temperature pure water spray cleaning work area.

[0039] Examples 1-4: Example 1:

[0040] This embodiment provides a process for preparing green tobacco leaves by low-temperature vacuum freeze-drying, including the following steps: Mature fresh leaves of the Golden Leaf variety harvested in Yunnan Province were selected and processed within 1.5 hours after harvesting to remove coarse, hard tobacco stems and damaged or rotten leaves.

[0041] The surface impurities are cleaned by spraying with 7℃ low-temperature pure water, and the surface free moisture is removed by centrifugal drying to control the overall moisture content of the tobacco leaves to 75%.

[0042] The pre-treated fresh tobacco leaves are evenly spread in a single layer in a food-grade stainless steel freeze-drying tray, with the thickness of the layer controlled at 25mm, ensuring that the tobacco leaves do not overlap or suffer any compression damage.

[0043] The tray with the material laid is sent into a vacuum freeze-drying chamber equipped with the chilled water docking and waste heat recovery module described in Preparation Example 1 and the dehumidification and dust removal component described in Preparation Example 2. The entire process is carried out in a sealed and light-proof manner.

[0044] First, the storage chamber is pre-cooled to -40℃, and then a constant temperature pre-freezing treatment is carried out. The pre-freezing temperature is -40℃ and the holding time is 75 minutes, so that the temperature of the center of the tobacco leaves drops to -32℃.

[0045] Start the primary sublimation drying process, control the vacuum degree of the freeze-drying chamber to 65 Pa, the cold trap temperature to -55℃, and maintain the material temperature at -5℃, and continuously sublimate and dry for 10 hours.

[0046] Adjust the vacuum level of the freeze-drying chamber to 35 Pa for secondary desorption and auxiliary drying. Slowly increase the temperature to 20°C at a rate of 2°C / h and maintain the temperature for desorption and drying for 4 hours.

[0047] After drying, the vacuum inside the chamber is slowly broken by high-purity nitrogen gas, and the material is discharged quickly at room temperature. It is then sealed and stored in light-proof aluminum foil bags to obtain the finished green tobacco leaves with a moisture content of 10%.

[0048] Example 2:

[0049] This embodiment provides a process for preparing green tobacco leaves by low-temperature vacuum freeze-drying, including the following steps: Mature fresh leaves of the Golden Leaf variety harvested in Yunnan are selected and processed within 1 hour of harvesting to remove coarse, hard stems and damaged or rotten leaves.

[0050] The surface impurities are cleaned by spraying with pure water at a low temperature of 4℃, and the surface free moisture is removed by centrifugal drying to control the overall moisture content of the tobacco leaves to 70%.

[0051] The pre-treated fresh tobacco leaves are evenly spread in a single layer in a food-grade stainless steel freeze-drying tray, with the thickness of the layer controlled at 20mm, ensuring that the tobacco leaves do not overlap or suffer any compression damage.

[0052] The tray with the material laid is sent into a vacuum freeze-drying chamber equipped with the chilled water docking and waste heat recovery module described in Preparation Example 1 and the dehumidification and dust removal component described in Preparation Example 2. The entire process is carried out in a sealed and light-proof manner.

[0053] First, the storage chamber is pre-cooled to -40℃, and then a constant temperature pre-freezing treatment is carried out. The pre-freezing temperature is -35℃, and the holding time is 60 minutes, so that the temperature of the center of the tobacco leaves drops to -30℃.

[0054] Start the primary sublimation drying process, control the vacuum degree of the freeze-drying chamber to 50 Pa, the cold trap temperature to -55℃, and maintain the material temperature at -10℃, and continuously sublimate and dry for 8 hours.

[0055] Adjust the vacuum level of the freeze-drying chamber to 20 Pa for secondary desorption and auxiliary drying. Slowly increase the temperature to 15°C at a rate of 1°C / h and maintain the temperature for desorption and drying for 3 hours.

[0056] After drying, the vacuum inside the chamber is slowly broken by high-purity nitrogen gas, and the material is discharged quickly at room temperature. It is then sealed and stored in light-proof aluminum foil bags to obtain the finished green tobacco leaves with a moisture content of 8%.

[0057] Example 3:

[0058] This embodiment provides a process for preparing green tobacco leaves by low-temperature vacuum freeze-drying, including the following steps: Mature fresh leaves of the Golden Leaf variety harvested in Yunnan are selected and processed within 2 hours after harvesting to remove coarse, hard tobacco stems and damaged or rotten leaves.

[0059] The surface impurities are cleaned by spraying with pure water at a low temperature of 10℃, and the surface free moisture is removed by centrifugal drying to control the overall moisture content of the tobacco leaves to 80%.

[0060] The pre-treated fresh tobacco leaves are evenly spread in a single layer in a food-grade stainless steel freeze-drying tray, with the thickness of the layer controlled at 30mm, ensuring that the tobacco leaves do not overlap or suffer any compression damage.

[0061] The tray with the material laid is sent into a vacuum freeze-drying chamber equipped with the chilled water docking and waste heat recovery module described in Preparation Example 1 and the dehumidification and dust removal component described in Preparation Example 2. The entire process is carried out in a sealed and light-proof manner.

[0062] First, the storage chamber is pre-cooled to -40℃, and then a constant temperature pre-freezing treatment is carried out. The pre-freezing temperature is -45℃, and the holding time is 90 minutes, so that the temperature of the center of the tobacco leaves drops to -35℃.

[0063] Start the primary sublimation drying process, control the vacuum degree of the freeze-drying chamber to 80 Pa, the cold trap temperature to -55℃, and maintain the material temperature at 0℃ for 12 hours of continuous sublimation drying.

[0064] Adjust the vacuum level of the freeze-drying chamber to 50 Pa for secondary desorption and auxiliary drying. Slowly increase the temperature to 25°C at a rate of 3°C / h and maintain the temperature for desorption and drying for 5 hours.

[0065] After drying, the vacuum inside the chamber is slowly broken by high-purity nitrogen gas, and the material is discharged quickly at room temperature. It is then sealed and stored in light-proof aluminum foil bags to obtain the finished green tobacco leaves with a moisture content of 12%.

[0066] Example 4:

[0067] This embodiment provides a process for preparing green tobacco leaves by low-temperature vacuum freeze-drying, including the following steps: Mature fresh leaves of the Golden Leaf variety harvested in Yunnan Province were selected and processed within 1.2 hours after harvesting to remove coarse, hard tobacco stems and damaged or rotten leaves.

[0068] The surface impurities are cleaned by spraying with pure water at 5℃, and the surface free moisture is removed by centrifugal drying, controlling the overall moisture content of the tobacco leaves to 78%.

[0069] The pre-treated fresh tobacco leaves are evenly spread in a single layer in a food-grade stainless steel freeze-drying tray, with the thickness of the spread controlled at 22mm, and the tobacco leaves are free from overlap and compression damage.

[0070] The tray with the material laid is sent into a vacuum freeze-drying chamber equipped with the chilled water docking and waste heat recovery module described in Preparation Example 1 and the dehumidification and dust removal component described in Preparation Example 2. The entire process is carried out in a sealed and light-proof manner.

[0071] First, the storage chamber is pre-cooled to -40℃, and then a constant temperature pre-freezing treatment is carried out. The pre-freezing temperature is -38℃, and the holding time is 80 minutes, so that the temperature of the center of the tobacco leaves drops to -31℃.

[0072] Start the primary sublimation drying process, control the vacuum degree of the freeze-drying chamber to 70 Pa, the cold trap temperature to -55℃, and maintain the material temperature at -2℃, and continuously sublimate and dry for 11 hours.

[0073] Adjust the vacuum level of the freeze-drying chamber to 40 Pa for secondary desorption and auxiliary drying. Slowly increase the temperature to 22℃ at a rate of 2.5℃ / h and maintain the temperature for desorption and drying for 3.5h.

[0074] After drying, the vacuum inside the chamber is slowly broken by high-purity nitrogen, and the material is discharged quickly at room temperature. It is then sealed and stored in light-proof aluminum foil bags to obtain the finished green tobacco leaves with a moisture content of 11%.

[0075] Comparative Examples 1-5: Comparative Example 1: Compared with Example 1, the difference is that the constant temperature pre-freezing and segmented vacuum freeze-drying process was not used. Instead, the pre-treated fresh tobacco leaves were directly sent into a conventional curing barn and dried with high-temperature hot air circulation at 55℃~70℃ until the moisture content was 10%. All other aspects were the same.

[0076] Comparative Example 2: Compared with Example 1, the difference is that the steps of pre-cooling the chamber to -40°C and pre-freezing at a constant temperature of -40°C are omitted. The fresh tobacco leaves at room temperature after being laid out are placed directly in the room temperature vacuum chamber, and the vacuum unit is directly started to vacuum and cool down for drying. Everything else is the same.

[0077] Comparative Example 3: Compared with Example 1, the difference is that in the secondary analytical drying stage, the temperature of the constant temperature analytical drying is increased to 40°C for drying and dehydration, while the rest are the same.

[0078] Comparative Example 4: Compared with Example 1, the difference is that after drying, the vacuum inside the chamber was broken by directly connecting it to the natural air in the workshop, and high-purity nitrogen was not used for vacuum breaking and isolation protection. All other aspects are the same.

[0079] Comparative Example 5: Compared with Example 1, the difference is that: an independent conventional commercial food freeze-drying equipment is used, the frozen water docking and waste heat recovery module described in Example 1 is not connected, and the exhaust end is not connected to the negative pressure closed direct-connection dehumidification and dust removal component described in Example 2, and the exhaust gas is directly discharged into the workshop environment. All other aspects are the same.

[0080] Test Examples 1-6: Test Example 1: The experimental subjects were the finished green tobacco leaves prepared according to the complete process flow of Examples 1, 2, 3, and 4. Sampling was conducted in a light-protected, temperature-controlled environment with relative humidity below 40%, and leaves were randomly selected from sealed aluminum foil bags corresponding to each batch of each example as test samples.

[0081] Fragments were cut from the leaf tip, middle, and base of each sample in each embodiment. After thorough mixing, approximately 5 grams of the test material were weighed and spread evenly in the sample pan of the halogen moisture analyzer. The heating temperature was set to 105°C, and the amount of free water evaporation was measured in constant weight mode. The final moisture content value when the instrument determined constant weight was recorded. Each embodiment was measured three times in parallel, and the arithmetic mean was taken.

[0082] Ten whole tobacco leaves were randomly selected from each embodiment before and after drying. Using calipers, five evenly distributed fixed points were selected on the surface of each tobacco leaf to measure its thickness. Image processing software was used to calculate the planar projected area of ​​the fresh leaf and the dried product through contour extraction. Based on the absolute changes in thickness and area before and after drying, the thickness shrinkage rate and area shrinkage rate of each sample were calculated.

[0083] Accurately weigh approximately 2 grams of whole, dry tobacco leaf sample and record the initial dry weight. Completely immerse the sample in a beaker containing room temperature deionized water and allow it to soak for 30 minutes to allow the water to fully penetrate the pores of the leaf. Remove the sample and hang it above filter paper to drip dry until no obvious droplets fall from the surface. Weigh the sample after rehydration. Calculate the rehydration ratio of the sample as the ratio of the rehydrated mass to the initial dry weight.

[0084] Table 1. Evaluation data on phase change dehydration efficiency and tissue shape preservation of the finished products in each embodiment.

[0085] Summarize: Figure 1 The horizontal axis represents four examples, the left vertical axis shows the final moisture content, thickness shrinkage rate, and area shrinkage rate, and the right vertical axis shows the rehydration ratio. Solid lines connect dark gray squares representing the final moisture content, dashed lines connect medium gray triangles representing thickness shrinkage rate, dotted lines connect light gray rhombuses representing area shrinkage rate, and dashed lines connect white circles representing the rehydration ratio.

[0086] According to the data in Table 1, the final moisture content of the finished products in the four embodiments ranged from 8.35% to 11.78%, with Example 1 having a moisture content of 10.12%. This range meets the material moisture requirements of the cigarette manufacturing production line. This indicates that by supplementing the latent heat of sublimation with a low-temperature radiant plate in a low-pressure environment, the phase transition sublimation of water molecules can be achieved. When using hot air for dehydration and drying, the migration of liquid water inside the leaves causes surface tension shrinkage. In this test, Example 3 had relatively high thickness shrinkage and area shrinkage rates, at 4.5% and 4.8%, respectively, while the shrinkage rates of all four embodiments were below 5%. The leaves after segmented vacuum drying maintained their appearance at harvest. The initial low-temperature gradient pre-freezing stage of the system caused free water in the plant tissue to cross the maximum crystallization zone, forming dispersed micro-ice crystals inside and outside the cells. Under these conditions, the crystals did not grow, and the plant's cellulose skeleton and cell wall membranes were not damaged by ice crystal puncture. After the microcrystals sublimated in the gaseous state in the subsequent vacuum, a porous network was formed inside the plant. This porous skeleton was preserved, increasing the surface area inside the dry matter. The recorded rehydration ratios ranged from 5.61 to 5.92, with Example 2, which had a lower moisture content, showing a rehydration ratio of 5.92, verifying the integrity of the porous structure. The pore structure provides a permeation path for the external liquid feedstock during the cigarette flavoring and additive process. When the process parameters varied within the set range, the drying indicators and morphological data remained stable, indicating the stability of the processing technology.

[0087] Test Example 2: The experimental subjects were the finished green tobacco leaves prepared according to Examples 1, 2, 3 and 4.

[0088] Turn on the high-precision colorimeter and calibrate it using a standard white board and black board. Randomly select 10 flat areas on both sides of the blades of each embodiment sample to measure color parameters, read the 'a' value in the color space, record the measurement data, and calculate the arithmetic mean as the evaluation index for the sample color. A negative 'a' value indicates that the color is more greenish.

[0089] Weigh 0.5 g of the ground, whole tobacco leaf powder sample into a centrifuge tube, add an appropriate amount of 80% acetone aqueous solution, and perform ultrasonic extraction under light-protected conditions. After centrifugation, collect the supernatant and place it in a cuvette of a UV-Vis spectrophotometer. Measure the absorbance at wavelengths of 663 nm and 645 nm, respectively. Calculate the absolute contents of chlorophyll a and chlorophyll b using the absorbance data; the sum of the two is the total chlorophyll content.

[0090] A simultaneous distillation-extraction apparatus was used. A suitable amount of chopped tobacco leaf sample was weighed and an internal standard solution was added for extraction to collect the organic phase rich in volatile components. The extract was dehydrated and concentrated before being injected into a gas chromatography-mass spectrometry (GC-MS) system for analysis. Volatile components were qualitatively identified based on a mass spectral library, and the absolute mass concentrations of target compounds (terpenes and aromatic alcohols) were calculated using the internal standard method.

[0091] Table 2. Evaluation data on chlorophyll and aromatic substance retention of finished products from each embodiment.

[0092] Summarize: Figure 2 (a) shows the changes in total chlorophyll content and color difference (a) values ​​for characterizing color in the samples of each embodiment. Figure 2 (b) The mass concentration distribution of trace volatile substances terpenes and aromatic alcohols in each sample is presented. The horizontal axis of the figure represents the four sample examples corresponding to the test. In (a), the left vertical axis of the figure shows the total chlorophyll content, and the right vertical axis corresponds to the color difference a value. The dark gray pentagrams connected by solid lines represent the total chlorophyll content, and the medium gray hexagons connected by dashed lines represent the color difference a value. In (b), the left vertical axis of the figure shows the terpene concentration, and the right vertical axis corresponds to the aromatic alcohol concentration. The crosses connected by dotted lines correspond to the terpene concentration, and the plus signs connected by dotted lines represent the aromatic alcohol concentration.

[0093] According to the data in Table 2, the color difference a values ​​measured in the four examples all fell within the range of -9.87 to -11.45, exhibiting a negative value, with Example 1 reaching a color difference a value of -11.45. Chlorosis in plant tissues is generally related to polyphenol oxidase enzymatic reactions induced by increased environmental temperature and chlorophyll degradation. The process operates at low temperatures, inhibiting the activity of polyphenol oxidase. The system utilizes a vacuum environment combined with high-purity nitrogen for pressure balancing, reducing the probability of oxygen contact within the system and blocking the oxidation reaction of chlorophyll, thus preserving the original color of the leaves. Spectroscopic measurements showed that the total chlorophyll content of the samples ranged from 1.95 to 2.31 mg / g, with Example 1 reaching 2.31 mg / g. This chemical indicator corresponds to the apparent color data. Terpenes and aromatic alcohols in tobacco leaves are mostly heat-sensitive and volatile organic compounds. The terpene concentrations in the four examples ranged from 129.5 to 145.6 μg / g, and the aromatic alcohol concentrations ranged from 78.4 to 89.2 μg / g, with Example 1 showing the highest retention rates. Under heating and baking conditions, these substances are prone to evaporation or breakage of internal chemical bonds. During drying, the system's radiant heat only compensates for the latent heat required for water sublimation, and the upper temperature limit of the dehydration process is below 25°C, failing to reach the boiling points of most organic aroma components. The sealed vacuum negative pressure chamber physically reduces the leakage of aromatic molecules. Example 1, as the baseline parameter combination, showed better data in terms of aroma concentration retention and color curing.

[0094] Test Example 3: The experimental subjects were the finished products prepared using the process of Example 1 and the finished products prepared using Comparative Example 1 (conventional high-temperature hot air circulation baking at 55°C to 70°C). Both processing procedures used freshly harvested tobacco leaves of the same variety and of equal quality as initial materials from the same batch.

[0095] After the drying process, the two batches of finished products are laid flat on an inspection table equipped with a standard light source (color temperature 6500K) for manual full-volume sorting. Leaves with grayish-brown patches on the surface (ashing), overall or partial color change from green to yellowish-brown (browning), carbonized and blackened leaf edges or veins (scorching), and physical breakage (broken) are picked out and weighed. The percentage of the mass of each type of defective leaf relative to the total dry matter mass of the batch is calculated to obtain a quantitative indicator of the defect rate.

[0096] Samples were randomly selected from the two groups of good products and subjected to low-temperature grinding. The absolute mass of core aroma-producing polyphenols such as chlorogenic acid and rutin in the powder was determined using high-performance liquid chromatography (HPLC). An equal amount of fresh leaves that had not undergone drying treatment was extracted simultaneously to determine the initial total polyphenol content. The total polyphenol retention rate was calculated by comparing the measured value of the dried product with the initial value of the fresh leaves.

[0097] Table 3. Comparative Evaluation Data of Overall Quality and Raw Material Utilization Rate between Example 1 and Comparative Example 1

[0098] Summarize: Figure 3 (a) shows the proportion of finished products with four types of appearance defects: ash, browning, scorching and breakage. The horizontal axis corresponds to the specific defect category, and the vertical axis is the defect quality percentage. The black asterisks connected by the black solid line represent the data trajectory of Example 1, and the light gray lower triangles connected by the black dashed line represent the data trajectory of Comparative Example 1. Figure 3 (b) The overall retention of the core aroma-producing polyphenols is presented. The horizontal axis corresponds to the two groups of test subjects, and the vertical axis is the percentage of retention relative to the fresh leaf baseline. The dark gray bars represent Example 1, and the light gray bars represent Comparative Example 1. The specific retention rate values ​​are marked above the bars.

[0099] According to the data in Table 3, Comparative Example 1, which used traditional hot air drying, exhibited a high proportion of material loss in terms of appearance quality, with a browning rate of 27.81%, an ash-covering rate of 12.56%, a breakage rate of 15.62%, and a scorching rate of 4.73%. In the hot air circulation environment of a conventional curing barn, the evaporation and migration of water within plant cells is a slow, transboundary thermodynamic process. During this period, the tobacco leaves are exposed to a relatively high temperature of 55℃ to 70℃ for an extended period. This environment provides a catalytic zone for polyphenol oxidase, leading to the destruction of the chlorophyll skeleton and the oxidation of phenolic substances, which macroscopically manifests as yellowing and browning of the leaves and localized gray spots. When hot air flows through the dense gaps between materials, uneven airflow distribution easily occurs, causing some leaf edges to scorch due to excessive water loss. The surface tension generated by the outward expulsion of water causes irreversible physical shrinkage of the leaves, resulting in stress concentration in the internal tissues, thus increasing the risk of breakage during handling and loading / unloading.

[0100] In the test results of Example 1, the browning rate was suppressed to 1.34%, the scorching rate was 0.00%, and the ash retention rate and breakage rate were at 0.12% and 2.45%, respectively. The phase change dehydration mechanism of low-temperature pre-freezing combined with low-pressure sublimation avoids the active temperature range of heat-sensitive enzymes, and the intervention of high-purity nitrogen breaks the oxidation chain. Free water absorbs latent heat of radiation in the solid state and becomes gaseous. When it is released from cell binding, it does not generate capillary contraction force, maintaining the physical flexibility of the original pore structure of the leaves and reducing the probability of dry leaf breakage. At the chemical composition level, the total polyphenol retention rate of Comparative Example 1 was 34.2%, while that of Example 1 reached 88.6%. The molecular bonds of polyphenolic aromatic substances are relatively sensitive to heat input, and continuous high-temperature baking causes thermal degradation of aromatic substances. The drying chamber temperature of Example 1 was controlled below 25°C throughout the process, and the energy provided by the micro-thermal radiation pipes was used to maintain the latent heat consumption of water molecule phase change, which was insufficient to destroy the chemical structure of organic matter. The vacuum negative pressure chamber forms a sealed physical boundary, preventing the volatile aroma from leaking out with the dehumidifying airflow.

[0101] Test Example 4: The experimental subjects were dehydrated tobacco leaves prepared using the processes of Examples 1 to 4 of this invention. Two sets of control samples were also introduced: Comparative Example 1 (commonly dried by hot air circulation at 65°C) and Comparative Example 2 (commonly freeze-dried product, i.e., after pre-freezing at -40°C, heating is achieved solely through heat conduction via contact with the vacuum chamber shelf, without the intervention of radiation plates or nitrogen pressure balancing).

[0102] Samples were randomly selected from each group and pretreated in a vacuum degassing station before being transferred to a surface area and pore size analyzer (BET analyzer). Adsorption and desorption isotherms of nitrogen were measured in a liquid nitrogen environment (77 K). The surface area parameter per unit mass of sample was calculated using a multi-point BET equation to assess the retention of the porous framework within the plant after dehydration.

[0103] Complete dry leaves from each group were selected and uniformly cut into standard rectangular strips 15 mm wide and 50 mm long, avoiding the midrib area. The strips were fixed at both ends to the fixtures of a computer-controlled universal testing machine, and a constant tensile rate was set for fracture testing. The system automatically recorded the maximum load during the tensile process and calculated the ultimate tensile strength (MPa) based on the cross-sectional area, serving as an indicator for evaluating the flexibility and resistance to mechanical damage of the dried product.

[0104] A high-precision three-phase power quality analyzer was connected in series in the main power supply circuit of the drying equipment corresponding to each process group to record the active power consumption throughout the entire cycle from the entry of fresh leaves into the drying chamber to the exit of the drying chamber after reaching the target moisture content. Combined with the absolute moisture mass removed in each batch, the unit dehydration energy consumption (kWh / kg) was calculated to quantify the economic viability of the process in industrial applications.

[0105] Table 4. Microstructure, mechanical properties, and energy consumption test and evaluation data of each embodiment and comparative example.

[0106] Summarize: Figure 4 (a) Reveals the micropore area of ​​each test object after molding and its corresponding macroscopic mechanical strength performance. The left vertical axis of the figure shows the BET specific surface area data, and the right vertical axis corresponds to the tensile strength value. The dark gray right-facing triangles connected by solid lines represent specific surface area, and the light gray left-facing triangles connected by dotted lines represent tensile strength. Figure 4 (b) The electrical energy consumed to remove one kilogram of moisture is presented in the form of a stem plot. The horizontal axis corresponds to all six test batches, and the vertical axis is the energy consumption index. The hollow square at the top of the dotted line marks the specific energy consumption location, and the specific measurement value is attached above.

[0107] According to the data in Table 4, the BET specific surface area measured in the four examples remained between 14.82 and 16.15 m². 2 The tensile strength ranged from 1.98 to 2.31 MPa, with a density between / g. Based on the performance of Comparative Example 1, the specific surface area of ​​the hot-air dried product was 1.24 m². 2The leaf has a tensile strength of 0.85 MPa and a measured energy consumption of 1.15 kWh / kg for dehydration. In a conventional heated environment where liquid water migrates outwards, water molecules are transported via the capillary network in the intercellular spaces. The evaporation interface gradually recedes into the tissue, and the surface tension generated by this phase transition stretches and binds the originally full cell walls together. The plant's original three-dimensional fibrous network collapses and densifies under this physical contraction, causing the micropores to close, resulting in leaves that become macroscopically dry, hard, and brittle, unable to withstand external tensile stress.

[0108] Conventional freeze-drying processes (Comparative Example 2), although achieving a specific surface area of ​​14.21 m², 2 While retaining the advantages of phase change dehydration in terms of tensile strength (1.76 MPa) and per kilogram, its unit dehydration energy consumption reaches 4.35 kWh / kg. The sublimation of ice crystals under low pressure requires continuous absorption of latent heat. Traditional freeze-drying equipment, limited by the heat conduction bottleneck in a vacuum environment, relies solely on the bottom shelf for heat transfer, easily leading to uneven heat distribution and obstructed transmission. This prolongs the entire dehydration cycle, resulting in high basic power consumption for maintaining the operation of the vacuum pump and refrigeration unit. The process design of this invention, while maintaining vacuum isolation, achieves non-contact compensation of latent heat through a low-temperature radiant plate, supplemented by micro-pressure difference disturbances from high-purity nitrogen, constructing a balanced pathway for heat penetration and water vapor escape.

[0109] The system's internal heat and mass transfer efficiency was optimized, resulting in a unit energy consumption of 1.76–1.92 kWh / kg for this embodiment. While this value is higher than the basic hot air energy consumption of Comparative Example 1, it reduces the freeze-drying electricity cost of Comparative Example 2. It maintains the original porous support framework of freeze-drying technology while approaching the cost of hot air drying. The loose and porous microstructure within the plant skeleton provides a buffer space for stress dispersion, endowing the dry matter with mechanical flexibility and exhibiting a certain degree of resistance to breakage during subsequent filament cutting and slicing processes.

[0110] Test Example 5: The experimental subjects were dehydrated dried sheets obtained by the processes of Examples 1, 2, 3 and 4. Comparative Example 1 (conventional 65℃ hot air circulation baking) and Comparative Example 2 (conventional -40℃ vacuum freeze drying) were introduced as reference control groups.

[0111] 2.0 g of each group of dry material samples was accurately weighed using an analytical balance and placed in a constant temperature water bath (maintained at 25°C) containing distilled water. After a 45-minute soaking period, the material was removed with tweezers and laid flat on quantitative filter paper. Free water droplets adhering to the surface were absorbed, and the sample was weighed again. The rehydration ratio (g / g) of each group was calculated by comparing the mass of the sample after water absorption with the initial dry mass. This ratio characterizes the physical water absorption and recovery capacity of the plant tissue's porous skeleton after dehydration.

[0112] Each batch of dried leaves was placed in a constant temperature and humidity chamber (temperature 22±1℃, relative humidity 60±2%) for 48 hours to equilibrate the material to a suitable moisture content for processing. The leaves were then handed over to the cigarette processing laboratory for shredding and rolling into uniform single-material cigarettes. Five nationally qualified smoking experts conducted blind tests in a standard sensory evaluation room, independently scoring "aroma fullness" and "smoke smoothness" using professional scales (each item has a maximum score of 40 points). Extreme values ​​were removed, and the arithmetic mean was used as the final evaluation index.

[0113] Table 5. Rehydration characteristics and sensory evaluation data of each embodiment and comparative example.

[0114] Summarize: Figure 5 (a) reflects the mass recombination ability of each group of dehydrated samples under specific water bath conditions. The horizontal axis corresponds to 6 test batches, and the vertical axis is the measured rehydration ratio. The light gray squares connected by dotted lines show the distribution of water absorption rate of each sample, and the specific measurement value is marked directly above each mark. Figure 5 (b) The sensory evaluation scores of the rolled cigarettes are presented. Since the scale ranges of the two indicators are the same, a single Y-axis system is used for comparison. The vertical axis represents the scores of the smoking experts (the full score is 40). The dark gray upper triangles connected by solid lines represent the aroma fullness score, and the hollow rhombuses connected by dashed lines correspond to the smoke delicacy score.

[0115] According to the data in Table 5, the test subjects in each group exhibited different characteristics in terms of physical moisture reconstitution and sensory quality. Comparative Example 1, using traditional hot air drying, performed at a relatively low level across all indicators, with a rehydration ratio of 1.83 g / g and aroma fullness and smoke fineness scores of 24.3 and 22.5, respectively. The higher temperature environment caused capillary contraction, resulting in physical densification of the tissue. This made it difficult for the dried plant fiber network to undergo secondary hydration and expansion. Simultaneously, the heat-induced degradation of organic matter led to off-flavors such as burnt and bitterness, affecting the vaping experience. Comparative Example 2, relying on low-pressure sublimation, increased the rehydration ratio to 4.35 g / g, but because its system was under high vacuum throughout the dehydration cycle, the lack of gaseous component hindrance allowed low-boiling-point aroma compounds such as alcohols and esters to escape along with the desorbed water vapor.

[0116] In terms of sensory perception, Comparative Example 2 scored only 31.6 for aroma fullness and 32.4 for smoke smoothness, indicating an overall weak smoke. The data from the Example groups showed stable retention characteristics, with the rehydration ratio of the four examples ranging from 4.62 to 4.91 g / g, and the smoke evaluation scores ranging from 35.6 to 37.2. Example 3 scored 37.1 for aroma fullness, while Example 1 scored 37.2 for smoothness.

[0117] The system incorporates low-temperature micro-radiative heating combined with a micro-positive pressure nitrogen compensation mechanism, which suppresses stress deformation of the outer wall when solid ice crystals detach from the cytoskeleton. The retained pores provide conditions for capillary level rise during immersion in water, ensuring uniform oxygen diffusion during subsequent rolling and combustion. Free aroma molecules are retained in situ in a controlled pressure field, resulting in smooth sensory feedback during the evaluation and absorption process.

[0118] Test Example 6: The experimental subjects were batches of dried leaves dehydrated and dried using the processes of Examples 1 to 4 of this invention. Comparative Example 1 (conventional 65℃ hot air circulation drying) and Comparative Example 2 (conventional -40℃ contact vacuum freeze drying) were also used as references. All test batches were fresh leaf raw materials from the same origin and harvested at the same maturity level.

[0119] Random samples were taken from multiple points in each batch of finished product, pulverized and sieved through a 40-mesh sieve, and then accurately weighed. The total water-soluble sugar content was determined using a continuous flow analyzer, and the absolute dry matter mass fraction of free nicotine was determined using gas chromatography (GC). This step aimed to evaluate the retention capacity of different dehydration mechanisms for heat-sensitive and highly volatile components.

[0120] The remaining intact dried leaves from each group were packed into standard corrugated cardboard boxes lined with polyethylene film, sealed, and then transferred to an artificial climate chamber. The climate chamber parameters were set to a temperature of 28±1℃ and a relative humidity of 75±2%, and operated continuously for 180 days to simulate a typical high-temperature and high-humidity storage environment.

[0121] Before and after 180 days, portable spectrophotometers were used to extract L, a, and b chromaticity space values ​​from fixed marked areas on the leaf surface, and the total color difference value was calculated. This is used to characterize the material's resistance to browning and pigment degradation during long-term storage. Simultaneously, the material mass is weighed and the moisture content is measured upon leaving the warehouse, and its equilibrium moisture regain relative to the initial storage period is calculated to reflect the moisture absorption tolerance of the dry matter skeleton.

[0122] Table 6. Data on the retention and long-term storage stability of chemical components in the examples and comparative examples.

[0123] Summarize: Figure 6 (a) reveals the retention status of the core water-soluble and volatile components inside the dry matter. The left vertical axis corresponds to the total water-soluble sugar content, and the right vertical axis corresponds to the free nicotine content. The dark gray circles connected by solid lines depict the fluctuation of the total sugar data, and the black crosses connected by dashed lines represent the distribution of the nicotine data. Figure 6(b) shows the physicochemical decay of the dry film after 180 days of environmental stress. The left vertical axis represents the total color difference shift in appearance, and the right vertical axis reflects the equilibrium moisture regain after the matrix absorbs environmental moisture. The light gray pentagram connected by the dotted line corresponds to the color difference index, and the hollow hexagon connected by the dotted line indicates the moisture regain index.

[0124] According to the data in Table 6, in terms of the retention effect of basic chemical components, Comparative Example 1 using the traditional hot air process showed different results, with a total water-soluble sugar content of 14.28% and a free nicotine content of 1.36%. The continuous heat input in the industrial baking environment induced the heterocyclic Maillard reaction between sugars and amino acids, leading to the consumption of sugars. The higher-temperature airflow acted as a carrier for the volatilization of free alkaloids such as nicotine. Comparative Example 2, using conventional freeze-drying, avoided the thermal degradation pathway by using low temperatures, maintaining the total sugar content at 23.41%. However, the open sublimation process under negative pressure reduced the partial pressure resistance at the gas-liquid interface, causing nicotine with a certain saturated vapor pressure to escape along with the sublimated water vapor, resulting in a final measured value of only 1.42%. Extending to long-term storage, this freeze-dried plant skeleton, without gaseous medium protection, exposed its environmental sensitivity.

[0125] Comparative Example 2 achieved an equilibrium moisture regain of 17.65% under high humidity conditions for 180 days. The internal microscopic open pores adsorbed water molecules from the air, and the high free water activity induced enzymatic oxidative browning within the tissue, resulting in storage color differences. The value reached 6.74. The four embodiments exhibited different physicochemical evolution trajectories under the synergistic effect of micro-pressure differential nitrogen disturbance and low-temperature radiation. The total sugar content remained stable in the range of 23.86%–24.52%, and the nicotine retention rate was 2.05%–2.18%. The high-purity nitrogen supplemented in the drying chamber established a microscopic pressure barrier during the dehydration stage, occupying pore channels during ice crystal sublimation and recession, inhibiting the outward diffusion of low-boiling-point aromatics and alkaloids. Moreover, the in-situ permeation of this inert gas formed a certain degree of physical passivation layer on the cell wall surface.

[0126] In subsequent warehousing verification, the moisture regain of Examples 1 to 4 was controlled within the processing range of 12.56% to 13.11%, which avoided the dryness and hardness caused by the structural densification of Comparative Example 1 (moisture regain was only 9.72%, and its heat-damaged matrix underwent pigment degradation after long-term storage). The water activity was 11.84, which also resisted the hygroscopic deliquescence of Comparative Example 2. The low water activity, combined with the residual nitrogen microenvironment inside the tissue, blocked the oxidation chain of pigment polyphenols, so that the color difference of the example remained stable between 3.16 and 3.75 after long-term storage.

Claims

1. A process for preparing green tobacco leaves by low-temperature vacuum freeze-drying, characterized in that, Includes the following steps: Remove coarse and hard tobacco stems and damaged or rotten leaves. Use low-temperature pure water spraying at 4℃~10℃ to clean surface impurities. Remove surface free moisture by centrifugal air drying to control the overall moisture content of the tobacco leaves to 70%~80%. The pre-treated fresh tobacco leaves are evenly spread in the freeze-drying tray; The trays with the material laid are sent into a vacuum freeze-drying chamber for constant temperature pre-freezing treatment, with a pre-freezing temperature of -35℃ to -45℃. Start the primary sublimation drying process, control the vacuum degree of the freeze-drying chamber to be 50-80 Pa, and control the material temperature to be maintained at -10℃ to 0℃, and continuously sublimate and dry for 8-12 hours; Adjust the vacuum level of the freeze-drying chamber to 20-50 Pa for secondary desorption and auxiliary drying, and slowly raise the temperature to 15-25℃ for constant temperature desorption and drying for 3-5 hours. After drying, the vacuum inside the silo is broken, the material is discharged, and it is sealed and stored in light-proof packaging to obtain the finished green tobacco leaves.

2. The process for preparing green tobacco leaves by low-temperature vacuum freeze-drying according to claim 1, characterized in that, In the primary sublimation drying process, the vacuum degree of the freeze-drying chamber is controlled at 65-70 Pa, the cold trap temperature is -55℃, the material temperature is maintained at -5℃ to -2℃, and the sublimation drying is carried out continuously for 10-11 hours. In the secondary desorption drying process, the vacuum degree of the freeze-drying chamber is adjusted to 35-40 Pa, and the temperature is slowly increased to 20-22°C at a rate of 1-3°C / h, and the desorption drying is carried out at a constant temperature for 3.5-4 hours.

3. The process for preparing green tobacco leaves by low-temperature vacuum freeze-drying according to claim 1, characterized in that, Prepare pure water and control its temperature within the range of 4°C to 10°C; use the low-temperature pure water at the set temperature to spray the cleaning area, thus forming a low-temperature pure water spray cleaning work area.

4. The process for preparing green tobacco leaves by low-temperature vacuum freeze-drying according to claim 1, characterized in that, The vacuum freeze-drying chamber is equipped with a chilled water inlet and waste heat recovery module, as well as a negative pressure sealed direct-connection dehumidification and dust removal component, and operates in a completely sealed and light-proof manner.

5. The process for preparing green tobacco leaves by low-temperature vacuum freeze-drying according to claim 4, characterized in that, The specific implementation method of the chilled water connection and waste heat recovery module is as follows: The inlet of the cascade low-temperature refrigeration unit is directly connected to the existing 7℃ and 12℃ central air conditioning chilled water supply network of the cigarette factory. A waste heat recovery device is installed to recover the low-temperature water vapor waste heat generated during the freeze-drying process and connect it to the fresh air preheating system in the cigarette factory workshop, forming a loop for the recycling of low-temperature water vapor waste heat.

6. The process for preparing green tobacco leaves by low-temperature vacuum freeze-drying according to claim 4, characterized in that, The specific implementation method of the negative pressure sealed direct-connection dehumidification and dust removal component is as follows: The freeze-drying chamber and Roots vacuum unit are set up using a negative pressure closed structure. The negative pressure end of the silo is directly connected to the existing dehumidification and dust removal duct of the cigarette factory workshop in an airtight manner, ensuring that the exhaust process is directly integrated into the workshop exhaust system under negative pressure, achieving zero overflow of dust and odor.

7. The process for preparing green tobacco leaves by low-temperature vacuum freeze-drying according to claim 1, characterized in that, Select mature fresh leaves of the Golden Leaf variety and complete the processing within 1-2 hours after harvesting to remove coarse and hard tobacco stems and damaged or rotten leaves; The surface impurities are cleaned by spraying with pure water at a low temperature of 5℃~7℃, and the surface free moisture is removed by centrifugal drying, so that the overall moisture content of the tobacco leaves is controlled at 75%~78%.

8. The process for preparing green tobacco leaves by low-temperature vacuum freeze-drying according to claim 1, characterized in that, The freeze-drying tray is a food-grade stainless steel freeze-drying tray. The pretreated fresh tobacco leaves are evenly spread in a single layer in a food-grade stainless steel freeze-drying tray, with the thickness of the layer controlled at 20-30mm, ensuring that the tobacco leaves do not overlap or suffer from compression damage.

9. The process for preparing green tobacco leaves by low-temperature vacuum freeze-drying according to claim 1, characterized in that, First, pre-cool the storage chamber to -40℃, then perform constant temperature pre-freezing treatment at -38℃ to -40℃ for 75 to 80 minutes, so that the temperature at the center of the tobacco leaves drops to -31℃ to -32℃.

10. The process for preparing green tobacco leaves by low-temperature vacuum freeze-drying according to claim 1, characterized in that, After drying, the vacuum inside the chamber is slowly broken by high-purity nitrogen gas, and the material is discharged quickly at room temperature. It is then sealed and stored in light-proof aluminum foil bags to obtain the finished green tobacco leaves with a moisture content of 10% to 11%.

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