A processing method for inhibiting browning and keeping crispness of dried gongcai by synergistically using vanillic acid and cysteine

Abstract

The application provides a processing method for inhibiting browning and keeping crispness of dried Gongcai by synergistically using vanillic acid and cysteine, and belongs to the field of agricultural product processing and storage and preservation. The method comprises the following steps: selecting dried Gongcai products prepared by dehydration and drying; immersing the Gongcai in a natural synergistic treatment liquid containing vanillic acid, L-cysteine and food-grade calcium salt for impregnation treatment; after draining, low-temperature hot air drying is performed at a hot air temperature of not higher than 50 DEG C until the water content is reduced to 9% to 10%; and finally, vacuum packaging and storage are performed, and cold storage is performed in the dark. In the low-temperature dehydration stage, vanillic acid is used as a specific substrate of polyphenol oxidase, and a quinone intermediate is generated in situ under the action of enzyme to irreversibly covalently combine with the active center of PPO, so that the enzyme is passivated; meanwhile, L-cysteine can remove excess quinone intermediates and complex copper ions. The application does not require high temperature or special physical field, and only uses conventional low-temperature hot air drying equipment, so that the shelf life is effectively prolonged and a certain crisp and tender taste can be ensured.

Description

Technical Field

[0001] This application relates to the field of agricultural product processing and storage preservation technology, and in particular to a processing method that utilizes vanillic acid and cysteine ​​to synergistically inhibit browning and maintain crispness in dried chrysanthemum greens. Background Technology

[0002] As a unique dehydrated vegetable variety in my country, dried gongcai (a type of dried vegetable) is highly favored for its distinctive crisp texture and flavor. However, dried gongcai is prone to discoloration (yellowing and browning) during long-term storage. The root cause lies in the fact that the polyphenol oxidase (PPO) remaining in the tissue is still active, continuously catalyzing the oxidation of endogenous polyphenols into quinones, which further polymerize to form dark pigments.

[0003] To solve this technical problem, those skilled in the art have conducted numerous explorations, and the currently available publicly available technical means can be mainly divided into two categories.

[0004] The first category involves chemical preservatives for color protection. Early methods often used sulfites (such as sodium metabisulfite) as strong reducing agents to directly inhibit browning. While effective, this method poses food safety risks, including excessive sulfur residue and the potential for allergic reactions, and has been gradually restricted or replaced. In recent years, natural extracts such as tea polyphenols, kojic acid, and ascorbic acid have been widely used for color protection in fruits and vegetables. However, current research generally confirms that the inhibition of PPO by natural antioxidants like tea polyphenols is a non-covalent, reversible complexation or competitive inhibition. During long-term storage, these inhibitor molecules are highly susceptible to desorption or oxidation, leading to a significant "rebound" in PPO activity after 60 days of storage, resulting in recurrence of browning in the later stages and failing to meet the requirements for long shelf life.

[0005] The second category involves physical heat or energy field enzyme inactivation technologies. For example, high-temperature blanching (temperature ≥80℃), microwave, radio frequency, or low-temperature plasma treatment can directly denature PPO proteins, thereby causing them to lose their catalytic activity. Chinese patent application CN202511939147.1 discloses a method for slowing down the browning of dried gongcai (a type of dried vegetable) based on compound microbial fermentation combined with dual-field synergistic treatment (low-temperature plasma and low-frequency alternating magnetic field) and LED pulsed light storage, achieving good color protection. However, these physical treatment methods have inherent technical defects: First, high-temperature treatment will destroy the pectin structure in the cell walls of gongcai, leading to β-elimination reactions and causing gongcai to lose its characteristic "crisp and tender" texture; second, the investment and operation and maintenance costs of special high-energy physical field equipment such as low-temperature plasma and alternating magnetic fields are extremely high, making it difficult to popularize and promote in small and medium-sized agricultural product processing enterprises.

[0006] Developing a low-cost method that can effectively inhibit polyphenol oxidase to extend shelf life while maintaining the crisp and tender texture of dried radish is a pressing technical problem that needs to be solved. Summary of the Invention

[0007] In view of the problems in the existing technology that reversible antioxidants are prone to desorption, leading to browning recurrence in the later stage of storage, and that high temperature and high energy physical field treatment will destroy the crisp and tender texture of dried gongcai and the equipment cost is high, the present invention provides a processing method that uses vanillic acid and cysteine ​​to synergistically inhibit browning and maintain crispness in dried gongcai. It can effectively inhibit polyphenol oxidase to extend shelf life, while ensuring a certain degree of crispness and tenderness in gongcai.

[0008] The method for inhibiting browning of dried gongcai (a type of dried vegetable) by synergistic inhibition of vanillic acid and cysteine ​​in this application includes the following steps: (1) Selecting dried gongcai that has been dehydrated and dried; (2) Synergistic immersion treatment: Immersing the dried gongcai from step (1) in a natural synergistic treatment liquid for immersion treatment, and draining the surface liquid after removal; wherein, the composition of the natural synergistic treatment liquid by weight percentage is: 0.10%-0.40% vanillic acid, 0.05%-0.20% L-cysteine, 0.05%-0.15% food-grade calcium salt, and the remainder is pure water; (3) Low-temperature hot air drying: Drying the gongcai after step (2) at a hot air temperature not exceeding 50°C until the moisture content of the gongcai drops to 9%-10%; (4) Packaging and storage: Packing the gongcai after step (3) into a food-grade packaging bag, vacuum sealing it, and storing it in a cold storage away from light.

[0009] Furthermore, the food-grade calcium salt is preferably calcium chloride or calcium lactate. In the color-protecting system of this invention, the addition of trace amounts of calcium salt has a dual technical function: First, it acts as a residual enzyme activity activator. An appropriate amount of calcium ions can activate the catalytic activity of residual PPO in the dried vegetable tissue at low temperatures (40-50℃), ensuring that vanillic acid can be converted to vanillin in the early stages of drying, providing sufficient intermediates for subsequent irreversible covalent bonding. Second, it acts as a cell wall reinforcing agent. Calcium ions can combine with pectic acid in the cell wall of the dried vegetable to form a stable "calcium bridge" structure, effectively inhibiting cell wall shrinkage during dehydration and supporting the preservation of the crispness of the dried vegetable after low-temperature drying.

[0010] Furthermore, the preferred hot air drying temperature in step (3) is 45℃, the wind speed is 0.8m / s, and the drying time is 60-90min. Through numerous experiments, the inventors discovered that 45℃ is the critical temperature equilibrium point for triggering irreversible covalent bonding reactions and preserving the pectin structure. If the temperature is higher than 50℃, although the enzymatic reaction rate increases, microscopic thermal damage begins to occur in the pectin structure of the *Gynostemma pentaphyllum* cell walls, affecting the taste; if the temperature is lower than 40℃, the micro-activity of PPO is insufficient in the early stages of dehydration, and the conversion rate of vanillic acid to quinone is too slow, leading to the loss of some vanillic acid with water vaporization, resulting in incomplete enzyme inactivation. A gentle wind speed of 0.8m / s ensures that the surface moisture of the *Gynostemma pentaphyllum* evaporates slowly, allowing the active ingredients in the color-protecting solution to fully penetrate and participate in the reaction in the early stages of drying; 60-90min is the actual industrial drying cycle determined based on the conventional thickness and moisture content of *Gynostemma pentaphyllum*.

[0011] Furthermore, in step (4), the oxygen permeability of the food-grade packaging bag is ≤200 cm⁻². 3 / (m 2 (day atm). In this invention, although the irreversible covalent binding of vanillic acid and PPO has fundamentally disrupted the catalytic conformation of the enzyme protein, excessive residual oxygen may still trigger trace amounts of non-enzymatic oxidation reactions (such as lipid peroxidation) during long-term storage. Controlling the oxygen permeability of the packaging bag to below 200 can maximize the isolation of external oxygen, providing a low-oxygen microenvironment for the long-term stability of the color, flavor, and nutrients of the dried vegetables.

[0012] Furthermore, the immersion treatment temperature is 20-30℃, and the immersion time is 8-15 minutes. This temperature range is chosen based on the universality of ambient industrial environments, eliminating the need for heating, saving energy, and effectively preventing heat-sensitive components in the color-protecting solution (such as cysteine ​​and calcium salts) from reacting prematurely during the immersion stage. The 8-15 minute time window is sufficient to allow the treatment solution to evenly wet the microporous structure of the dried radish surface, promoting the full adhesion of vanillic acid and cysteine ​​to the surface of the radish, while avoiding excessive water absorption by the radish due to excessive immersion time, which would affect the efficiency of subsequent low-temperature drying.

[0013] Furthermore, the preferred temperature for the low-temperature hot air drying is 40-45℃, and the wind speed is 0.5-1.0 m / s. Limiting the temperature to 40-45℃ ensures a mild and controllable reaction, providing sufficient time for the remaining PPO enzyme microactivity to complete the complete reaction chain of "substrate oxidation - quinone generation - covalent grafting"; the wind speed of 0.5-1.0 m / s creates a gentle air circulation, preventing the surface of the dried vegetables from drying and hardening due to excessive wind speed, thus preventing the internal moisture from being properly released and affecting the texture of the final product.

[0014] Furthermore, the temperature of the cold storage is 3-5℃. Strictly controlling the storage temperature within the low-temperature range of 3-5℃ can further inhibit the recovery of micro-activity of trace amounts of incompletely inactivated enzymes that may exist inside the dried vegetables at a physical level. At the same time, it reduces the respiration rate and moisture migration rate of the internal microenvironment of the dried vegetables, ensuring that the packaged product maintains a high degree of stability in color, crispness, and flavor during its shelf life of several months.

[0015] The mechanism of action of this application: The inventors discovered that during the initial stage of low-temperature dehydration of dried gongcai (a type of dried vegetable) at temperatures not exceeding 50°C, the remaining moisture and oxygen within the vegetable can maintain a certain degree of weak catalytic activity of polyphenol oxidase (PPO), while this temperature range is insufficient to cause thermal damage to the gongcai pectin. If the specific substrate vanillic acid is introduced into the system at this point, the remaining PPO will catalyze the oxidation of vanillic acid during dehydration, generating a highly reactive vanillin intermediate in situ. Since this reaction occurs within the microenvironment of the PPO's active site, the generated vanillin will directly and irreversibly covalently bind to the key free sulfhydryl groups of the PPO's active site, thereby destroying the enzyme's catalytic active site at the molecular structural level.

[0016] Furthermore, to enhance the selectivity of the reaction and suppress side reactions, this invention incorporates L-cysteine. L-cysteine ​​effectively removes excess free vanillin from the system, preventing non-specific oxidative damage to the endogenous nutrients of the dried chrysanthemum. Simultaneously, the sulfhydryl group of L-cysteine ​​can complex with free copper ions in the system, reducing the auxiliary catalytic efficiency of copper ions on the PPO active site. Through these dual mechanisms, vanillic acid and L-cysteine ​​form a synergistic effect of irreversible covalent bonding and auxiliary complexation inhibition. This combination overcomes the limitations of reversible inhibition by conventional antioxidants and eliminates the need for expensive heating or specialized physical field equipment.

[0017] The beneficial effects of this application are: 1. This invention utilizes the active quinone intermediate generated by the in-situ oxidation of residual PPO during the low-temperature drying stage at 40-45℃ of vanillic acid, which undergoes irreversible covalent bonding with the key groups of the PPO active center; at the same time, L-cysteine ​​removes excess active quinone and complexes copper ions in the PPO active center, forming a synergistic effect of irreversible covalent bonding and auxiliary complexation inhibition, fundamentally eliminating the "browning rebound" problem in the later storage period caused by desorption and consumption of conventional antioxidants (such as tea polyphenols).

[0018] 2. This invention eliminates the need for the 8-12 hour microbial fermentation process and the energy-intensive special high-energy physics equipment such as low-temperature plasma and alternating magnetic fields required in existing technologies. It achieves streamlined production using only conventional room-temperature impregnation tanks and standard hot-air drying equipment, significantly reducing equipment investment and operating energy consumption, and shortening the process cycle. Because the entire process is carried out at a low temperature not exceeding 50℃, the damage to the pectin structure of the dried vegetable cell walls is avoided, extending the product's shelf life while minimizing harm to its characteristic crisp and tender texture. Detailed Implementation

[0019] The embodiments of this application will now be described in more detail with reference to the examples. While embodiments of this application are shown in the examples, it should be understood that this application can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to make this application more thorough and complete, and to fully convey the scope of this application to those skilled in the art.

[0020] Example 1 (1) Select dried gongcai (a type of vegetable) that has been dehydrated and dried.

[0021] (2) Synergistic impregnation treatment: Prepare a treatment solution (containing 0.25% vanillic acid, 0.10% L-cysteine, and 0.10% anhydrous calcium chloride). Immerse at room temperature (25°C) for 10 minutes.

[0022] (3) Low temperature hot air drying: After draining, put it into a hot air drying oven at 45℃ and 0.8m / s for 70 minutes until the moisture content is 9.5%.

[0023] (4) Packaging and storage: 100g / bag is packed into a high-barrier PA / PE composite vacuum bag (oxygen permeability ≤180 cm). 3 / (m 2 ·day·atm), vacuum sealed, and stored in a cold storage at 4℃ away from light.

[0024] Example 2 The difference between this embodiment and Example 1 lies in the concentration of components in the natural synergistic treatment solution: the treatment solution is prepared with 0.15% vanillic acid, 0.08% L-cysteine, and 0.08% anhydrous calcium chloride; the immersion temperature is 28°C for 12 minutes; the drying temperature is 42°C, the air velocity is 0.6 m / s, and the drying time is 80 minutes. The remaining steps and parameters are the same as in Example 1.

[0025] Example 3 The difference between this embodiment and Example 1 lies in the concentration of components and the types of calcium salts in the natural synergistic treatment solution: the treatment solution is prepared with 0.40% vanillic acid, 0.20% L-cysteine, and 0.15% calcium lactate; the immersion temperature is 30°C for 8 minutes; the drying temperature is 40°C, the air velocity is 0.5 m / s, and the drying time is 90 minutes. The remaining steps and parameters are the same as in Example 1.

[0026] Example 4 The difference between this embodiment and Example 1 lies in the concentration of components and the types of calcium salts in the natural synergistic treatment solution: the treatment solution is prepared with 0.10% vanillic acid, 0.05% L-cysteine, and 0.05% anhydrous calcium chloride; the immersion temperature is 20°C and the time is 15 min; the drying temperature is 44°C, the air velocity is 1.0 m / s, and the drying time is 65 min. The remaining steps and parameters are the same as in Example 1.

[0027] Example 5 The difference between this embodiment and Embodiment 1 is that the types of calcium salts in the natural synergistic treatment solution are different: the treatment solution is prepared with 0.25% vanillic acid, 0.10% L-cysteine, and 0.10% calcium lactate; the remaining steps and parameters are the same as in Embodiment 1.

[0028] Example 6 The difference between this embodiment and Example 1 lies in the component concentrations and impregnation conditions of the natural synergistic treatment solution: the treatment solution was prepared with 0.35% vanillic acid, 0.15% L-cysteine, and 0.12% anhydrous calcium chloride; the impregnation temperature was 22°C for 11 minutes; the drying temperature was 43°C, the air velocity was 0.9 m / s, and the drying time was 75 minutes. The remaining steps and parameters were the same as in Example 1.

[0029] Comparative Example 1 (Conventional tea polyphenol infusion) The treatment solution was changed to a 0.5% aqueous solution of tea polyphenols. Everything else was exactly the same as in Example 1.

[0030] Comparative Example 2 (conventional high-temperature drying to inactivate enzymes) Without soaking in any liquid, directly dry with hot air at 85°C for 70 minutes until the moisture content is 9.5%. The remaining packaging and storage are the same as in Example 1.

[0031] Comparative Example 3 (Contains vanillic acid only, without L-cysteine) The treatment solution contained only 0.25% vanillic acid and 0.10% calcium chloride. The rest was completely consistent with Example 1.

[0032] Comparative Example 4 (containing only L-cysteine, excluding vanillic acid) contained only 0.10% L-cysteine ​​and 0.10% calcium chloride. The rest was completely consistent with Example 1.

[0033] The residual activity of polyphenol oxidase (PPO) and sensory crispness were evaluated in samples of dried mustard greens stored for 90 days. The specific methods and results are as follows: PPO residual activity determination: PPO residual activity was determined according to the catechol method. The rate of enzyme reaction was expressed as the rate of change of absorbance at a wavelength of 420 nm, and the percentage of activity of the stored sample relative to the initial sample was calculated.

[0034] Sensory evaluation method for crispness: An evaluation panel of 10 sensory evaluators was formed. Samples were randomly numbered and left at room temperature for 30 minutes, followed by a blind sample test. The evaluation index was "sensory crispness score," using a 10-point scale: 8-10 points for "crisp and tender, with a crisp breaking sensation when chewing"; 5-7 points for "relatively crisp texture, slightly chewy"; 2-4 points for "soft texture, noticeably chewy"; and 0-1 points for "soft and mushy texture, no crispness." The arithmetic mean of all evaluators' scores was taken as the final sensory crispness score (rounded to one decimal place). The results are shown in Table 1.

[0035] Table 1 Comparison of key quality indicators after 90 days of storage Results analysis: The experimental data in Table 1 show that Comparative Example 1 (tea polyphenol immersion group) verified the technical limitations of conventional reversible inhibitors in long-term storage: after 90 days, PPO activity rebounded to 54.8%, indicating that non-covalent inhibition is insufficient to meet the long-term color protection requirements of dried gongcai (a type of dried vegetable). Comparative Example 3 (vanillic acid treatment group) lacked the scavenging effect of L-cysteine ​​on free quinone intermediates, resulting in a residual enzyme activity of only 11.8%, failing to reach the optimal inactivation level. This indicates that the synergistic effect of vanillic acid and L-cysteine ​​is key to achieving efficient enzyme inactivation. Comparative Example 4 (containing only L-cysteine, without vanillic acid) showed a residual PPO activity of 36.2%, significantly higher than the example group, indicating that relying solely on the complexation of copper ions by L-cysteine ​​cannot effectively inactivate PPO; irreversible covalent binding induced by vanillic acid as a specific substrate is the core pathway. Although Comparative Example 2 (high-temperature drying group) reduced enzyme activity to 17.5%, its sensory crispness score was only 4.1, indicating that while high-temperature treatment can inhibit enzyme activity, it severely damages the pectin structure of the dried mustard greens, sacrificing its commercial texture value. In contrast, Examples 1 to 6, using different concentration ratios (covering the boundary and median values ​​of vanillic acid 0.10%-0.40%, L-cysteine ​​0.05%-0.20%, and calcium salt 0.05%-0.15%), all achieved excellent and stable results. The residual PPO activity was inhibited in the extremely low range of 5.9%-7.5%, and the sensory crispness score was maintained at 7.2-7.6.

[0036] The above experimental data fully demonstrate that the synergistic treatment method of vanillic acid and L-cysteine ​​described in this invention can achieve irreversible permanent inactivation of polyphenol oxidase under low temperature conditions, effectively solving the technical problems of easy failure of reversible inhibitors and damage to texture by high temperature and physical field treatment in the prior art. While extending the shelf life, it retains a certain degree of crisp and tender taste of dried radish, and the process is simple and the equipment cost is low, which has significant industrial application value.

[0037] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A processing method for inhibiting browning and maintaining crispness of dried dried radish using vanillic acid and cysteine ​​synergistically, characterized in that, Includes the following steps: (1) Select dried gongcai (a type of vegetable) that has been dehydrated and dried; (2) Synergistic soaking treatment: Soak the dried gongcai from step (1) in a natural synergistic treatment solution for soaking treatment, and then drain the surface liquid after taking it out; The composition of the natural synergistic treatment liquid by weight percentage is: 0.10%-0.40% vanillic acid, 0.05%-0.20% L-cysteine, 0.05%-0.15% food-grade calcium salt, and the remainder is pure water; (3) Low-temperature hot air drying: The dried vegetables after step (2) are dried at a low-temperature hot air temperature not exceeding 50°C until the moisture content of the dried vegetables drops to 9%-10%; (4) Packaging and storage: The dried vegetables after step (3) are packed into food-grade packaging bags, vacuumed and sealed, and stored in a cold storage away from light.

2. The processing method according to claim 1, which utilizes vanillic acid and cysteine ​​to synergistically inhibit browning and maintain crispness in dried dried radish, is characterized in that... The food-grade calcium salt is calcium chloride or calcium lactate.

3. The processing method according to claim 1, which utilizes vanillic acid and cysteine ​​to synergistically inhibit browning and maintain crispness in dried dried radish, is characterized in that... In step (3), the hot air drying temperature is 45℃, the wind speed is 0.8m / s, and the drying time is 60-90min.

4. The processing method according to claim 1, which utilizes vanillic acid and cysteine ​​to synergistically inhibit browning and maintain crispness in dried dried radish, is characterized in that... In step (4), the oxygen permeability of the food-grade packaging bag is ≤200 cm⁻². 3 / (m 2 ·day·atm).

5. The processing method according to claim 1, which utilizes vanillic acid and cysteine ​​to synergistically inhibit browning and maintain crispness in dried dried radish, is characterized in that... The immersion temperature for the immersion treatment is 20-30℃, and the immersion time is 8-15 minutes.

6. The processing method according to claim 1, which utilizes vanillic acid and cysteine ​​to synergistically inhibit browning and maintain crispness in dried dried radish, is characterized in that... The temperature of the low-temperature hot air drying is 40-45℃ and the wind speed is 0.5-1.0m / s.

7. The processing method according to claim 1, which utilizes vanillic acid and cysteine ​​to synergistically inhibit browning and maintain crispness in dried dried radish, is characterized in that... The temperature of the cold storage is 3-5℃.

Images