Prebiotic coating freeze-dried fruit and method of making same

Abstract

The present application relates to the technical field of functional food processing, and particularly relates to a prebiotic coating freeze-dried fruit and a preparation method thereof. The prebiotic coating freeze-dried fruit comprises a freeze-dried fruit base and a prebiotic coating layer coated on the surface of the freeze-dried fruit base, wherein the prebiotic coating layer comprises, by weight, 30-50 parts of fructooligosaccharide, 20-40 parts of galactooligosaccharide, 5-15 parts of resistant dextrin, 3-8 parts of modified chitosan agent and 1-5 parts of plant extract. The present application forms a uniform and stable prebiotic functional coating layer on the surface of the freeze-dried fruit by combining a vacuum freeze-drying technology with a multi-layer spraying coating process. The prebiotic content of the obtained product is greater than or equal to 15 g / 100 g, the water activity is less than or equal to 0.3, and the brittleness retention rate is greater than or equal to 95%. The product retains the original flavor and nutritional ingredients of the fruit, and at the same time, the product is endowed with good intestinal health regulation function.

Description

Technical Field

[0001] This invention relates to the field of freeze-dried fruit technology, specifically to a prebiotic-coated freeze-dried fruit and its preparation method. Background Technology

[0002] Freeze-dried fruits are popular with consumers because they retain the original color, flavor, nutrients, and crisp texture of the fruit. Vacuum freeze-drying technology, which sublimates water under low temperature and low pressure conditions, effectively avoids the degradation of heat-sensitive nutrients and has become one of the mainstream technologies for high-quality fruit processing.

[0003] Prebiotics, as a class of indigestible food components, can play a role in regulating gut health by selectively stimulating the growth and activity of beneficial bacteria in the gut. Common prebiotics include fructooligosaccharides, galactooligosaccharides, and inulin, which can be selectively fermented and utilized by intestinal bifidobacteria and lactic acid bacteria. However, most prebiotic products currently available are in powder, liquid, or capsule form, lacking convenient, snack-like consumption formats.

[0004] Chitosan is the only alkaline polysaccharide found in nature, possessing excellent film-forming properties, biocompatibility, and antibacterial activity. Ferulic acid is a widely distributed phenolic acid compound in plants, exhibiting antioxidant and anti-inflammatory biological activities. Studies have shown that chitosan-grafted ferulic acid copolymers possess synergistic antioxidant and antibacterial activities. β-Cyclodextrin has a hollow structure with an internal hydrophobic and external hydrophilic component, allowing it to form inclusion complexes with hydrophobic functional molecules, thus improving its water solubility and stability.

[0005] Currently, there are no systematic reports on coating prebiotics onto the surface of freeze-dried fruits to create composite products that combine snack-like textures with probiotic functions. Existing surface coating technologies for freeze-dried fruits mostly use traditional coating materials such as syrups and honey, which suffer from problems such as low prebiotic content, poor stability, and excessive sweetness. Therefore, developing a prebiotic-coated freeze-dried fruit with high prebiotic content, a crisp texture, and stable activity, and its preparation method, has broad market prospects and application value. Summary of the Invention

[0006] In view of the deficiencies of the prior art, the purpose of this invention is to provide a prebiotic-coated freeze-dried fruit and its preparation method, so as to solve the problems mentioned in the background art.

[0007] The present invention solves the technical problem by adopting the following technical solution: This invention provides a prebiotic-coated freeze-dried fruit, comprising a freeze-dried fruit base and a prebiotic coating covering its surface, wherein the prebiotic coating comprises the following components by weight: 30-50 parts of fructooligosaccharide, 20-40 parts of galactooligosaccharide, 5-15 parts of resistant dextrin, 3-8 parts of modified chitosan agent, 1-5 parts of plant extract, and 2-6 parts of food-grade binder.

[0008] Preferably, the freeze-dried fruit base is fruit slices or fruit granules that have undergone vacuum freeze-drying, selected from at least one of apples, strawberries, mangoes, bananas, peaches, pears, and blueberries, with a moisture content of ≤5% and a water activity of ≤0.25 after freeze-drying.

[0009] Preferably, the plant extract comprises, by weight, 2-5 parts of green tea extract, 1-3 parts of rosemary extract, and 1-2 parts of licorice extract; wherein the green tea extract contains ≥50% tea polyphenols, the rosemary extract contains ≥20% rosmarinic acid, and the licorice extract contains ≥30% glycyrrhizic acid.

[0010] Preferably, the modified chitosan agent is prepared by the following steps: S1: Dissolve chitosan in a 1-3% (w / w) acetic acid solution to prepare a 2-5% (w / w) chitosan solution. Filter to remove insoluble matter to obtain a chitosan acetic acid solution. S2: According to the molar ratio of chitosan to ferulic acid of (2-5):1, ferulic acid is dissolved in anhydrous ethanol, N,N'-dicyclohexylcarbodiimide and 4-dimethylaminopyridine are added, and the solution is activated at room temperature for 30-60 min to obtain an activated ester solution. S3: Slowly add the activated ester solution dropwise to the chitosan acetate solution, stir and react at 40-60℃ for 4-8 hours. After the reaction is complete, precipitate with acetone, filter, wash and dry to obtain ferulic acid grafted chitosan. S4: Dissolve ferulic acid-grafted chitosan in deionized water to prepare a solution with a mass fraction of 2-4%, add 5-10% β-cyclodextrin (by mass of chitosan), stir and encapsulate at 40-50℃ for 1-3 hours, and freeze-dry to obtain the modified chitosan agent.

[0011] Preferably, the degree of deacetylation of the chitosan in step S1 is 85-95%, and the viscosity-average molecular weight is 5 × 10⁻⁶. 4 -2×10 5 Da; In step S2, the molar ratio of ferulic acid to chitosan is 1:3, the molar ratio of N,N'-dicyclohexylcarbodiimide to ferulic acid is (1-1.5):1, and the molar ratio of 4-dimethylaminopyridine to ferulic acid is (0.3-0.6):1.

[0012] This invention also provides a method for preparing prebiotic-coated freeze-dried fruit, comprising the following steps: S100: Fruit pretreatment: After washing, peeling and pitting fresh fruit, cut it into slices with a thickness of 3-8mm or granules with a side length of 8-15mm, and prefreeze it at -18~-25℃ for 4-8 hours to obtain pre-frozen fruit; S200: Vacuum freeze-drying: Pre-frozen fruit is placed in a vacuum freeze dryer and processed in two stages, sublimation drying and desorption drying, under conditions of vacuum degree ≤20Pa and cold trap temperature ≤-45℃, to obtain freeze-dried fruit base material, with the moisture content after freeze-drying controlled to ≤5%; S300: Prebiotic Coating Solution Preparation: Add fructooligosaccharides, galactooligosaccharides, resistant dextrin, modified chitosan agent, and plant extracts to deionized water according to the specified ratio, stir and dissolve in a water bath at 50-70℃ to prepare a coating solution with a solid content of 15-30%, and then use it after homogenization and degassing treatment. S400: Coating and molding: The coating liquid is evenly applied to the surface of the freeze-dried fruit base material using a multi-layer spraying process. The amount of each spray is 3-8% of the weight of the freeze-dried fruit base material. A total of 2-4 layers are sprayed. After each layer is sprayed, it is dried in hot air at 40-60℃ for 5-15 minutes to allow the coating to adhere and cure. S500: Gradient drying: Place the coated freeze-dried fruit in a vacuum drying oven and dry it at 40-50℃ for 1-3 hours until the water activity is ≤0.3; S600: Nitrogen-filled packaging: After drying, freeze-dried fruits with prebiotic coating are cooled to room temperature in an environment with relative humidity ≤30%, and then packaged with nitrogen to obtain the finished product.

[0013] Preferably, in step S200, the sublimation drying stage conditions are: temperature -10~5℃, vacuum degree 10-20Pa, time 8-15h; the desorption drying stage conditions are: temperature 25-45℃, vacuum degree 5-10Pa, time 4-8h.

[0014] Preferably, in step S300, the homogenization conditions are a pressure of 20-40 MPa and a temperature of 50-60°C; the degassing is performed using vacuum degassing with a vacuum degree of -0.08 to -0.095 MPa and a time of 15-30 min.

[0015] Preferably, in step S400, the spraying method is high-pressure airless spraying or ultrasonic atomization spraying, the spraying pressure is 0.2-0.5MPa, and the coating liquid temperature is 45-55℃.

[0016] Preferably, in step S500, the heating rate of the gradient drying is 1-3℃ / h.

[0017] This invention also provides an application of prebiotic-coated freeze-dried fruit, which is used as a functional snack food or food ingredient to prepare food or health food with the effects of regulating intestinal flora, promoting the proliferation of probiotics, and improving constipation.

[0018] Compared with the prior art, the present invention has the following beneficial effects: Synergistic Effect of Modified Chitosan Grafting and Inclusion: This invention uses chitosan as a matrix and introduces phenolic hydroxyl and carboxyl groups through ferulic acid grafting modification, endowing chitosan with stronger antioxidant and antibacterial activities. Furthermore, through β-cyclodextrin inclusion, the hydrophobic cavities of cyclodextrin are used to load lipophilic functional factors, improving the water solubility of chitosan and the stability of the functional factors. The combination of ferulic acid-grafted chitosan and β-cyclodextrin inclusion complex significantly enhances the film-forming properties and antioxidant activity of the coating solution.

[0019] Synergistic Probiotic Effect of Prebiotic Complex Coating: This invention employs a compound of four prebiotics—fructooligosaccharides, galactooligosaccharides, inulin, and resistant dextrin—covering different degrees of polymerization (short-chain, medium-chain, and long-chain) to achieve selective proliferation of beneficial bacteria throughout the entire intestinal tract. Fructooligosaccharides and galactooligosaccharides can be rapidly utilized by Bifidobacteria, while resistant dextrin acts on the distal colon, creating a cascade fermentation effect that effectively enhances intestinal health regulation.

[0020] The antioxidant barrier of plant extracts: The tea polyphenols in green tea extract, the rosmarinic acid in rosemary extract, and the glycyrrhizic acid in licorice extract form a natural antioxidant network that can effectively inhibit Maillard reaction and oxidative deterioration of the prebiotic coating during storage, while giving the product natural anti-corrosion and preservation capabilities.

[0021] Multi-layer spraying and gradient drying process ensures quality: This invention uses a multi-layer spraying process to build a prebiotic coating layer by layer, combined with gradient drying technology, to avoid problems such as coating cracking and reduced fruit crispness, ensuring that the crispness retention rate of the finished product is ≥95%. Compared with the traditional syrup impregnation method, the coating of this invention is evenly distributed, the prebiotic loading can be precisely controlled, and the crisp structure of the freeze-dried fruit is not damaged.

[0022] Nutritional value and ease of consumption: The prebiotic-coated freeze-dried fruit obtained by this invention has a prebiotic content of ≥15g / 100g, which can be used as a source of daily dietary fiber. At the same time, it retains the natural flavor and nutrients of the fruit, and has both snack attributes and functionality, meeting the modern consumer's dual needs for "deliciousness + health". Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to specific examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1 A prebiotic-coated freeze-dried apple includes a freeze-dried apple slice base and a prebiotic coating on its surface. The prebiotic coating contains, by weight, 40 parts of fructooligosaccharide, 30 parts of galactooligosaccharide, 10 parts of resistant dextrin, 5 parts of modified chitosan agent, and 3 parts of plant extract.

[0025] Preparation of modified chitosan agent: S1: A product with a degree of deacetylation of 90% and a viscosity-average molecular weight of 1×10⁻⁶. 5 10g of chitosan from Da was dissolved in 500mL of a 2% (w / w) acetic acid solution. The insoluble matter was removed by filtration to obtain a chitosan acetic acid solution. S2: Take 2.5g of ferulic acid and dissolve it in 50mL of anhydrous ethanol according to the molar ratio of chitosan to ferulic acid 3:1. Add 2.8g of DCC and 0.5g of DMAP and activate at room temperature for 40min to obtain an activated ester solution. S3: The activated ester solution was slowly added dropwise to the chitosan acetate solution, and the mixture was stirred at 50°C for 6 hours. After the reaction was completed, the mixture was precipitated with acetone, filtered, washed, and dried to obtain ferulic acid-grafted chitosan. S4: Dissolve ferulic acid-grafted chitosan in deionized water to prepare a 3% (w / w) solution, add 8% (w / w) of β-cyclodextrin (w / w) of chitosan, stir and encapsulate at 45°C for 2 hours, and freeze-dry to obtain the modified chitosan agent.

[0026] The plant extracts, by weight, include: 3 parts green tea extract (tea polyphenol content ≥55%), 1.5 parts rosemary extract (rosmarinic acid content ≥22%), and 1 part licorice extract (glycyrrhizic acid content ≥32%).

[0027] Preparation method of prebiotic-coated freeze-dried apples: S100: Apple pretreatment: Wash, peel and core fresh Fuji apples, cut them into 5mm thick slices, and pre-freeze at -22℃ for 6 hours; S200: Vacuum freeze-drying: Pre-frozen apple slices are placed in a vacuum freeze dryer with a vacuum degree of 15 Pa, a cold trap temperature of -48℃, a sublimation drying stage temperature of -5~0℃ for 12 hours, a desorption drying stage temperature of 30-40℃ for 6 hours, and a water content of 4.2% and a water activity of 0.22 after freeze-drying. S300: Prebiotic coating solution preparation: Add fructooligosaccharides, galactooligosaccharides, resistant dextrin, modified chitosan agent, and plant extracts to deionized water, stir and dissolve in a 60℃ water bath to prepare a coating solution with a solid content of 22%, and homogenize at 30MPa and 55℃, and degas at a vacuum degree of -0.09MPa for 20min. S400: Coating and forming: Ultrasonic atomization spraying is used, the spraying pressure is 0.35MPa, the coating liquid temperature is 50℃, the amount of a single spray is 5% of the weight of the freeze-dried apple slices, a total of 3 layers are sprayed, and each layer is dried in hot air at 50℃ for 10 minutes after spraying. S500: Gradient drying: Place the coated freeze-dried apple slices in a vacuum drying oven and dry them at 45℃ for 2 hours with a heating rate of 2℃ / h until the water activity is 0.28. S600: Nitrogen-filled packaging: Cool to room temperature in an environment with relative humidity ≤30%, then fill with nitrogen to obtain the product.

[0028] Example 2 A prebiotic-coated freeze-dried strawberry includes a freeze-dried strawberry granule base and a prebiotic coating on its surface. The prebiotic coating contains, by weight, 30 parts of fructooligosaccharide, 20 parts of galactooligosaccharide, 5 parts of resistant dextrin, 3 parts of modified chitosan agent, and 1 part of plant extract.

[0029] In the preparation of the modified chitosan agent, the degree of deacetylation of chitosan was 85%, and the molar ratio of ferulic acid to chitosan was 2:1. The plant extracts were 2 parts green tea extract, 1 part rosemary extract, and 1 part licorice extract. The preparation method was the same as in Example 1, except that in S100, strawberries were cut into 8mm pieces and pre-frozen at -18℃ for 4 hours; in S200, they were sublimated and dried for 10 hours and then desorbed and dried for 4 hours; in S300, the solid content was 15%; and in S400, 3% was sprayed in a single coat and two coats were sprayed in total. The prebiotic content was 16.2g / 100g, the water activity was 0.26, and the crispness retention rate was 96.5%.

[0030] Example 3 A prebiotic-coated freeze-dried mango includes a freeze-dried mango slice base and a prebiotic coating on its surface. The prebiotic coating contains, by weight, 50 parts of fructooligosaccharides, 40 parts of galactooligosaccharides, 15 parts of resistant dextrin, 8 parts of modified chitosan agent, and 5 parts of plant extract.

[0031] In the preparation of the modified chitosan agent, the degree of deacetylation of chitosan was 95%, and the molar ratio of ferulic acid to chitosan was 5:1. The plant extracts were 5 parts green tea extract, 3 parts rosemary extract, and 2 parts licorice extract. The preparation method was the same as in Example 1, with adjustments made to the following steps: in S100, the mango was cut into 6mm slices and pre-frozen at -25℃ for 8 hours; in S200, it was sublimated and dried for 15 hours and then desorbed and dried for 8 hours; in S300, the solid content was 30%; and in S400, 8% was sprayed in a single coat, with a total of 4 layers. The prebiotic content was 18.5g / 100g, the water activity was 0.24, and the brittleness retention rate was 97.2%.

[0032] Scale settings To verify the inventiveness of the modified chitosan agent and prebiotic coating formulation of this invention, the following 6 comparative examples were set up, all referring to the formulation and preparation process of Example 1, with only key parameters changed: Comparative Example 1: No prebiotic coating added, otherwise the same as Example 1.

[0033] Comparative Example 2: No modified chitosan agent was added to the prebiotic coating; otherwise, it was the same as in Example 1.

[0034] Comparative Example 3: Unmodified chitosan powder was added to replace the modified chitosan agent, and the rest was the same as in Example 1.

[0035] Comparative Example 4: No plant extracts were added to the prebiotic coating; otherwise, it was the same as in Example 1.

[0036] Comparative Example 5: The β-cyclodextrin inclusion step in S4 was omitted in the preparation of the modified chitosan agent, and the rest was the same as in Example 1.

[0037] Comparative Example 6: The prebiotic coating contains only a single prebiotic, and the rest is the same as in Example 1.

[0038] Performance testing Performance tests were conducted on the prebiotic-coated freeze-dried fruits prepared in Examples 1-3 and Comparative Examples 1-6: 1. Prebiotic content and water activity test The total dietary fiber content (representing prebiotic content) was determined according to GB 5009.88-2014, and the water activity was determined according to GB5009.238-2016. The results are shown in Table 1 below: 2. Antioxidant performance test The antioxidant activity of the product was determined using the DPPH free radical scavenging method, and the results are shown in Table 2 below: 3. In vitro probiotic proliferation promotion effect test An in vitro simulated intestinal fermentation system was used. The product sample was co-cultured with human fecal suspension for 24 hours, and the changes in the number of Bifidobacteria and Lactobacilli were detected. The results are shown in Table 3 below (expressed as log CFU / mL): 4. Storage stability test The product was stored at 25℃ and 60% relative humidity for 90 days. The retention rate of prebiotic content and sensory score (out of 10) were measured. The results are shown in Table 4 below: Results analysis: Regarding prebiotic content and water activity: The prebiotic content in Examples 1-3 reached 16.2-18.5 g / 100 g, significantly higher than that in Comparative Example 1, indicating that the coating process effectively loaded the prebiotic components. The water activity in Comparative Examples 2-3 increased to 0.32-0.35, while the brittleness retention rate decreased to 89.6-91.2%, indicating that the film-forming and water-repellent properties of the modified chitosan agent are crucial for maintaining product quality. The prebiotic content in Comparative Example 6 was slightly lower, indicating that the synergistic loading efficiency of different prebiotics was higher.

[0039] Regarding antioxidant properties: The DPPH scavenging rates of Examples 1-3 reached 72.3-82.1%, significantly better than Comparative Example 4, indicating that the compounding of plant extracts significantly enhanced the antioxidant activity of the products. The antioxidant properties of Comparative Examples 2-3 were lower than those of Example 1, indicating that the ferulic acid grafted component in the modified chitosan agent also contributed to a certain degree of antioxidant activity. The antioxidant properties of Comparative Example 5 were lower than those of Example 1, indicating that β-cyclodextrin inclusion can protect the activity of ferulic acid and improve its stability.

[0040] Regarding the probiotic proliferation-promoting effect: After 24 hours of culture, the Bifidobacterium and Lactobacillus in Examples 1-3 reached 9.5-10.1 and 9.2-9.8 log CFU / mL, respectively, significantly higher than Comparative Example 1, indicating that the prebiotic coating effectively exerted its probiotic effect. The proliferation-promoting effect of Comparative Examples 2-3 was lower than that of Example 1, indicating that the modified chitosan agent not only has a film-forming protective effect but also synergistically promotes probiotic proliferation due to the prebiotic activity of chitosan itself. Comparative Example 6 showed the worst proliferation-promoting effect, indicating that the tiered fermentation effect of the four prebiotic combinations is superior to that of a single prebiotic, achieving probiotic promotion throughout the entire intestinal tract.

[0041] Regarding storage stability: The 90-day prebiotic content retention rates of Examples 1-3 were all higher than 91.8%, significantly better than Comparative Examples 2-3, indicating that the film-forming oxygen barrier effect of the modified chitosan agent and the antioxidant effect of the plant extracts jointly ensured the storage stability of the prebiotics. The retention rate of Comparative Example 4 was 86.3%, lower than that of Example 1, indicating that the antioxidant effect of the plant extracts helped inhibit oxidative degradation during storage. The retention rate of Comparative Example 5 was 85.2%, lower than that of Example 1, indicating that β-cyclodextrin inclusion can improve the stability of functional components and extend the product shelf life.

[0042] In summary, this invention, through the synergistic design of modified chitosan agents and prebiotic composite coatings, produces freeze-dried fruits with prebiotic coatings that exhibit excellent performance in terms of prebiotic content, water activity control, crispness retention, antioxidant activity, probiotic proliferation promotion, and storage stability. The comparative examples fully verify the necessity of the grafting modification and cyclodextrin inclusion steps in the modified chitosan agents, the synergistic effect of the plant extract compound, and the superiority of the prebiotic composite formulation, demonstrating outstanding inventiveness and practical value.

[0043] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0044] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A prebiotic-coated freeze-dried fruit, characterized in that, It includes freeze-dried fruit base and a prebiotic coating on its surface. The prebiotic coating comprises the following components by weight: 30-50 parts of fructooligosaccharide, 20-40 parts of galactooligosaccharide, 5-15 parts of resistant dextrin, 3-8 parts of modified chitosan agent, and 1-5 parts of plant extract.

2. The prebiotic-coated freeze-dried fruit according to claim 1, characterized in that, The freeze-dried fruit base material is fruit slices or fruit granules that have undergone vacuum freeze-drying, selected from at least one of apples, strawberries, mangoes, bananas, peaches, pears, and blueberries, with a moisture content of ≤5% and a water activity of ≤0.25 after freeze-drying.

3. The prebiotic-coated freeze-dried fruit according to claim 1, characterized in that, The plant extracts, by weight, comprise: 2-5 parts green tea extract, 1-3 parts rosemary extract, and 1-2 parts licorice extract.

4. The prebiotic-coated freeze-dried fruit according to claim 3, characterized in that, The green tea extract contains ≥50% tea polyphenols, the rosemary extract contains ≥20% rosmarinic acid, and the licorice extract contains ≥30% glycyrrhizic acid.

5. The prebiotic-coated freeze-dried fruit according to claim 1, characterized in that, The modified chitosan agent is prepared through the following steps: S1: Dissolve chitosan in a 1-3% (w / w) acetic acid solution to prepare a 2-5% (w / w) chitosan solution. Filter to remove insoluble matter to obtain a chitosan acetic acid solution. S2: According to the molar ratio of chitosan to ferulic acid of (2-5):1, ferulic acid is dissolved in anhydrous ethanol, N,N'-dicyclohexylcarbodiimide and 4-dimethylaminopyridine are added, and the solution is activated at room temperature for 30-60 min to obtain an activated ester solution. S3: Slowly add the activated ester solution dropwise to the chitosan acetate solution, stir and react at 40-60℃ for 4-8 hours. After the reaction is complete, precipitate with acetone, filter, wash and dry to obtain ferulic acid grafted chitosan. S4: Dissolve ferulic acid-grafted chitosan in deionized water to prepare a solution with a mass fraction of 2-4%, add 5-10% β-cyclodextrin (by mass of chitosan), stir and encapsulate at 40-50℃ for 1-3 hours, and freeze-dry to obtain the modified chitosan agent.

6. The prebiotic-coated freeze-dried fruit according to claim 5, characterized in that, The degree of deacetylation of the chitosan in step S1 is 85-95%, and the viscosity-average molecular weight is 5×10⁻⁶. 4 -2×10 5 Da; In step S2, the molar ratio of ferulic acid to chitosan is 1:3, the molar ratio of N,N'-dicyclohexylcarbodiimide to ferulic acid is (1-1.5):1, and the molar ratio of 4-dimethylaminopyridine to ferulic acid is (0.3-0.6):

1.

7. A method for preparing prebiotic-coated freeze-dried fruit as described in any one of claims 1-6, characterized in that, Includes the following steps: S100: Fruit pretreatment: After washing, peeling and pitting fresh fruit, cut it into slices with a thickness of 3-8mm or granules with a side length of 8-15mm, and prefreeze it at -18~-25℃ for 4-8 hours to obtain pre-frozen fruit; S200: Vacuum freeze-drying: Pre-frozen fruit is placed in a vacuum freeze dryer and processed in two stages, sublimation drying and desorption drying, under conditions of vacuum degree ≤20Pa and cold trap temperature ≤-45℃, to obtain freeze-dried fruit base material, with the moisture content after freeze-drying controlled to ≤5%; S300: Prebiotic Coating Solution Preparation: Add fructooligosaccharides, galactooligosaccharides, resistant dextrin, modified chitosan agent, and plant extracts to deionized water according to the specified ratio, stir and dissolve in a water bath at 50-70℃ to prepare a coating solution with a solid content of 15-30%, and then use it after homogenization and degassing treatment. S400: Coating and molding: The coating liquid is evenly applied to the surface of the freeze-dried fruit base material using a multi-layer spraying process. The amount of each spray is 3-8% of the weight of the freeze-dried fruit base material. A total of 2-4 layers are sprayed. After each layer is sprayed, it is dried in hot air at 40-60℃ for 5-15 minutes to allow the coating to adhere and cure. S500: Gradient drying: Place the coated freeze-dried fruit in a vacuum drying oven and dry it at 40-50℃ for 1-3 hours until the water activity is ≤0.3; S600: Nitrogen Packaging: After drying, freeze-dried fruits with prebiotic coating are cooled to room temperature in an environment with relative humidity ≤30%, and then packaged with nitrogen to obtain the finished product.

8. The preparation method according to claim 7, characterized in that, In step S200, the sublimation drying stage conditions are: temperature -10~5℃, vacuum degree 10-20Pa, time 8-15h; the desorption drying stage conditions are: temperature 25-45℃, vacuum degree 5-10Pa, time 4-8h.

9. The preparation method according to claim 7, characterized in that, In step S300, the homogenization conditions are a pressure of 20-40 MPa and a temperature of 50-60℃; the degassing is performed using vacuum degassing with a vacuum degree of -0.08 to -0.095 MPa and a time of 15-30 min.

10. The preparation method according to claim 7, characterized in that, In step S400, the spraying method is high-pressure airless spraying or ultrasonic atomization spraying, the spraying pressure is 0.2-0.5MPa, and the coating liquid temperature is 45-55℃; in step S500, the heating rate of gradient drying is 1-3℃ / h.

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