A drying method of hibiscus flowers

Abstract

The application discloses a drying method of hibiscus flowers. The method comprises the following steps: 1) enzyme inactivation and color protection: the hibiscus flowers are sequentially immersed in an acidic citric acid-vitamin C solution and a solution added with corn stem and leaf water extract, so as to efficiently inactivate oxidase and protect color; 2) composite film construction: a hibiscus leaf extract solution is used as a natural film-forming matrix, is compounded with hydroxypropyl methyl cellulose and the like to form a film-forming solution, the hibiscus flowers are immersed and coated, and a supercritical CO2 drying technology is used to form a uniform and porous protective film on the petal surface; and 3) hot air drying: two-stage gradient temperature and humidity control is adopted for final drying. The application can significantly inhibit enzyme browning and non-enzyme browning in the drying process, effectively reduce the loss of functional components such as anthocyanins and flavonoids, solve the problems of petal blackening, brittleness, fragility and the like caused by traditional drying, and the finished product is full in shape, bright in color, and green in the whole process.

Description

Technical Field

[0001] This invention belongs to the field of food processing technology, and specifically relates to a method for drying hibiscus flowers. Background Technology

[0002] Hibiscus flowers are rich in flavonoids, polyphenols, vitamins, and other functional substances, giving them high nutritional and medicinal value. However, fresh hibiscus flowers have a high water content and delicate tissues, making them susceptible to enzymatic browning, microbial contamination, and loss of functional components, which seriously affects their quality.

[0003] There are five commonly used drying techniques in the preparation of dried flowers: physical drying, hot air drying, heat pump drying, vacuum freeze drying, and microwave drying. Among them, hot air drying is widely used due to its advantages such as simple operation, low cost, wide applicability, and good versatility. However, it is more damaging to the quality of flowers, accelerating the decomposition of pigments such as anthocyanins, or causing anthocyanins and vitamin C to undergo oxidation or non-enzymatic browning reactions, resulting in flowers with an overly dark color, reduced content of active ingredients, and significant nutrient loss; it is also prone to uneven drying. At the same time, hot air can cause the pyrolysis of the gelatinous substance in the cell walls, causing the petals to become too brittle or charred, with incomplete shapes, or even becoming brittle and easily broken.

[0004] This solution employs a three-step pretreatment process: "enzyme inactivation → color protection → hydrophilic membrane coating to block functional substances," combined with precise hot air drying. This process efficiently removes moisture while maximizing the retention of hibiscus flower functional substances, thereby enhancing the color and quality of the dried product. Summary of the Invention

[0005] Technical Problems to be Solved: This technical solution addresses the core pain points in existing hibiscus flower processing. Through the synergistic effect of acidic inactivation and chelation of natural components, it achieves a dual effect of enzyme inactivation and color protection, blocking the oxidative browning pathway at its source, reducing nutrient loss, and solving the problem of enzymatic browning caused by polyphenol oxidase. Furthermore, this solution combines the construction of a natural composite protective film with a segmented drying process, replacing the cell wall to withstand heat, regulating the rate of water migration, avoiding damage to petal tissues from high temperatures, maintaining morphological integrity and color, and solving the problem of petal quality damage during drying. This solution balances the retention of active ingredients, dehydration efficiency, and stability.

[0006] Technical solution: A method for drying hibiscus flowers, the steps of which are as follows: S1: After gently washing the hibiscus flowers and draining the surface water, soak them in a citric acid-vitamin C compound solution. After a short soaking, take them out, rinse them, and drain them to obtain the acid-treated hibiscus flowers. S2: Add water to corn stems and leaves and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH with citric acid-vitamin C compound solution. Add corn stem and leaf extract and mix evenly to obtain compound color protection solution. Soak the acid-treated hibiscus flowers in the compound color protection solution. After the process is completed, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add water to hibiscus leaves and beat at high speed to form a homogenate. Extract by stirring in a water bath. Filter to obtain hibiscus leaf extract. Add hydroxypropyl methylcellulose and sodium citrate, stir to dissolve, then add glycerol, continue stirring to mix, and adjust pH to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution, and intermittently turned during the process. The immersed hibiscus flowers are then removed, left to stand, and then dried with supercritical CO2 to obtain preliminarily dried hibiscus flowers. S5: The initially dried hibiscus flowers undergo two stages of hot air drying to obtain dried hibiscus flowers.

[0007] Furthermore, in step S1, the pH of the citric acid-vitamin C composite solution is 2.5-3.5, and the mass ratio of citric acid to vitamin C is (1-2):(1-2); the short soaking time is 1-2 minutes.

[0008] Furthermore, in step S2, the mass ratio of corn stems and leaves to water is 1:(1-2); the pH is adjusted to 5.5-6.0; the mass ratio of corn stem and leaf extract to citric acid-vitamin C complex solution is 1:(1-2); and the soaking time is 2-3 hours.

[0009] Furthermore, in step S3, the mass ratio of hibiscus leaves to water is 1:(5-8); the water bath stirring temperature is 50-60℃, the stirring speed is 100-200 rpm, and the water bath stirring time is 30-40 min; the addition amounts of hydroxypropyl methylcellulose, sodium citrate, and glycerol are 1.0-1.5 wt.%, 0.3-0.5 wt.%, and 0.5-0.8 wt.% of the hibiscus leaf extract, respectively; the stirring speed is 150-250 rpm, and the stirring time is 20-30 min; the pH is adjusted to 5.5-6.5.

[0010] Furthermore, in step S4, the immersion time is 2-5 min; the standing time is 3-5 min; the supercritical CO2 drying temperature is 30-40℃, the pressure is 10-15 MPa, the CO2 flow rate is 0.8-1.2 L / min, and the time is 5-10 min.

[0011] Furthermore, the conditions for the two-stage hot air drying in step S5 are as follows: the temperature of the first stage is 50-55℃, the relative humidity is 15%-20%, and the drying time is 4-5h; the temperature of the second stage is 55-60℃, the relative humidity is 20-25%, and the drying time is 2-3h.

[0012] The dried hibiscus flowers prepared by the drying method described above.

[0013] Beneficial effects: This invention first rapidly disrupts the spatial structure of the enzyme in an acidic environment, dissociating copper ions at the active center and inactivating the enzyme. Then, it simultaneously utilizes citric acid and metallothionein from corn stem and leaf water extract to chelate copper ions, blocking the recovery of enzyme activity. Meanwhile, flavonoids and polyphenols simultaneously scavenge free radicals and inhibit oxidative browning, achieving a synergistic effect of enzyme inactivation and color protection.

[0014] The polysaccharides and pectin-like substances in hibiscus leaves of this invention have natural adhesiveness and film-forming properties. When combined with high-temperature resistant HPMC, they form a porous membrane structure with a "molecular sieve effect." Water molecules can evaporate through the micropores of the membrane, meeting the dehydration requirements of hot air drying. Meanwhile, large molecular functional substances such as flavonoids and anthocyanins are blocked by the membrane and cannot be lost with water vapor. At the same time, the membrane layer adheres tightly to the surface of the petals and preferentially bears the heat of the hot air during the drying process, avoiding direct heating of the petal cell walls that would lead to pyrolysis of the gums. This maintains the integrity of the petal shape and prevents brittleness and scorching. Glycerin, as a plasticizer, can improve the flexibility of the membrane layer and prevent the membrane layer from cracking and falling off during the drying process.

[0015] This invention employs brief supercritical CO2 drying. In an environment with zero surface tension, the film-forming liquid coating the surface of the petals can solidify uniformly and smoothly, forming a continuous, transparent, crack-free, and more fluffy film with higher porosity. This film can better lock in the fragrance of the flowers and prevent oxidation and physical damage. The high diffusivity of supercritical CO2 allows it to instantly penetrate into every corner of the petals and the film layer, achieving simultaneous and rapid dehydration and solidification inside and out. This avoids the "shelling" or uneven stress problems that may occur when drying from the surface to the inside, ensuring that the film layer structure is intact and adheres firmly and lastingly.

[0016] This invention employs a segmented drying mode, adapted to the tissue structure and protective film characteristics of hibiscus flowers. First, it efficiently removes internal water in a low-humidity environment, and enhanced dehumidification can reduce browning caused by moisture. In the later stage, it reduces the moisture gradient between the inside and outside of the petals, avoiding rapid moisture migration that could lead to tissue damage, thus balancing storage stability and quality.

[0017] Compared to traditional hot air drying, this invention significantly reduces pigment decomposition and browning by using synergistic enzyme inactivation and color protection, natural membrane barrier, and segmented drying. It also improves the retention rate of flavonoids and polyphenols, while solving the problems of petals being brittle, charred, and fragile, thus maintaining their morphological integrity. All raw materials are natural, with no risk of exogenous residues, and are highly compatible with hibiscus flowers, making it environmentally friendly and safe. Attached Figure Description

[0018] Figure 1 These are images of the appearance of hibiscus flowers from Example 1 and those dried directly with hot air without pretreatment; Figure 2 This is a graph showing the relative conductivity results of the hibiscus flower samples from Example 5 and the comparative example. Detailed Implementation

[0019] To provide a clearer and more thorough understanding of the technical solution of this invention, the invention will be further described below with reference to the accompanying drawings and embodiments. The following embodiments are illustrative of the invention, but the invention is not limited to these embodiments: Example 1

[0020] A method for drying hibiscus flowers, the steps of which are as follows: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Add 100g of corn stems and leaves to 100g of water and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 100g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain compound color-protecting solution. Soak the acid-treated hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add 250g of water to 50g of hibiscus leaves and beat at high speed to form a homogenate. Extract at 200rpm in a 50℃ water bath for 30min. Filter to obtain hibiscus leaf extract. Add 2g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of hibiscus leaf extract and stir at 200rpm to dissolve. Then add 1g of glycerol and continue stirring for 20min to mix. Adjust the pH to 6.0 to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, and turned over once every 30 seconds during the process. The immersed hibiscus flowers are then removed and left to stand for 3 minutes. Subsequently, they are dried by supercritical CO2 at a temperature of 35°C, a pressure of 12 MPa, a CO2 flow rate of 1 L / min, and a time of 5 minutes to obtain preliminarily dried hibiscus flowers. S5: The preliminarily dried hibiscus flowers undergo two stages of hot air drying. The first stage has a temperature of 50℃, a relative humidity of 15%, and a drying time of 5 hours. The second stage has a temperature of 55℃, a relative humidity of 20%, and a drying time of 3 hours, resulting in dried hibiscus flowers. Example 2

[0021] A method for drying hibiscus flowers, the steps of which are as follows: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Add 100g of corn stems and leaves to 100g of water and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 200g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain compound color-protecting solution. Soak the acid-treated hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add 250g of water to 50g of hibiscus leaves and beat at high speed to form a homogenate. Extract at 200rpm in a 50℃ water bath for 30min. Filter to obtain hibiscus leaf extract. Add 2g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of hibiscus leaf extract and stir at 200rpm to dissolve. Then add 1g of glycerol and continue stirring for 20min to mix. Adjust the pH to 6.0 to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, and turned over once every 30 seconds during the process. The immersed hibiscus flowers are then removed and left to stand for 3 minutes. Subsequently, they are dried by supercritical CO2 at a temperature of 35°C, a pressure of 12 MPa, a CO2 flow rate of 1 L / min, and a time of 5 minutes to obtain preliminarily dried hibiscus flowers. S5: The preliminarily dried hibiscus flowers undergo two stages of hot air drying. The first stage has a temperature of 50℃, a relative humidity of 15%, and a drying time of 5 hours. The second stage has a temperature of 55℃, a relative humidity of 20%, and a drying time of 3 hours, resulting in dried hibiscus flowers. Example 3

[0022] A method for drying hibiscus flowers, the steps of which are as follows: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Add 100g of corn stems and leaves to 100g of water and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 100g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain compound color-protecting solution. Soak the acid-treated hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add 50g of hibiscus leaves to 400g of water and beat at high speed to form a homogenate. Extract at 50℃ water bath and 200rpm for 30min. Filter to obtain hibiscus leaf extract. Add 2g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of hibiscus leaf extract and stir at 200rpm to dissolve. Then add 1g of glycerol and continue stirring for 20min to mix. Adjust the pH to 6.0 to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, and turned over once every 30 seconds during the process. The immersed hibiscus flowers are then removed and left to stand for 3 minutes. Subsequently, they are dried by supercritical CO2 at a temperature of 35°C, a pressure of 12 MPa, a CO2 flow rate of 1 L / min, and a time of 5 minutes to obtain preliminarily dried hibiscus flowers. S5: The preliminarily dried hibiscus flowers undergo two stages of hot air drying. The first stage has a temperature of 50℃, a relative humidity of 15%, and a drying time of 5 hours. The second stage has a temperature of 55℃, a relative humidity of 20%, and a drying time of 3 hours, resulting in dried hibiscus flowers. Example 4

[0023] A method for drying hibiscus flowers, the steps of which are as follows: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Add 100g of corn stems and leaves to 100g of water and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 100g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain compound color-protecting solution. Soak the acid-treated hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add 250g of water to 50g of hibiscus leaves and beat at high speed to form a homogenate. Extract at 200rpm in a 50℃ water bath for 30min. Filter to obtain hibiscus leaf extract. Add 3g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of hibiscus leaf extract and stir at 200rpm to dissolve. Then add 1g of glycerol and continue stirring for 20min to mix. Adjust the pH to 6.0 to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, and turned over once every 30 seconds during the process. The immersed hibiscus flowers are then removed and left to stand for 3 minutes. Subsequently, they are dried by supercritical CO2 at a temperature of 35°C, a pressure of 12 MPa, a CO2 flow rate of 1 L / min, and a time of 5 minutes to obtain preliminarily dried hibiscus flowers. S5: The preliminarily dried hibiscus flowers undergo two stages of hot air drying. The first stage has a temperature of 50℃, a relative humidity of 15%, and a drying time of 5 hours. The second stage has a temperature of 55℃, a relative humidity of 20%, and a drying time of 3 hours, resulting in dried hibiscus flowers. Example 5

[0024] A method for drying hibiscus flowers, the steps of which are as follows: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Add 100g of corn stems and leaves to 100g of water and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 100g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain compound color-protecting solution. Soak the acid-treated hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add 250g of water to 50g of hibiscus leaves and beat at high speed to form a homogenate. Extract at 200rpm in a 50℃ water bath for 30min. Filter to obtain hibiscus leaf extract. Add 2g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of hibiscus leaf extract and stir at 200rpm to dissolve. Then add 1g of glycerol and continue stirring for 20min to mix. Adjust the pH to 6.0 to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, and turned over once every 30 seconds during the process. The immersed hibiscus flowers are then removed and left to stand for 3 minutes. Subsequently, they are dried by supercritical CO2 at a temperature of 35°C, a pressure of 12 MPa, a CO2 flow rate of 1 L / min, and a time of 10 minutes to obtain preliminarily dried hibiscus flowers. S5: The preliminarily dried hibiscus flowers undergo two stages of hot air drying. The first stage has a temperature of 50℃, a relative humidity of 15%, and a drying time of 5 hours. The second stage has a temperature of 55℃, a relative humidity of 20%, and a drying time of 3 hours, resulting in dried hibiscus flowers. Comparative Example 1

[0025] The difference between this comparative example and Example 1 is that it does not involve brief immersion in a citric acid-vitamin C complex solution at pH 3.0, as detailed below: S1: Gently wash the hibiscus flowers and drain the surface water. Prepare a solution of citric acid and vitamin C in a 1:1 ratio. Add 100g of corn stems and leaves to 100g of water and blend at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 100g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain a compound color-protecting solution. Soak the hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain the color-protected hibiscus flowers. S2: Add 250g of water to 50g of hibiscus leaves and beat at high speed to form a homogenate. Extract at 200rpm in a 50℃ water bath for 30min. Filter to obtain hibiscus leaf extract. Add 2g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of hibiscus leaf extract and stir at 200rpm to dissolve. Then add 1g of glycerol and continue stirring for 20min to mix. Adjust the pH to 6.0 to obtain film-forming solution. S3: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, and turned over once every 30 seconds during the process. The immersed hibiscus flowers are then removed and left to stand for 3 minutes. Subsequently, they are dried by supercritical CO2 at a temperature of 35°C, a pressure of 12 MPa, a CO2 flow rate of 1 L / min, and a time of 5 minutes to obtain preliminarily dried hibiscus flowers. S4: The preliminarily dried hibiscus flowers undergo two stages of hot air drying. The first stage has a temperature of 50℃, a relative humidity of 15%, and a drying time of 5 hours. The second stage has a temperature of 55℃, a relative humidity of 20%, and a drying time of 3 hours, resulting in dried hibiscus flowers. Comparative Example 2

[0026] The difference between this comparative example and Example 1 is that corn stem and leaf extract is not added, as detailed below: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Adjust the pH to 5.5 with citric acid-vitamin C compound solution. Continue to soak the acid-treated hibiscus flowers for 2 hours, then remove them, rinse and drain to obtain color-protected hibiscus flowers. S3: Add 250g of water to 50g of hibiscus leaves and beat at high speed to form a homogenate. Extract at 200rpm in a 50℃ water bath for 30min. Filter to obtain hibiscus leaf extract. Add 2g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of hibiscus leaf extract and stir at 200rpm to dissolve. Then add 1g of glycerol and continue stirring for 20min to mix. Adjust the pH to 6.0 to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, and turned over once every 30 seconds during the process. The immersed hibiscus flowers are then removed and left to stand for 3 minutes. Subsequently, they are dried by supercritical CO2 at a temperature of 35°C, a pressure of 12 MPa, a CO2 flow rate of 1 L / min, and a time of 5 minutes to obtain preliminarily dried hibiscus flowers. S5: The preliminarily dried hibiscus flowers undergo two stages of hot air drying. The first stage has a temperature of 50℃, a relative humidity of 15%, and a drying time of 5 hours. The second stage has a temperature of 55℃, a relative humidity of 20%, and a drying time of 3 hours, resulting in dried hibiscus flowers. Comparative Example 3

[0027] The difference between this comparative example and Example 1 is that it does not involve soaking in the film-forming solution, as detailed below: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Add 100g of corn stems and leaves to 100g of water and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 100g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain compound color-protecting solution. Soak the acid-treated hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Supercritical CO2 drying is carried out at a temperature of 35℃, a pressure of 12MPa, a CO2 flow rate of 1L / min, and a time of 5min to obtain preliminarily dried hibiscus flowers. S4: The preliminarily dried hibiscus flowers undergo two stages of hot air drying. The first stage has a temperature of 50℃ and a relative humidity of 15% for 5 hours; the second stage has a temperature of 55℃ and a relative humidity of 20% for 3 hours, resulting in dried hibiscus flowers. Comparative Example 4

[0028] The difference between this comparative example and Example 1 is that hibiscus leaf extract is not added, as detailed below: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Add 100g of corn stems and leaves to 100g of water and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 100g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain compound color-protecting solution. Soak the acid-treated hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add 2g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of water, stir at 200rpm to dissolve, then add 1g of glycerol, continue stirring for 20min to mix well, and adjust the pH to 6.0 to obtain the film-forming solution; S4: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, and turned over once every 30 seconds during the process. The immersed hibiscus flowers are then removed and left to stand for 3 minutes. Subsequently, they are dried by supercritical CO2 at a temperature of 35°C, a pressure of 12 MPa, a CO2 flow rate of 1 L / min, and a time of 5 minutes to obtain preliminarily dried hibiscus flowers. S5: The preliminarily dried hibiscus flowers undergo two stages of hot air drying. The first stage has a temperature of 50℃, a relative humidity of 15%, and a drying time of 5 hours. The second stage has a temperature of 55℃, a relative humidity of 20%, and a drying time of 3 hours, resulting in dried hibiscus flowers. Comparative Example 5

[0029] The difference between this comparative example and Example 1 is that supercritical CO2 drying is not performed, as detailed below: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Add 100g of corn stems and leaves to 100g of water and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 100g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain compound color-protecting solution. Soak the acid-treated hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add 250g of water to 50g of hibiscus leaves and beat at high speed to form a homogenate. Extract at 200rpm in a 50℃ water bath for 30min. Filter to obtain hibiscus leaf extract. Add 2g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of hibiscus leaf extract and stir at 200rpm to dissolve. Then add 1g of glycerol and continue stirring for 20min to mix. Adjust the pH to 6.0 to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, turning them over once every 30 seconds. After immersion, the hibiscus flowers are removed and left to stand for 3 minutes to obtain the treated hibiscus flowers. S5: The treated hibiscus flowers are subjected to two-stage hot air drying. The first stage has a temperature of 50℃ and a relative humidity of 15% for 5 hours. The second stage has a temperature of 55℃ and a relative humidity of 20% for 3 hours to obtain dried hibiscus flowers. Comparative Example 6

[0030] The difference between this comparative example and Example 1 is that the two-stage hot air drying is not performed, as detailed below: S1: After gently washing the hibiscus flowers and draining the surface water, immerse them in a citric acid-vitamin C compound solution with a pH of 3.0. After a brief soaking of 1 minute, remove them. Prepare a solution of citric acid and vitamin C in a 1:1 ratio, rinse and drain to obtain acid-treated hibiscus flowers. S2: Add 100g of corn stems and leaves to 100g of water and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH to 5.5 with 100g of citric acid-vitamin C compound solution. Add 100g of corn stem and leaf extract and mix well to obtain compound color-protecting solution. Soak the acid-treated hibiscus flowers in the compound color-protecting solution. After 2 hours, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add 250g of water to 50g of hibiscus leaves and beat at high speed to form a homogenate. Extract at 200rpm in a 50℃ water bath for 30min. Filter to obtain hibiscus leaf extract. Add 2g of hydroxypropyl methylcellulose and 0.6g of sodium citrate to 200g of hibiscus leaf extract and stir at 200rpm to dissolve. Then add 1g of glycerol and continue stirring for 20min to mix. Adjust the pH to 6.0 to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution for 3 minutes, and turned over once every 30 seconds during the process. The immersed hibiscus flowers are then removed and left to stand for 3 minutes. Subsequently, they are dried by supercritical CO2 at a temperature of 35°C, a pressure of 12 MPa, a CO2 flow rate of 1 L / min, and a time of 5 minutes to obtain preliminarily dried hibiscus flowers. S5: The preliminarily dried hibiscus flowers are then dried by hot air drying at a temperature of 50℃ and a relative humidity of 15% for 8 hours to obtain dried hibiscus flowers. Performance testing

[0031] 1. Appearance evaluation: An evaluation team will score the color and integrity of the dried hibiscus flowers.

[0032] The results are shown in Table 1. The scores for color and integrity of the examples were significantly higher than those of the comparative examples, demonstrating the overall superiority of this technical solution. Constructing a physical protective film (film-forming solution) is the first and most important line of defense in maintaining the sensory quality of the product; comparative example 3 scored the lowest. Furthermore, comparing comparative examples 1 and 2, the acidic pretreatment and the key steps of inhibiting browning with corn stem and leaf extract are crucial in determining whether the product color can achieve excellent results. Figure 1 As shown, compared with hibiscus flowers dried directly with hot air without pretreatment, the appearance and texture of hibiscus flowers under the two drying methods are significantly different. The color of Example 1 is more vibrant, with no obvious browning or shrinkage, and the texture is fluffy and crisp. In contrast, the hibiscus flower sample dried directly with hot air without pretreatment shows obvious browning and severe fading.

[0033] Table 1 Appearance rating of dried hibiscus flowers

[0034] 2. Detection of active ingredient content The vitamin C content was determined in accordance with GB 5009.86—2016 "National Food Safety Standard: Determination of Ascorbic Acid in Food"; the total flavonoid content was determined in accordance with SN / T4592—2016 "Determination of Total Flavonoids in Exported Food"; and the proanthocyanidin content was determined by spectrophotometry.

[0035] During the drying process, due to moisture loss, sugars concentrate and crystallize, causing cells to shrink and rupture. Cellular contents spill out, leading to oxidative degradation of vitamin C, total flavonoids, and proanthocyanidins due to increased contact with external oxygen. The results are shown in Table 2. Comparative Example 1 lacked a crucial step for rapid enzyme inactivation, and incomplete enzyme activity inhibition exacerbated the oxidative loss of active ingredients. Comparative Example 2 lacked the synergistic effect of metal ion chelation and natural antioxidants, resulting in a decreased retention rate of antioxidant components (especially proanthocyanidins). Comparative Example 3's petals lost their physical protective layer and were directly exposed to hot air, leading to significant losses of heat-sensitive components (such as vitamin C) and volatile / oxidizable components during drying. Overall, the examples showed higher retention rates of active ingredients, while the comparative examples suffered from decreased active ingredient content due to the absence of various process steps.

[0036] Table 2. Content of active ingredients in dried hibiscus flowers

[0037] 3. Relative conductivity Dried hibiscus flowers (0.1 g, uncrushed) were soaked in 20 mL of deionized water in an Erlenmeyer flask. The solution was shaken at 100 rpm for 15 min at room temperature. After filtration, the conductivity R1 was measured using a conductivity meter. The extract and flowers were then boiled for 15 min, cooled to room temperature with running water, and the conductivity R2 was measured. The percentage ratio of R1 to R2 was calculated as the relative conductivity.

[0038] Cell membrane permeability reflects the integrity and permeability of petal cell membranes. A higher relative electrolyte leakage rate indicates a lower ability of the cell membrane to retain intracellular solutes and electrolytes. Changes in cell membrane permeability were studied by measuring the relative conductivity of different samples. The relative conductivity of each sample is shown below. Figure 2 As shown, Comparative Example 3, lacking core physical protection, suffered direct thermal damage and dehydration stress to the cell membrane, leading to significant ion leakage and the highest predicted conductivity. Comparative Example 1, lacking the rapid enzyme inactivation step, experienced stronger enzymatic browning and oxidative stress in subsequent processes, resulting in severe membrane system damage and a substantial increase in conductivity. The synergistic color protection, composite protective film, and optimized drying process in Example 5 effectively maintained the integrity of hibiscus cells and mitigated drying damage. The application of the film-forming solution is the most critical factor in maintaining cell membrane integrity, while other steps provide important synergistic support from chemical and physical kinetic perspectives.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention, or modify them into equivalent embodiments, without departing from the spirit and technical essence of the present invention. Therefore, any simple modifications, equivalent substitutions, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention, without departing from the content of the technical solutions of the present invention, shall still fall within the scope of protection of the present invention.

Claims

1. A method for drying hibiscus flowers, characterized in that, The steps of this method are as follows: S1: After gently washing the hibiscus flowers and draining the surface water, soak them in a citric acid-vitamin C compound solution. After a short soaking, take them out, rinse them, and drain them to obtain the acid-treated hibiscus flowers. S2: Add water to corn stems and leaves and stir at high speed to form a homogenate. Filter to obtain corn stem and leaf extract. Adjust the pH with citric acid-vitamin C compound solution. Add corn stem and leaf extract and mix evenly to obtain compound color protection solution. Soak the acid-treated hibiscus flowers in the compound color protection solution. After the process is completed, take them out, rinse and drain to obtain color-protected hibiscus flowers. S3: Add water to hibiscus leaves and beat at high speed to form a homogenate. Extract by stirring in a water bath. Filter to obtain hibiscus leaf extract. Add hydroxypropyl methylcellulose and sodium citrate, stir to dissolve, then add glycerol, continue stirring to mix, and adjust pH to obtain film-forming solution. S4: After color protection, the hibiscus flowers are immersed in the film-forming solution, and intermittently turned during the process. The immersed hibiscus flowers are then removed, left to stand, and then dried with supercritical CO2 to obtain preliminarily dried hibiscus flowers. S5: The initially dried hibiscus flowers undergo two stages of hot air drying to obtain dried hibiscus flowers.

2. The method for drying hibiscus flowers according to claim 1, characterized in that: In step S1, the pH of the citric acid-vitamin C composite solution is 2.5-3.5, and the mass ratio of citric acid to vitamin C is (1-2):(1-2); the short soaking time is 1-2 min.

3. The method for drying hibiscus flowers according to claim 1, characterized in that: In step S2, the mass ratio of corn stems and leaves to water is 1:(1-2); the pH is adjusted to 5.5-6.0; the mass ratio of corn stem and leaf extract to citric acid-vitamin C compound solution is 1:(1-2); and the soaking time is 2-3 hours.

4. The method for drying hibiscus flowers according to claim 1, characterized in that: In step S3, the mass ratio of hibiscus leaves to water is 1:(5-8); the water bath stirring temperature is 50-60℃, the stirring speed is 100-200 rpm, and the water bath stirring time is 30-40 min; the addition amounts of hydroxypropyl methylcellulose, sodium citrate, and glycerol are 1.0-1.5 wt.%, 0.3-0.5 wt.%, and 0.5-0.8 wt.% of the hibiscus leaf extract, respectively; the stirring speed is 150-250 rpm, and the stirring time is 20-30 min; the pH is adjusted to 5.5-6.

5.

5. The method for drying hibiscus flowers according to claim 1, characterized in that: In step S4, the immersion time is 2-5 minutes; the standing time is 3-5 minutes; the supercritical CO2 drying temperature is 30-40℃, the pressure is 10-15 MPa, the CO2 flow rate is 0.8-1.2 L / min, and the time is 5-10 minutes.

6. The method for drying hibiscus flowers according to claim 1, characterized in that: The conditions for the two-stage hot air drying in step S5 are as follows: the temperature of the first stage is 50-55℃, the relative humidity is 15%-20%, and the drying time is 4-5h; the temperature of the second stage is 55-60℃, the relative humidity is 20-25%, and the drying time is 2-3h.

7. Dried hibiscus flowers prepared by the drying method according to any one of claims 1-6.

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