Tilia miqueliana leaf vegetable production process

Abstract

The invention relates to the technical field of tilia miqueliana leaf vegetable processing, in particular to a tilia miqueliana leaf vegetable making technology which comprises the following steps: S1, raw material pretreatment; s2, blanching; s3, flavone color protection modification treatment; s4, oxalic acid removal and softening treatment; s5, dehydrating and shaping; s6, freezing and packaging; according to the technical scheme, a gradient pressure reduction mode is adopted for dewatering and shaping, moisture on the surfaces of the leaves is removed at a low vacuum degree, then the vacuum degree and the temperature are increased to remove internal free moisture, leaf shrinkage, deformation and cell damage caused by a single dewatering mode are avoided, the moisture content after dewatering is controlled to be 60%-70%, the subsequent freezing and packaging effect is guaranteed, and the production cost is reduced. And the fresh and tender texture of the leaves can be maintained. In the freezing and packaging stage, formation of ice crystals in the leaves is reduced through low-temperature quick freezing, damage of the ice crystals to cell structures is avoided, and high-vacuum-degree packaging is matched, oxygen is isolated, and oxidative deterioration and microorganism breeding are inhibited; and the shelf life of the product is further prolonged through low-temperature refrigeration storage.

Description

Technical Field

[0001] This invention relates to the field of Nanjing linden leaf vegetable processing technology, specifically a Nanjing linden leaf vegetable processing method. Background Technology

[0002] Nanjing linden, a precious tree species unique to my country, boasts tender leaves rich in flavonoids, vitamin C, and other nutrients. Its delicate texture gives it high nutritional value and development potential. In recent years, with the increasing demand for natural and healthy ingredients, Nanjing linden leaves have gradually gained market attention. However, the large-scale processing technology for Nanjing linden leaves is still immature, and many problems urgently need to be addressed.

[0003] In traditional leafy vegetable processing, the raw materials are mostly washed using conventional rinsing methods, which are difficult to completely remove residual impurities and fine contaminants from the surface of the leaves and are prone to damaging the leaves. Improper parameter control during the blanching process often leads to the loss of nutrients and darkening of the color of the leaves, while failing to effectively reduce the oxalic acid content in the leaves. Color protection treatments mostly use conventional color-protecting agents, which have limited color protection effects. Leafy vegetables are prone to browning during processing and storage, affecting sensory quality. If oxalic acid is not removed, there will be bitterness and other problems when eating, which will affect the eating experience.

[0004] Therefore, developing a processing method for Nanjing linden leaves that can effectively retain nutrients, improve sensory quality, reduce oxalic acid content, and extend shelf life has become an urgent technical problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and propose a processing method for Nanjing linden leaves.

[0006] The specific technical solution is as follows: A processing method for Nanjing linden leaves includes the following steps:

[0007] S1. Raw material pretreatment: Select fresh, disease-free, and old stem and yellow leaves of Nanjing linden, remove impurities and residual petioles, rinse 2-3 times with warm water at 30-35℃ using the bubble rinsing method, with a bubble rinsing pressure of 0.1-0.15MPa. After rinsing, drain until there are no obvious water droplets on the surface of the leaves, and set aside for later use.

[0008] S2. Blanching: Place the pre-treated Nanjing linden leaves into the blanching liquid, control the blanching temperature at 80-85℃ and the blanching time at 40-60s, and turn the leaves over during the blanching process once every 30 seconds; after blanching, immediately remove them and place them in an ice water bath at 0-4℃ to cool rapidly to room temperature for 1-2 minutes, then remove them and drain the water for later use;

[0009] S3. Flavonoid Color Protection Modification Treatment: Blanched and cooled young leaves of Nanjing linden are placed in a composite color protection modification solution and soaked using an intermittent ultrasonic-assisted penetration method. This method is suitable for the thin and brittle characteristics of Nanjing linden leaves, avoids continuous ultrasonic damage to the leaves, and promotes rapid penetration of the color protection agent, improving the uniformity and durability of the color protection. The composite color protection modification solution consists of 0.3-0.5% ascorbic acid, 0.2-0.4% citric acid, 0.1-0.2% bamboo leaf flavonoid extract, and the balance being deionized water. The ultrasonic parameters are: frequency 20-30kHz, power 100-150W, ultrasonic for 30 seconds, pause for 30 seconds, soaking temperature 25-30℃, soaking time 20-30 minutes, during which the leaves are gently turned over once every 10 minutes. After soaking, rinse 1-2 times with deionized water, each rinse for 10-15 seconds, drain the water, and set aside.

[0010] S4. Oxalic Acid Removal and Softening Treatment: Young leaves of Nanjing linden, after flavonoid color-protecting modification, are placed in a removal and softening solution. A segmented pH control method is used for gentle removal and softening, taking advantage of the tight binding of oxalic acid to the fibers in Nanjing linden leaves. This achieves efficient oxalic acid removal and gentle fiber softening in synergy, avoiding damage to the leaf structure. The treatment temperature is maintained at 30-35℃, with slow stirring at a speed of 20-30 r / min to ensure full penetration of the removal and softening solution. After treatment, the leaves are removed, drained, and set aside.

[0011] S5. Dehydration and shaping: Place the tender leaves of Nanjing linden that have been softened by removing oxalic acid into a vacuum dehydration device, control the vacuum degree to be 0.06-0.08MPa and the dehydration temperature to be 40-45℃, and use a gradient decompression method during the dehydration process. Stop dehydration when the moisture content of the leaves is 60-70%. After dehydration is completed, take them out and let them cool naturally to room temperature for later use.

[0012] S6. Frozen Packaging: Place the dehydrated Nanjing linden leaves into a quick-freezing device, control the quick-freezing temperature at -35 to -40℃ and the quick-freezing time at 15-20 minutes. After the core temperature of the leaves drops below -18℃, immediately place them into food-grade composite packaging material and vacuum them to a vacuum degree of 0.09-0.1MPa to complete the vacuum packaging. After packaging, store them in a low-temperature cold storage at -18±2℃.

[0013] As a further technical solution, the harvesting time for the tender leaves of Nanjing linden in step S1 is from May to June each year. When harvesting, select tender shoots and leaves with a length of 5-8cm and a thickness of 3-8mm. After harvesting, pack them in a low-temperature preservation box and control the transportation temperature at 4-8℃.

[0014] As a further technical solution, the blanching solution in step S2 is a mixture of deionized water, salt, and EDTA-2Na, wherein the mass concentration of salt is 0.1-0.2% and the mass concentration of EDTA-2Na is 0.01-0.02%.

[0015] As a further technical solution, the preparation method of the composite color-protecting and modifying liquid in step S3 is as follows: first, heat deionized water to 40-45℃, then add ascorbic acid and citric acid in sequence, stirring at a speed of 60-80 r / min until completely dissolved; then add bamboo leaf flavonoid extract, continue stirring for 20-30 min, keep warm for 30 min, cool to room temperature, and filter with a 0.22μm sterile filter membrane to remove impurities before use; the bamboo leaf flavonoid extract and the Nanjing linden leaf flavonoid components work synergistically to improve color-protecting stability and reduce the oxidative loss of flavonoid components.

[0016] As a further technical solution, the preparation method of the bamboo leaf flavonoid extract is as follows: Select fresh bamboo leaves, remove impurities and old leaves, pulverize to 40-60 mesh, add 8-10 times the mass of 70-75% ethanol solution, extract twice at 50-55℃ and ultrasonic power 180-200W, each extraction for 40-50 min, combine the two extracts, concentrate under reduced pressure until there is no ethanol odor, and obtain a concentrated solution; add 3-4 times the volume of deionized water to the concentrated solution, stir evenly, let stand for 12-16 h, centrifuge at 5000-5500 r / min for 15-20 min, take the supernatant, purify by adsorption through macroporous resin, elute with 65-70% ethanol solution, collect the eluent, concentrate again under reduced pressure to a thick paste, vacuum dry to a water content ≤5%, pulverize and pass through an 80-100 mesh sieve to obtain the bamboo leaf flavonoid extract;

[0017] Vacuum drying requires a vacuum level of 0.08-0.1 MPa and a temperature of 50-55℃.

[0018] As a further technical solution, the segmented pH control in step S4 is performed using food-grade sodium bicarbonate solution and citric acid solution, wherein the mass concentration of sodium bicarbonate solution is 5% and the mass concentration of citric acid solution is 3%, and the drop rate during adjustment is 1-2 drops / s to maintain stable pectinase activity.

[0019] As a further technical solution, the segmented pH control is as follows: in the first stage, the pH value is adjusted to 6.0-6.2 from 0 to 5 minutes; in the second stage, the pH value is adjusted to 6.5-7.0 from 5 to 15 minutes; and in the third stage, the pH value is adjusted to 6.8-7.0 from 15 to 20 minutes.

[0020] As a further technical solution, the food-grade pectinase mentioned in step S4 has an enzyme activity of 5000-8000 U / g. Before adding it to the de-softening solution, the pectinase is diluted 10 times with deionized water at 35℃ and allowed to stand for 15-20 minutes to ensure that the enzyme activity is fully activated. After activation, it is slowly added to the de-softening solution and stirred evenly to avoid excessive local enzyme concentration, which could lead to excessive local softening of the leaves.

[0021] As a further technical solution, the softening solution in step S4 is composed of 0.1-0.2% sodium bicarbonate, 0.2-0.3% trehalose, 0.05-0.1% food-grade pectinase, and the remainder deionized water, with a liquid-to-solid ratio of 6:1 (mL:g).

[0022] As a further technical solution, the specific parameters of the gradient dehydration in step S5 are as follows: the first stage is 0-3 min with a vacuum degree of 0.06 MPa and a dehydration temperature of 40°C, which initially removes the surface moisture of the leaves; the second stage is 3-8 min with a vacuum degree of 0.08 MPa and a dehydration temperature of 45°C, which removes the free water inside the leaves; the leaf moisture content is monitored in real time during the dehydration process, and dehydration is stopped immediately when it reaches 60-70% to avoid excessive dehydration that would cause the leaves to shrink and the taste to deteriorate.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] In the raw material pretreatment stage of this invention, the optimal harvesting time and specifications for Nanjing linden leaves are clearly defined. Combined with low-temperature preservation and transportation, the freshness and nutritional integrity of the raw materials are guaranteed from the source. A 30-35℃ warm water bubble rinse is used. By rationally controlling the rinsing pressure and frequency, impurities and residues on the leaf surface are thoroughly removed while avoiding leaf damage. Simultaneously, warm water rinsing reduces the dissolution of nutrients. The blanching stage uses a composite blanching solution composed of deionized water, salt, and EDTA-2Na. Salt increases the osmotic pressure of the leaves, helping to remove some oxalic acid, while EDTA-2Na chelates metal ions, inhibiting oxidative browning. Precise control of blanching temperature and time, along with regular turning, kills microorganisms on the leaf surface while avoiding nutrient loss and texture deterioration due to overheating. Immediately after blanching, rapid cooling in an ice water bath quickly terminates the thermal reaction, effectively maintaining the bright green color and tender texture of the leaves, laying a good foundation for subsequent processing.

[0025] In the flavonoid color-protecting modification treatment, the composite color-protecting modification solution is composed of ascorbic acid, citric acid, and bamboo leaf flavonoid extract. Ascorbic acid can inhibit the activity of polyphenol oxidase and reduce pigment degradation. Citric acid can adjust the pH value of the system and enhance the stability of ascorbic acid. Bamboo leaf flavonoid extract can not only synergistically protect the color but also enrich the nutritional components of leafy vegetables. Combined with intermittent ultrasonic-assisted penetration, the cavitation effect and mechanical vibration of ultrasound promote the rapid penetration of the color-protecting agent into the interior of the leaves, improve the uniformity and durability of color protection, and effectively solve the browning problem in the processing and storage of leafy vegetables. The oxalic acid removal and softening treatment uses a compound softening solution containing sodium bicarbonate, trehalose, and food-grade pectinase. Sodium bicarbonate reacts with oxalic acid to promote its decomposition and removal. Trehalose protects the leaf cell structure and reduces nutrient loss. Food-grade pectinase specifically decomposes pectin in the leaves, achieving moderate softening. Segmented pH control optimizes the conditions for oxalic acid removal and pectinase catalysis by gradually adjusting the pH value of the reaction environment. Combined with gentle stirring, this ensures thorough oxalic acid removal and uniform softening, solving the bitterness problem of leafy vegetables while maintaining the elasticity and taste of the leaves.

[0026] The dehydration and shaping process in this invention employs a gradient decompression method. First, a lower vacuum level is used to remove surface moisture from the leaves, then the vacuum level and temperature are increased to remove internal free water. This avoids leaf shrinkage, deformation, and cell damage caused by a single dehydration method, controlling the moisture content after dehydration to 60-70%. This ensures both the effectiveness of subsequent freezing and packaging and maintains the tender texture of the leaves. During the freezing and packaging stage, rapid freezing at low temperatures reduces the formation of ice crystals inside the leaves, preventing damage to cell structure. Combined with high-vacuum packaging, oxygen is isolated, inhibiting oxidative deterioration and microbial growth. Low-temperature cold storage further extends the product's shelf life, ensuring consumers can enjoy consistently high-quality Nanjing linden leaves for an extended period. The entire process is interconnected and synergistic, optimizing the entire process from raw materials to finished product. This ultimately achieves excellent results for Nanjing linden leaves, including high nutrient retention, vibrant green color, tender texture, low oxalic acid content, and long shelf life, effectively solving many problems inherent in existing processes. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the processing steps for Nanjing linden leaf vegetables. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0029] This invention provides a processing method for Nanjing linden leaves, comprising the following steps:

[0030] S1. Raw material pretreatment: Select fresh, disease-free, and old stem and yellow leaves of Nanjing linden, remove impurities and residual petioles, rinse 2-3 times with warm water at 30-35℃ using the bubble rinsing method, with a bubble rinsing pressure of 0.1-0.15MPa. After rinsing, drain until there are no obvious water droplets on the surface of the leaves, and set aside for later use.

[0031] S2. Blanching: Place the pre-treated Nanjing linden leaves into the blanching liquid, control the blanching temperature at 80-85℃ and the blanching time at 40-60s, and turn the leaves over during the blanching process once every 30 seconds; after blanching, immediately remove them and place them in an ice water bath at 0-4℃ to cool rapidly to room temperature for 1-2 minutes, then remove them and drain the water for later use;

[0032] S3. Flavonoid Color Protection and Modification Treatment: Blanched and cooled Nanjing linden leaves are placed in a composite color protection and modification solution and soaked using an intermittent ultrasonic-assisted penetration method. The composite color protection and modification solution consists of 0.3-0.5% ascorbic acid, 0.2-0.4% citric acid, 0.1-0.2% bamboo leaf flavonoid extract, and the balance being deionized water. The ultrasonic parameters are: frequency 20-30kHz, power 100-150W, ultrasonic treatment for 30 seconds, pause for 30 seconds, soaking temperature 25-30℃, and soaking time 20-30 minutes, during which the leaves are gently turned over once every 10 minutes. After soaking, rinse with deionized water 1-2 times, each rinse for 10-15 seconds, drain the water, and set aside.

[0033] S4. Oxalic acid removal and softening treatment: The tender leaves of Nanjing linden that have been modified with flavonoid color protection are placed in the removal and softening solution, and a segmented pH control method is used for gentle removal and softening treatment; the treatment temperature is maintained at 30-35℃, and the leaves are slowly stirred at a speed of 20-30 r / min; after the treatment is completed, the leaves are removed, drained, and set aside.

[0034] S5. Dehydration and shaping: Place the tender leaves of Nanjing linden that have been softened by removing oxalic acid into a vacuum dehydration device, control the vacuum degree to be 0.06-0.08MPa and the dehydration temperature to be 40-45℃, and use a gradient decompression method during the dehydration process. Stop dehydration when the moisture content of the leaves is 60-70%. After dehydration is completed, take them out and let them cool naturally to room temperature for later use.

[0035] S6. Frozen Packaging: Place the dehydrated Nanjing linden leaves into a quick-freezing device, control the quick-freezing temperature at -35 to -40℃ and the quick-freezing time at 15-20 minutes. After the core temperature of the leaves drops below -18℃, immediately place them into food-grade composite packaging material and vacuum them to a vacuum degree of 0.09-0.1MPa to complete the vacuum packaging. After packaging, store them in a low-temperature cold storage at -18±2℃.

[0036] In this invention, the preferred time for harvesting the tender leaves of Nanjing linden in step S1 is May to June each year. When harvesting, tender shoots and leaves with a length of 5-8cm and a thickness of 3-8mm are preferred. After harvesting, the leaves are placed in a low-temperature preservation box and the transportation temperature is controlled at 4-8℃. This selection and processing method can ensure the freshness and quality of the raw materials to the greatest extent, laying the foundation for the subsequent production of high-quality Nanjing linden leaf vegetables.

[0037] In step S1, the preferred water temperature is 30-35℃. This temperature range can effectively remove impurities and residues from the leaf surface without damaging the nutrients and taste of the leaves. The preferred number of bubble rinsings is 2-3 times, and the preferred rinsing pressure is 0.1-0.15MPa. Multiple rinsings with appropriate pressure can ensure the cleanliness of the leaves while avoiding damage to the leaves due to excessive pressure.

[0038] The blanching solution in step S2 is preferably a mixture of deionized water, salt, and EDTA-2Na, wherein the salt concentration is preferably 0.1-0.2% and the EDTA-2Na concentration is preferably 0.01-0.02%. Salt enhances the blanching effect, and EDTA-2Na helps protect the color; the two work synergistically to improve the quality of leafy vegetables. The blanching temperature is preferably 80-85℃, which effectively kills microorganisms on the leaf surface, removes some oxalic acid, and avoids nutrient loss and deterioration in taste caused by high temperatures. The blanching time is preferably 40-60 seconds; too short a time will not achieve the above effects, while too long a time will affect the color and texture of the leaves. The ice water bath cooling temperature is preferably 0-4℃, and the cooling time is preferably 1-2 minutes. Rapid cooling allows the leaves to quickly stop the heat treatment, maintaining good color and taste.

[0039] The preferred method for preparing the composite color-protecting and modifying solution in step S3 is as follows: First, heat deionized water to 40-45℃, then add ascorbic acid and citric acid sequentially, stirring at 60-80 r / min until completely dissolved; then add bamboo leaf flavonoid extract, continue stirring for 20-30 min, keep warm for 30 min, cool to room temperature, and filter through a 0.22 μm sterile filter membrane to remove impurities before use. This preparation method ensures that all components are fully dissolved and mixed, improving the stability and color-protecting effect of the composite color-protecting and modifying solution.

[0040] The preferred method for preparing the bamboo leaf flavonoid extract is as follows: Select fresh bamboo leaves, remove impurities and old leaves, pulverize to 40-60 mesh, add 8-10 times the mass of 70-75% ethanol solution, extract twice at 50-55℃ and ultrasonic power 180-200W, each extraction for 40-50 min, combine the two extracts, concentrate under reduced pressure until there is no ethanol odor, and obtain a concentrated solution; add 3-4 times the volume of deionized water to the concentrated solution, stir evenly, let stand for 12-16 h, centrifuge at 5000-5500 r / min for 15-20 min, take the supernatant, purify by adsorption through macroporous resin, elute with 65-70% ethanol solution, collect the eluent, concentrate again under reduced pressure to a thick paste, vacuum dry to a water content ≤5%, pulverize and pass through an 80-100 mesh sieve to obtain the bamboo leaf flavonoid extract; the vacuum degree of vacuum drying is 0.08-0.1 MPa and the temperature is 50-55℃. This preparation method can efficiently extract flavonoids from bamboo leaves with high purity, thus better exerting its color-protecting and modifying effects.

[0041] In step S3, the ultrasonic frequency is preferably 20-30kHz and the power is preferably 100-150W. Such ultrasonic parameters can promote the penetration of the composite color-protecting modification liquid into the interior of the leaf and improve the color protection effect. The soaking temperature is preferably 25-30℃ and the soaking time is preferably 20-30min. Suitable temperature and time can ensure that the color-protecting modification reaction is fully carried out.

[0042] The segmented pH adjustment in step S4 preferably uses a food-grade sodium bicarbonate solution and a citric acid solution, wherein the mass concentration of the sodium bicarbonate solution is 5% and the mass concentration of the citric acid solution is 3%, and the dropping rate during adjustment is 1-2 drops / s. Segmented pH adjustment can provide a suitable reaction environment for oxalic acid removal and softening treatment, improving the treatment effect; controlling the dropping rate can avoid damage to the leaves caused by sudden pH changes.

[0043] The preferred parameters for segmented pH adjustment are as follows: in the first stage (0-5 min), adjust the pH to 6.0-6.2; in the second stage (5-15 min), adjust the pH to 6.5-7.0; and in the third stage (15-20 min), adjust the pH to 6.8-7.0. This segmented adjustment method can gradually optimize the reaction conditions, ensuring thorough oxalic acid removal and appropriate leaf softening.

[0044] The food-grade pectinase mentioned in step S4 preferably has an enzyme activity of 5000-8000 U / g. Before adding it to the softening solution, the pectinase should be diluted 10 times with deionized water at 35℃ and allowed to stand for 15-20 minutes. Pectinase with suitable enzyme activity can effectively decompose pectin in leaves, achieving a softening effect; dilution and standing treatment can activate the activity of pectinase and improve reaction efficiency.

[0045] The softening solution described in step S4 is preferably composed of 0.1-0.2% sodium bicarbonate, 0.2-0.3% trehalose, 0.05-0.1% edible-grade pectinase, and the remainder deionized water, with a liquid-to-solid ratio of 6:1 (mL:g). Sodium bicarbonate assists in the removal of oxalic acid, trehalose protects the nutrients and palatability of the leaves, and edible-grade pectinase achieves softening. The three work synergistically to improve the treatment effect; a suitable liquid-to-solid ratio ensures that the leaves are fully in contact with the softening solution.

[0046] In step S4, the preferred treatment temperature is 30-35℃, which provides suitable conditions for the softening removal reaction and increases the reaction rate; the preferred stirring speed is 20-30 r / min, which allows the blades to contact the softening removal liquid evenly and ensures consistent treatment results.

[0047] The preferred parameters for the gradient dehydration in step S5 are: First stage (0-3 min): vacuum degree 0.06 MPa, dehydration temperature 40℃, to initially remove surface moisture from the leaves; Second stage (3-8 min): vacuum degree 0.08 MPa, dehydration temperature 45℃, to remove internal free water from the leaves. This gradient dehydration method avoids problems such as shrinkage and deformation of leaves due to excessively rapid dehydration, while effectively removing moisture to ensure the quality and shelf life of leafy vegetables.

[0048] In step S5, dehydration is stopped when the leaf moisture content is 60-70%. This moisture content ensures the taste of the leafy vegetables and provides good conditions for subsequent frozen packaging.

[0049] In step S6, the quick-freezing temperature is preferably -35 to -40℃ and the quick-freezing time is preferably 15-20 minutes. Rapid freezing can reduce the formation of ice crystals in leafy vegetables, avoid damage to leaf cells by ice crystals, and maintain the quality of leafy vegetables. The vacuum degree of vacuum packaging is preferably 0.09-0.1MPa. A good vacuum environment can prevent leafy vegetables from oxidizing and deteriorating, and extend the shelf life. The storage temperature is controlled at -18±2℃, which can maintain the quality of leafy vegetables for a long time.

[0050] The processing technology for Nanjing linden leaves provided by this invention, through the coordinated operation and parameter optimization of each step, can effectively preserve the nutritional components of Nanjing linden leaves, improve their color, taste and texture, reduce oxalic acid content, and extend shelf life, thus solving the problems of nutrient loss, poor color, poor taste, high oxalic acid content and short shelf life that exist in traditional processing technology.

[0051] To further illustrate the present invention, the following detailed description is provided through the examples and comparative examples.

[0052] Example 1:

[0053] S1. Raw Material Pretreatment: Select tender leaves of Nanjing linden harvested in May each year. Choose tender shoots and leaves 5cm in length and 3mm in thickness, ensuring the leaves are fresh, free from pests and diseases, and free from old stems and yellowing. After harvesting, pack them in low-temperature preservation boxes, controlling the transportation temperature at 4℃. Upon arrival at the processing site, remove impurities and residual petioles from the leaves, and rinse twice with 30℃ warm water using an air bubble rinse method at a pressure of 0.1MPa. After rinsing, drain until there are no obvious water droplets on the leaf surface, and set aside for use.

[0054] S2. Blanching: Prepare the blanching solution, which is a mixture of deionized water, salt, and EDTA-2Na, with a salt concentration of 0.1% and an EDTA-2Na concentration of 0.01%. Place the pre-treated Nanjing linden leaves into the blanching solution, controlling the blanching temperature at 80℃ and the blanching time at 40 seconds. During blanching, turn the leaves once every 30 seconds. Immediately after blanching, remove the leaves and place them in a 0℃ ice water bath to rapidly cool to room temperature for 1 minute. Drain the water and set aside.

[0055] S3. Flavonoid Color Protection and Modification Treatment: First, prepare bamboo leaf flavonoid extract. Select fresh bamboo leaves, remove impurities and old leaves, and pulverize to 40 mesh. Add 8 times the mass of 70% ethanol solution and extract twice at 50℃ and ultrasonic power 180W, 40 min each time. Combine the two extracts and concentrate under reduced pressure until there is no ethanol odor to obtain a concentrated solution. Add 3 times the volume of deionized water to the concentrated solution, stir evenly, let stand for 12 h, centrifuge at 5000 r / min for 15 min, take the supernatant, purify by adsorption through macroporous resin, elute with 65% ethanol solution, collect the eluent, concentrate again under reduced pressure to a thick paste, and vacuum dry at 0.08 MPa and 50℃ until the water content is ≤5%. Pulverize and pass through an 80-mesh sieve to obtain bamboo leaf flavonoid extract. To prepare the composite color-protecting and modifying solution, first heat deionized water to 40℃, then add ascorbic acid and citric acid sequentially, stirring at 60 r / min until completely dissolved. Next, add the prepared bamboo leaf flavonoid extract, ensuring the composite color-protecting and modifying solution contains 0.3% ascorbic acid, 0.2% citric acid, and 0.1% bamboo leaf flavonoid extract, with deionized water as the remainder. Continue stirring for 20 min, maintain the temperature for 30 min, cool to room temperature, and filter through a 0.22 μm sterile membrane to remove impurities. Place blanched and cooled Nanjing linden leaves into the composite color-protecting and modifying solution and soak them using intermittent ultrasonic-assisted penetration. The ultrasonic parameters are: frequency 20 kHz, power 100 W, 30 s ultrasonic treatment followed by a 30 s pause; soaking temperature 25℃; soaking time 20 min, gently turning the leaves every 10 min. After soaking, rinse once with deionized water for 10 s, drain, and set aside.

[0056] S4. Oxalic Acid Removal and Softening Treatment: Prepare a removal and softening solution consisting of 0.1% sodium bicarbonate, 0.2% trehalose, 0.05% edible-grade pectinase, and the remainder deionized water, with a liquid-to-solid ratio of 6:1 (mL:g). The edible-grade pectinase has an enzyme activity of 5000 U / g. Before adding it to the removal and softening solution, dilute the pectinase 10 times with deionized water at 35℃ and let it stand for 15 minutes. Place the young leaves of Nanjing linden, which have undergone flavonoid color-protecting modification, into the removal and softening solution. Use a segmented pH control method for gentle removal and softening treatment. The segmented pH control is achieved using edible-grade sodium bicarbonate solution and citric acid solution, with a mass concentration of 5% for sodium bicarbonate solution and 3% for citric acid solution, and a drop rate of 1 drop / s during adjustment. In the first stage (0-5 min), the pH was adjusted to 6.0; in the second stage (5-15 min), the pH was adjusted to 6.5; and in the third stage (15-20 min), the pH was adjusted to 6.8. The treatment temperature was maintained at 30℃, and the mixture was slowly stirred at 20 rpm. After treatment, the leaves were removed, drained, and set aside.

[0057] S5. Dehydration and Shaping: The tender leaves of Nanjing linden, after oxalic acid removal and softening treatment, are placed in a vacuum dehydration device. A gradient dehydration method is used: the first stage (0-3 minutes) uses a vacuum of 0.06 MPa and a dehydration temperature of 40°C to initially remove surface moisture from the leaves; the second stage (3-8 minutes) uses a vacuum of 0.08 MPa and a dehydration temperature of 45°C to remove internal free moisture from the leaves. Dehydration is stopped when the leaf moisture content reaches 60%. After dehydration, the leaves are removed and allowed to cool naturally to room temperature for later use.

[0058] S6. Frozen Packaging: Place the dehydrated Nanjing linden leaves into a quick-freezing device, control the quick-freezing temperature at -35℃ and the quick-freezing time at 15 minutes. After the center temperature of the leaves drops below -18℃, immediately place them into food-grade composite packaging material and vacuum them to a vacuum degree of 0.09MPa to complete the vacuum packaging. After packaging, store them in a low-temperature cold storage at -18±2℃.

[0059] Example 2:

[0060] S1. Raw Material Pretreatment: Select tender leaves of Nanjing linden harvested in May and June each year. Choose tender shoots and leaves with a length of 6.5cm and a thickness of 5.5mm, ensuring the leaves are fresh, free from pests and diseases, and free from old stems and yellowing. After harvesting, pack them in low-temperature preservation boxes, controlling the transportation temperature at 6℃. Upon arrival at the processing site, remove impurities and residual petioles from the leaves, and rinse twice with 32℃ warm water using an air bubble rinse method at a pressure of 0.12MPa. After rinsing, drain until there are no obvious water droplets on the leaf surface, and set aside for use.

[0061] S2. Blanching: Prepare the blanching solution, which is a mixture of deionized water, salt, and EDTA-2Na, with a salt concentration of 0.15% and an EDTA-2Na concentration of 0.015%. Place the pre-treated Nanjing linden leaves into the blanching solution, controlling the blanching temperature at 82℃ and the blanching time at 50 seconds. During blanching, turn the leaves once every 30 seconds. Immediately after blanching, remove the leaves and place them in a 2℃ ice water bath to rapidly cool to room temperature for 1.5 minutes. Drain the water and set aside.

[0062] S3. Flavonoid Color Protection and Modification Treatment: First, prepare bamboo leaf flavonoid extract. Select fresh bamboo leaves, remove impurities and old leaves, and pulverize to 50 mesh. Add 9 times the mass of 72% ethanol solution and extract twice at 52℃ and ultrasonic power 190W, 45 min each time. Combine the two extracts and concentrate under reduced pressure until there is no ethanol odor to obtain a concentrated solution. Add 3.5 times the volume of deionized water to the concentrated solution, stir evenly, let stand for 14 h, centrifuge at 5200 r / min for 17 min, take the supernatant, purify by adsorption through macroporous resin, elute with 67% ethanol solution, collect the eluent, concentrate again under reduced pressure to a thick paste, and vacuum dry at 0.09 MPa and 52℃ until the water content is ≤5%. Pulverize and pass through a 90-mesh sieve to obtain bamboo leaf flavonoid extract. To prepare the composite color-protecting and modifying solution, first heat deionized water to 42℃, then add ascorbic acid and citric acid sequentially, stirring at 70 r / min until completely dissolved. Next, add the prepared bamboo leaf flavonoid extract, ensuring the composite color-protecting and modifying solution contains 0.4% ascorbic acid, 0.3% citric acid, and 0.15% bamboo leaf flavonoid extract, with deionized water as the remainder. Continue stirring for 25 min, maintain the temperature for 30 min, cool to room temperature, and filter through a 0.22 μm sterile filter membrane to remove impurities. Place blanched and cooled Nanjing linden leaves into the composite color-protecting and modifying solution and soak them using intermittent ultrasonic-assisted penetration. The ultrasonic parameters are: frequency 25 kHz, power 120 W, 30 s ultrasonic treatment followed by a 30 s pause; soaking temperature 27℃; soaking time 25 min, gently turning the leaves every 10 min. After soaking, rinse twice with deionized water for 12 s each time, drain, and set aside.

[0063] S4. Oxalic Acid Removal and Softening Treatment: Prepare a removal and softening solution consisting of 0.15% sodium bicarbonate, 0.25% trehalose, 0.07% edible-grade pectinase, and the remainder deionized water, with a liquid-to-solid ratio of 6:1 (mL:g). The edible-grade pectinase has an enzyme activity of 6500 U / g. Before adding it to the removal and softening solution, dilute the pectinase 10 times with deionized water at 35℃ and let it stand for 17 minutes. Place the young leaves of Nanjing linden, which have undergone flavonoid color-protecting modification, into the removal and softening solution. Use a segmented pH control method for gentle removal and softening treatment. The segmented pH control is achieved using edible-grade sodium bicarbonate solution and citric acid solution, with a mass concentration of 5% for sodium bicarbonate solution and 3% for citric acid solution, and a drop rate of 1.5 drops / s during adjustment. In the first stage (0-5 min), the pH was adjusted to 6.1; in the second stage (5-15 min), the pH was adjusted to 6.7; and in the third stage (15-20 min), the pH was adjusted to 6.9. The treatment temperature was maintained at 32℃, with slow stirring at 25 rpm throughout the process. After treatment, the leaves were removed, drained, and set aside for later use.

[0064] S5. Dehydration and Shaping: The tender leaves of Nanjing linden, after oxalic acid removal and softening treatment, are placed in a vacuum dehydration device. A gradient dehydration method is used: the first stage (0-3 minutes) uses a vacuum of 0.06 MPa and a dehydration temperature of 40°C to initially remove surface moisture from the leaves; the second stage (3-8 minutes) uses a vacuum of 0.08 MPa and a dehydration temperature of 45°C to remove internal free moisture from the leaves. Dehydration is stopped when the leaf moisture content reaches 65%. After dehydration, the leaves are removed and allowed to cool naturally to room temperature for later use.

[0065] S6. Frozen Packaging: Place the dehydrated Nanjing linden leaves into a quick-freezing device, control the quick-freezing temperature at -37℃ and the quick-freezing time at 17 minutes. After the center temperature of the leaves drops below -18℃, immediately place them into food-grade composite packaging material and vacuum them to a vacuum degree of 0.095MPa to complete the vacuum packaging. After packaging, store them in a low-temperature cold storage at -18±2℃.

[0066] Example 3:

[0067] S1. Raw Material Pretreatment: Select tender leaves of Nanjing linden harvested in June each year. Choose tender shoots and leaves 8cm in length and 8mm in thickness, ensuring the leaves are fresh, free from pests and diseases, and free from old stems and yellowing. After harvesting, pack them in low-temperature preservation boxes, controlling the transportation temperature at 8℃. Upon arrival at the processing site, remove impurities and residual petioles from the leaves, and rinse them three times with 35℃ warm water using an air bubble rinse method at a pressure of 0.15MPa. After rinsing, drain until there are no obvious water droplets on the leaf surface, and set aside for use.

[0068] S2. Blanching: Prepare the blanching solution, which is a mixture of deionized water, salt, and EDTA-2Na, with a salt concentration of 0.2% and an EDTA-2Na concentration of 0.02%. Place the pre-treated Nanjing linden leaves into the blanching solution, controlling the blanching temperature at 85℃ and the blanching time at 60 seconds. During blanching, turn the leaves once every 30 seconds. Immediately after blanching, remove the leaves and place them in a 4℃ ice water bath for 2 minutes to rapidly cool to room temperature. Drain the water and set aside.

[0069] S3. Flavonoid Color Protection and Modification Treatment: First, prepare bamboo leaf flavonoid extract. Select fresh bamboo leaves, remove impurities and old leaves, and pulverize to 60 mesh. Add 10 times the mass of 75% ethanol solution, and extract twice at 55℃ and ultrasonic power 200W, 50 min each time. Combine the two extracts and concentrate under reduced pressure until there is no ethanol odor to obtain a concentrated solution. Add 4 times the volume of deionized water to the concentrated solution, stir evenly, let stand for 16 h, centrifuge at 5500 r / min for 20 min, take the supernatant, purify by adsorption through macroporous resin, elute with 70% ethanol solution, collect the eluent, concentrate again under reduced pressure to a thick paste, and vacuum dry at 0.1 MPa and 55℃ until the water content is ≤5%. Pulverize and pass through a 100-mesh sieve to obtain bamboo leaf flavonoid extract. To prepare the composite color-protecting and modifying solution, first heat deionized water to 45℃, then add ascorbic acid and citric acid sequentially, stirring at 80 r / min until completely dissolved. Next, add the prepared bamboo leaf flavonoid extract, ensuring the composite color-protecting and modifying solution contains 0.5% ascorbic acid, 0.4% citric acid, and 0.2% bamboo leaf flavonoid extract, with deionized water as the remainder. Continue stirring for 30 min, maintain the temperature for 30 min, and after cooling to room temperature, filter through a 0.22 μm sterile filter membrane to remove impurities. Place the blanched and cooled Nanjing linden leaves into the composite color-protecting and modifying solution and soak them using intermittent ultrasonic-assisted penetration. The ultrasonic parameters are: frequency 30 kHz, power 150 W, 30 s ultrasonic treatment followed by a 30 s pause; soaking temperature 30℃; soaking time 30 min, gently turning the leaves every 10 min. After soaking, rinse twice with deionized water for 15 s each time, drain, and set aside.

[0070] S4. Oxalic Acid Removal and Softening Treatment: Prepare a removal and softening solution consisting of 0.2% sodium bicarbonate, 0.3% trehalose, 0.1% food-grade pectinase, and the remainder deionized water, with a liquid-to-solid ratio of 6:1 (mL:g). The food-grade pectinase has an enzyme activity of 8000 U / g. Before adding it to the removal and softening solution, dilute the pectinase 10 times with deionized water at 35℃ and let it stand for 20 minutes. Place the young leaves of Nanjing linden, which have undergone flavonoid color-protecting modification, into the removal and softening solution. Use a segmented pH control method for gentle removal and softening treatment. The segmented pH control is achieved using food-grade sodium bicarbonate solution and citric acid solution, with a mass concentration of 5% for sodium bicarbonate solution and 3% for citric acid solution, and a drop rate of 2 drops / s during adjustment. In the first stage (0-5 minutes), adjust the pH to 6.2; in the second stage (5-15 minutes), adjust the pH to 7.0; and in the third stage (15-20 minutes), adjust the pH to 7.0. The processing temperature was maintained at 35℃, and the mixture was slowly stirred at a speed of 30r / min. After processing, the leaves were removed, drained, and set aside for later use.

[0071] S5. Dehydration and Shaping: The tender leaves of Nanjing linden, after oxalic acid removal and softening treatment, are placed in a vacuum dehydration device. A gradient dehydration method is used: the first stage (0-3 minutes) uses a vacuum of 0.06 MPa and a dehydration temperature of 40°C to initially remove surface moisture from the leaves; the second stage (3-8 minutes) uses a vacuum of 0.08 MPa and a dehydration temperature of 45°C to remove internal free moisture from the leaves. Dehydration is stopped when the leaf moisture content reaches 70%. After dehydration, the leaves are removed and allowed to cool naturally to room temperature for later use.

[0072] S6. Frozen Packaging: Place the dehydrated Nanjing linden leaves into a quick-freezing device, control the quick-freezing temperature at -40℃ and the quick-freezing time at 20 minutes. After the center temperature of the leaves drops below -18℃, immediately place them into food-grade composite packaging material and vacuum them to a vacuum degree of 0.1MPa to complete the vacuum packaging. After packaging, store them in a low-temperature cold storage at -18±2℃.

[0073] Comparative Example 1:

[0074] The manufacturing process of Example 2 is adopted, except that: in step S3, the flavonoid color-protecting modification treatment is not performed, and after blanching and cooling, the oxalic acid removal and softening treatment in step S4 is performed directly.

[0075] Comparative Example 2:

[0076] The manufacturing process of Example 2 is adopted, except that: in step S4, the segmented pH control method is not used, and the pH value is directly adjusted to 6.7 before oxalic acid removal and softening treatment is performed, and no food-grade pectinase is added to the removal and softening solution.

[0077] test:

[0078] Experiment 1: Sensory quality evaluation experiment;

[0079] Ten professional sensory evaluators were selected to conduct sensory quality evaluations on the Nanjing linden leaves prepared in Examples 1-3 and Comparative Examples 1-2 according to sensory evaluation standards. Evaluation indicators included color, taste, and texture, with each indicator having a maximum score of 10 points, for a total score of 30 points. Color evaluation standards: 8-10 points for bright green, uniform leaf color without yellowing or browning; 5-7 points for relatively bright green leaves with slight yellowing or browning; 0-4 points for significantly yellowed or browned leaves. Taste evaluation standards: 8-10 points for tender taste without bitterness; 5-7 points for relatively tender taste with slight bitterness; 0-4 points for rough taste with significant bitterness. Texture evaluation standards: 8-10 points for soft, elastic texture without stiffness; 5-7 points for relatively soft texture with slight stiffness; 0-4 points for stiff texture without elasticity. The average score from 10 evaluators was taken as the final sensory quality score. The experimental results are as follows:

[0080] Table 1

[0081] Group Color score Taste score Quality Score Total Score Example 1 8.5 8.2 8.3 25.0 Example 2 9.2 9.0 9.1 27.3 Example 3 8.8 8.6 8.7 26.1 Comparative Example 1 5.3 7.8 8.0 21.1 Comparative Example 2 8.9 6.2 5.8 20.9

[0082] The experimental results show that the total sensory quality scores of Examples 1-3 are all higher than those of Comparative Examples 1-2, indicating that the Nanjing linden leaf vegetable processing technology provided by the present invention can effectively improve the sensory quality of leaf vegetables.

[0083] Comparative Example 1, lacking flavonoid color-protecting modification treatment, had a leaf color score of only 5.3, significantly lower than Examples 1-3. The ascorbic acid, citric acid, and bamboo leaf flavonoid extract in the flavonoid color-protecting modification treatment have a synergistic color-protecting effect, effectively inhibiting the activity of polyphenol oxidase in the leaves, preventing browning, and maintaining the leaves' bright green color. Comparative Example 1, lacking this treatment step, was prone to browning during subsequent processing and storage, resulting in poor color and affecting the overall sensory quality.

[0084] Comparative Example 2, lacking a segmented pH control method and without adding edible-grade pectinase to the softening solution, scored only 6.2 in taste and 5.8 in texture, both significantly lower than Examples 1-3. The segmented pH control method provides a suitable reaction environment for oxalic acid removal and softening, ensuring thorough removal and appropriate leaf softening. Edible-grade pectinase effectively breaks down pectin in the leaves, resulting in soft leaves with a tender texture. Comparative Example 2 lacked these key treatments, leading to incomplete oxalic acid removal, a bitter taste, stiff leaf texture, poor elasticity, and unsatisfactory sensory quality.

[0085] Experiment 2: Nutrient retention and oxalic acid content determination test;

[0086] Nutritional composition determination: The content of flavonoids in Nanjing linden leaves prepared in Examples 1-3 and Comparative Examples 1-2 was determined by high performance liquid chromatography; the content of vitamin C was determined by spectrophotometry.

[0087] Oxalic acid content determination: The oxalic acid content in Nanjing linden leaves prepared in Examples 1-3 and Comparative Examples 1-2 was determined by potassium permanganate titration. The results are as follows:

[0088] Table 2

[0089] Group Flavonoid content (mg / 100g) Vitamin C content (mg / 100g) Oxalic acid content (mg / 100g) Example 1 125.3 38.6 85.2 Example 2 142.5 42.8 72.6 Example 3 130.8 40.2 78.5 Comparative Example 1 98.7 32.5 73.1 Comparative Example 2 138.6 39.5 120.8

[0090] The experimental results show that Examples 1-3 are superior to Comparative Examples 1-2 in terms of flavonoid content, vitamin C content, and oxalic acid content, indicating that the Nanjing linden leaf vegetable processing technology provided by the present invention can effectively retain the nutritional components of leaf vegetables and reduce oxalic acid content.

[0091] Comparative Example 1, lacking flavonoid color-protecting modification treatment, had a flavonoid content of only 98.7 mg / 100g and a vitamin C content of only 32.5 mg / 100g, both significantly lower than Examples 1-3. The ultrasonic-assisted penetration method in the flavonoid color-protecting modification treatment promotes the penetration of nutrients from the composite color-protecting modification solution into the leaf interior. Simultaneously, ascorbic acid and citric acid protect vitamin C from oxidative decomposition, and bamboo leaf flavonoid extract increases the flavonoid content in the leaves. Comparative Example 1 lacked this treatment, resulting in low nutrient retention.

[0092] Comparative Example 2, lacking a segmented pH control method and without adding food-grade pectinase to the softening solution, had an oxalic acid content as high as 120.8 mg / 100g, significantly higher than Examples 1-3. Segmented pH control optimizes the reaction conditions for oxalic acid removal, improving removal efficiency; food-grade pectinase breaks down pectin in leaf cell walls, making oxalic acid easier to release and remove. Comparative Example 2 lacked these key treatments, resulting in incomplete oxalic acid removal and a high oxalic acid content in the leaves, affecting the safety and taste of the leafy vegetables.

[0093] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not describe all details exhaustively, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification.

Claims

1. A processing method for Nanjing linden leaves, characterized in that, Includes the following steps: S1. Raw material pretreatment: Select fresh, disease-free, and old stem and yellow leaves of Nanjing linden, remove impurities and residual petioles, rinse 2-3 times with warm water at 30-35℃ using the bubble rinsing method, with a bubble rinsing pressure of 0.1-0.15MPa. After rinsing, drain until there are no obvious water droplets on the surface of the leaves, and set aside for later use. S2. Blanching: Place the pre-treated Nanjing linden leaves into the blanching liquid, control the blanching temperature at 80-85℃ and the blanching time at 40-60s. During the blanching process, turn the leaves over once every 30 seconds. After blanching, immediately remove the leaves and place them in an ice water bath at 0-4℃ to cool rapidly to room temperature for 1-2 minutes. After removing the leaves, drain the water and set aside. S3. Flavonoid Color Protection and Modification Treatment: Blanched and cooled Nanjing linden leaves are placed in a composite color protection and modification solution and soaked using an intermittent ultrasonic-assisted penetration method. The composite color protection and modification solution consists of 0.3-0.5% ascorbic acid, 0.2-0.4% citric acid, 0.1-0.2% bamboo leaf flavonoid extract, and the balance being deionized water. The ultrasonic parameters are: frequency 20-30kHz, power 100-150W, ultrasonic treatment for 30 seconds, pause for 30 seconds, soaking temperature 25-30℃, and soaking time 20-30 minutes, during which the leaves are gently turned over once every 10 minutes. After soaking, rinse with deionized water 1-2 times, each rinse for 10-15 seconds, drain the water, and set aside. S4. Oxalic acid removal and softening treatment: The tender leaves of Nanjing linden that have been modified with flavonoid color protection are placed in the removal and softening solution, and a segmented pH control method is used for gentle removal and softening treatment; the treatment temperature is maintained at 30-35℃, and the leaves are slowly stirred at a speed of 20-30 r / min; after the treatment is completed, the leaves are removed, drained, and set aside. S5. Dehydration and shaping: Place the tender leaves of Nanjing linden that have been softened by removing oxalic acid into a vacuum dehydration device, control the vacuum degree to be 0.06-0.08MPa and the dehydration temperature to be 40-45℃, and use a gradient decompression method during the dehydration process. Stop dehydration when the moisture content of the leaves is 60-70%. After dehydration is completed, take them out and let them cool naturally to room temperature for later use. S6. Frozen Packaging: Place the dehydrated Nanjing linden leaves into a quick-freezing device, control the quick-freezing temperature at -35 to -40℃ and the quick-freezing time at 15-20 minutes. After the core temperature of the leaves drops below -18℃, immediately place them into food-grade composite packaging material and vacuum them to a vacuum degree of 0.09-0.1MPa to complete the vacuum packaging. After packaging, store them in a low-temperature cold storage at -18±2℃.

2. The processing method for Nanjing linden leaves according to claim 1, characterized in that, The harvesting time for the tender leaves of Nanjing linden in step S1 is from May to June each year. When harvesting, select tender shoots and leaves with a length of 5-8cm and a thickness of 3-8mm. After harvesting, pack them in a low-temperature preservation box and control the transportation temperature at 4-8℃.

3. The processing method for Nanjing linden leaves according to claim 1, characterized in that, The blanching solution mentioned in step S2 is a mixture of deionized water, salt, and EDTA-2Na, wherein the mass concentration of salt is 0.1-0.2% and the mass concentration of EDTA-2Na is 0.01-0.02%.

4. The processing method for Nanjing linden leaves according to claim 1, characterized in that, The preparation method of the composite color-protecting and modifying liquid in step S3 is as follows: first, heat deionized water to 40-45℃, then add ascorbic acid and citric acid in sequence, stirring at a speed of 60-80 r / min until completely dissolved; then add bamboo leaf flavonoid extract, continue stirring for 20-30 min, keep warm for 30 min, cool to room temperature, and filter with a 0.22μm sterile filter membrane to remove impurities before use.

5. The processing method for Nanjing linden leaves according to claim 4, characterized in that, The preparation method of the bamboo leaf flavonoid extract is as follows: Select fresh bamboo leaves, remove impurities and old leaves, pulverize to 40-60 mesh, add 8-10 times the mass of 70-75% ethanol solution, extract twice at 50-55℃ and ultrasonic power 180-200W, each extraction for 40-50 min, combine the two extracts, concentrate under reduced pressure until there is no ethanol odor, and obtain a concentrated solution; add 3-4 times the volume of deionized water to the concentrated solution, stir evenly, let stand for 12-16 h, centrifuge at 5000-5500 r / min for 15-20 min, take the supernatant, purify by adsorption through macroporous resin, elute with 65-70% ethanol solution, collect the eluent, concentrate again under reduced pressure to a thick paste, vacuum dry to a water content ≤5%, pulverize and pass through an 80-100 mesh sieve to obtain the bamboo leaf flavonoid extract; Vacuum drying requires a vacuum level of 0.08-0.1 MPa and a temperature of 50-55℃.

6. The processing method for Nanjing linden leaves according to claim 1, characterized in that, The segmented pH adjustment in step S4 uses food-grade sodium bicarbonate solution and citric acid solution for adjustment, wherein the mass concentration of sodium bicarbonate solution is 5% and the mass concentration of citric acid solution is 3%, and the drop acceleration rate during adjustment is 1-2 drops / s.

7. The processing method for Nanjing linden leaves according to claim 1, characterized in that, The segmented pH control is as follows: in the first stage (0-5 min), adjust the pH value to 6.0-6.2; in the second stage (5-15 min), adjust the pH value to 6.5-7.0; and in the third stage (15-20 min), adjust the pH value to 6.8-7.

0.

8. The processing method for Nanjing linden leaves according to claim 1, characterized in that, The food-grade pectinase mentioned in step S4 has an enzyme activity of 5000-8000 U / g. Before adding the softening solution, the pectinase is diluted 10 times with deionized water at 35℃ and allowed to stand for 15-20 minutes.

9. The processing method for Nanjing linden leaves according to claim 1, characterized in that, The softening solution described in step S4 consists of 0.1-0.2% sodium bicarbonate, 0.2-0.3% trehalose, 0.05-0.1% food-grade pectinase, and the remainder deionized water, with a liquid-to-solid ratio of 6:1 (mL:g).

10. The processing method for Nanjing linden leaves according to claim 1, characterized in that, The specific parameters for gradient dehydration in step S5 are as follows: the first stage is 0-3 min with a vacuum degree of 0.06 MPa and a dehydration temperature of 40°C, which initially removes moisture from the leaf surface; the second stage is 3-8 min with a vacuum degree of 0.08 MPa and a dehydration temperature of 45°C, which removes free water from the inside of the leaf.

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