Preparation method of high-solubility prune powder suitable for external fruits and inferior fruits of prunes

Abstract

The invention relates to a preparation method of high-solubility prune powder suitable for external fruits and inferior fruits of prunes, and the preparation method comprises the following steps: S1, coating the surfaces of the prunes with a bacillus subtilis bacterial suspension, fermenting, and completing softening treatment to obtain softened prunes; the bacillus subtilis bacterial suspension is a bacterial suspension obtained by fermentation induction of an inducing substance; the inducing substance comprises prune peel; s2, pulping the softened prune to obtain prune pulp, then inoculating lactobacillus casei and lactobacillus plantarum, and performing primary fermentation to obtain primary prune fermentation liquor; s3, performing ultrasonic-assisted treatment on the primary prune fermentation liquor, and then performing secondary fermentation to obtain secondary prune fermentation liquor; and S4, performing freeze drying on the secondary prune fermentation liquor, and performing crushing so as to obtain the prune powder. The problems that in the prior art, stubborn peels and scar tissues of secondary fruits and outer fruits of prunes are difficult to effectively degrade, so that the solubility of the prune powder is poor, the taste is rough, and nutrients and flavor substances are lost can be solved.

Description

Technical Field

[0001] This invention relates to the field of fermentation technology, and in particular to a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums. Background Technology

[0002] Prunes, rich in anthocyanins, polyphenols, and flavonoids—natural antioxidants—and possessing a unique flavor, are recognized as a healthy fruit and are highly favored by consumers. However, during their growth, prunes are susceptible to pests, diseases, and windy, sandy climates. Their skin often develops an excessively thick waxy layer and numerous lignified scars, and the skin itself is prone to significant thickening, fibrosis, and lignification. Furthermore, mechanical impacts during the transportation process after harvesting exacerbate the fruit's appearance damage. In the fresh food market, there are high requirements for the appearance, size, and smoothness of the prunes' surface. Therefore, in the commercial grading process, a large number of plums that do not have fresh consumption value, such as those with thick, hard skin, scars, or substandard sizes, will be screened out. These are called "out-of-grade plums" (generally referring to plums with severe and small deformities, scars covering more than 10% of the whole fruit surface, uneven maturity, and relatively severe mechanical damage) and "second-grade plums" (generally referring to plums with scars covering less than 10% of the whole fruit surface area, less deformed, basically mature, and with less mechanical damage). If these are discarded directly, it will not only cause a huge waste of resources, but also restrict the improvement of the overall efficiency of the industry.

[0003] In conventional technologies, fermenting secondary and terrestrial fruits into fruit powder that is easy to store and transport and has higher added value is a promising high-value utilization path. However, for plums, since the plum peel is the richest part of the plum in nutrients, containing a large amount of anthocyanins, polyphenols, and other substances, and the plum peel is inconvenient to remove and difficult to discard, although secondary and terrestrial plums can also be fermented into powder, the unique physical and chemical properties of their peel often result in poor quality plum powder prepared by existing processes, seriously affecting its value transformation.

[0004] Specifically, because the secondary and outer fruits of plums are relatively thick and have locally lignified and fibrous skin and hard scar tissue, conventional crushing and grinding methods often cannot fully crush and destroy them in one go. Repeated processing is required, which is not only energy-intensive and inefficient, but may also exacerbate the loss of heat-sensitive nutrients due to frictional heat generation, affecting the quality of plum powder.

[0005] Meanwhile, the prune material obtained after conventional crushing often still contains a large number of hard particles and fiber bundles that have not been fully broken down. During the subsequent fermentation process, these structures often have a large amount of nutrients attached to them and are highly resistant to microbial enzymatic hydrolysis. They are difficult to be effectively degraded within the conventional fermentation time. Their particle size often overlaps with that of components such as fruit pulp fibers, making it difficult to remove them by methods such as sieving or fermentation filtration (filtration may lead to problems such as reduced product yield). This results in a contradiction between the existing technology and the preservation of the nutritional components of prune powder and the guarantee of the solubility and taste of prune powder. If conventional fermentation methods are used, the decomposition treatment intensity or time is insufficient, and these hard particles and fiber bundles cannot be eliminated. This will directly lead to poor solubility of the final prune powder and a large amount of insoluble matter with a rough texture after reconstitution. However, if the fermentation time is significantly extended, a large number of different types of bacteria are piled up for fermentation, or the intensity of related mechanical / chemical treatments is increased in order to pursue full decomposition, it will lead to excessive processing and loss of nutrients. This will cause the basic nutrients such as sugar in plums to be excessively consumed by microorganisms, and may produce excessive amounts of acid or other metabolic byproducts. Correspondingly, heat-sensitive functional components, such as anthocyanins, vitamins, and polyphenols, will also be severely degraded due to prolonged heat or biochemical environments, resulting in an unbalanced flavor in the final plum powder and a decline in both product yield and nutritional value.

[0006] In summary, there is an urgent need for a fermentation process that can effectively preserve the nutritional components of plums while improving the taste and solubility of plum powder, so as to achieve efficient and high-value utilization of low-value plum raw materials. Summary of the Invention

[0007] (a) Technical problems to be solved

[0008] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a method for preparing highly soluble plum powder applicable to the outer and secondary fruits of plums. It solves the problem in the existing plum fermentation technology that it is difficult to effectively degrade the stubborn peel and scar tissue of the secondary and outer fruits of plums, resulting in poor solubility of plum powder, rough taste, and loss of nutrients and flavor substances.

[0009] (II) Technical Solution

[0010] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0011] This invention provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums, comprising the following steps:

[0012] S1: Coat the surface of plums with a Bacillus subtilis suspension and ferment under the fermentation conditions of Bacillus subtilis to complete the softening treatment of the plum surface and obtain softened plums;

[0013] The Bacillus subtilis suspension is a Bacillus subtilis suspension obtained by fermentation induced by an inducing substance; the inducing substance is at least one of plum peel, pectin, cellulose, hemicellulose, lignin, and ferulic acid.

[0014] S2: Pulping softened prunes to obtain prune pulp; Inoculating the prune pulp with a mixed bacterial suspension containing Lactobacillus casei and Lactobacillus plantarum, and carrying out primary fermentation to obtain primary prune fermentation broth;

[0015] S3: The primary prune fermentation broth is subjected to ultrasonic-assisted treatment, and then secondary fermentation is carried out to obtain the secondary prune fermentation broth;

[0016] S4: Freeze-dry and pulverize the secondary plum fermentation liquid to obtain plum powder.

[0017] According to a preferred embodiment of the present invention, in S1, a Bacillus subtilis suspension is coated on the surface of the plums to be fermented by spraying or soaking; the fermentation conditions of Bacillus subtilis are: fermentation in an oxygen-rich environment or air environment at 30-40°C and humidity not less than 85% for 1.5-6 hours to complete the softening treatment.

[0018] According to a preferred embodiment of the present invention, in S1, the viable count of Bacillus subtilis in the Bacillus subtilis suspension is not less than 5 × 10⁻⁶. 7 CFU / mL; pH of Bacillus subtilis suspension was 6-7.5; volume-to-mass ratio of Bacillus subtilis suspension to prunes to be treated was 1:20-1:8; the inducing substance was at least one of the following: pericarp of prune secondary fruit, pericarp of prune tertiary fruit, cellulose, hemicellulose, lignin, and ferulic acid.

[0019] According to a preferred embodiment of the present invention, in S1, the Bacillus subtilis suspension is prepared by the following method:

[0020] S11: Activation and expansion: Inoculate Bacillus subtilis strain into liquid culture medium, shake and culture to obtain seed culture;

[0021] S12: Induction culture: The seed culture is transferred to an induction medium containing inducing substances and induced for a certain period of time to obtain the induced bacterial culture;

[0022] S13: Preparation of bacterial suspension: Centrifuge the induced bacterial solution, collect the bacterial cells and resuspend them to obtain Bacillus subtilis bacterial suspension;

[0023] The inducing substance is the carbon source in the induction medium, and its addition amount is 10wt%-60wt% of the total mass of the induction medium; the induction culture time is not less than 12 hours, and the OD of the bacterial solution after induction is... 600 The value was 0.8-1.2; the bacterial concentration of Bacillus subtilis in the suspension was 1×10⁻⁶.8 -5×10 9 CFU / mL.

[0024] According to a preferred embodiment of the present invention, in S1, before the softening treatment, the plums need to undergo a first low-temperature plasma treatment; during the first low-temperature plasma sterilization treatment, the discharge voltage is 60-120kV, the frequency is 10-15kHz, the treatment distance is 3-10cm, and the surface temperature of the plums does not exceed 60℃.

[0025] According to a preferred embodiment of the present invention, in S2, the softened plums are first pitted to obtain pitted plums; the pitted plums are pulped to obtain plum pulp, and then the plum pulp is subjected to a second low-temperature plasma treatment to obtain plum pulp without live bacteria; during the second low-temperature plasma sterilization treatment, the discharge voltage is 10-60kV, the frequency is 5-15MHz, the treatment distance is 2-5cm, and the surface temperature of the plum pulp does not exceed 60℃.

[0026] According to a preferred embodiment of the present invention, in S2, the ratio of *Lactobacillus casei* to *Lactobacillus plantarum* is 2:1-1.5, and the OD value of the mixed bacterial suspension is... 600 The concentration was 0.8 ± 0.02; the volume ratio of the mixed bacterial suspension to the prune pulp was 3-4:100; and the pH of the prune pulp was 5.5-6.5.

[0027] According to a preferred embodiment of the present invention, the method for preparing the mixed bacterial suspension is as follows:

[0028] Lactobacillus casei and Lactobacillus plantarum strains were activated separately. The activated mixed strains were inoculated into MRS broth medium and cultured at 37-38℃ for 24 hours. After culture, the bacterial cells were collected, resuspended in sterile physiological saline, and the OD of the bacterial suspension was adjusted. 600 The concentration was increased to 0.8 ± 0.02, resulting in a mixed bacterial suspension for fermentation.

[0029] Before inoculating the mixed bacterial suspension into plum pulp, the mixed bacterial suspension is first subjected to pulsed magnetic field pre-activation treatment with a magnetic field strength of 3-8 mT, a pulse frequency of 10-15 Hz, and a treatment time of 5-10 min.

[0030] According to a preferred embodiment of the present invention, both the primary fermentation in S2 and the secondary fermentation in S3 are isothermal anaerobic fermentations, with a fermentation temperature of 36-39°C; the primary fermentation time is 7-8 hours, and the secondary fermentation time is 15-19 hours. Both fermentations are carried out under shaking, with a total fermentation time of 22-26 hours.

[0031] According to a preferred embodiment of the present invention, in S3, ultrasonic-assisted treatment is performed under the same environmental conditions as primary fermentation; the ultrasonic-assisted treatment is intermittent ultrasonic-assisted treatment, with an ultrasonic frequency of 22-30kHz, a power of 80-100W, a treatment time of 5-15min, and an intermittent mode of working for 1-3 seconds and stopping for 2-6 seconds; after the ultrasonic treatment is completed, the plum pulp is restored to the fermentation conditions, and the oscillating fermentation continues until the total fermentation time is over.

[0032] (III) Beneficial Effects

[0033] The beneficial effects of this invention are as follows: This invention provides a method for preparing highly soluble plum powder applicable to the outer and secondary fruits of plums. By employing a softening treatment method involving coating the surface of the plums to be fermented with an induced Bacillus subtilis suspension, the method restricts and enriches the surface of the skin and scars of the secondary and outer fruits of plums through induced culture of Bacillus subtilis, which can decompose cellulose, lignin, waxes, and other substances in the plum peel. Through microbial activity and enzymatic hydrolysis, the hard scars and fibrous peel defects on the surface of the secondary and outer fruits of plums are rapidly softened. Compared with existing technologies, this invention, through softening treatment, decomposes and softens substances on the surface of the secondary and outer fruits of plums that may affect the taste in advance, avoiding the formation of a large amount of poorly soluble, rough-tasting insoluble matter during subsequent fermentation. On this basis, the softening treatment can make the originally hard tissues that are difficult to break into softer and looser, thereby significantly reducing the difficulty and energy consumption of subsequent mechanical pulping, making the pulping more thorough, forming a more homogeneous and delicate prune pulp, further improving the fermentation effect during subsequent fermentation, making the prune pulp ferment more thoroughly, improving the quality of the final prune powder, and avoiding the appearance of insoluble substances with a rough texture.

[0034] Meanwhile, the method of the present invention can effectively control the decomposition during softening to the surface of the plum fruit, avoid consuming the core flavor substances and nutrients such as organic acids and soluble sugars inside the plum, avoid using violent physical or chemical softening methods, and retain the anthocyanins and other nutrients of the plum secondary fruit, outer fruit skin and inside to the maximum extent, thus ensuring its nutritional value.

[0035] Furthermore, this invention specifically selects Lactobacillus casei and Lactobacillus plantarum as a compound fermentation agent for subsequent fermentation. The metabolic complementarity of these two lactic acid bacteria can efficiently complete the main lactic acid fermentation and acid production, while also synergistically and deeply decomposing the pretreated materials, synergistically regulating the flavor, taste and acidity of the final product, and converting more macromolecular substances into soluble components, further improving the solubility of the plum powder obtained by this invention.

[0036] Finally, this invention also utilizes the physical force generated by the ultrasonic cavitation effect to break down and disperse any fibrous aggregates that may exist in the fermentation system and enhance the mass transfer effect by carrying out primary and secondary fermentation and ultrasonic-assisted treatment in between. This stimulates the metabolism of Lactobacillus casei and Lactobacillus plantarum and the transformation of related substances, thereby improving the fermentation effect of this invention without significantly prolonging the fermentation cycle, and further improving the solubility and flavor of the plum powder of this invention. Attached Figure Description

[0037] Figure 1 This is a secondary plum fermentation broth prepared according to Example 1 of a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums according to the present invention;

[0038] Figure 2 The secondary plum fermentation broth prepared in Comparative Example 1 is a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums according to the present invention.

[0039] Figure 3 This is a plum powder block prepared according to Example 1 of a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums according to the present invention. Detailed Implementation

[0040] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0041] First, it should be noted that the "solubility" in this invention can be understood as reconstitution property. It does not refer to the chemical property of whether a substance can be dispersed in a solvent in the form of molecules or ions. Rather, it refers to the property of whether the fruit powder can be quickly wetted and dispersed after being mixed with water to form a uniform, stable, smooth-tasting suspension or colloid without visible rough particles or precipitates.

[0042] This invention provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums, comprising the following steps:

[0043] S1: Softening treatment: A Bacillus subtilis suspension is coated on the surface of the plum, and the surface of the plum is softened under the fermentation conditions of Bacillus subtilis to obtain softened plum.

[0044] The Bacillus subtilis suspension is a bacterial suspension of Bacillus subtilis obtained by fermentation induced by an inducing substance. The inducing substance is at least one of plum peel, pectin, cellulose, hemicellulose, lignin, and ferulic acid.

[0045] S2: Primary fermentation: The softened plums are pulped and sterilized to inactivate the Bacillus subtilis, resulting in plum pulp. Then, a mixed bacterial suspension containing Lactobacillus casei and Lactobacillus plantarum is inoculated into the plum pulp for primary fermentation to obtain primary plum fermentation broth.

[0046] S3: Secondary fermentation: The primary prune fermentation broth is subjected to ultrasonic-assisted treatment, and then secondary fermentation is carried out to obtain secondary prune fermentation broth.

[0047] S4: Powdering: Freeze-dry and pulverize the secondary plum fermentation liquid to obtain plum powder.

[0048] In S1, Bacillus subtilis naturally possesses the inherent potential to efficiently secrete various hydrolases and oxidoreductases, especially the ability to secrete enzyme systems targeting the main components of plant cell walls (pectin, cellulose, hemicellulose), lignin, and their related linkages, and exhibits good adaptability. This invention uses prune peel, cellulose, lignin, and ferulic acid for induction culture, allowing Bacillus subtilis to adapt to the prune peel environment in advance and enhancing its ability to produce targeted enzyme systems such as cellulase, hemicellulase, and ferulic acid esterase. When Bacillus subtilis is coated on the surface of prune fruit, without other nutrients, the induced Bacillus subtilis can quickly adapt to the prune peel environment, continuously secreting a complex enzyme system that efficiently decomposes substances such as thick, hard prune peel and scars, effectively degrading hard substances such as scars and lignin on the prune surface, softening the prune peel, and reducing the possibility of insoluble coarse particles in the final prune fruit powder product.

[0049] Meanwhile, in S1, because Bacillus subtilis is an aerobic or facultative anaerobic bacterium, this growth characteristic allows it to rapidly reproduce and metabolize in the oxygen-rich environment on the fruit surface, quickly establishing a dominant population and initiating the softening process, resulting in rapid and efficient softening. During its vigorous growth, reproduction, and metabolic activities, Bacillus subtilis not only continuously provides fresh, highly active extracellular enzymes but also alters the pH of the local microenvironment through the organic acids it produces. This helps to disrupt the cell wall structure and exert a certain biophysical loosening effect on the epidermal tissue, further enhancing the softening effect.

[0050] Furthermore, Bacillus subtilis exhibits excellent biocompatibility, producing no fungal toxins or other harmful metabolites, and can be effectively and thoroughly eliminated in subsequent sterilization steps, ensuring the safety and purity of the process of this invention. In addition, since this invention only coats the surface of plums with Bacillus subtilis, its activity is strictly confined to the plum skin, especially the surface layer, during the shorter fermentation time. This minimizes the premature consumption of core flavor components such as organic acids, anthocyanins, and polyphenols, as well as soluble sugars and other nutrients within the pulp, providing a higher-quality substrate for subsequent lactic acid fermentation and preventing a decline in the nutritional value of the plum powder.

[0051] In S2, during the primary fermentation, after inoculating with Lactobacillus casei and Lactobacillus plantarum, these two bacteria can quickly start up in the prune pulp and enter a vigorous growth and reproduction period. The number of bacteria increases logarithmically, achieving effective expansion of the strain. The acids and enzymes produced by their active metabolism can also perform preliminary degradation of the prune pulp, forming the basic flavor.

[0052] At the end of primary fermentation, the *Lactobacillus casei* and *Lactobacillus plantarum* cells (hereinafter referred to as cells) in the primary prune fermentation broth are in the logarithmic growth phase, exhibiting high cell activity and environmental responsiveness. At this stage, ultrasonic-assisted treatment (S3) is performed. Appropriate ultrasonic cavitation energy can improve the permeability of the cell membranes of logarithmic-phase lactic acid bacteria, promoting intracellular and extracellular substance exchange and stimulating metabolic activity, resulting in more vigorous metabolism. Simultaneously, ultrasound can break up any small bacterial flocs that may form in the primary prune fermentation broth, improving the homogeneity of the fermentation substrate and cells, eliminating mass transfer interfaces, increasing the contact area between cells and substrate, and enhancing mass transfer efficiency. Furthermore, ultrasonic-assisted treatment can further break down and disperse plant cell fragments, fiber / lignocellulose aggregates, or large molecular flocs that have not been fully decomposed after primary fermentation. This directly refines the particles, exposing bound phenols, flavonoids, and other functional components encapsulated in these substances, as well as new enzyme action sites, improving the decomposition and transformation of these substances during secondary fermentation. This process avoids the formation of insoluble, coarse particles while further enhancing the flavor and nutritional value of the prune powder.

[0053] In S3, based on the high uniformity, high mass transfer efficiency, and high cell activity brought about by ultrasound assistance, secondary fermentation can further reduce the soluble sugar content in the system through continuous and efficient lactic acid fermentation, converting it into metabolites such as organic acids. This reduces the proportion of small-molecule soluble sugars in the prune powder and increases the organic acid content. Simultaneously, the enzyme systems secreted by these active cells (such as glycosidases and esterases) synergistically work with the acidic environment generated by fermentation to more effectively hydrolyze bound polyphenols, flavonoids, and other large-molecule nutrients, converting them into free, more bioavailable small-molecule active substances. This further decomposes any potentially stubborn components, ultimately resulting in a significant refinement of the particle size of the solids in the secondary prune fermentation broth, a reduction in the degree of macromolecular polymerization, and an increase in hydrophilic groups. This particle refinement and improved hydrophilicity effectively enhance the wettability, solubility (reconstituteability), and flowability of the prune powder of this invention. In addition, the reduction of soluble sugar content and the increase of the proportion of small molecule soluble components, such as organic acids and free phenols, can also affect the drying effect in the subsequent freeze-drying process, regulate the micro-crystallization behavior of the dried powder, reduce the proportion of highly hygroscopic amorphous sugars (or small molecule soluble sugars), and further improve the solubility, anti-caking properties and storage stability of plum powder.

[0054] In step S4, the fermentation broth is freeze-dried and pulverized to avoid the loss of heat-sensitive nutrients, such as anthocyanins, polyphenols, vitamins, and flavor compounds, which may be caused by heat drying. Since the preceding steps S1 to S3 have sufficiently broken down the large molecules and insoluble fiber components in the prunes into small molecules and soluble substances, and decomposed most of the hard particles that could lead to a rough texture, the prune powder of this invention itself has good solubility. After this freeze-drying and pulverization, a prune powder with fluffy, fine, uniform, and free-flowing particles can be obtained. When it comes into contact with water, the water can quickly wet the particles and rehydrate rapidly to form a uniform, stable, and smooth suspension.

[0055] Furthermore, it should be noted that both *Lactobacillus casei* and *Lactobacillus plantarum* of the present invention are lactic acid bacteria that can be used for probiotic fermentation. The present invention does not limit the specific species of *Bacillus subtilis*. However, it should be clearly stated that the selected *Bacillus subtilis* strain should be non-toxic, non-pathogenic, and free of transferable drug-resistant plasmids, or in other words, a generally recognized safe strain. Commonly used *Bacillus subtilis* model strain 168, *Bacillus subtilis* ATCC6633, and *Bacillus subtilis* DE111, etc., can all be used.

[0056] Bacillus subtilis is generally believed to possess the ability to decompose cellulose, hemicellulose, pectin, and ferulic acid esters (through the secretion of hydrolytic enzymes such as cellulase, hemicellulase, pectinase, and ferulic acid esterase). These enzymes work synergistically to effectively decompose the cellulose skeleton, hemicellulose network, and pectin layer in plum epidermis and scar tissue, severing the ferulic acid ester bonds between lignin and polysaccharides and disintegrating the complex framework structure formed by lignin and other structures. This breaks down stubborn lignin-containing tissues into loose, isolated particles or fragments. Through this multi-target enzymatic hydrolysis, even without directly degrading lignin macromolecules, it can thoroughly disintegrate the complex structures formed by lignin and other substances, causing these originally dense tissues to dissociate into loose particles or fragments, thus achieving a significant softening effect. Some Bacillus subtilis strains can also secrete oxidoreductases such as lignin peroxidase, manganese peroxidase, or laccase, possessing the ability to directly decompose lignin and achieving an even more thorough softening effect.

[0057] Regardless of whether the strain has the ability to directly degrade lignin, it can achieve the basic effect of softening the outer skin and facilitating subsequent processing. The lignin fragments and other substances generated during the softening process will be further refined or transformed in the subsequent pulping and fermentation processes, and will not affect the product quality and safety of the plum powder.

[0058] Finally, it is important to note that when using Bacillus subtilis, which is not approved for use as a probiotic, complete sterilization is required in S2 to ensure the inactivation of Bacillus subtilis and its spores. When using Bacillus subtilis that can be used as a probiotic, and assuming no contamination from other microorganisms, sterilization in S2 may be omitted or only simple sterilization may be necessary. However, appropriate fermentation conditions must be adjusted to ensure that subsequent lactic acid bacteria can be properly introduced, multiply, and ferment. Considering the practical process difficulty, contamination from other microorganisms, and competition in subsequent fermentation, sterilization in S2 is preferable to inactivate Bacillus subtilis, its spores, and other microorganisms, preventing them from entering subsequent fermentation and the product.

[0059] Preferably, in S1, the surface of the plums to be fermented is coated with a Bacillus subtilis suspension by spraying or soaking to ensure that the plum surface, especially scars and dents, is in effective contact with the bacterial solution.

[0060] Preferably, in S1, the fermentation conditions for Bacillus subtilis are: fermentation in an oxygen-rich environment or air environment at 30-40℃ and humidity not lower than 85% for 1-12 hours, preferably 1.5-6 hours, and more preferably 4-6 hours, to complete the softening treatment. The specific fermentation time can be adjusted according to the actual condition of the plums to avoid excessive fermentation time leading to excessive decomposition of the plum surface and consumption of nutrients, or insufficient fermentation time leading to incomplete softening.

[0061] Preferably, in S1, the number of viable Bacillus subtilis in the Bacillus subtilis suspension is not less than 5 × 10⁻⁶. 7 More preferably, the concentration of Bacillus subtilis cells in the Bacillus subtilis suspension is 1×10⁻⁶ CFU / mL. 8 -5×10 9 CFU / mL. The pH of the Bacillus subtilis suspension is 6-7.5. The high concentration and suitable pH of the Bacillus subtilis suspension allow the large number of viable bacteria to quickly adapt to and directly and rapidly decompose the substances on the plum skin after coating, thus softening the plum and ensuring a fast softening rate. Simultaneously, the absence of other nutrient sources in the suspension forces the Bacillus subtilis to decompose the plum skin, further ensuring the softening effect.

[0062] The volume-to-mass ratio of Bacillus subtilis suspension to the plums to be treated is 1:20-1:8. Ensure sufficient suspension for coating; avoid uneven coverage and poor softening due to an insufficient ratio, and avoid uneconomical use due to an excessively high ratio.

[0063] Preferably, in S1, the Bacillus subtilis suspension is prepared by the following method:

[0064] S11: Activation and Propagation: Inoculate food-grade, or rather, generally recognized as safe, Bacillus subtilis strain into liquid culture medium and culture with shaking to obtain seed culture. Shaking culture refers to conventional machine shaking, and the culture time is determined according to actual needs.

[0065] S12: Induction Culture: The seed culture is transferred to an induction medium containing inducing substances and induced for a certain period of time to obtain the induced bacterial culture. The induction culture time should be no less than 12 hours to ensure that the bacteria can grow sufficiently and complete enzyme synthesis and accumulation. The OD value of the induced bacterial culture... 600 The value is 0.8-1.2, which means that the bacteria are in the vigorous growth phase from the late logarithmic phase to the early stationary phase. At this time, the secondary metabolism of the bacteria (such as the production of certain enzymes) is often more active. After centrifugation and resuspension to remove metabolic byproducts from the culture medium, stable functional bacteria with a specific metabolic state can be obtained.

[0066] S13: Preparation of bacterial suspension: Centrifuge the induced bacterial solution, collect the bacterial cells and resuspend them to obtain Bacillus subtilis bacterial suspension.

[0067] The inducing substance is the carbon source in the induction medium, specifically at least one selected from the pericarp of the second and third fruits of the plum, cellulose, hemicellulose, lignin, and ferulic acid. The amount of the inducing substance added is 10wt%-60wt%, more preferably 30wt%-50wt%, of the total mass of the induction medium. Using the corresponding target degradants as a carbon source, *Bacillus subtilis* is induced to grow. When the strain grows in a medium containing these target degradants, its metabolic pathways for synthesizing related degradative enzyme systems, such as cellulase, hemicellulase, lignin peroxidase, and ferulic acid esterase, are specifically activated and enhanced. The resulting bacterial suspension carries highly targeted enzymes. When it comes into contact with the similarly composed plum pericarp, it can rapidly initiate and efficiently degrade the target substance. Compared to using uninduced ordinary strains or directly adding multiple commercial enzymes, it not only acts faster, is more targeted, and is lower in cost, but also further accelerates the softening rate through microbial activity. Other materials in the induction medium can be selected from conventional materials for cultivation.

[0068] Preferably, in step S1, before the softening treatment, the plums need to undergo a first low-temperature plasma treatment. This low-temperature plasma treatment can etch away the wax and other coatings on the plum surface while simultaneously sterilizing it, allowing the subsequently coated Bacillus subtilis suspension to adhere more evenly and firmly to the peel. It also enables the secreted enzymes and other substances to more effectively contact and act on the corresponding substrate, thus improving the softening efficiency.

[0069] Preferably, in S1, during the first low-temperature plasma treatment, the discharge voltage is 60-120kV, the treatment distance is 3-10cm, and the frequency is 10-15kHz to ensure the etching effect. During the first low-temperature plasma treatment, the surface temperature of the plum does not exceed 60℃ to avoid damage to nutrients.

[0070] Of course not. In actual operation, methods such as soaking in chemical agents can also be used for sterilization, such as soaking in sodium hypochlorite solution.

[0071] Preferably, in step S2, the softened plums are first pitted to obtain pitted plums. After softening, conventional pulping methods can be used to fully crush and homogenize the plums. Specific pulping process parameters, such as blade speed and time, need to be confirmed based on actual equipment conditions. The goal is to obtain plum pulp with thoroughly broken cells and a concentrated and uniform particle size distribution. This invention will not elaborate further.

[0072] Preferably, in step S2, after obtaining the pitted prunes, the pitted prunes are pulped to obtain prune pulp. Then, the prune pulp undergoes a second low-temperature plasma treatment to obtain prune pulp free of live bacteria. This second low-temperature plasma treatment further alters the permeability of the prune cells in the prune pulp, making it easier for intracellular contents (such as flavor compounds, pigments, and nutrient molecules) that were not fully released during pulping to diffuse out during subsequent fermentation and dissolution, thus improving the nutrient dissolution rate and bioavailability. Simultaneously, the etching effect of plasma further increases the total surface area and surface activity of the solid particles in the prune pulp, providing more and easier attachment sites for subsequent inoculation with *Lactobacillus casei* and *Lactobacillus plantarum*, which is beneficial for bacterial colonization and further enhances the biocatalytic efficiency of subsequent fermentation. Furthermore, the low-temperature plasma treatment enhances the hydrophilicity of the modified particle surface, further improving the wettability, dispersibility, and solubility of the obtained prune powder. Of course, the second low-temperature plasma can also completely inactivate Bacillus subtilis and its spores, as well as a very small amount of environmental bacteria that may be introduced during the pulping process, providing a basis for the rapid reproduction of the two subsequent lactobacilli and ensuring the safety of subsequent lactic acid bacteria fermentation and plum powder.

[0073] Preferably, during the second low-temperature plasma sterilization treatment, the discharge voltage is 10-60kV, the frequency is 5-15MHz, and the treatment distance is 2-5cm to ensure effective alteration of plum cell permeability and sterilization effect. The surface temperature of the plum pulp also does not exceed 60℃ to avoid nutrient loss.

[0074] Preferably, in S2, the ratio of Lactobacillus casei to Lactobacillus plantarum is 2:1-1.5. This ratio helps to form a population structure dominated by Lactobacillus casei, which produces acid quickly and is highly acid-resistant, in the early stage of fermentation, quickly establish an acidic environment, and start fermentation. At the same time, it ensures that Lactobacillus plantarum, which has broad-spectrum substrate utilization and aroma-producing characteristics, accounts for a considerable proportion, together constructing a rich flavor and ensuring the further decomposition of stubborn tissues.

[0075] Preferably, in S2, the OD in the mixed bacterial suspension 600 The pH of the mixed bacterial suspension and prune pulp was 0.8 ± 0.02, with a volume ratio of 3-4:100 to ensure the activity and inoculation quantity of both types of lactic acid bacteria, and to ensure that the inoculated cells could achieve rapid start-up and shorten the fermentation lag period. The pH of the prune pulp was 5.5-6.5 to ensure rapid fermentation start-up.

[0076] Preferably, the method for preparing the mixed bacterial suspension is as follows: Lactobacillus casei and Lactobacillus plantarum strains are activated separately. The activated mixed strains are inoculated into a culture medium (such as conventional MRS broth medium) and cultured at 37-38℃ for 24 hours. After culture, the bacterial cells are collected, resuspended in sterile physiological saline, and the OD of the bacterial suspension is adjusted. 600 The concentration was increased to 0.8 ± 0.02, resulting in a mixed bacterial suspension for fermentation.

[0077] Preferably, before inoculating the mixed bacterial suspension into the prune pulp, the mixed bacterial suspension is pre-activated by a pulsed magnetic field with a magnetic field strength of 3-8 mT, a pulse frequency of 10-15 Hz, and a treatment time of 5-10 min. This pulsed magnetic field pre-activation treatment significantly activates the activity of lactic acid bacteria, promotes the activity of intracellular metabolic enzymes, and allows the lactic acid bacteria to attach to and colonize the surface of the prune pulp particles modified by low-temperature plasma. This enables the two lactobacillus bacterial groups to initiate growth and acid production metabolism more quickly after inoculation, achieving a higher substrate adhesion rate, thereby significantly shortening the fermentation lag phase and improving the initial efficiency of primary fermentation.

[0078] Preferably, both the primary fermentation in S2 and the secondary fermentation in S3 are isothermal anaerobic fermentations, with a fermentation temperature of 36-39℃. This ensures the continuity and efficiency of the fermentation process, avoiding fluctuations in fermentation rate or changes in the metabolic pathways of lactic acid bacteria caused by temperature and oxygen fluctuations. The primary fermentation time is 7-8 hours, sufficient for the inoculated lactic acid bacteria to complete logarithmic growth. The secondary fermentation time is 15-19 hours, allowing for deep transformation and flavor accumulation. This ensures that the components in the prunes are fully degraded and soluble sugars are effectively converted into functional components such as organic acids, while avoiding problems such as excessive consumption of nutrients, flavor imbalance, or low production efficiency caused by excessively long fermentation cycles. Both fermentations are carried out under conventional shaking culture mode, with a total fermentation time of 22-26 hours.

[0079] Preferably, in step S3, ultrasonic-assisted treatment is performed under the same environmental conditions as the primary fermentation. The ultrasonic-assisted treatment is intermittent, with an ultrasonic frequency of 22-30 kHz, a power of 80-100 W, and a treatment time of 5-15 minutes. The intermittent mode involves a 1-3 second working cycle followed by a 2-6 second pause. This intermittent ultrasonic-assisted treatment allows for the use of ultrasound to break down / disperse materials, enhance mass transfer, and stimulate the growth and metabolism of lactic acid bacteria. Furthermore, when the ultrasound stops, it ensures uniform heat distribution within the primary prune fermentation broth, preventing cumulative damage to heat-sensitive nutrients and lactic acid bacteria activity caused by localized overheating from continuous ultrasound.

[0080] Preferably, in S3, after the ultrasonic-assisted treatment is completed, the plum pulp is restored to the fermentation conditions and the fermentation continues with shaking until the total fermentation time is over.

[0081] Preferably, the freeze-drying conditions in S4 are as follows: the fermented secondary plum broth is transferred to an ultra-low temperature freezer and pre-frozen at -75-85°C for 24 hours, then processed in a vacuum freeze dryer at -40-50°C to obtain plum powder blocks; after the plum powder blocks are dried to a moisture content of less than 6%, they are removed and pulverized to obtain the finished plum powder product. The freeze-drying method completely avoids the problems of degradation of heat-sensitive substances caused by the presence of liquid water and high temperatures in traditional heat drying, thus preserving the integrity of heat-sensitive nutrients such as anthocyanins, polyphenols, and vitamins, as well as the rich volatile flavor compounds produced by lactic acid bacteria fermentation.

[0082] Preferably, in step S4, after pulverization, large particles resulting from incomplete pulverization are removed by sieving or other methods to obtain plum powder with a particle size not exceeding a certain value. This further ensures the uniformity of the particle size in the obtained plum powder, guaranteeing the consistency of the fruit powder product. The particle size of the removed particles needs to be determined based on the specific pulverization process or grading standards. Furthermore, these removed large particles can be added to other batches of the pulverization process or directly subjected to secondary pulverization / grinding to ensure their size meets the standards.

[0083] In S4, it should be noted that this invention fundamentally alters the composition and structure of prune powder through the synergistic treatments in S1-S3, including bio-softening of stubborn tissues (such as peel and scars), deep fermentation degradation, and a second low-temperature plasma physical modification. These treatments significantly increase the content of soluble organic acids, free phenols, and other small molecules in the prune powder, while greatly reducing and softening the size and quantity of insoluble fiber aggregates and hard particles. The particle size control in the pulverizing process in S4 is based on optimization for conventional food powder processing requirements, rather than being a necessary prerequisite for achieving the "high solubility" of this invention. This invention does not impose special restrictions on the particle size of the pulverized prune powder; those skilled in the art can make conventional adjustments based on the final product's reconstituted texture, flowability, and other requirements.

[0084] However, to ensure the prune powder maintains consistently high solubility, the particle size is preferably controlled to pass through an 80-200 mesh standard sieve, or in other words, the particle size is 75-180 micrometers. More preferably, its particle size distribution D90 does not exceed 150 micrometers. Within this particle size range, the powder exhibits both good flowability and rapid wetting and settling speed. Combined with the inherent characteristics of the prune powder of this invention—strong hydrophilicity of the particle surface, high proportion of soluble components, and minimal insoluble residue—it can be rapidly rehydrated at this conventional grinding fineness, forming a uniform, stable, and smooth suspension, thereby comprehensively achieving excellent solubility, flavor, and storage stability.

[0085] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present invention can be understood more clearly and thoroughly, and that the scope of the present invention can be fully conveyed to those skilled in the art.

[0086] Example 1

[0087] This embodiment provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums, including the following steps:

[0088] Raw material preparation: Pick plums (President plums) that are 80% ripe from a plum plantation in Yining, Xinjiang. Take the outer and second-grade plums with scars and thickened and hardened skin, wash them and set them aside.

[0089] S1: Softening Treatment: The plum fruit underwent a first low-temperature plasma treatment with the following parameters: discharge voltage 100kV, frequency 12kHz, treatment distance 5cm, and treatment time 3 minutes. During treatment, the fruit surface temperature did not exceed 50℃. After activation and expansion, Bacillus subtilis was transferred to an induction medium containing plum pericarp powder and ferulic acid in a 7:3 mass ratio, accounting for 25% of the carbon source weight. After 24 hours of induction culture, the induced bacterial suspension was obtained. The bacterial cells were collected by centrifugation and resuspended in sterile physiological saline. The pH of the bacterial suspension was adjusted to 6.5, and the viable Bacillus subtilis concentration was adjusted to 1×10⁻⁶. 9 CFU / mL was used to obtain a Bacillus subtilis suspension. Prunes were immersed in the suspension for 5 seconds and then placed in a constant temperature and humidity chamber at 35°C and 90% for 3.5 hours to obtain softened prunes.

[0090] S2: Primary Fermentation: Softened prunes are pitted to obtain pitted prunes. The pitted prunes are mixed with pure water at a mass ratio of 2:1 and pulped at 1500 rpm for 2 minutes to obtain prune pulp. This prune pulp is placed in a dielectric barrier discharge low-temperature plasma device and treated for 3 minutes at a discharge voltage of 50kV, a frequency of 10MHz, and a treatment distance of 3cm to obtain prune pulp without viable bacteria. *Lactobacillus casei* and *Lactobacillus plantarum* are mixed at a quantity ratio of 2:1, cultured in MRS broth medium, and then resuspended in sterile physiological saline to prepare a mixed bacterial suspension. This suspension is then pre-activated with a pulsed magnetic field at a magnetic field strength of 5mT, a pulse frequency of 10Hz, and a treatment time of 10 minutes. This mixed bacterial suspension is inoculated into the prune pulp at an inoculum rate of 3% (v / v) and subjected to isothermal anaerobic fermentation at 37℃ under shaking conditions for 7 hours to obtain primary prune fermentation broth.

[0091] S3: Secondary Fermentation: The primary prune fermentation broth was placed in an ultrasonic instrument and ultrasonically treated for 5 minutes at a frequency of 25kHz, a power of 90W, and a water temperature of 37℃, using an intermittent mode of 2 seconds on and 4 seconds off. After ultrasonication, the material was returned to a constant-temperature anaerobic fermentation environment at 37℃ with vibration for another 15 hours, bringing the total fermentation time to 24 hours, to obtain the secondary prune fermentation broth. Figure 1 As shown.

[0092] S4: Powdering: After pre-freezing the secondary plum fermentation broth at -80℃ for 24 hours, transfer it to a vacuum freeze dryer and dry it under conditions of cold trap temperature -55℃ and vacuum degree below 10Pa until the moisture content is ≤6%. Remove the dried product, as shown below. Figure 3 As shown, the prune powder is crushed and passed through an 80-mesh standard sieve. The material passing through the sieve is collected to obtain the prune powder product of this embodiment.

[0093] Example 2

[0094] This embodiment provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums. The difference from Embodiment 1 is that in S2, the inoculation ratio of the mixed bacterial suspension of Lactobacillus casei and Lactobacillus plantarum is 2:1.2.

[0095] Example 3

[0096] This embodiment provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums. The difference from Embodiment 1 is that, in S1, the parameters for the first low-temperature plasma treatment are adjusted to: discharge voltage 80kV, frequency 15kHz, and treatment distance 8cm. In S2, the parameters for the second low-temperature plasma treatment of the plum pulp are adjusted to: discharge voltage 40kV, frequency 15MHz, and treatment distance 5cm.

[0097] Example 4

[0098] This embodiment provides a method for preparing highly soluble plum powder suitable for plum outer fruit and secondary fruit. The difference from Embodiment 1 is that the inducing substance is a mixture of plum outer fruit powder, microcrystalline cellulose, hemicellulose and ferulic acid in a mass ratio of 4:2:2:2, accounting for 55% of the carbon source weight, and the softening treatment time is 5.5 hours.

[0099] Example 5

[0100] This embodiment provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums. The difference from Embodiment 1 is that the first and second low-temperature plasma treatments are not performed, and the plum pulp is sterilized in S2 by treating it at 95°C for 3 minutes.

[0101] Example 6

[0102] This embodiment provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums. The difference from Embodiment 1 is that, in S2, no pulsed magnetic field pretreatment is performed.

[0103] Comparative Example 1

[0104] This comparative example provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums. The difference from Example 1 is that in S1, the *Bacillus subtilis* suspension is not coated. The secondary plum fermentation broth obtained in S3 is as follows... Figure 2 As shown.

[0105] Comparative Example 2

[0106] This comparative example provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums. The difference from Example 1 is that in S2, only Lactobacillus plantarum is used, and in S3, the secondary fermentation time is extended to 24 hours.

[0107] Comparative Example 3

[0108] This comparative example provides a method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums. The difference from Example 1 is that ultrasonic-assisted treatment is not performed in S3.

[0109] Prune powder was prepared using the methods of Examples 1-3 and Comparative Examples 1-3, respectively. The finished products were tested for particle size (using a laser particle size analyzer, unit: micrometer), wetting time (unit: second), solubility (using centrifugation method, unit: %), total phenols (using Folin-Ciocalteu method, unit: mgGAE / g), total flavonoids (using colorimetric method, unit: mgRE / g), and angle of repose (unit: degree). Each group was repeated 3 times, and the mean ± standard deviation was taken. The results are shown in Table 1.

[0110] Table 1: Parameters of the prune powder in each embodiment and comparative example

[0111]

[0112] Based on the above test results, it can be found that Example 1, due to the use of induced Bacillus subtilis for targeted bio-softening of the plum skin, combined with two different low-temperature plasma treatments, pulsed magnetic field pre-activation of lactic acid bacteria, and ultrasonic-assisted fermentation, can effectively reduce the possibility of rough, insoluble substances appearing in the plum powder. Figure 1 As shown, no obvious insoluble particles were observed on the bottle wall. Without the addition of dispersants, stabilizers, or other additives, the final plum powder product exhibited excellent overall performance in terms of particle size, wetting and dissolution rate, complete dissolution, retention of functional components (total phenols and total flavonoids), and powder flowability (angle of repose).

[0113] Compared to Example 1, Example 2 changed the inoculation ratio of Lactobacillus casei and Lactobacillus plantarum, which affected the metabolic balance of the microbial community in the fermentation system. The broad-spectrum decomposition ability was insufficient, the synergistic degradation efficiency of lactic acid bacteria decreased, and the metabolite spectrum was reduced. The solubility, total phenol and total flavonoid content of the product were slightly reduced, while the particle size was slightly increased and the angle of repose was slightly increased.

[0114] Compared to Example 1, Example 3 adjusted the parameters (voltage, distance, and frequency) of the two low-temperature plasma treatments. By further optimizing the plasma treatment effect at a lower voltage and a higher discharge frequency, the etching and cell permeability alteration effects were improved and made gentler. This better balanced the activation and sterilization of the material surface with the permeability modification of the slurry, thus creating more favorable conditions for subsequent fermentation. The resulting product had finer particle size, higher solubility, slightly increased total phenol and total flavonoid content, and a smaller angle of repose.

[0115] Compared to Example 1, Example 4 used a higher proportion and more complex inducer to induce Bacillus subtilis and extended the softening time. Through stronger induction and a longer action time, Bacillus subtilis produced a more efficient and targeted enzyme system, achieving more thorough surface softening and degradation. The various indicators of its fruit powder product were further comprehensively improved compared to Example 1, especially in solubility, total phenolic content, and total flavonoid content.

[0116] Compared to Example 1, Example 5 omits the two low-temperature plasma treatments and replaces them with traditional high-temperature sterilization. This results in the loss of the non-thermal modification effect of plasma, leading to a decrease in softening effect. Not only does it fail to improve the permeability and hydrophilicity of the plum material, but the high temperature also damages heat-sensitive components and alters the properties of the slurry, severely impairing subsequent fermentation efficiency and final product quality. Consequently, the particle size of the product increases significantly, while the solubility, total phenol and total flavonoid content decrease drastically, and the angle of repose increases.

[0117] Compared to Example 1, Example 6 omits the pre-activation treatment with a pulsed magnetic field. Without pre-activation, the fermentation start-up rate and metabolic activity of the two lactic acid bacteria are insufficient, resulting in inadequate substrate conversion within the same timeframe. This affects the product's fineness, soluble component content, and powder flowability, leading to a significant decrease in particle size, solubility, total phenol and total flavonoid content, and an increased angle of repose.

[0118] Compared to Example 1, Comparative Example 1 omits the Bacillus subtilis coating softening treatment, lacking a specific biological softening step targeting the thick, hard skin and scars of the plum fruit. Subsequent processes cannot effectively treat these stubborn impurities, such as... Figure 2As shown, the secondary prune fermentation broth contained a large amount of insoluble coarse matter that could not be effectively degraded. Ultimately, under the same freeze-drying and pulverizing process, the quality of the prune powder product was severely degraded, with the largest particle size, the worst solubility, the lowest total phenol and total flavonoid content, and the largest angle of repose.

[0119] Compared to Example 1, Comparative Example 2 used only Lactobacillus plantarum for single-strain fermentation, lacking the synergistic effect of Lactobacillus casei. The fermentation capacity of the single strain was limited, and even with extended fermentation time, the advantages of the compound strain in acid production, degradation and transformation could not be realized, resulting in insufficient fermentation depth. The particle size, solubility, total phenol and total flavonoid content of its product were significantly lower than those of Example 1.

[0120] Compared to Example 1, Comparative Example 3, due to the elimination of ultrasonic-assisted treatment in S3, lacked the crushing, homogenizing, and enhanced mass transfer effects of ultrasonic cavitation, resulting in limited efficiency and depth of secondary fermentation. This affected the final degradation degree of the plums and the full release of related functional components, leading to a significant decrease in the solubility, total phenol and total flavonoid content, and increased particle size of the product.

[0121] 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 them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing highly soluble plum powder suitable for the outer and secondary fruits of plums, characterized in that, Includes the following steps: S1: Coat the surface of plums with a Bacillus subtilis suspension and ferment under the fermentation conditions of Bacillus subtilis to complete the softening treatment of the plum surface and obtain softened plums; The Bacillus subtilis suspension is a Bacillus subtilis suspension obtained by fermentation induced by an inducing substance; the inducing substance is at least one of plum peel, pectin, cellulose, hemicellulose, lignin, and ferulic acid. S2: The softened prunes are pulped to obtain prune pulp; a mixed bacterial suspension containing Lactobacillus casei and Lactobacillus plantarum is inoculated into the prune pulp for primary fermentation to obtain primary prune fermentation broth; S3: The primary prune fermentation broth is subjected to ultrasonic-assisted treatment, and then secondary fermentation is carried out to obtain secondary prune fermentation broth; S4: Freeze-dry and pulverize the secondary plum fermentation liquid to obtain plum powder.

2. The method for preparing highly soluble prune powder as described in claim 1, characterized in that, In S1, the Bacillus subtilis suspension is coated onto the surface of the plums to be fermented by spraying or soaking. The fermentation conditions for Bacillus subtilis are: fermentation in an oxygen-rich environment or air environment at 30-40℃ and humidity not less than 85% for 1.5-6 hours to complete the softening treatment.

3. The method for preparing highly soluble prune powder as described in claim 1, characterized in that, In S1, the number of viable Bacillus subtilis bacteria in the Bacillus subtilis suspension is not less than 5 × 10⁻⁶. 7 CFU / mL; the pH of the Bacillus subtilis suspension is 6-7.5; the volume-to-mass ratio of the Bacillus subtilis suspension to the prunes to be treated is 1:20-1:8; the inducing substance is at least one of the following: the pericarp of the prune secondary fruit, the pericarp of the prune tertiary fruit, cellulose, hemicellulose, lignin, and ferulic acid.

4. The method for preparing highly soluble prune powder as described in claim 1, characterized in that, In S1, the Bacillus subtilis suspension is prepared by the following method: S11: Activation and expansion: Inoculate Bacillus subtilis strain into liquid culture medium, shake and culture to obtain seed culture; S12: Induction culture: The seed culture is transferred to an induction medium containing an inducing substance and induced for a certain period of time to obtain the induced bacterial culture; S13: Preparation of bacterial suspension: Centrifuge the induced bacterial solution, collect the bacterial cells and resuspend them to obtain the Bacillus subtilis bacterial suspension; The inducing substance is a carbon source in the induction medium, and its addition amount is 10wt%-60wt% of the total mass of the induction medium; the induction culture time is not less than 12 hours, and the OD of the bacterial solution after induction is... 600 The value is 0.8-1.2; the concentration of Bacillus subtilis cells in the Bacillus subtilis suspension is 1×10⁻⁶. 8 -5×10 9 CFU / mL.

5. The method for preparing highly soluble prune powder as described in claim 1, characterized in that, In S1, before the softening treatment, the plums need to undergo a first low-temperature plasma treatment. During the first low-temperature plasma sterilization treatment, the discharge voltage is 60-120kV, the frequency is 10-15kHz, the treatment distance is 3-10cm, and the surface temperature of the plums does not exceed 60℃.

6. The method for preparing highly soluble prune powder as described in claim 1 or 5, characterized in that, In step S2, the softened plums are first pitted to obtain pitted plums; the pitted plums are then pulped to obtain plum pulp, and the plum pulp is subjected to a second low-temperature plasma treatment to obtain plum pulp without live bacteria; during the second low-temperature plasma sterilization treatment, the discharge voltage is 10-60kV, the frequency is 5-15MHz, the treatment distance is 2-5cm, and the surface temperature of the plum pulp does not exceed 60℃.

7. The method for preparing highly soluble prune powder as described in claim 1, characterized in that, In S2, the ratio of *Lactobacillus casei* to *Lactobacillus plantarum* is 2:1-1.5, and the OD value of the mixed bacterial suspension is... 600 The concentration is 0.8 ± 0.02; the volume ratio of the mixed bacterial suspension to the prune pulp is 3-4:100; the pH of the prune pulp is 5.5-6.

5.

8. The method for preparing highly soluble prune powder as described in claim 1 or 7, characterized in that, The method for preparing the mixed bacterial suspension is as follows: Lactobacillus casei and Lactobacillus plantarum strains were activated separately. The activated mixed strains were inoculated into MRS broth medium and cultured at 37-38℃ for 24 hours. After culture, the bacterial cells were collected, resuspended in sterile physiological saline, and the OD of the bacterial suspension was adjusted. 600 The concentration was increased to 0.8 ± 0.02, resulting in a mixed bacterial suspension for fermentation. Before inoculating the mixed bacterial suspension into plum pulp, the mixed bacterial suspension is first subjected to pulsed magnetic field pre-activation treatment with a magnetic field strength of 3-8 mT, a pulse frequency of 10-15 Hz, and a treatment time of 5-10 min.

9. The method for preparing highly soluble prune powder as described in claim 1, characterized in that, Both the primary fermentation in S2 and the secondary fermentation in S3 are constant-temperature anaerobic fermentations, with a fermentation temperature of 36-39℃. The primary fermentation time is 7-8 hours, and the secondary fermentation time is 15-19 hours. Both fermentations are carried out under shaking, with a total fermentation time of 22-26 hours.

10. The method for preparing highly soluble prune powder as described in claim 1 or 9, characterized in that, In S3, the ultrasonic-assisted treatment is carried out under the same environmental conditions as the primary fermentation. The ultrasonic-assisted treatment is an intermittent ultrasonic-assisted treatment with an ultrasonic frequency of 22-30kHz, a power of 80-100W, a treatment time of 5-15min, and an intermittent mode of working for 1-3 seconds and stopping for 2-6 seconds. After the ultrasonic treatment is completed, the plum pulp is restored to the fermentation conditions and the fermentation continues with shaking until the total fermentation time is over.

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