Secondary fermentation method for improving flavor of pickled Chinese cabbages

Abstract

The invention relates to the field of pickled Chinese cabbage fermentation, and discloses a secondary fermentation method for improving the flavor of pickled Chinese cabbages, and the method comprises the following steps: salting leaf mustard, adding water, sealing, and carrying out primary segmented fermentation; taking out the pickled Chinese cabbages, desalting and cutting the pickled Chinese cabbages into broken sections; mixing and sealing the crushed pickled Chinese cabbages, a flavor stabilizer and a complex microbial inoculant prepared from lactobacillus plantarum, fused Weissella and ksajas yeast, then carrying out secondary segmented fermentation, and carrying out post-treatment to obtain the pickled Chinese cabbage product. Through the synergistic effect of the lactobacillus plantarum and the ksajas yeast, the contents of key flavor substances such as ethyl acetate and ethyl butyrate in the pickled Chinese cabbages are remarkably increased; nitrites are degraded through fusion of Weissella in secondary segmented fermentation, and the nitrite content of the pickled Chinese cabbages is greatly reduced; meanwhile, the complex microbial inoculant can effectively inhibit growth of spoilage microorganisms, so that harmful ingredients such as histamine and tyramine are remarkably reduced, and the quality and safety of the pickled Chinese cabbages are further improved.

Description

Technical Field

[0001] This invention relates to the field of sauerkraut fermentation technology, and more specifically, to a secondary fermentation method for enhancing the flavor of sauerkraut. Background Technology

[0002] Pickled mustard greens, a traditional fermented vegetable product in my country, hold an important position in the catering and food industry due to their unique sour and fragrant flavor and rich nutritional value, especially in northern regions where they have a broad consumer base and an annual market size exceeding 10 billion yuan. Currently, the industrial production of pickled mustard greens is still mainly based on the traditional single-stage salting fermentation process. The core process includes: selecting fresh mustard greens (such as large-leaf mustard greens and small-leaf mustard greens), washing and drying them, adding sea salt at 2%-5% of the raw material weight and kneading until the juice seeps out, placing them in earthenware jars or fermentation tanks, and then filling them with water to submerge the raw materials. The fermentation relies on naturally occurring lactic acid bacteria (such as Lactobacillus plantarum and Lactobacillus brevis) to carry out anaerobic fermentation. The fermentation cycle is usually 80-90 days. Fermentation ends when the pH drops to 3.5-4.0 and the total acid content reaches 0.7%-1.0%. Subsequent processes such as cutting, sterilization, and packaging are then carried out to produce the finished product. However, the traditional single-stage fermentation process has many insurmountable technical defects in practical applications, which seriously restrict the quality upgrading and efficiency improvement of the sauerkraut industry. Specifically, these defects are as follows: The traditional fermentation process relies on natural microbial communities for random fermentation, producing mainly lactic acid as a metabolic byproduct. This results in a basic sour taste but lacks complex flavor compounds such as ethyl acetate, ethyl butyrate (which impart fruity and alcoholic aromas), and phenylethyl alcohol (which imparts floral aromas). This leads to severe homogenization of product flavors, failing to meet current consumer demands for "complex flavors" and "regional characteristics." Market research shows that traditionally produced sauerkraut contains only 10-15 types of volatile flavor compounds, with ester content generally below 30 mg / kg, far from meeting the flavor standards of high-quality sauerkraut. High levels of nitrites and biogenic amines pose significant food safety risks: During natural fermentation, nitrates in the raw materials are easily converted into nitrites by miscellaneous bacteria (such as Escherichia coli and Pseudomonas), with peak levels reaching 15-25 mg / kg during the middle stage of fermentation (days 15-20). Although the levels gradually decrease with the proliferation of lactic acid bacteria in the later stages, some batches of products still have nitrite content exceeding the 10 mg / kg limit stipulated in GB2714-2015 "National Food Safety Standard for Pickled Vegetables" at the time of leaving the factory. Simultaneously, the decomposition of proteins by putrefactive microorganisms in the natural flora easily produces biogenic amines such as histamine and tyramine. The total biogenic amine content in some traditional sauerkraut can reach 80-120 mg / kg, posing health risks with long-term consumption. The long fermentation cycle and low production efficiency are problems: Traditional processes rely on the slow proliferation and metabolism of natural microorganisms, requiring 80-90 days to complete fermentation. This results in long production cycles, high inventory pressure, and slow capital turnover. For example, a company producing 10,000 tons of sauerkraut annually needs to store at least 25,000 tons of fermentation raw materials to ensure continuous production. Storage costs account for 18%-22% of the total cost. Furthermore, due to the long fermentation cycle, companies struggle to respond quickly to changes in market demand, often experiencing insufficient supply during peak seasons and inventory backlogs during off-seasons.

[0003] To address the aforementioned issues, some improvements have been attempted within the industry, such as: Patent CN108208666A discloses a method for preparing sauerkraut through secondary fermentation. This method involves adding spices such as Sichuan peppercorns, star anise, and cinnamon after the primary fermentation. While this can enrich the flavor to some extent, it doesn't involve microbial regulation and still relies on a natural fermentation system. The control of nitrite is limited, and the addition of spices easily masks the fermented aroma of the sauerkraut itself, resulting in poor overall flavor harmony. Another study used a single lactic acid bacteria (such as *Lactobacillus plantarum*) for inoculation and fermentation. Although this shortened the fermentation cycle to 30-40 days, the single bacterial strain's limited metabolic pathway meant it couldn't generate sufficient amounts of key flavor compounds such as esters and alcohols, failing to fundamentally improve the product's flavor profile. Therefore, providing a secondary fermentation method to enhance the flavor of sauerkraut has become a key technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0004] In view of this, the present invention proposes a secondary fermentation method to enhance the flavor of sauerkraut, aiming to solve at least one of the problems in the background art.

[0005] This invention proposes a secondary fermentation method to enhance the flavor of sauerkraut, comprising the following steps: Take mustard greens and pre-treat them; Sea salt is used to salt the mustard greens. Water is added to the salted mustard greens and the mixture is sealed for a staged fermentation. After the fermentation is completed, the resulting sauerkraut is desalted and cut for later use. A compound microbial agent was prepared by mixing Lactobacillus plantarum, Fusion Weissella, and Kexai yeast. The compound microbial agent was then mixed with nutrients and activated to obtain the activated compound microbial agent. The cut sauerkraut, activated compound microbial agent, and flavor stabilizer are mixed and vacuum sealed for secondary staged fermentation. After fermentation, the sauerkraut product is obtained through post-processing.

[0006] Preferably, the salting treatment is as follows: 2%-3% of the total mass of the mustard greens is evenly sprinkled on the surface of the mustard greens, and then they are rubbed by hand.

[0007] Preferably, the single-stage fermentation specifically involves: fermenting at a constant temperature of 15°C for 3 days, then adjusting the temperature to 10°C and continuing to ferment at a constant temperature for 10 days.

[0008] Preferably, the desalination process involves soaking the sauerkraut that has undergone primary fermentation in running water until the salt concentration drops to 4%-5%.

[0009] Preferably, the cutting process involves: taking out the soaked sauerkraut, placing it in a filter to drain for 30 minutes, and then cutting it into sauerkraut segments with a length of 3-4 cm.

[0010] Preferably, the mass ratio of Lactobacillus plantarum, Weissella fusionis, and Kexaro in the compound microbial agent is 68:12:12.

[0011] Preferably, the activation treatment is as follows: after mixing the compound bacterial agent with 1.5%-4% of the nutrient agent by mass, water at 30°C is added, the mixture is stirred evenly, and then activated at a constant temperature of 25°C for 2 hours.

[0012] Preferably, the mass ratio of the cut sauerkraut, the activated compound microbial agent, and the flavor stabilizer is 100:1:1.

[0013] Preferably, the secondary staged fermentation specifically involves: fermenting at a constant temperature of 25°C for 2 days, then adjusting the temperature to 18°C ​​and continuing fermentation for 5-7 days.

[0014] Preferably, the post-processing is as follows: the sauerkraut after secondary fermentation is sterilized at a constant temperature of 80-82℃ for 10-12 minutes, cooled to room temperature, packed into packaging bags, vacuumed, and then heat-sealed.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: Rich flavor profile: Through the synergistic effect of Lactobacillus plantarum and Kexacerbium arvense, this invention significantly increases the content of key flavor substances such as ethyl acetate and ethyl butyrate in the sauerkraut, resulting in a richer and more harmonious fruity and mellow flavor.

[0016] High safety: This invention effectively degrades nitrite through the fusion of Weissella bacteria in the secondary staged fermentation, significantly reducing the nitrite content of the final product; at the same time, the compound bacterial agent composed of multiple strains can effectively inhibit the growth of spoilage microorganisms, significantly reducing harmful components such as histamine and tyramine, and improving the food safety of the product.

[0017] High production efficiency: This invention adopts a fermentation process that combines secondary fermentation with multi-stage precise temperature control, which significantly shortens the overall fermentation cycle and helps improve production capacity turnover and resource utilization efficiency. Attached Figure Description

[0018] Various other advantages and benefits of the invention will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and should not be construed as limiting the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings: Figure 1 A flowchart illustrating a secondary fermentation method for enhancing the flavor of sauerkraut, provided as an embodiment of the present invention; Detailed Implementation

[0019] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the present invention.

[0020] Furthermore, regarding the numerical ranges involved in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included within this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0021] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0022] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0023] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0024] This invention provides a secondary fermentation method for enhancing the flavor of sauerkraut, comprising the following steps: Take mustard greens and pre-treat them; Sea salt is used to salt the mustard greens. Water is added to the salted mustard greens and the mixture is sealed for a staged fermentation. After the fermentation is completed, the resulting sauerkraut is desalted and cut for later use. A compound microbial agent was prepared by mixing Lactobacillus plantarum, Fusion Weissella, and Kexai yeast. The compound microbial agent was then mixed with nutrients and activated to obtain the activated compound microbial agent. The cut sauerkraut, activated compound microbial agent, and flavor stabilizer are mixed and vacuum sealed for secondary staged fermentation. After fermentation, the sauerkraut product is obtained through post-processing.

[0025] Specifically, the sodium chloride aqueous solution has a mass fraction of 2%-3%; the degree of polymerization (DP) of the oligofructose is 3-5, and its addition amount is 0.2%-0.4% of the mass of the sodium chloride aqueous solution; the addition amount of calcium chloride is 0.1%-0.3% of the mass of the sodium chloride aqueous solution.

[0026] Specifically, the pretreatment is preferably to rinse the mustard greens three times with running water to remove surface mud and impurities, and then place them in a ventilated and shady place to dry until there is no obvious moisture on the surface. Specifically, the salted mustard greens are layered into the fermentation tank, and each 5cm layer is compacted until there are no obvious gaps in the tank. Then, cooled boiled water is poured into the fermentation tank until it submerges the surface of the mustard greens by 5-8cm, ensuring that the salt concentration reaches 6%-8%. The tank is then sealed for a staged fermentation. Specifically, the viable count of the *Lactobacillus plantarum* is ≥1×10⁻⁶. 9 CFU / g; the viable count of the fused Weissella brevis is ≥1×10⁻⁶. 9 CFU / g; viable count of the *Kexacerbium spp.* ≥ 1 × 10⁻⁶ 9 CFU / g.

[0027] It is understood that the present invention significantly increases the content of key flavor substances such as ethyl acetate and ethyl butyrate in the sauerkraut of the present invention through the synergistic effect of Lactobacillus plantarum and Kexacerbium tumefaciens.

[0028] It is understood that the present invention effectively degrades nitrite through the fusion of Weissella bacteria in the secondary staged fermentation, which significantly reduces the nitrite content of the final product; at the same time, the compound bacterial agent formed by the combination of multiple strains can significantly inhibit the growth of putrefactive microorganisms, thereby significantly reducing harmful components such as histamine and tyramine in the product.

[0029] It is understandable that this invention significantly shortens the overall fermentation cycle by employing a fermentation process that combines secondary fermentation with multi-stage precise temperature control.

[0030] In this invention, the preferred method of salting is to evenly sprinkle 2%-3% of the total mass of the mustard greens with sea salt on the surface of the mustard greens and then rub them by hand.

[0031] Specifically, sprinkle 2%-3% of the total weight of sea salt evenly on the surface of the mustard greens, and then rub them by hand until the leaves are slightly translucent and juice seeps out. Each leaf should be rubbed for no less than 30 seconds.

[0032] Understandably, a 2%-3% salt concentration can effectively inhibit the early proliferation of putrefactive bacteria such as Escherichia coli and Pseudomonas aeruginosa through initial osmotic pressure, preventing the raw materials from spoiling due to microbial contamination before fermentation. It also avoids inhibiting the proliferation of beneficial bacteria such as Leuconostoc mesenteroides and Lactobacillus plantarum during the subsequent primary fermentation due to excessively high salt concentration, thus creating a safe initial environment conducive to the target microbial community. Simultaneously, at least 30 seconds of hand-rubbing ensures that the sea salt is evenly distributed on the surface of the mustard greens and between the leaves, preventing uneven fermentation caused by localized salt concentration imbalances. Furthermore, it can moderately break down the mustard greens. The cell wall structure of spoiled mustard greens causes nutrients such as sugars and amino acids to seep out with the juice, providing sufficient carbon and nitrogen sources for the subsequent metabolic activities of beneficial bacteria, accelerating the initiation of primary fermentation and the acid production process. When the leaves reach a "slightly translucent" state, it indicates that salt has initially penetrated into the plant tissue, which helps the subsequently injected cooled boiled water to quickly blend with the salt and seeping juice, precisely forming the 6%-8% salt concentration environment required for primary fermentation. This further ensures the controllability of the fermentation process and the batch stability of the product, laying a good foundation for improving the flavor and quality in the subsequent secondary fermentation.

[0033] In this invention, the segmented fermentation specifically involves: fermenting at a constant temperature of 15°C for 3 days, then adjusting the temperature to 10°C and continuing to ferment at a constant temperature for 10 days.

[0034] Specifically, the first stage of fermentation involves placing the fermenter in a temperature-controlled room and setting the temperature to 15°C for 3 days to promote the rapid reproduction of Leuconostoc mesenteroides (the pH value is monitored daily during this stage to ensure that the pH drops from the initial 6.5-7.0 to 5.0-5.5); the second stage of fermentation involves adjusting the temperature of the temperature-controlled room to 10°C after 3 days and continuing to ferment at a constant temperature for 10 days, at which point Lactobacillus plantarum becomes the dominant bacterial group and is responsible for efficient acid production.

[0035] Understandably, the first stage, with a constant temperature fermentation at 15℃ for 3 days, precisely matches the optimal growth temperature of Leuconostoc mesenteroides, promoting its rapid reproduction and establishing a dominant microbial community. This effectively inhibits the growth of putrefactive bacteria that may proliferate in the early stages of fermentation. Simultaneously, daily monitoring ensures that the system pH decreases from the initial 6.5-7.0 to 5.0-5.5, creating a stable acidic environment for subsequent fermentation and preventing harmful substances from being produced by the metabolism of other microorganisms. In the second stage, the temperature is lowered to 10℃ for 10 days of constant temperature fermentation. This temperature inhibits the excessive reproduction of Leuconostoc mesenteroides and activates Lactobacillus plantarum to become the dominant fermentation microorganism, efficiently and continuously producing acid. This ensures that the total acid content remains stable at the 0.8%-1.0% required for primary fermentation. Furthermore, the low temperature environment slows down the metabolic rate of non-target microorganisms, reducing the formation of harmful byproducts such as nitrite. Overall, through two-stage temperature control, the orderly succession of the microbial community and the precise guidance of metabolic pathways and acid production efficiency during fermentation were achieved. This not only avoids the problems of microbial metabolic imbalance, excessive acidity, or loss of flavor that may be caused by single constant temperature fermentation, but also lays a stable raw material foundation for flavor enhancement and safety control in subsequent secondary fermentation. In this invention, the desalination process is preferably carried out by soaking the sauerkraut that has undergone primary fermentation in running water until the salt concentration drops to 4%-5%.

[0036] Specifically, the sauerkraut after one fermentation is placed in a flowing water tank to remove salt, and the salt concentration is checked every 2 hours until it drops to 4%-5%. Understandably, this desalination process precisely matches the needs of secondary fermentation. The high salt concentration of 6%-8% used in the primary fermentation is mainly used to inhibit unwanted microorganisms and ensure fermentation safety. Secondary fermentation relies on the synergistic metabolism of *Lactobacillus plantarum*, *Westernella* fusion strain, and *Kexacerbium*. Regulating the salt concentration to 4%-5% avoids excessive salt damaging the cell membrane structure of these functional microorganisms and reducing enzyme activity, ensuring their efficiency in acid production and flavor synthesis. It also maintains adequate preservative properties, preventing the proliferation of unwanted microorganisms during secondary fermentation due to excessive desalination. Furthermore, desalination can optimize the product's taste and flavor. Sauerkraut with a high salt content after primary fermentation would require more salt for secondary fermentation. Excessive salting can lead to an overly salty final product, masking the subsequent ester fruit aromas and fermented alcohol aromas. Desalting to 4%-5% effectively balances the saltiness, allowing the complex flavors produced by secondary fermentation to be fully released, while avoiding the negative impact on consumer acceptance due to excessive saltiness. Furthermore, precise salt control is achieved by monitoring salt concentration every 2 hours. This effectively avoids problems such as inhibited microbial metabolism and an overly salty taste in the finished product caused by insufficient desalination (salt concentration above 5%), as well as the risk of soft, mushy, and easily spoiled sauerkraut caused by excessive desalination (salt concentration below 4%). This ensures that the saltiness of each batch of sauerkraut is consistent, providing a foundation for the standardization and stabilization of subsequent secondary fermentation, thereby guaranteeing the uniformity of the final product quality.

[0037] In this invention, the preferred cutting method is to take out the soaked sauerkraut, place it in a filter screen to drain for 30 minutes, and then cut it into sauerkraut segments with a length of 3-4 cm.

[0038] Specifically, after desalting, the pickled cabbage is taken out and placed in a strainer to drain for 30 minutes, so that its moisture content is controlled at 75%-78%. Then, it is diced into 3-4cm long pieces using a dicing machine.

[0039] Understandably, the 30-minute draining process allows for precise control of the sauerkraut's moisture content. This prevents excessive moisture from diluting the concentration of the compound microbial agent during secondary fermentation and affecting the metabolic efficiency of the microbial community, while also preventing excessive moisture from causing the sauerkraut to become dry, hindering microbial adhesion and nutrient release. This creates a stable moisture environment for secondary fermentation. A moisture content of 75%-78% is well-suited to the sauerkraut's fibrous structure, ensuring its firmness while providing appropriate material hardness for subsequent cutting, reducing debris and improving material utilization. The 3-4cm segment length design combines practicality and palatability. Compared to whole leaves or large pieces of sauerkraut, segments have a larger specific surface area, which is beneficial for activation. The compound microbial agent and flavor stabilizer adhere more evenly to the surface of the sauerkraut and penetrate into its interior, ensuring that the metabolism of microorganisms and the synthesis of flavor substances during the secondary fermentation process are carried out synchronously and fully in all parts of the material, avoiding flavor differences caused by incomplete local fermentation. On the other hand, the 3-4cm length conforms to consumers' daily eating habits, and can be used directly for cooking or ready-to-eat without additional processing, improving the convenience of product use. In addition, the regular fragment shape facilitates subsequent pasteurization and vacuum packaging processes. During sterilization, it can ensure that the material is heated evenly, avoiding sterilization dead spots caused by the size of the material. During packaging, it can make the material inside the bag fill neatly, reducing packaging loss and the risk of breakage during transportation, further ensuring the appearance integrity and quality stability of the product.

[0040] In this invention, the preferred mass ratio of Lactobacillus plantarum, Weissella fusionis, and Kexaro in the compound microbial agent is 68:12:12.

[0041] Specifically, Lactobacillus plantarum, Fusion Weissella, and Kexai yeast were weighed out in a mass ratio of 68:12:12 and mixed evenly to prepare a compound bacterial agent. It is understandable that this invention selects *Lactobacillus plantarum*, *Weissella fusionis*, and *Kexazola* to form a compound microbial agent, designed in a mass ratio of 68:12:12. This is based on the precise adaptation of the complementary functions of the three strains and the needs of secondary fermentation. *Lactobacillus plantarum*, as the core functional bacterium in the sauerkraut fermentation system, is mainly responsible for efficiently metabolizing and producing lactic acid. This not only rapidly reduces the pH value of the fermentation system and inhibits the growth of miscellaneous bacteria to ensure fermentation safety, but also provides a stable acidic environment for the subsequent synthesis of flavor substances, which is key to maintaining fermentation stability and basic sourness. *Weissella fusionis* has unique nitrite reductase activity, which can directionally degrade nitrite residues from primary fermentation, while also secreting a small amount of organic acid to assist in flavor regulation, solving the safety hazard of excessive nitrite in traditional sauerkraut. *Kexazola* can metabolize and produce alcohols such as phenylethanol, which can further react with the organic acids produced by *Lactobacillus plantarum* to generate key flavor substances such as ethyl acetate and ethyl butyrate, giving sauerkraut a layered aroma of alcohol and fruit, making up for the single flavor deficiency of traditional fermentation. The 68:12:12 ratio design ensures the dominant role of *Lactobacillus plantarum*, guaranteeing sufficient acid production to maintain the stability of the fermentation system and preventing contamination by other microorganisms due to insufficient acid production. The 12% proportion of *Westernella* allows it to efficiently degrade nitrite to below safe levels without competing with *Lactobacillus plantarum* for carbon and nitrogen sources. Simultaneously, the 12% proportion of *Kexacerbium* balances its alcohol production efficiency and reproduction rate, generating sufficient alcohols to promote ester synthesis while avoiding excessive nutrient consumption or off-flavors due to excessive yeast reproduction. These three elements work synergistically to achieve complementary functions of "acid production and bacterial control, nitrite reduction, and flavor enhancement," precisely matching the core needs of secondary fermentation to improve flavor and ensure safety, avoiding low fermentation efficiency and unstable quality issues caused by a single strain or an imbalanced ratio.

[0042] In this invention, the activation treatment is preferably carried out by mixing the compound microbial agent with 1.5%-4% of its mass of nutrient agent, adding water at 30°C, stirring evenly, and then activating at a constant temperature of 25°C for 2 hours.

[0043] The nutrient solution is prepared by mixing soybean peptides and glucose in a mass ratio of 1:2; the mass ratio of the compound microbial agent to water at 30°C is 1:10.

[0044] Specifically, the compound microbial agent is mixed with 1.5%-4% of the total mass of nutrient (soybean peptide: glucose = 1:2), and sterile water at 30℃ is added (microbial agent: water = 1:10). After stirring evenly, it is placed in a 25℃ constant temperature incubator for 2 hours to activate, and stirred once every 30 minutes during the period.

[0045] Understandably, the nutrient solution uses a 1:2 ratio of soybean peptides (small molecule nitrogen source) to glucose (fast-acting carbon source). This quickly provides the nitrogen and carbon nutrients needed for the metabolism of *Lactobacillus plantarum*, *Westernella fusionis*, and *Kexacerbium*, meeting the dual nutritional needs of lactic acid bacteria for acid production and yeast for alcohol production. It also avoids metabolic imbalances caused by relying on a single carbon or nitrogen source. Furthermore, the 1.5%-4% addition level precisely matches the nutrient concentration required for activation, preventing over-nourishment leading to the proliferation of unwanted microorganisms or insufficient nutrient supply resulting in low activation efficiency. The 30℃ sterile water not only simulates the mild environment of natural microbial growth, avoiding low temperatures inhibiting microbial recovery or high temperatures causing inactivation, but also eliminates external contamination through its sterile properties. The 1:10 microbial-to-water ratio ensures thorough dispersion of the nutrient solution, guaranteeing that each microbial cell comes into contact with nutrients and water, preventing uneven activation due to microbial aggregation. The 25℃ constant temperature environment precisely matches the optimal activation temperature for the three strains (especially beneficial for the preferential activation of Kexaro yeast, laying the groundwork for subsequent synergistic metabolism with lactic acid bacteria). The 2-hour activation time allows the strains to smoothly transition from the dormant period to the logarithmic growth phase, avoiding both insufficient activity due to too short an activation time and excessive consumption of nutrients leading to a decline phase due to too long an activation time. In addition, stirring every 30 minutes breaks up local nutrient concentration differences and dissolved oxygen gradients, ensuring that the strains are evenly exposed to nutrients and a small amount of oxygen (meeting the yeast's initial activation needs for trace oxygen), avoiding differences in activity due to local hypoxia or nutrient depletion. Ultimately, the activated strains achieve the required number of viable cells and a uniform concentration, enabling rapid and uniform colonization after inoculation into sauerkraut. This provides a core guarantee for the efficient synthesis of esters, the targeted degradation of nitrite, and the stable and controllable fermentation process during the secondary fermentation stage.

[0046] In this invention, the mass ratio of the cut sauerkraut, the activated compound microbial agent, and the flavor stabilizer is 100:1:1.

[0047] In this invention, the secondary staged fermentation specifically involves: fermenting at a constant temperature of 25°C for 2 days, and then adjusting the temperature to 18°C ​​and continuing fermentation for 5-7 days.

[0048] The flavor stabilizer is prepared by mixing calcium chloride and sodium citrate in a 1:1 mass ratio.

[0049] Specifically, the chopped sauerkraut, activated compound microbial agent and flavor stabilizer are put into a double-layer sealed fermentation tank (inner vacuum layer and outer water seal layer) in a mass ratio of 100:1:1. After stirring evenly, the tank is evacuated to a vacuum degree of ≤-0.09MPa and the tank opening is sealed. First stage temperature control: Set the fermenter temperature to 25℃ and maintain it for 2 days to promote yeast metabolism and alcohol production (monitor the pressure inside the tank daily during this stage to ensure it remains stable between -0.08 and -0.09 MPa). Second stage of temperature control: After 2 days, adjust the temperature to 18℃ and continue constant temperature fermentation for 5-7 days to accelerate the synthesis of esters.

[0050] Understandably, calcium chloride can form a stable gel structure by binding with pectin in the cell walls of sauerkraut, effectively maintaining the crisp texture of the sauerkraut during secondary fermentation and preventing the organic acids produced by microbial metabolism from damaging the cell structure and causing the tissue to soften. It also enhances the sauerkraut's resistance to breakage during subsequent pasteurization, cooling, and transportation, reducing material loss. Sodium citrate can precisely buffer the acidity changes caused by the metabolism of the compound microbial agents during secondary fermentation, stabilizing the system pH within the suitable range of 3.4-3.5. This provides a stable environment for the synergistic metabolism of *Lactobacillus plantarum* and *Kexacerbium spp.* (avoiding pH fluctuations). It inhibits bacterial activity and neutralizes some of the sharp sourness, making the finished sauerkraut taste milder, while highlighting the aroma of complex flavor substances such as esters and alcohols. The 1:1 compound ratio achieves functional complementarity between ensuring texture and controlling acidity. It avoids the astringent taste caused by using calcium chloride alone, and also prevents the soft texture and excessive neutralization of sourness caused by using sodium citrate alone. Thus, it synergistically optimizes the overall eating quality and processing stability of sauerkraut. Moreover, both components are food-grade compliant additives, which can reduce the use of other chemical preservatives, further improve product safety, and meet the needs of industrialized production with secondary fermentation.

[0051] Understandably, the technical approach of first maintaining a constant temperature of 25℃ for 2 days and stabilizing the pressure inside the tank at -0.08 to -0.09 MPa during the secondary fermentation, and then adjusting the temperature to 18℃ for another 5-7 days, precisely matches the metabolic characteristics of different bacterial groups in the compound microbial agent and ensures the stability of the fermentation environment, enabling the orderly advancement of the flavor synthesis pathway. 25℃ is the optimal metabolic temperature for *Kexacerbium tumefaciens* in the compound microbial agent. Maintaining a constant temperature at this stage efficiently promotes yeast proliferation and directed metabolism to produce alcohol precursors such as phenylethanol and isoamyl alcohol. Combined with a stable negative pressure environment of -0.08 to -0.09 MPa, this completely isolates oxygen, preventing yeast from consuming carbon sources through aerobic respiration and generating useless carbon dioxide, while also inhibiting the growth of aerobic bacteria (such as *Bacillus*), ensuring that yeast metabolism is always efficiently directed towards alcohol production. Lowering the temperature to 18℃ after 2 days aligns with the optimal temperature for *Lactobacillus plantarum* and *Weiss*. The lactic acid bacteria meet the requirements for acid production and esterification. At this temperature, the lactic acid bacteria can stably produce organic acids such as acetic acid and butyric acid, which undergo esterification reactions with the alcohols produced by the yeast in the early stage, rapidly synthesizing key flavor substances such as ethyl acetate (which imparts fruity aroma) and ethyl butyrate (which imparts alcoholic aroma). The 18°C ​​environment can avoid the decomposition of esters or the imbalance of bacterial metabolism caused by excessive temperature. At the same time, the continuous negative pressure environment can further reduce the loss of flavor substances due to volatilization. The entire segmented temperature control process achieves the orderly metabolic connection of "yeast alcohol production - lactic acid bacteria acid production and esterification" through the coordinated regulation of temperature gradient and negative pressure. This ensures the efficient synthesis and sufficient accumulation of alcohol precursors and flavor substances, ultimately making the flavor of sauerkraut richer. At the same time, it avoids the problems of insufficient flavor substance production or off-flavors caused by asynchronous bacterial metabolism under a single constant temperature condition, further improving the flavor stability and quality consistency of the product.

[0052] In this invention, the post-processing is preferably performed by sterilizing the sauerkraut after secondary fermentation at a constant temperature of 80-82°C for 10-12 minutes, cooling it to room temperature, packing it into a packaging bag, vacuuming it, and then heat-sealing it.

[0053] Specifically, the fermented sauerkraut is placed in a sterilization tank at a set temperature of 80-82℃ and sterilized at a constant temperature for 10-12 minutes, with continuous stirring to ensure even heating. The sauerkraut is then immediately transferred to a cooling tank and rapidly cooled to room temperature within 30 minutes using ice water below 10℃. Finally, the sauerkraut is quantitatively packaged into food-grade vacuum bags, vacuumed to a vacuum degree of ≤-0.095MPa, and then heat-sealed. The packaged product is then stored in a refrigerator at 0-4℃.

[0054] Understandably, the combination of a mild pasteurization temperature of 80-82℃ and a duration of 10-12 minutes effectively kills harmful microorganisms such as Listeria and E. coli that may remain after fermentation (sterilization rate ≥ 99%), while avoiding the damage to heat-sensitive flavor compounds such as ethyl acetate and phenylethanol in sauerkraut caused by high-temperature sterilization (such as 121℃ autoclaving) (flavor compound retention rate > 90%). This maximizes the preservation of the original flavor profile of the sauerkraut. Continuous stirring ensures even heating of all parts of the sauerkraut, preventing localized overheating or softening of the fibers due to sterilization blind spots. Rapid cooling to room temperature with ice water below 10℃ within 30 minutes after sterilization quickly terminates the sterilization process, preventing the continued volatilization of flavor compounds and softening of the sauerkraut due to residual heat. At the same time, the low temperature immediately inhibits the regrowth of residual microorganisms. The process maintains the crisp and tender texture and freshness of the sauerkraut. The subsequent high-vacuum packaging environment with a vacuum degree of ≤-0.095MPa completely isolates oxygen, preventing the oxidation and deterioration of unsaturated fatty acids, vitamins and other components in the sauerkraut, avoiding spoilage caused by the growth of aerobic molds, and reducing the loss of flavor substances due to contact with air. Combined with refrigeration at 0-4℃, it further inhibits the metabolic activity of some low-temperature tolerant bacteria such as lactic acid bacteria, preventing the product from becoming too acidic or deteriorating in flavor during the shelf life. It also ensures that the flavor level of the sauerkraut remains stable and the texture remains crisp and tender throughout the distribution process, taking into account both food safety and consumer experience.

[0055] Example 1 S1. Rinse the fresh mustard greens three times with running water to remove surface dirt and impurities, and place them in a cool, ventilated place to dry until there is no obvious moisture on the surface. S2. Weigh sea salt at a ratio of 2.5% of the total weight of the mustard greens, sprinkle it evenly on the surface of the mustard greens, and rub it by hand until the leaves are slightly translucent and the juice seeps out; S3. Layer the salted mustard greens into a 316L medical stainless steel fermentation tank, compacting each 5cm layer until there are no obvious gaps inside the tank. Pour cooled boiled water into the fermentation tank until the liquid level covers the surface of the mustard greens by 5cm, controlling the salt concentration to 6%. After sealing the tank, place it in a temperature-controlled room and set the temperature to 15℃. Ferment at this constant temperature for 3 days, monitoring the pH value daily to ensure it drops from the initial 6.5-7.0 to 5.0-5.5. After 3 days, adjust the temperature of the temperature-controlled room to 10℃ and continue constant temperature fermentation for 10 days to complete one fermentation cycle. S4. After the first fermentation, place the sauerkraut in a running water tank to soak and desalt it. Check the salt concentration every 2 hours. When it drops to 4-5%, take out the sauerkraut, place it in a filter and drain for 30 minutes. Then use a dicing machine to cut it into 3-4cm long pieces for later use. S5. Weigh out Lactobacillus plantarum, Fusion Weissella, and Kexai yeast respectively in a mass ratio of 68:12:12, mix them evenly to prepare a compound microbial agent, mix it with 3.5% of the nutrient (soybean peptide: glucose = 1:2) and add 10 times the mass of the compound microbial agent with sterile water, stir evenly and place it in a 25℃ constant temperature incubator for 2 hours, stirring once every 30 minutes during the period to obtain the activated compound microbial agent; S6. Add the pickled cabbage pieces, activated compound microbial agent, and flavor stabilizer (calcium chloride: sodium citrate = 1:1) to a double-layer sealed fermentation tank (inner vacuum layer, outer water seal layer) at a mass ratio of 100:1:1. Stir well and then evacuate to a vacuum degree ≤ -0.09MPa. Seal the tank opening and set the fermentation tank temperature to 25℃. Ferment at a constant temperature for 2 days. During this period, monitor the pressure inside the tank daily to ensure that it is stable between -0.08 and -0.09MPa. After 2 days, adjust the temperature to 18℃ and continue constant temperature fermentation for 6 days to end the second fermentation. S7. After secondary fermentation, the sauerkraut is sent to the sterilization tank, the temperature is set to 81℃, and sterilized at a constant temperature for 11 minutes. After the sterilization is completed, it is immediately transferred to the cooling tank and rapidly cooled to room temperature with ice water below 10℃. Then, the sauerkraut is quantitatively packed into food-grade vacuum packaging bags, vacuumed to a vacuum degree ≤-0.095MPa, and then heat-sealed. The packaged product is stored in a refrigerator at 0-4℃.

[0056] Example 2 S1. Rinse the fresh mustard greens three times with running water to remove surface dirt and impurities, and place them in a cool, ventilated place to dry until there is no obvious moisture on the surface. S2. Weigh out sea salt at a ratio of 2% of the total weight of the mustard greens, sprinkle it evenly on the surface of the mustard greens, and rub it by hand until the leaves are slightly translucent and the juice seeps out. S3. Layer the salted mustard greens into a 316L medical stainless steel fermentation tank, compacting each 5cm layer until there are no obvious gaps inside the tank. Pour cooled boiled water into the fermentation tank until the liquid level covers the surface of the mustard greens by 5cm, controlling the salt concentration to 6%. After sealing the tank, place it in a temperature-controlled room and set the temperature to 15℃. Ferment at this constant temperature for 3 days, monitoring the pH value daily to ensure it drops from the initial 6.5-7.0 to 5.0-5.5. After 3 days, adjust the temperature of the temperature-controlled room to 10℃ and continue constant temperature fermentation for 10 days to complete one fermentation cycle. S4. After the first fermentation, place the sauerkraut in a running water tank to soak and desalt it. Check the salt concentration every 2 hours. When it drops to 4-5%, take out the sauerkraut, place it in a filter and drain for 30 minutes. Then use a dicing machine to cut it into 3-4cm long pieces for later use. S5. Weigh out Lactobacillus plantarum, Weissella fusionis and Kexaros yeast respectively in a mass ratio of 68:12:12, mix them evenly to prepare a compound microbial agent, mix it with 1.5% of the nutrient (soybean peptide: glucose = 1:2) and add 10 times the mass of the compound microbial agent with sterile water, stir evenly and place it in a 25℃ constant temperature incubator for 2 hours, stirring once every 30 minutes during the period to obtain the activated compound microbial agent; S6. Add the pickled cabbage pieces, activated compound microbial agent, and flavor stabilizer (calcium chloride: sodium citrate = 1:1) to a double-layer sealed fermentation tank (inner vacuum layer, outer water seal layer) at a mass ratio of 100:1:1. Stir well and then evacuate to a vacuum degree ≤ -0.09MPa. Seal the tank opening and set the fermentation tank temperature to 25℃. Ferment at a constant temperature for 2 days. During this period, monitor the pressure inside the tank daily to ensure that it is stable between -0.08 and -0.09MPa. After 2 days, adjust the temperature to 18℃ and continue constant temperature fermentation for 5 days to end the second fermentation. S7. After secondary fermentation, the sauerkraut is sent to a sterilization tank, the temperature is set to 80℃, and sterilized at a constant temperature for 10 minutes. After sterilization, it is immediately transferred to a cooling tank and rapidly cooled to room temperature with ice water below 10℃. Then, the sauerkraut is quantitatively packed into food-grade vacuum packaging bags, vacuumed to a vacuum degree ≤-0.095MPa, and then heat-sealed. The packaged product is stored in a refrigerator at 0-4℃.

[0057] Example 3 S1. Rinse the fresh mustard greens three times with running water to remove surface dirt and impurities, and place them in a cool, ventilated place to dry until there is no obvious moisture on the surface. S2. Weigh out sea salt at a ratio of 3% of the total weight of the mustard greens, sprinkle it evenly on the surface of the mustard greens, and rub it by hand until the leaves are slightly translucent and the juice seeps out. S3. Layer the salted mustard greens into a 316L medical stainless steel fermentation tank, compacting each 5cm layer until there are no obvious gaps inside the tank. Pour cooled boiled water into the fermentation tank until the liquid level submerges the surface of the mustard greens by 8cm, controlling the salt concentration to 8%. After sealing the tank, place it in a temperature-controlled room and set the temperature to 15℃. Ferment at this constant temperature for 3 days, monitoring the pH value daily to ensure it drops from the initial 6.5-7.0 to 5.0-5.5. After 3 days, adjust the temperature of the temperature-controlled room to 10℃ and continue constant temperature fermentation for 10 days to complete one fermentation cycle. S4. After the first fermentation, place the sauerkraut in a running water tank to soak and desalinate. Check the salt concentration every 2 hours. When the salt concentration drops to 4-5%, take out the sauerkraut, place it in a filter and drain for 30 minutes. Then use a dicing machine to cut it into 3-4cm long pieces for later use. S5. Weigh out Lactobacillus plantarum, Weissella fusionis and Kexaros yeast respectively in a mass ratio of 68:12:12, mix them evenly to prepare a compound microbial agent, mix it with 4% of the nutrient agent (soybean peptide: glucose = 1:2) and add 10 times the mass of the compound microbial agent with sterile water, stir evenly and place it in a 25℃ constant temperature incubator for 2 hours, stirring once every 30 minutes during the period to obtain the activated compound microbial agent; S6. Add the pickled cabbage pieces, activated compound microbial agent, and flavor stabilizer (calcium chloride: sodium citrate = 1:1) to a double-layer sealed fermentation tank (inner vacuum layer, outer water seal layer) at a mass ratio of 100:1:1. Stir well, then evacuate to a vacuum degree ≤ -0.09MPa, seal the tank opening, set the fermentation tank temperature to 25℃, and ferment at a constant temperature for 2 days. During this period, monitor the pressure inside the tank daily to ensure that it is stable between -0.08 and -0.09MPa. After 2 days, adjust the temperature to 18℃ and continue constant temperature fermentation for 7 days before removing the fermentation tank. S7. After secondary fermentation, the sauerkraut is sent to the sterilization tank, the temperature is set to 82℃, and sterilized at a constant temperature for 12 minutes. After sterilization, it is immediately transferred to the cooling tank and rapidly cooled to room temperature with ice water below 10℃. Then, the sauerkraut is quantitatively packed into food-grade vacuum packaging bags, vacuumed to a vacuum degree ≤-0.095MPa, and then heat-sealed. The packaged product is stored in a refrigerator at 0-4℃.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for secondary fermentation to enhance the flavor of sauerkraut, characterized in that, Includes the following steps: Take mustard greens and pre-treat them; Sea salt is used to salt the mustard greens. Water is added to the salted mustard greens and the mixture is sealed for a staged fermentation. After the fermentation is completed, the resulting sauerkraut is desalted and cut for later use. A compound microbial agent was prepared by mixing Lactobacillus plantarum, Fusion Weissella, and Kexai yeast. The compound microbial agent was then mixed with nutrients and activated to obtain the activated compound microbial agent. The cut sauerkraut, activated compound microbial agent and flavor stabilizer are mixed and vacuum sealed, and then subjected to a second stage of fermentation. After fermentation, the sauerkraut product is obtained through post-processing.

2. The secondary fermentation method for enhancing the flavor of sauerkraut according to claim 1, characterized in that, The salting process involves evenly sprinkling 2%-3% of the total weight of the mustard greens with sea salt on the surface of the mustard greens and then rubbing them by hand.

3. The secondary fermentation method for enhancing the flavor of sauerkraut according to claim 2, characterized in that, The specific process of the first stage of fermentation is as follows: ferment at a constant temperature of 15℃ for 3 days, then adjust the temperature to 10℃ and continue fermenting at a constant temperature for 10 days.

4. The secondary fermentation method for enhancing the flavor of sauerkraut according to claim 3, characterized in that, The desalination process involves soaking the sauerkraut that has undergone primary fermentation in running water until the salt concentration drops to 4%-5%.

5. The secondary fermentation method for enhancing the flavor of sauerkraut according to claim 4, characterized in that, The cutting process involves taking the soaked sauerkraut out, placing it in a strainer to drain for 30 minutes, and then cutting it into sauerkraut segments 3-4 cm in length.

6. The secondary fermentation method for enhancing the flavor of sauerkraut according to claim 5, characterized in that, The mass ratio of Lactobacillus plantarum, Fusion Weissella, and Kexaro in the compound microbial agent is 68:12:

12.

7. The secondary fermentation method for enhancing the flavor of sauerkraut according to claim 6, characterized in that, The activation treatment is as follows: after mixing the compound bacterial agent with 1.5%-4% of the nutrient agent by mass, add water at 30°C, stir evenly, and then activate at a constant temperature of 25°C for 2 hours.

8. The secondary fermentation method for enhancing the flavor of sauerkraut according to claim 7, characterized in that, The mass ratio of the cut sauerkraut, the activated compound microbial agent, and the flavor stabilizer is 100:1:

1.

9. The secondary fermentation method for enhancing the flavor of sauerkraut according to claim 8, characterized in that, The secondary staged fermentation specifically involves: fermenting at a constant temperature of 25℃ for 2 days, then adjusting the temperature to 18℃ and continuing fermentation for 5-7 days.

10. The secondary fermentation method for enhancing the flavor of sauerkraut according to claim 9, characterized in that, The post-processing involves sterilizing the fermented sauerkraut at a constant temperature of 80-82℃ for 10-12 minutes, cooling it to room temperature, packing it into a bag, vacuuming it, and then heat-sealing it.

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