Improved three-section type tobacco leaf baking method based on vein shaping

Abstract

The invention discloses an improved three-section tobacco leaf curing method based on vein shaping, and aims to solve the problems of complicated tobacco leaf temperature control points, inaccurate curing process judgment, unstable curing quality and poor quality uniformity in the prior art on the basis of traditional classical three-section curing. According to the method, 44 DEG C, 55 DEG C and 66 DEG C serve as boundary temperature points to divide a yellowing period, a color fixing period and a stem drying period, three vein shaping monitoring intervals of 40-44 DEG C, 50-55 DEG C and 60-66 DEG C and corresponding auxiliary points are additionally arranged, vein shape changes serve as a baking process judgment basis, and baking nodes are accurately anchored; meanwhile, four standing balance points are arranged, the wet bulb temperature control requirements of all stages are defined, and seamless connection with a traditional baking process is achieved. According to the method, temperature overlapping and complex setting of all stages are avoided through the temperature nodes, overtemperature roasting in the stem drying stage is avoided, the vein shaping characteristics serve as objective monitoring indexes, seamless connection with a traditional technology is achieved by additionally arranging the standing balance points, and the roasting stability of different batches is improved.

Description

Technical Field

[0001] This invention belongs to the field of tobacco processing technology, and is particularly applicable to the standardized curing operation of tobacco leaves in intensive curing barns and traditional curing barns. Specifically, it relates to an improved three-stage tobacco curing method based on leaf vein shaping. Background Technology

[0002] Tobacco curing is a crucial step in determining tobacco quality. The classic three-stage curing process (yellowing stage, color fixing stage, and drying stage) has been used in the tobacco industry for many years due to its relatively simple operation. This process typically uses 42℃, 54℃, and 68℃ as segmented temperature nodes, controlling temperature changes to achieve tobacco leaf dehydration, yellowing, and the formation of internal quality.

[0003] However, existing technologies have three core problems:

[0004] Firstly, the segmented temperature nodes are set unreasonably. One issue is the weak representativeness and overlap of the stage temperatures: the existing process sets the boundary between the yellowing and color-fixing stages at 42℃. At this point, the leaves are only 80-90% yellow, the midrib is still green, and the petioles and auricles are green, which does not match the meaning of "yellowing complete" or actual needs. Objectively, the 42-45℃ range represents a difference and overlap in physiological changes between the late yellowing stage and the early color-fixing stage. 42-45℃ is a crucial period for the complete yellowing of leaves and veins, making it highly susceptible to quality defects such as "baking green," "ashing," or "steaming," increasing the technical difficulty of the curing operation. The existing segmentation method only focuses on the continuity of time and temperature, ignoring the overlapping characteristics of physiological changes in tobacco leaves. Especially in the late yellowing stage and the early color-fixing stage, there is significant stage confusion, with blurred boundaries that are difficult to distinguish, leading to a disconnect between theory and practice and a lack of clear basis for curing operations. Secondly, the key temperature nodes are not set properly: the temperature stabilization time at the 54℃ key node for color fixing is generally insufficient, affecting the formation of oil and aroma in tobacco leaves; the drying stage ends at 68℃, which is very easy to cause the tobacco leaves to "burn red" due to regional high temperature or subjective high temperature and rapid drying, resulting in the precipitation of permeable red pigments in the tobacco leaves, and other off-flavors (or burnt flavor) appearing in the tobacco leaf evaluation. High temperature accelerates the loss of aroma substances, affecting the quality of tobacco leaves.

[0005] Secondly, there are deviations in the process indicators: Traditional processes use leaf color (yellow, dry) and leaf shape changes (bending and curling, rolling) as core indicators, failing to incorporate leaf veins and petioles, which exhibit more stable physiological changes and provide clearer indicators, into the monitoring system. The indicators for the curing process mainly rely on subjective judgments such as leaf color and water loss status, commonly using subjective assessments or descriptions of yellowing degree ("8-10% yellow") and dehydration degree (dry leaves, dry midribs, etc.). This lack of objective and precise quantitative reference standards makes it easy for different curing personnel to make judgment errors, operational deviations, or technical disputes due to differences in experience and training. This results in uneven quality within the same batch of tobacco leaves or failure to achieve the desired effect. Furthermore, the internal and external indicators such as yellowing, drying, aroma, and oil content are not consistent, failing to fully leverage the quality advantages of the tobacco leaves. The objective characteristics of petiole changes have not been utilized, and the correspondence between leaf shape changes and temperature (e.g., small rolls at 46-49℃, large rolls at 52-54℃) deviates from the intuitive understanding of tobacco farmers. For example, in the actual curing process, small rolls have slightly curled edges and small or slightly curled leaves, with the leaf shape resembling a large cylinder; while large rolls are where the leaves are further dried, the whole body shrinks, and the leaves may appear as small cylinders, shrinking even more tightly and smaller. Therefore, small rolls are described as slightly curled large cylinders, and large rolls are deeply curled small cylinders. This description deviates from the cognitive habits of tobacco farmers and is prone to operational misjudgment.

[0006] Thirdly, the setting of temperature and humidity control points and the rate of temperature rise and stabilization are complex and irregular, difficult to remember, and challenging to promote, train, and master. Traditional processes use ten temperature control points: 36℃, 38℃, 40℃, 42℃, 46℃, 48℃, 50℃, 54℃, 60℃, and 68℃. There is no clear logical connection between these temperature points, and the rate of temperature rise varies greatly, starting slowly, then accelerating, and finally slowing down. Temperature and humidity control is difficult to master or set. This makes it difficult for tobacco farmers to remember and master, increasing the difficulty of technical training and promotion, and easily leading to temperature control errors in actual operation. Furthermore, the preheating and drying stages lack control: existing research often neglects temperature control below 36℃ during preheating and fails to clarify the temperature and humidity control standards for the pre-curing preparation stage; the drying stage only focuses on the final temperature and lacks a refined control mechanism linked to leaf vein changes.

[0007] Leaf vein morphology changes significantly during tobacco curing, and its characteristics are highly indicative of the curing process. However, current technologies lack a systematic monitoring system to address the correlation between vein morphology changes and the curing process, failing to use vein shaping characteristics as a core indicator of curing stage transitions. This results in a lack of scientific basis for controlling the curing process, making it difficult to meet the standardized and precise curing demands of the modern tobacco industry. Therefore, a clearly segmented and precisely indicative tobacco curing process is urgently needed to overcome the shortcomings of existing technologies. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of existing three-stage tobacco curing processes, such as overlapping and ambiguous segments and subjective indicators, and to provide an improved three-stage tobacco curing process based on leaf vein shaping characteristics. This process achieves precise control of the curing process by optimizing temperature segment nodes, constructing a leaf vein morphology monitoring system, simplifying the setting of multi-level temperature control points, and implementing supporting control strategies (static balancing, constant-rate heating, and coordinated temperature control). This upgrades the complete three-stage curing process and promotes the rapid application of the technology.

[0009] Technical solution of the present invention: An improved three-stage tobacco curing method based on leaf vein shaping uses 44℃, 55℃, and 66℃ as the dividing temperature points for the three-stage curing. Specifically, the stages are: 44℃ and below for the yellowing stage, 44℃-55℃ for the color fixing stage, and 55℃-66℃ for the dry vein stage. Based on the dividing temperature points, three leaf vein shaping monitoring intervals are set: 40-44℃, 50-55℃, and 60-66℃. The changes in leaf vein morphology within each interval are used as the basis for judging the curing process.

[0010] Preferably, the heating rate during the entire baking process is 0.6-1℃ / h.

[0011] Preferably, the wet-bulb temperature control requirements for each stage are as follows: during the yellowing stage at 44℃ and below, the wet-bulb temperature is controlled at 36℃±1℃; during the color-fixing stage at 44℃-55℃, the wet-bulb temperature is controlled at 38℃±1℃; and during the drying stage at 55℃-66℃, the wet-bulb temperature is controlled at 40℃±1℃. Adjustments are made based on the wet-bulb temperatures of 40℃, 50℃, and 60℃ corresponding to 36℃, 38℃, and 40℃, respectively.

[0012] Preferably, the method further includes three leaf vein shaping auxiliary points; the leaf vein shaping auxiliary points are 40℃, 50℃, and 60℃, which correspond to the formation of three key periods for leaf vein shaping at 40-44℃, 50-55℃, and 60-66℃.

[0013] Further preferred leaf vein shaping standards in the 40-44℃ range are as follows: the needle-like branches turn from green to white, the auricles turn from green to yellow, and the edges dry and curl; when the temperature stabilizes at 44℃, the branches are completely white, the auricles turn yellow, the leaves are completely yellow, the main veins are soft and shiny and not easily broken, the 5-8 cm section of the main vein on the back of the petiole end is full, swollen and shiny, the petiole harvested section turns black, sunken and scabs, and the leaf margins are dry and curved within a 2-3 cm range to form a "curled edge but not curled body" shape.

[0014] Further preferred leaf vein shaping standards in the 50-55℃ range are: branch veins contracted, auricles completely dry, main vein curved, obvious indentation on the front, and loss of pubescence and purplish discoloration on the back; when the temperature stabilizes at 55℃, the main vein, i.e. the leaf tip and middle part of the front, is sunken into a groove shape, and the epidermis at the junction of the main and branch veins on the back is slightly wrinkled and initial punctate or segmental purplish discoloration appears; the petiole is bent and drooping in the 3 cm section, but not lying flat, and the leaf veins are in a "curled and wrapped" shape with "upper head down, middle waist bent, and side curled edges".

[0015] Further optimized, the leaf vein shaping standard in the 60-66℃ range is as follows: the grooves on the leaf tip and middle of the main vein on the front are further sunken into a slit shape, and intermittent collapsed purple segments appear on the back and gradually extend and connect; when the temperature stabilizes at 66℃, the main vein on the front is sunken inward to form a slit, the purple segments on the back are connected and connected, and the main vein is completely dry. The leaves are evenly curled and slightly stretched, the leaf tips droop again, and there is no moisture in the observation window of the drying room.

[0016] Preferably, the method further includes setting four static equilibrium points of 30℃, 36℃, 42℃, and 48℃ during the baking process, and stabilizing each equilibrium point for 4-12 hours to achieve a smooth temperature transition.

[0017] Preferably, the method further includes multiple categories of settling and balancing times during the baking process, wherein: the temperature stabilization time for the three segmented temperature points (44℃, 55℃, 66℃) is 16-24 h; and the temperature for the three leaf vein auxiliary temperature points (40℃, 50℃, 60℃) is preferably 12-15 h.

[0018] Preferably, a preheating temperature of 30℃ is set based on the initial ignition temperature, i.e., the room temperature of the baking room, for preheating management; the temperature stabilization time at 44℃ and 55℃ is respectively included in the previous baking stage; the period before 44℃ is the yellowing period, of which the time at 30-44℃ is 68-75 hours; the color fixing period at 44-55℃ is 48-55 hours; the drying period at 55-66℃ is 35-45 hours; the average time per batch is 150-190 hours (30-66℃); and the total baking management time after ignition is 160-210 hours / batch.

[0019] The beneficial effects of this invention are as follows: 1. Clearly defined stages: First, it solves the problem of overlapping temperatures between the later yellowing stage and the early color-fixing stage in traditional processes, avoiding confusion. Using 44℃ breaks the awkward situation where 42℃ is insufficient for yellowing (80-90% yellow) and insufficient greening of the leaf veins. The dividing point between yellowing and color fixing is extended to 44℃, at which point the tobacco leaves reach 90-100% yellow, and the main veins fade, clearly distinguishing the later yellowing stage from the early color-fixing stage. Simultaneously, it eliminates the confusion caused by the overlap between the two stages, reducing the difficulty of judging and operating during baking, and decreasing the probability of quality defects such as "baking green" and "ash residue." A 55℃ limit is set to highlight the basic drying of the leaves and the completion of color fixing, with the veins showing initial purplish discoloration on the front and back, visually and objectively indicating the completion of color fixing and the true entry into the drying stage. A 66℃ limit is set to achieve the final drying stage, reducing excessive consumption of reddening or aroma compounds caused by high temperatures, which is more conducive to improving the quality of the tobacco leaves. The setting of the 55℃ color-fixing endpoint and the 66℃ drying endpoint creates a clear stage division, avoiding confusion in the process. Experimental data shows that the proportion of top-grade tobacco leaves increased by 8%-12%, while the proportion of flue-cured tobacco decreased by 5%-8%. Secondly, based on practical experience, the starting temperature control and ending temperature points were supplemented and adjusted. A preheating temperature control point was added, setting 30℃ as the starting temperature for curing, achieving a hot start for biochemical changes in the curing barn and incorporating it into the entire curing process management. Through 4-6 hours of temperature stabilization and low-speed fan operation, the problem of poor cold start performance under natural temperatures was solved, effectively mitigating temperature or humidity differences between upper and lower layers or within the same layer caused by uneven tobacco loading, ensuring consistency within the curing barn, and simultaneously promoting the conversion and aggregation of starch-hydrolyzing enzymes, thus reducing starch content. The maximum drying temperature was adjusted from 68℃ to 66℃, which will significantly reduce the proportion of flue-cured tobacco, reduce the loss of aroma compounds, and contribute to an overall improvement in the aroma and flavor of the tobacco leaves.

[0020] 2. Precise indication: This invention uses leaf vein shaping characteristics as an objective monitoring indicator, replacing the traditional subjective judgment based on experience with leaf yellowing (degree of yellowing) and drying (hooked tips, curled edges, small curls, large curls, etc.). It incorporates important descriptions such as "swelling and glossing, scab formation at wounds; bending and drooping, purplish veins on the back; purple veins running through the leaf, and a smooth leaf shape" into key indicators during the baking stage. The yellowing, color fixation, and drying of leaf veins during baking have broader and deeper significance than leaf indications. This invention uses the yellowing and shaping of leaf veins during baking as indicators, providing observable and indicative descriptions of important characteristic changes or morphological features, essentially eliminating the phenomena of green inclusions and vein bleeding. For example, at the end of the yellowing stage (44℃), the main vein is clearly visible: the back of the vein is swollen and shiny, and the petiole wound is scabbed over; at the end of the color fixing stage (55℃), the main vein is clearly visible: the petiole is bent and turned upside down, the veins on the front are grooved, and the veins on the back are purplish; at the end of the drying stage (66℃), the main vein is clearly visible: the purple veins are connected, and the whole leaf is stretched out, etc. At the same time, this invention uses 66℃ as the endpoint temperature for drying the leaves, which can fundamentally eliminate the phenomenon of reddening the tobacco. These can more dynamically and accurately describe the classic changes in the baking process, which is conducive to mastering the process. This allows baking personnel of different skill levels to accurately control the baking process, improve the baking quality, and reduce the risk of baking special tobacco leaves. The indicators are objective and accurate, improving the baking quality: the morphological changes of leaf veins and petioles are introduced as core monitoring indicators, and their objective stability is better than traditional leaf characteristics, effectively reducing judgment bias; the 44℃ dividing point and the 55℃ stable temperature optimization can reduce the proportion of green and mixed tobacco, and the 66℃ drying control reduces the loss of aroma substances, significantly improving the oil content and aroma quality of tobacco leaves. The uniformity of tobacco quality in the same batch is improved by more than 15%.

[0021] 3. Strong compatibility: Seamless integration with traditional processes is achieved by adding a static equilibrium point. On one hand, this invention introduces the concept of static equilibrium for the first time, proposing a constant-rate heating, stepped humidity control, and static equilibrium method, breaking away from the cumbersome traditional variable-speed heating operation. Through 10 points (segment points, auxiliary points, and static equilibrium points) for delayed heat preservation, static equilibrium temperature stabilization ensures uniformity in the curing barn. "Stability" achieves "uniformity," realizing uniformity between different tobacco loading layers and between different areas within the same layer, laying the foundation for the next stage. Control over the crucial yellowing stage (36-44℃) is strengthened, with 4 temperature control points (36℃, 40℃, 42℃, 44℃), completely solving the problems of uneven and insufficient yellowing. It also considers the overlapping temperature range of yellowing and color fixing in the original curing process, with 2 temperature control points (42℃, 44℃) for this overlapping temperature range, taking into account the physiological changes during the early stages of yellowing and color fixing. The variable-speed heating process in the curing process is eliminated, and the variable-speed heating of 0.3-0.5℃ / h and 1℃ / h is unified to 0.6-1℃ / h. Different settling times are set at different temperature points to ensure that the changes in the leaves are consistent with the temperature description, eliminating the impact of excessively rapid heating on insufficient state tracking, and also making it easy to use and remember. On the other hand, this invention does not require modification of existing curing barn equipment, lowering the threshold for technology promotion. The coordinated control of setting points, temperature stabilization time, and heating rate is easy to explain, understand, and operate. Thirdly, the overall stage duration and total duration are effectively controlled, which is conducive to high-efficiency curing and reduces excessive energy consumption in some tobacco areas. The setting of the above temperature points and temperature stabilization time also greatly improves the chemical composition, which is conducive to reducing starch, promoting sugar conversion, increasing oil content, and enhancing aroma (for details, please refer to relevant technical data on tobacco curing).

[0022] 4. Highly efficient dissemination, clear temperature division rules, easy to remember and operate: The core segmented temperature points of 44℃, 55℃, and 66℃ are distributed in an arithmetic sequence. Combined with the hierarchical setting of auxiliary points of 40℃, 50℃, and 60℃ (integers and in an arithmetic sequence) and equilibrium points of 30℃, 36℃, 42℃, and 48℃ (multiples of 6 from 5 to 8), the problem of confusing traditional temperature points is solved. Tobacco farmers can quickly master the technology through "progressive rules + mnemonic rhymes," reducing training and promotion costs. It is easy to understand and remember, accelerates the application of the technology, and provides strong support for improving the quality and efficiency of the tobacco industry.

[0023] Figure and Table Description Figure 1 Temperature-time curves for the improved three-stage baking process based on leaf vein shaping.

[0024] Figure 2 A standard atlas of tobacco leaf morphology for the leaf vein shaping monitoring interval.

[0025] Figure 3 The key monitoring segment during the yellowing period at 40-44℃ is the petiole turning dark and forming a scab.

[0026] Figure 4 The key monitoring segment during the yellowing period at 40-44℃ is characterized by the expansion and brightening of the main vein.

[0027] Figure 5 The key monitoring segment during the yellowing period at 40-44℃ is characterized by hooked leaf tips and curled leaf edges.

[0028] Figure 6 The key monitoring segment for the color fixation period at 50-55℃ is the arched scroll.

[0029] Figure 7 The key monitoring section for the color fixation period at 50-55℃ is the groove head collapsing.

[0030] Figure 8 The key monitoring segment for color fixation at 50-55℃ is the purplish-purple pulsation point.

[0031] Figure 9 The key monitoring segment during the 60-66℃ dry rib stage is the purplish vein penetration.

[0032] Figure 10 The key monitoring period during the dry vegetative stage at 60-66℃ is when the leaf veins begin to unfold.

[0033] Figure 11 The key monitoring segment during the drying process at 60-66℃ is characterized by pulse contraction and gap formation. Detailed Implementation

[0034] The technical solution of the present invention will be further explained and described below with reference to specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention should be determined by the contents of the claims. Modifications and substitutions made by those skilled in the art to the technical solution of the present invention without creative effort all fall within the scope of protection of the present invention.

[0035] Table 1. Hierarchical Relationship of the "3+3+4" Multi-level Temperature Control System

[0036] Refer to Table 1.

[0037] First aspect: The three-stage baking process is redefined based on temperature node optimization. Abandoning the traditional three-stage baking process with temperature segments of 42℃, 54℃, and 68℃, a new three-stage baking dividing temperature of 44℃, 55℃, and 66℃ has been selected, achieving a clear division of each stage. The specific segments are as follows: Yellowing period: 30℃ (preheating start) - 44℃. The core objective is to promote the degradation of chlorophyll in tobacco leaves and the manifestation and increase of carotenoids and xanthophyll, so as to achieve uniform and thorough yellowing of leaves (including veins) and fading of the main veins to white. Color fixation period: The temperature is controlled at 44℃-55℃. The core objective is to fix the yellow pigment in the tobacco leaves, dehydrate and prevent the color from turning brown, while promoting the transformation of the aroma substances in the tobacco leaves. Drying stage: The temperature is controlled at 55℃-66℃. The core objective is to achieve rapid drying of the main vein of the tobacco leaves, further promote the transformation and preservation of aroma substances, ensure that the moisture content of the tobacco leaves meets the standard, and ensure the stability of subsequent storage.

[0038] The optimization of the above temperature nodes not only avoids the temperature overlap of each stage (42-45℃ late yellowing stage and early color fixing stage, 54-55℃ late color fixing stage and early drying stage), but also avoids overheating during the drying stage (the original limit of 68 degrees Celsius could easily cause some high-temperature areas in the curing barn). This improves the operability of the process without affecting the quality of tobacco curing.

[0039] The second aspect: ten automatic control setting points based on multi-level temperature control points. Based on the core segmentation, a "3+3+4" multi-level temperature control system is constructed, covering key nodes of the entire baking process, ensuring both regularity and process continuity: Three segmented temperature points: 44℃ (yellowing to color fixation boundary), 55℃ (color fixation to dry rib boundary), and 66℃ (dry rib endpoint), serve as the core basis for stage transitions; Three auxiliary points for leaf vein shaping: 40℃, 50℃, and 60℃, which correspond to the key nodes of leaf vein morphology changes in each stage, forming three key periods for leaf vein shaping: 40-44℃, 50-55℃, and 60-66℃. Four static equilibrium temperature control points: 30℃ (preheating), 36℃, 42℃, and 48℃, are used to achieve temperature and humidity balance in each stage, reduce temperature difference in the curing barn, and ensure uniform changes in tobacco leaves during the yellowing and color-fixing periods. Third aspect: Construction of an auxiliary monitoring index system based on leaf vein shaping Based on the aforementioned segmented temperature points, a leaf vein morphology grading and segmentation identification system is constructed using three "leaf vein shaping auxiliary segments or auxiliary points." This system allows for objective judgment of the baking process by monitoring changes in leaf vein morphology (focusing on the front and back of the midrib, petiole, leaf arch, and leaf margin). The specific auxiliary temperature ranges and corresponding leaf vein shaping characteristics are as follows: 1. 40-44℃ (Key Monitoring Period for Yellowing): This stage is characterized by the complete yellowing of the veins and the initial formation of leaf morphological changes. Key features include completely yellowing of the leaves, veins, and midrib, with dry, curled leaf edges. This process continues until the temperature stabilizes at 44℃. The veins turn completely white, the auricles turn completely yellow, and the leaves turn completely yellow. The midrib is swollen and shiny: the midrib (leaf part) is soft and shiny and not easily broken; the midrib (5-8 cm section) on the back of the petiole tip is obviously full, swollen and shiny. Darkening and scab formation in petioles: The harvested petiole cross-section shows a noticeable darkening in color and is sunken and scab-like; Leaf Edges with Small Curling Pattern: The leaf edges (within 2-3 cm) dry, bend, and shrink to form "curled edges." The leaf tip and edges curl up, and the entire leaf veins on the back show hooked patterns without curling the body, resembling a "ruyi" (a traditional Chinese scepter symbolizing good fortune). Characteristic mnemonic: "Full yellow, hooked, ruyi-shaped flowers, soft veins, swollen back, and scab-covered head." See details... Figure 3-5 .

[0040] 2.50-55℃ (Key Monitoring Period for Color Fixation): This stage is characterized by branch vein contraction and midrib morphology settling. Key features include: a distinctly sunken, grooved midrib; a purplish-brown, curved back; and bent, drooping petioles. This process continues until the temperature stabilizes at 55℃. The main vein groove is purplish-red: the branch veins further shrink, the auricles are completely dry, the leaves are completely dry, the main vein is clearly grooved on the front, and the epidermis in the middle of the back (where the main veins and branch veins connect) is slightly wrinkled. Irregular purplish-red (transdermal purplish-red spots or patches, not exceeding the thickness of the leaf veins) phenomenon begins to appear. Petiole bent and head drooping: The petiole (about 3 cm section) is obviously bent and drooping but not lying flat (the contact point of the tobacco cord or cigarette clip); The leaves curl and contract like a scroll painting: the veins further contract, the auricles are completely dry, the leaves are simultaneously fully dry, the edges contract, and the veins exhibit a "curled-up, bent-over, and side-curled" appearance, resembling a scroll painting. A characteristic mnemonic: "Bent-over leaves, grooved purple stem, and inverted stem." See details... Figure 6-8 .

[0041] 3.60-66℃ (Key Monitoring Segment During the Drying Stage): This stage is the period of main vein drying and morphological stabilization. Key characteristics include intermittent collapse and purplish-blue discoloration on the back of the main vein, gradually extending and connecting, until the main vein shrinks and completely dries. This process ends when the temperature stabilizes at 66℃. The main vein is visibly contracted on the front, with the middle section sunken to form a slit-like shape. The purplish-red section on the back connects to form a line, and the main vein is wrinkled and completely dried. Main veins curl and then relax: The leaves are evenly curled and slightly relaxed, and the leaf tips have slightly returned to a drooping state (the degree of curling decreases, and the overall leaf length stretches slightly after the tobacco leaves dry). Upon close inspection, there is no fog, and a fragrant aroma is detected: the observation window of the curing barn returns to a moisture-free state, and the odor from the chimney and exhaust vents increases (the smell of fuel and the aroma of tobacco leaves are prominent). A characteristic mnemonic: "High heat, fragrant aroma, veins gradually drying, purple veins connecting, leaves unfolding again." See details... Figure 9-11 .

[0042] The fourth aspect: a synergistic temperature control method combining static equilibrium, constant-rate heating, and stepwise humidity control.

[0043] To achieve compatibility and optimization with traditional processes, four static equilibrium points of 30℃, 36℃, 42℃, and 48℃ were added to the above temperature system. Stable temperature at each equilibrium point ensures a smooth transition in baking temperature. Different static temperature stabilization times were used at the ten temperature control points (three boundary temperature points, three leaf vein shaping auxiliary points, and four static equilibrium points) to extend the temperature control time and achieve static equilibrium of temperature, humidity, and morphological changes within the drying chamber. This system essentially covers the ten temperature control points of the original three-stage process, but uses simplified methods such as constant-rate heating and staged humidity control, making it easier to understand and remember. Combined with morphological description indicators at the end of each stage, it compensates for the deficiencies of traditional processes, such as incomplete yellowing, morphological misunderstanding, and the omission of key indicators like petiole collapse and purplish midrib.

[0044] 1. Static balancing: The temperature stabilization time for the three segmented temperature points (44℃, 55℃, 66℃) should be 15-18h; the temperature stabilization time for the three leaf vein auxiliary temperature points (40℃, 50℃, 60℃) should be 10-12h; the temperature stabilization time for the preheating temperatures (30℃, 36℃) should be 4-5h; and the temperature stabilization time for the other two static balancing points (42℃, 48℃) should be 8-10h.

[0045] 2. Constant heating rate: The heating rate remains constant at 0.6-1℃ / h throughout the entire baking process. For the other heating processes at the 10 points, the heating rate is uniformly set at 0.6-1℃ / h; the specific heating rate for different regions can be determined through experimentation.

[0046] 3. Segmented Humidity Control: Wet-bulb temperature is controlled in segments. For the yellowing stage (44℃ and below), the wet-bulb temperature is controlled at 36℃±1℃; for the color-fixing stage (44℃-55℃), the wet-bulb temperature is controlled at 38℃±1℃; for the drying stage (55℃-66℃), the wet-bulb temperature is controlled at 40℃±1℃, not exceeding 41℃. In other words, the wet-bulb temperature increases in a stepwise manner from the beginning to the end of each stage. Adjustments are made based on wet-bulb temperatures of 36℃, 38℃, and 40℃ at 40℃, 50℃, and 60℃, respectively.

[0047] 4. Coordinated temperature control: With 30℃ as the preheating temperature, the stabilization time at 44℃ and 55℃ are respectively divided into the previous stage. The yellowing period (30-44℃) lasts about 65 hours; the color fixing period (44-55℃) lasts 50 hours; the drying period (55℃) lasts 40 hours; the average duration of a single batch is 160 hours; and the total time for different parts of the tobacco leaves should be 140-180 hours.

[0048] Fifthly, methods for disseminating and promoting the technology. To rapidly popularize this baking technique, a three-dimensional dissemination system combining "mnemonic devices + visualization + hands-on training" should be established. 1. Simple and easy to remember: The leaf vein shaping characteristics of each key period are extracted into a popular mnemonic rhyme, combined with the "4, 5, 6" progressive rule of temperature points to reduce the difficulty of memorization; Ten temperature points, segmented points: 44℃, 55℃, 66℃; auxiliary points: 40℃, 50℃, 60℃; equilibrium points: 30℃, 36℃, 42℃, 48℃, which are 5 times, 6 times, 7 times and 8 times 6; stable temperature time: 18, 12, 10h; heating rate 1 / h; wet bulb control: 36℃±1℃, 38℃±1℃ and 40℃±1℃.

[0049] 2. Create standard morphological atlases and operation videos of tobacco leaves (leaf blade + veins + petiole) at various temperature points to visually demonstrate the characteristics of each stage; when conducting practical training, use changes in leaf vein morphology as the core assessment indicator to replace the traditional subjective judgment standard and improve the training effect.

[0050] Example 1 1. Experimental materials and equipment Experimental materials: Selected tobacco leaves from the middle section of the Yunyan 87 variety, with uniform maturity; Test equipment: Standard dense drying room, equipped with intelligent temperature control system (temperature accuracy ±0.5℃) and humidity sensor.

[0051] 2. Implementation Steps 2.1 Loading and preheating: Bundle the tobacco leaves according to the standard (about 15 kg of fresh leaves per bundle), hang them evenly in the curing barn, close the doors and windows of the curing barn, start the fan equipment at low speed, and raise the temperature to 30℃ at a rate of 0.8℃ / h. Let it stand for 5 hours to balance, ensuring that the temperature in the curing barn is uniform, and prepare the tobacco leaves for post-curing.

[0052] 2.2 Yellowing period control (≤44℃): Ignite and heat up to 36℃ at a rate of 1℃ / h, and let it stand at 36℃ for 5 hours to equilibrate; continue to heat up to 44℃ at a rate of 0.8℃ / h, and let it stand at 40℃ and 42℃ for 10 hours each, and enter the critical monitoring period of 40-44℃. Stabilize at 44℃ for about 18 hours. In the later stage, observe the leaf vein morphology at any time. Control the wet bulb at 35-37℃. When the wet bulb does not exceed 37℃ at 44℃, the yellowing period ends when the branch veins are completely white, the auricles turn yellow, the leaf margins curl, and the petiole cross-section turns black, sunken, and scab-like. This stage lasts for 60-72 hours.

[0053] 2.3 Color fixation period control (44℃-55℃): The temperature is increased from 44℃ to 55℃ at a rate of 0.8℃ / h, and then allowed to stand at 48℃, 50℃, and 55℃ for 8h, 10h, and 18h ​​respectively. When the wet bulb stabilizes at 37-39℃, the critical monitoring period of 50-55℃ begins. When the petiole collapses and the leaf is curled up, the petiole (head) is obviously bent, the midrib is clearly grooved on the upper surface of the leaf, the middle part of the leaf and the tip of the leaf, and the corresponding position on the lower surface shows dotted or segmented purplish spots. The color fixation period ends. This stage lasts for 48-55h.

[0054] 2.4 Drying stage control (55℃-66℃): Increase the temperature from 55℃ to 66℃ at a rate of 0.8℃ / h. Allow the temperature to stand at 60℃ and 66℃ for 10h and 18h ​​respectively, and then enter the critical monitoring stage of 60-66℃. Stabilize the temperature until the purple veins on both sides of the main vein are connected, the main vein on the front side shrinks and the veins are completely dry, the leaf tip droops again (the bending of the leaf veins is relieved), and there is no water vapor in the observation window. Then turn off the equipment and let it cool naturally to room temperature before smoke appears. This stage lasts for 40-45h.

[0055] After baking, the leaves are completely yellow without any smoke, the main veins are completely dry and elastic, the leaves are bright in color and elastic, and they are rich in oil, without any signs of reddening or scorching.

[0056] 3. Effect Verification Comparative experiments show that, compared with the traditional three-stage process (Comparative Example 1), the proportion of high-grade tobacco using the process of this invention is 74%, which is 6% higher than the traditional process (68%); the orange-yellow tobacco rate is 5% higher than the traditional process, and there is no obvious spoiled or red-cured tobacco; compared with other comparative examples (Comparative Examples 2-7), the proportion of high-grade tobacco is 8-15% higher than the traditional process, and the proportion of green and mixed tobacco is 8-12% lower.

[0057] Comparative Example 1: Traditional Three-Stage Process: Ten temperature points were set: 36℃, 38℃, 40℃, 42℃, 46℃, 48℃, 50℃, 54℃, 60℃, and 68℃. 42℃, 54℃, and 68℃ were key observation periods. At 42℃, the leaves were visually judged to be 80-90% yellow, with curled tips and edges; at 46-48℃, the main veins were completely yellow, and small curls appeared; at 54℃, large curls appeared, and the leaves were completely dry; at 68℃, the leaves were completely dry. The detailed process is as follows: Yellowing stage: After ignition, the temperature is increased to 36-38℃ at a rate of 1℃ per hour, with a dry-wet difference of 1-2℃. The fan runs at low speed, and the tobacco leaves begin to turn yellow, reaching 50-60% yellow. The leaf tips in the high-temperature layer turn yellow, and the leaves wilt and soften. The temperature is increased to 40℃ at a rate of 1℃ per 2 hours, with a wet-bulb temperature of 36-37℃. The fan runs at low speed, and the leaves turn yellow, reaching 70-80% yellow. The main vein softens, and the leaves in the high-temperature layer turn completely yellow, with the secondary veins slightly bluish. The temperature is increased to 42℃ at a rate of 1℃ per 2 hours, with a wet-bulb temperature of around 37℃. The fan runs at high speed to prolong the temperature stabilization time and increase the degree of yellowing. The leaves in the high-temperature layer turn yellow, reaching 90-100% yellow (yellow leaves with bluish veins), with the main vein softening and the leaf tips drying to about 3cm.

[0058] Color-fixing stage: Increase the temperature by 1°C every 2-3 hours to 46-48°C, with a wet-bulb temperature of 37-38°C and the fan running at high speed. Stabilize the temperature until the tobacco leaves are 100% yellow (all leaves and veins are yellow) and more than 1 / 3 of the leaves are dry (leaf tips and edges are dry); increase the temperature by 1°C every 1-2 hours to 53-54°C, with a wet-bulb temperature of 39-40°C and the fan running at low speed, and stabilize the temperature until all leaves in the chamber are completely dry.

[0059] Drying stage: Start from 54℃ and increase the temperature to 65-68℃ at a rate of 1℃ per hour, with the wet bulb temperature at 41℃ and the fan running at low speed; the dry bulb temperature should not exceed 70℃ and the wet bulb temperature should not exceed 41℃ to avoid the tobacco leaves turning red, and all the tobacco leaves in the kiln should be completely dry.

[0060] The results showed that Comparative Example 1, as the traditional benchmark process, adopted a "multi-temperature point variable-speed heating" mode, setting 10 key temperature nodes, and using "visual observation of leaf yellowing degree and rolling state" and "touch of midrib dryness" as the basis for stage transitions. Its curing results showed that the proportion of high-grade tobacco was 68%, the orange-yellow tobacco rate was 83%, and there were no obvious signs of damaged or reddened tobacco. Although it could meet the basic curing requirements, it had significant limitations: First, the heating rate was unstable (1℃ / 1-3h), leading to uneven distribution of temperature and humidity in the curing barn, and significant differences in post-maturity between the high-temperature layer and the middle and lower layers of tobacco leaves; second, the stage transition judgment relied on subjective experience, and standards such as "80-90% yellow" and "tip curling" lacked quantitative basis, easily leading to improper timing of the transition due to judgment errors; third, the final temperature of the dry rib stage (65-68℃) was close to the reddening critical value, requiring strict control of the wet-bulb temperature (≤41℃), making operation difficult. Example 1, by optimizing the heating mode and quantifying the judgment indicators, increased the proportion of high-quality tobacco by 6% and the rate of orange-yellow tobacco by 5%, directly demonstrating the shortcomings of traditional processes in precise control and standardized operation.

[0061] Comparative Example 2: Based on Example 1, only the temperature nodes were optimized, without introducing leaf vein monitoring indicators. Specific parameters were: 30℃, 36℃, 40℃, 42℃, 44℃, 48℃, 50℃, 55℃, 60℃, and 66℃. The detailed scheme is as follows: 1. After ignition, raise the temperature to 30℃ at a rate of 1℃ per hour, with the fan running at low speed for about 4 hours, until the temperature reaches 36℃, with a dry-wet difference of 1-2℃. The tobacco leaves begin to turn yellow, and the leaves wilt and soften. Raise the temperature to 40℃ at a rate of 1℃ per 2 hours, with a wet-bulb temperature of 36-37℃. With the fan running at low speed, the leaves in the middle layer will be 70-80% yellow, with the main veins softening. The leaves in the high-temperature layer will be completely yellow, with the secondary veins slightly bluish. Raise the temperature to 42℃ at a rate of 1℃ per 2 hours, with a wet-bulb temperature of about 37℃. With the fan running at high speed, extend the temperature stabilization time. The leaves in the high-temperature layer will reach 90-100% yellow, with the main veins softening and the leaf tips drying for about 3cm. Raise the temperature to 44℃ at a rate of 1℃ per 2-3 hours, with a wet-bulb temperature of 37℃. With the fan running at high speed, stabilize the temperature until the tobacco leaves are 100% yellow (completely yellow leaves and secondary veins) and about 1 / 3 dry (leaf tips and edges dry).

[0062] 2. Color fixation stage: Increase the temperature by 1°C every 1-2 hours to 55°C, wet bulb temperature 38-39°C, fan running at low speed. Extend the time and stabilize the temperature until all blades in the chamber are completely dry.

[0063] 3. Drying stage: Start from 55℃ and increase the temperature to 66℃ at a rate of 1℃ per hour. The wet bulb temperature is 40-41℃. The fan runs at low speed to avoid aroma loss and color fading.

[0064] The results showed that Comparative Example 2 used the optimized temperature nodes of Example 1, but removed the monitoring of leaf vein shaping characteristics, reverting to the traditional leaf condition observation method. The results indicated that curing personnel tended to prematurely switch the heat at 42℃, resulting in insufficient time for the critical yellowing stage at 40-44℃. This led to problems such as incomplete greening of the upper leaves, floating green on the main veins, and dull leaf auricle color. The core reason for this phenomenon is that the traditional leaf observation method cannot accurately capture the complete process of internal substance transformation in tobacco leaves. At 42℃, although the leaves reach 90-100% yellow, the veins are not completely white, and the petiole cross-section has not formed a scab. Switching the heat at this point interrupts the yellowing process, leading to insufficient transformation of substances such as starch and chlorophyll. In contrast, the leaf vein monitoring indicators in Example 1 (completely white veins, scab formation on the petiole cross-section, etc.) are essentially a quantitative representation of the internal maturity of the tobacco leaves, avoiding the defects caused by premature heat switching due to subjective experience. This highlights the crucial role of accurate monitoring indicators in ensuring curing quality.

[0065] Comparative Example 3: Based on Example 1, other stages can refer to the traditional process, and the drying end point is set at 70°C.

[0066] The results showed that Comparative Example 3, by raising the final temperature of the dry-bulb stage to 70℃, far exceeding the traditional upper limit of 68℃, resulted in a nearly 30% increase in the proportion of red-cured tobacco and serious defects such as darkening and reddening of the leaf tissue. From the perspective of the curing mechanism, when the dry-bulb temperature reaches 70℃, if the wet-bulb temperature exceeds 42℃, it will accelerate the oxidative browning of the leaf tissue cells, leading to color deterioration and the formation of discoloration in the tobacco leaves. Although high temperatures can shorten the dry-bulb stage time by 5-8 hours, the economic benefits outweigh the economic gains at the cost of quality. This result confirms the scientific validity of controlling the dry-bulb temperature below 70℃ in the traditional process, and also indicates that setting the final dry-bulb stage temperature of 66℃ in Example 1 is the optimal choice that balances dry-bulb efficiency and quality assurance, effectively avoiding the risk of red-curing caused by high temperatures.

[0067] Comparative Example 4: Except for the "yellowing-color fixation" dividing point being set at 42℃, the color fixation period was initiated at 42℃ based on traditional experience (e.g., leaves turning 80-90% yellow), and all other procedures were carried out according to the scheme of this invention.

[0068] The results showed that Comparative Example 4, which advanced the "yellowing-color fixation" boundary to 42℃, still exhibited problems such as dull tobacco leaf surface, bluish veins, and mixed colors, despite having other parameters consistent with Example 1. There are three main reasons for this: First, at 42℃, the yellowing process of the tobacco leaves was not complete, and the main veins still contained green. Entering the color fixation period prematurely would result in unconverted chlorophyll residue, forming bluish veins. Second, the constant-rate heating mode (0.8℃ / h) in Example 1 heated the leaves faster than the traditional process after 42℃, and the premature switching of the heat further amplified the temperature and humidity differences between the upper and lower layers of the curing barn, easily leading to poor dehumidification in the upper high-temperature zone. Third, there were significant differences in the judgment and understanding of the heat switching among the curing personnel. Poor dehumidification in some areas resulted in higher wet-bulb temperatures in others, causing the tobacco leaves to brown in the early stages of color fixation, resulting in a dull surface and some bluish veins after curing, exhibiting a phenomenon of being both green and mixed. The main veins near the petiole contained green, while the leaf tip was mixed with colors and lacked brightness. This result shows that precise control of the heat transfer point is the core of the baking process. In Example 1, delaying the heat transfer point to 44°C and verifying it with leaf vein index can ensure sufficient yellowing, laying the foundation for quality improvement in the subsequent color fixing stage. It also highlights the limitations of the subjective timing of the heat transfer in the traditional process.

[0069] Comparative Example 5: Except for the "end temperature of the drying period" being adjusted from 66℃ to 68℃, all other procedures were carried out in accordance with the scheme of this invention.

[0070] The results showed that, although increasing the drying endpoint temperature from 66℃ to 68℃ in Comparative Example 5 shortened the curing time by 5-8 hours, it resulted in problems such as loss of aroma compounds, thinner and lighter leaves, and reduced yield (32-50g / 100 leaves). From the perspective of material transformation, aroma precursors in tobacco leaves (such as carotenoids and polyphenols) are easily volatilized and decomposed at high temperatures. Compared with 66℃, the rate of aroma compound loss was significantly increased at 68℃, resulting in more pronounced aroma loss in the tobacco leaves, manifested as a strong aroma in the tobacco outside the curing barn. At the same time, high temperatures accelerated the evaporation of leaf moisture, leading to excessive shrinkage of leaf mesophyll cells, a decrease in leaf thickness and weight, and ultimately affecting yield. The traditional drying endpoint temperature (68℃) also carries similar risks. However, setting the drying endpoint at 66℃ in Example 1 can maximize the preservation of aroma compounds and leaf morphology while ensuring thorough drying, achieving a balance between quality and yield, further demonstrating the optimization potential of the traditional process in temperature setting.

[0071] Comparative Example 6: The temperature settings, heating rate, and wet-bulb control were exactly the same as in Example 1. However, in judging the stage transition and the duration of temperature stabilization, the leaf vein shaping characteristics were not used. Instead, the traditional, vague leaf observation method was used: to judge the end of the yellowing period, the leaves were checked to see if they were "9-10% yellow" and if the leaf tips were "hooked and curled"; to judge the end of the color fixation period, the leaves were checked to see if they were "small curled" or "large curled"; to judge the end of the dry vein period, the main vein was touched to see if it was brittle and easily broken.

[0072] The results showed that Comparative Example 6 used the temperature and humidity control parameters of Example 1, but reverted to the traditional fuzzy observation method to judge the stage transition. This resulted in significant differences in the switching time between different operators (a total time difference of nearly 20 hours), ash buildup on the tobacco leaves after curing, veining, and significant differences in quality between upper and lower layers. This phenomenon fully exposed the core shortcomings of the traditional process: the lack of quantitative standards for judging stage transitions, and descriptions such as "9-10% yellow" and "small rolls" are highly subjective, easily leading to cognitive biases among operators with different experience levels, resulting in chaotic heating rhythms. In contrast, the leaf vein shaping characteristics of Example 1 (such as the groove in the main vein and the penetration of purple veins) provided an objective and quantifiable basis for judging stage transitions, effectively reducing human operational differences and ensuring consistent curing progress in each layer of tobacco leaves within the curing barn. This is one of the key reasons why Example 1 improved quality stability compared to the traditional process. Furthermore, the traditional observation method requires frequent opening of the curing barn for observation, which easily disrupts the temperature and humidity balance, further aggravating curing defects. Quantitative indicators can reduce the frequency of observation and ensure a stable curing environment.

[0073] Comparative Example 7: A traditional three-stage variable-speed heating mode is used, but the temperature nodes of this invention (44℃, 55℃, 66℃) are forcibly applied. The settling time at 30℃, 36℃, 42℃, 48℃, etc., is eliminated or significantly shortened.

[0074] The results showed that Comparative Example 7, using the temperature nodes of Example 1 but employing the traditional three-stage variable-speed heating mode and eliminating or shortening the multi-point settling balancing time, exhibited significant differences in the quality of tobacco leaves between the upper and lower layers of the curing barn. This result verifies the scientific validity of the "constant-speed heating + multi-point settling" system in Example 1: the traditional variable-speed heating mode (1℃ / 1-3h) easily leads to uneven temperature conduction within the curing barn, while multi-point settling balancing allows each layer of tobacco leaves to fully adapt to temperature changes, ensuring synchronized yellowing and color fixing processes; even with reasonable temperature nodes, eliminating settling cannot compensate for the uneven distribution of temperature and humidity fields. Traditional processes often suffer from significant quality differences between upper and lower layers of tobacco leaves due to the lack of a uniformity control mechanism. Example 1, through the combination of constant-speed heating and settling balancing, effectively solves this problem, simplifies the operation process, reduces reliance on operator experience, and highlights the dual advantages of process optimization.

[0075] The comparisons of the various comparative examples with the traditional three-stage process and Example 1 show that the core influencing factors on tobacco curing quality include: precise setting of temperature nodes, stability of heating rate, quantitative judgment criteria for stage transitions, and uniformity of temperature and humidity fields within the curing barn. The core flaw of the traditional three-stage process lies in the instability of the heating mode and the subjective nature of stage transitions, leading to poor quality consistency and a low proportion of high-grade tobacco. Example 1 optimizes temperature and humidity uniformity through "constant-rate heating + multi-point static placement," quantifies stage transition standards based on leaf vein shaping characteristics, and precisely controls key temperature nodes (especially the 44℃ transition and 66℃ drying endpoint), effectively avoiding defects such as premature greening, excessive reddening, and aroma loss, thus achieving quality improvement. The negative results of the various comparative examples further confirm the necessity of these optimization measures, providing crucial experimental evidence for the standardization and precision improvement of tobacco curing processes.

Claims

1. An improved three-stage tobacco curing method based on leaf vein shaping, characterized in that, The three-stage baking process is divided into three temperature ranges: 44℃, 55℃, and 66℃. Specifically, the ranges are: 44℃ and below is the yellowing stage, 44℃-55℃ is the color-fixing stage, and 55℃-66℃ is the drying stage. Based on these temperature ranges, three leaf vein shaping monitoring intervals are set: 40-44℃, 50-55℃, and 60-66℃. The changes in leaf vein morphology within each interval are used as the basis for judging the baking process.

2. The improved three-stage tobacco curing method based on leaf vein shaping according to claim 1, characterized in that, The heating rate during the entire baking process is 0.6-1℃ / h.

3. The improved three-stage tobacco curing method based on leaf vein shaping according to claim 1, characterized in that, The wet-bulb temperature control requirements for each stage are as follows: during the yellowing stage at 44℃ and below, the wet-bulb temperature should be controlled at 36℃±1℃; during the color-fixing stage at 44℃-55℃, the wet-bulb temperature should be controlled at 38℃±1℃; and during the drying stage at 55℃-66℃, the wet-bulb temperature should be controlled at 40℃±1℃. Adjustments should be made based on the wet-bulb temperatures of 40℃, 50℃, and 60℃, which correspond to 36℃, 38℃, and 40℃, respectively.

4. The improved three-stage tobacco curing method based on leaf vein shaping according to claim 1, characterized in that, The method also includes three leaf vein shaping auxiliary points; the leaf vein shaping auxiliary points are 40℃, 50℃, and 60℃, which correspond to the three key periods for leaf vein shaping: 40-44℃, 50-55℃, and 60-66℃.

5. The improved three-stage tobacco curing method based on leaf vein shaping according to claim 4, characterized in that, The standard for leaf vein shaping in the 40-44℃ range is as follows: the needle-like branches turn from green to white, the auricles turn from green to yellow, and the edges dry and curl. When the temperature stabilizes at 44℃, the branches are completely white, the auricles turn yellow, the leaves are completely yellow, the main veins are soft and shiny and not easily broken, the 5-8 cm section of the main vein on the back of the petiole end is full, swollen and shiny, the petiole is blackened and sunken with scabs at harvest, and the leaf margins are dry and curved within a 2-3 cm range, forming a "curled edge but not curled body" shape.

6. The improved three-stage tobacco curing method based on leaf vein shaping according to claim 4, characterized in that, The standard for leaf vein shaping in the 50-55℃ range is as follows: branch veins shrink, auricles are completely dry, main veins are curved, the upper surface is obviously sunken, and the underside has lost its hairs and is purplish-red. When the temperature stabilizes at 55℃, the main vein, i.e. the leaf tip and middle part of the leaf, is sunken into a groove on the upper surface, and the epidermis at the junction of the main and branch veins on the underside is slightly wrinkled and has initial punctate or segmental purplish-red spots. The petiole is bent and drooping in the 3 cm section, but not lying flat. The leaf veins are in the shape of "upper head down, middle waist bent, and side curled up" and "curled up and wrapped".

7. The improved three-stage tobacco curing method based on leaf vein shaping according to claim 4, characterized in that, The standard for leaf vein shaping in the 60-66℃ range is as follows: the grooves on the leaf tip and middle of the main vein on the front side are further sunken into a slit shape, and intermittent collapsed purple segments appear on the back side and gradually extend and connect; when the temperature stabilizes at 66℃, the main vein on the front side is sunken inward to form a slit, the purple segments on the back side are connected and connected, and the main vein is completely dry. The leaves are evenly curled and slightly stretched, the leaf tips droop again, and there is no moisture in the observation window of the drying room.

8. The improved three-stage tobacco curing method based on leaf vein shaping according to claim 1, characterized in that, The method also includes setting four static equilibrium points of 30℃, 36℃, 42℃, and 48℃ during the baking process, and stabilizing each equilibrium point for 4-12 hours to achieve a smooth temperature transition.

9. The improved three-stage tobacco curing method based on leaf vein shaping according to claim 1, characterized in that, The method also includes a settling and equilibration time during the baking process, wherein: the settling and equilibration time for the three dividing temperature points of 44℃, 55℃, and 66℃ is 16-24h; and the settling and equilibration time for the three leaf vein auxiliary temperature points of 40℃, 50℃, and 60℃ is 12-15h.

10. The improved three-stage tobacco curing method based on leaf vein shaping according to claim 1, characterized in that, Based on the initial ignition temperature (room temperature of the drying room), a preheating temperature of 30℃ is set for preheating management. The temperature stabilization time at 44℃ and 55℃ is included in the previous baking stage. The period before 44℃ is the yellowing period, during which the time at 30-44℃ is 68-75 hours; the color fixing period is 48-55 hours at 44-55℃; and the drying period is 35-45 hours at 55-66℃. The average duration of a single batch is 150-190 hours (30-66℃). The total baking management time after ignition is 160-210 hours per batch.

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