Preserved tender walnuts and preparation method thereof

Abstract

The invention discloses tender preserved walnuts and a preparation method thereof, and belongs to the technical field of food processing. The method sequentially comprises the steps of raw material pretreatment, cleaning and color protection, ultrasonic deastringency treatment, ultrasonic-assisted sugaring, curing and drying, and cooling and packaging. The core of the method is that a synergistic process combining ultrasonic deastringency and ultrasonic-assisted sugar soaking is innovatively adopted, and the method specifically comprises the following steps: firstly, performing deastringency treatment on tender walnuts in a deastringency solution at 20-40 DEG C by using ultrasonic waves, then performing short-time high-temperature sugar boiling, and performing auxiliary sugar soaking in a sugar solution at 15-25 DEG C and with the sugar degree of 55-70% by using ultrasonic waves. According to the method, the processing period is greatly shortened to several hours from several days in a traditional process, oxidation browning of polyphenol is effectively inhibited, loss of water-soluble nutrient substances is reduced, the bitter taste is thoroughly removed, the water content of the finally obtained tender candied walnuts is larger than or equal to 25%, and the tender candied walnuts are natural in color, soft and glutinous in taste and pure in flavor.

Description

Technical Field

[0001] This application belongs to the field of food processing, and specifically relates to a method for preparing candied fruit. Background Technology

[0002] The larval damage caused by the walnut tussock moth (Atrijuglans hetaohei) is a key factor restricting walnut production. Because the larvae bore into the fruit and grow, causing the fruit to turn black and shrivel, the infestation rate can reach 90%, significantly reducing walnut yield and economic value (Lü Shuxian, Fu Bing, Wei Chunsheng, et al. Occurrence patterns and control measures of the walnut tussock moth [J]. Horticulture and Seedlings, 2023, 43(08):76-77). This pest is distributed in walnut-producing areas such as North China, Northwest China, Southwest China, and Central South China, and is prevalent in Hebei, Beijing, Shanxi, Shaanxi, Henan, and other places (Meng Shubiao. Occurrence patterns and integrated management techniques of the walnut tussock moth [J]. Hebei Fruit Trees, 2022, (01):37+39). The peak period for pests and diseases is from May to June each year, when walnuts are not yet mature. If immature walnuts can be harvested and utilized in advance, the impact of pests and diseases can be reduced, and the amount of fertilizer and pesticides used can also be decreased.

[0003] Immature walnuts have not yet formed complete kernels, and their shells have a low degree of lignification, maintaining a crisp and tender texture with a noticeable astringency, a sweet and sour taste, and the unique flavor of walnuts. The key to making candied fruit from immature walnuts lies in removing bitterness and introducing sweetness while preserving their unique crisp texture, refreshing flavor, and high-value nutrients to the maximum extent. Current sugar-preserving processes mainly follow two paths: one is traditional low-temperature, long-term sugar preservation, where the treated tender walnuts are placed in a low-concentration sugar solution and soaked at room temperature for several days, relying on the concentration difference to allow the sugar to slowly penetrate; the other is high-temperature, short-time sugar preservation, which uses a high-concentration sugar solution at a higher temperature (e.g., 80-100℃) for cooking or soaking, accelerating sugar penetration and water evaporation through heat, and is less time-consuming.

[0004] However, both of these mainstream sugar-preserving processes have inherent drawbacks when processing young walnuts, particularly their severe damage to heat-sensitive and easily oxidized nutrients such as polyphenols. On one hand, polyphenol oxidation leads to color deterioration and loss of antioxidants: young walnuts are rich in polyphenols, which are important antioxidants and also affect the product's color. During low-temperature, long-term sugar-preserving, prolonged exposure to the aqueous and oxygen environments easily leads to enzymatic browning under the action of enzymes such as polyphenol oxidase (PPO), causing the walnuts to change from milky white or light yellow to grayish-brown, resulting in a decline in product appearance and a reduction in antioxidant activity. In high-temperature, short-time sugar-preserving, although high temperatures can quickly inactivate enzymes and inhibit enzymatic browning, the intense heat directly triggers non-enzymatic browning (such as the Maillard reaction and caramelization), similarly causing a darker, more muted color. Furthermore, high temperatures directly degrade heat-sensitive polyphenols, reducing their activity. Secondly, it causes a significant loss of water-soluble nutrients: whether through prolonged soaking or high-temperature cooking, the prolonged and extensive contact between the sugar-soaked medium (sugar solution) and the walnut tissue creates conditions for the diffusion and dissolution of intracellular water-soluble nutrients (such as vitamin C, B vitamins, amino acids, minerals, and some soluble polyphenols) into the sugar solution. These nutrients are discarded with the processing waste liquid, resulting in a substantial reduction in the nutritional value of the final product.

[0005] In summary, existing sugar-preserving processes for tender walnuts face a dilemma: achieving both color and nutrient retention while simultaneously enhancing flavor and efficacy is difficult. Prioritizing penetration (low temperature for a long time or high temperature for a short time) inevitably leads to color deterioration and nutrient loss. Conversely, excessively shortening processing time or lowering temperature to preserve color and freshness results in insufficient sugar penetration, bland product, and shortened shelf life. Therefore, there is an urgent need to develop a novel sugar-preserving technology that can achieve highly efficient sugar penetration in a very short time while effectively inhibiting oxidation and reducing nutrient leaching, thus overcoming the core bottleneck in the processing of tender walnut preserves. Summary of the Invention

[0006] The purpose of this application is to address the problems mentioned in the background section by providing a candied tender walnut and its preparation method. This method aims to achieve efficient sugaring while minimizing oxidative discoloration, reducing nutrient loss, and preserving the unique crisp texture and fresh flavor of tender walnuts through an innovative ultrasound-assisted de-astringency and sugar-preserving process.

[0007] To achieve the above objectives, this application adopts the following technical solution: a candied walnut fruit and its preparation method, characterized in that the method comprises the following steps in sequence:

[0008] (1) Raw material pretreatment: Harvest within the suitable harvesting period of 30 to 60 days after the walnut flowers bloom, and complete the high-pressure steaming and dehulling treatment within 4 hours after harvesting to obtain tender walnuts. Use a knife to make surface slits on the tender walnuts, with a slit length of about 4 mm to 8 mm and a depth of about 1 mm to 2 mm.

[0009] (2) Cleaning and color protection: Place the tender walnuts treated above into a color protection agent aqueous solution with a concentration of 0.1%~0.5% and soak for 10~30 minutes. Then clean them to remove impurities and inhibit surface browning.

[0010] (3) De-astringency treatment: Place the cleaned tender walnuts in a de-astringency solution at a temperature of 20~40℃ and use ultrasonic-assisted immersion treatment. After the treatment is completed, take them out, rinse them gently with clean water, and drain the surface water.

[0011] (4) Ultrasonic-assisted sugar soaking: This step includes:

[0012] Sugar boiling: Immerse the tender walnuts, after removing the astringency, in a sugar solution at 90-95℃ with a sugar content of 35%-45%, and boil for 10-30 minutes;

[0013] Sugar syrup adjustment: Remove the tender walnuts, add sugar to the sugar syrup to adjust its sugar content to 55%~70%, and pre-cool to 15~25℃;

[0014] Ultrasonic sugaring: The tender walnuts that have been boiled in sugar are re-immersed in the adjusted sugar solution and sugared under ultrasonic assistance. During the sugaring process, the sugar solution should be kept circulating.

[0015] (5) Ripening and drying: After sugaring, take out the tender walnuts and drain off the excess sugar liquid. First, place them in a clean environment with a temperature of 30~50℃ and a relative humidity of 50%~60% for 2~4 hours to ripen. Then transfer them to a forced-air drying oven and dry them at 50~70℃ until the surface is dry and the internal moisture content meets the standard.

[0016] (6) Cooling and packaging: Cool the dried product to room temperature and seal it in an inert gas environment.

[0017] The color-protecting agent is any one of ascorbic acid and citric acid, or a mixture of both.

[0018] The deastringent solution is a buffer solution containing 0.5% to 2.0% citric acid or malic acid and food-grade tanninase.

[0019] The sugar source in the sugar solution is selected from any one or more of granulated sugar, trehalose, fructose syrup, and isomaltooligosaccharide.

[0020] Optionally, the sugar solution may also contain 0.1% to 0.5% by weight of a natural flavoring agent, which may be selected from any one of honey, osmanthus extract or rose extract.

[0021] Optionally, the sugar solution also includes a preservative and a pH adjuster. The preservative is potassium sorbate, added at 0.1% of the sugar solution mass; the pH adjuster is either citric acid or malic acid, added at 0.1% to 0.3% of the sugar solution mass. Further, the inert gas used in the cooling and packaging steps is nitrogen or a mixture of carbon dioxide and nitrogen.

[0022] The tender walnut preserves prepared in this application have a water content of ≥25%, a soft and glutinous texture, and a unique walnut flavor.

[0023] The candied fruit preparation method proposed in this application, based on ultrasonic deastringency and ultrasonic-assisted sugar extraction, solves the long-standing technical bottlenecks in traditional candied fruit preparation, such as long production cycle, poor flavor, and color deterioration, through the synergy and innovation of specific process steps. Compared with existing technologies, this application can bring the following significant benefits:

[0024] (1) It achieves efficient astringency removal, fundamentally improving flavor quality. Traditional processes rely on slow penetration and enzymatic hydrolysis through salting or prolonged soaking to remove astringency, which is time-consuming and has unstable effects. This application utilizes the cavitation effect of ultrasound to generate strong mechanical shearing force and micro-jet, instantly destroying the oil cell layer and cell wall on the surface of the peel, allowing bitter substances to dissolve quickly and fully. This physical field-assisted astringency removal method not only shortens the astringency removal time from the traditional 24-30 hours to tens of minutes, but also avoids the risk of loss of fruit nutrients and microbial growth that may be caused by prolonged water immersion, laying a solid foundation for the finished product to obtain pure fruit aroma and sweetness.

[0025] (2) Significantly shortens the sugar-soaking period and effectively prevents oxidative browning, maintaining the bright color of the product. Traditional sugar-soaking relies on the natural diffusion of high-concentration sugar solution, which usually takes several days or even weeks, easily leading to enzymatic browning of the fruit due to prolonged exposure to air, resulting in a dull color. This application introduces ultrasonic assistance in the low-temperature sugar-soaking stage. The cavitation effect greatly enhances the mass transfer efficiency of the sugar solution, allowing sugar molecules to quickly and evenly penetrate into the fruit, shortening the long sugar-soaking period to several hours. At the same time, because the entire sugar-soaking process is significantly shortened and the ultrasonic action can be carried out in a sealed container, the contact between the fruit and oxygen is minimized, thereby effectively inhibiting the activity of polyphenol oxidase, allowing the finished product to maintain the bright color of fresh fruit, solving the common problem of "dark color" in traditional candied fruit.

[0026] (3) A unique texture regulation mechanism of "rapid sugar penetration and high-quality shape preservation" is formed. The micro-stirring effect of ultrasound ensures the uniformity of sugar penetration, avoiding the "hard outside and wet inside" or soft and rotten phenomenon caused by uneven penetration in traditional processes. This application uses a process combination of "ultrasound deastringency-high temperature sugar cooking-ultrasound-assisted sugar soaking" to first open cell channels with ultrasound and heat treatment, and then maintain the efficient and gentle penetration of sugar with an ultrasonic field. This regulation mechanism allows the fruit to absorb sugar quickly while maintaining its cell structure. The final product can achieve a high water content (≥25%) and has a soft, palatable, plump and non-wrinkled texture.

[0027] (4) The overall process is energy-saving and safe, and better meets the requirements of modern food processing. The core process steps of this application are all carried out under low or medium temperature conditions (ultrasonic deastringency and ultrasonic sugar extraction can both be completed at room temperature to 60°C), avoiding the risks of high energy consumption, fructose caramelization, and the generation of harmful substances (such as hydroxymethylfurfural) that may be caused by traditional high-temperature long-term cooking. As a non-thermal physical processing technology, ultrasonic treatment itself leaves no chemical additive residues, ensuring the food safety of the product. The significant reduction in the entire production cycle also means a reduction in energy consumption and an increase in production efficiency, which has good prospects for industrial application. Detailed Implementation

[0028] Unless otherwise specified, all reagents and instruments used in the following examples are commercially available products.

[0029] Example 1: A type of candied walnut and its preparation method

[0030] (1) Raw material pretreatment: Harvest within the suitable harvesting period of 45 days after the walnut flowers bloom, and complete the high-pressure steaming and dehulling treatment within 4 hours after harvesting to obtain tender walnuts. Use a knife to make surface slits on the tender walnuts, with a slit length of about 6 mm and a depth of about 1.5 mm.

[0031] (2) Cleaning and color protection: Place the tender walnuts in a 0.3% ascorbic acid color protection solution and soak for 20 minutes, then rinse them with clean water.

[0032] (3) De-astringency treatment: Place the cleaned tender walnuts in a de-astringency solution at a temperature of 30°C (the de-astringency solution is a buffer solution containing 1.0% citric acid and food-grade tannin enzyme), and use ultrasonic assisted immersion treatment with a power of 300W for 30 minutes. After the treatment is completed, take them out, rinse them gently with clean water, and drain the surface water.

[0033] (4) Ultrasonic-assisted sugar curdling:

[0034] Sugar cooking: Immerse the tender walnuts, after removing the astringency, in a 93°C sugar solution with a sugar content of 40% and cook for 20 minutes.

[0035] Sugar syrup adjustment and ultrasonic sugaring: Remove the tender walnuts with a strainer, add white sugar to the sugar syrup to adjust the sugar content to 60%, and pre-cool to 20°C. Re-immerse the cooked walnuts in the adjusted sugar syrup and sugar-soak them for 5 hours under ultrasonic conditions at 300W and 20°C, keeping the sugar syrup circulating during the process.

[0036] (5) Ripening and drying: After sugaring, the tender walnuts are taken out and excess sugar liquid is drained. They are placed in a clean environment with a temperature of 40°C and a relative humidity of 55% for 3 hours to ripen. Then they are transferred to a forced-air drying oven and dried at 60°C until the surface is dry and the internal moisture content meets the standard (approximately 35% moisture content).

[0037] (6) Cooling and packaging: Cool the dried product to room temperature and seal it in a nitrogen-protected environment.

[0038] Example 2: A type of candied walnut and its preparation method

[0039] (1) Raw material pretreatment: Harvest within the suitable harvesting period of 45 days after the walnut flowers bloom, and complete the high-pressure steaming and dehulling treatment within 4 hours after harvesting to obtain tender walnuts. Use a knife to make surface slits on the tender walnuts, with a slit length of about 6 mm and a depth of about 1.5 mm.

[0040] (2) Cleaning and color protection: Place the tender walnuts in a 0.3% ascorbic acid color protection solution and soak for 20 minutes, then rinse them with clean water.

[0041] (3) Deastringency treatment: The cleaned tender walnuts are placed in a deastringency solution at a temperature of 30°C (the deastringency solution is a buffer solution containing 1.0% citric acid and food-grade tannin enzyme), and ultrasonic assisted soaking treatment with a power of 350W is used for 20 minutes.

[0042] (4) Ultrasonic-assisted sugar curdling:

[0043] Sugar cooking: Immerse the tender walnuts, after removing the astringency, in a 93°C sugar solution with a sugar content of 40% and cook for 20 minutes.

[0044] Sugar syrup adjustment and ultrasonic sugaring: Remove the tender walnuts with a strainer, add white sugar to the sugar syrup to adjust the sugar content to 60%, and pre-cool to 20°C. Re-immerse the cooked walnuts in the adjusted sugar syrup and sugar-soak them for 3 hours under ultrasonic conditions at 300W and 20°C, keeping the sugar syrup circulating during the process.

[0045] (5) Ripening and drying: After sugaring, the tender walnuts are taken out and excess sugar liquid is drained. They are placed in a clean environment with a temperature of 40°C and a relative humidity of 55% for 3 hours to ripen. Then they are transferred to a forced-air drying oven and dried at 60°C until the surface is dry and the internal moisture content meets the standard (approximately 35% moisture content).

[0046] (6) Cooling and packaging: Cool the dried product to room temperature and seal it in a nitrogen-protected environment.

[0047] Example 3: A type of candied walnut and its preparation method

[0048] The only difference between this embodiment and embodiment 1 is that in step (3) desiccation, the ultrasonic desiccation temperature is 20°C, and the other process parameters and steps are the same as in embodiment 1.

[0049] Example 4: A type of candied walnut and its preparation method

[0050] The only difference between this embodiment and embodiment 1 is that in step (4) ultrasonic-assisted sugar soaking, the sugar content of the sugar solution after adjustment is 70%, and the sugar soaking temperature is 30℃. The other process parameters and steps are the same as in embodiment 1.

[0051] Example 5: A type of candied walnut and its preparation method

[0052] The only difference between this embodiment and embodiment 1 is that in step (4) ultrasonic-assisted sugaring, the sugar source is a compound of white sugar and trehalose in a mass ratio of 1:1. The other process parameters and steps are the same as in embodiment 1.

[0053] Example 6: A type of candied walnut and its preparation method

[0054] The difference between this embodiment and Embodiment 1 is that:

[0055] In step (3) deastringency treatment, the ultrasonic power is 200W.

[0056] In step (4), the ultrasonic-assisted sugar curdling process uses a power of 200W.

[0057] The remaining process parameters and steps are the same as in Example 1.

[0058] Example 7: A type of candied walnut and its preparation method

[0059] The difference between this embodiment and Embodiment 1 is that:

[0060] In step (3) desiccation treatment, the ultrasonic impregnation time is 15 minutes.

[0061] In step (4), the ultrasonic-assisted sugar-soaking process takes 2 hours.

[0062] The remaining process parameters and steps are the same as in Example 1.

[0063] Example 8: A type of candied walnut and its preparation method

[0064] The difference between this embodiment and Embodiment 1 is that:

[0065] In step (3) deabrasion treatment, the ultrasonic deabrasion temperature is 40°C.

[0066] In step (4), the temperature of ultrasonic-assisted sugar curdling is 25°C.

[0067] The remaining process parameters and steps are the same as in Example 1.

[0068] Comparative Example 1: A type of candied walnut and its preparation method

[0069] (1) Raw material pretreatment: Same as in Example 1.

[0070] (2) Cleaning and color protection: Same as in Example 1.

[0071] (3) De-astringency treatment: Soak the cleaned tender walnuts in room temperature (25°C) water for 24 hours (without using ultrasound).

[0072] (4) Sugar soaking: Immerse the tender walnuts after removing the bitterness in a white sugar solution with a sugar content of 40% at 93°C and cook for 20 minutes. Then, soak them in sugar at room temperature for 72 hours (without using ultrasound).

[0073] (5) Curing and drying: Same as in Example 1.

[0074] (6) Cooling and packaging: Same as in Example 1.

[0075] Comparative Example 2: A type of candied walnut and its preparation method

[0076] (1) Raw material pretreatment: Same as in Example 1.

[0077] (2) Cleaning and color protection: Same as in Example 1.

[0078] (3) De-abrasion treatment: Same as in Example 1 (using ultrasound).

[0079] (4) Sugar soaking: Sugar cooking is the same as in Example 1. After cooking, soak in sugar at room temperature for 72 hours (without using ultrasound).

[0080] (5) Curing and drying: Same as in Example 1.

[0081] (6) Cooling and packaging: Same as in Example 1.

[0082] Comparative Example 3: A type of candied walnut and its preparation method

[0083] (1) Raw material pretreatment: Same as in Example 1.

[0084] (2) Cleaning and color protection: Same as in Example 1.

[0085] (3) De-astringency treatment: Same as Comparative Example 1 (soaking at room temperature, without using ultrasound).

[0086] (4) Sugar curdling: Sugar boiling is the same as in Example 1. After boiling, ultrasonic-assisted sugar curdling is performed (under the same conditions as in Example 1).

[0087] (5) Curing and drying: Same as in Example 1.

[0088] (6) Cooling and packaging: Same as in Example 1.

[0089] Comparative Example 4: A type of candied walnut and its preparation method

[0090] The difference between this comparative example and Example 1 is as follows:

[0091] In step (3) deacidification, the ultrasonic power is 150W.

[0092] In step (4), the ultrasonic-assisted sugar curdling process uses a power of 150W.

[0093] The remaining process parameters and steps are the same as in Example 1.

[0094] Comparative Example 5: A type of candied walnut and its preparation method

[0095] The difference between this comparative example and Example 1 is as follows:

[0096] In step (3) desiccation treatment, the ultrasonic impregnation time is 5 minutes.

[0097] In step (4), the ultrasonic-assisted sugar-soaking process takes 1 hour.

[0098] The remaining process parameters and steps are the same as in Example 1.

[0099] Characterization:

[0100] (1) Moisture content: Refer to the direct drying method in standard GB5009.3-2016.

[0101] (2) Sensory evaluation: Blind evaluation was conducted by 10 people aged 20-40. The evaluation dimensions included: (1) Appearance: Natural color, no local or large-scale discoloration. The overall condition is full and there is no obvious shrinkage (1-10 points); (2) Flavor: Sweet and sour ratio, bitterness, and aroma coordination (1-10 points); (3) Texture: Moderately soft and firm, soft and chewy with a certain elasticity (1-10 points); The test results were averaged.

[0102] Sample number Moisture content (%) Appearance (1~10 points) Flavor (1-10 points) Taste (1-10 points) Example 1 31.5 9.5 9.5 9.5 Example 2 30.8 9.2 9.3 9.3 Example 3 33.7 9.7 9.4 9.4 Example 4 30.9 9.3 9.0 8.8 Example 5 31.3 9.4 9.6 9.4 Example 6 29.2 8.5 8.5 8.5 Example 7 26.5 8.0 8.0 8.0 Example 8 28.6 8.2 8.8 8.8 Comparative Example 1 23.3 6.0 5.0 6.0 Comparative Example 2 25.8 7.0 7.5 7.0 Comparative Example 3 26.9 8.5 6.0 8.0 Comparative Example 4 24.7 7.5 7.0 7.0 Comparative Example 5 23.4 7.0 6.5 6.5

[0103] The comparison of the above embodiments with the comparative examples fully demonstrates that the ultrasonic deacidification and ultrasonic-assisted sugar extraction synergistic process provided in this application has achieved significant progress in the following aspects compared with the traditional process (Comparative Example 1) or the process that only adopts some innovative points (Comparative Examples 2 and 3):

[0104] While significantly shortening the production cycle (from several days to several hours), the product quality is effectively guaranteed and even improved. As can be seen from Examples 1 to 5, the product obtained in this application can stably maintain a high moisture content (>30%), has a natural and bright appearance (score>9.0), a pure flavor without significant bitterness (score>9.0), and a soft and palatable texture (score>9.0).

[0105] The ultrasonic deastringency removal and ultrasonic-assisted sugar infusion processes exhibit a significant synergistic effect, and neither can be omitted. While Comparative Example 2 (ultrasonic deastringency removal only) improved the flavor, the long sugar infusion period led to color deterioration; Comparative Example 3 (ultrasonic sugar infusion only) ensured efficiency and color, but incomplete deastringency removal resulted in significant flavor defects. Only by combining the two processes (as in Example 1) can the comprehensive goals of high efficiency, excellent color, good flavor, and superior taste be achieved simultaneously.

[0106] The process parameters in this application have an optimal range. Although Examples 6-8 are superior to traditional processes, their effects are significantly inferior to those of Preferred Examples 1-5, which proves the rationality and necessity of the parameter ranges (such as ultrasonic power and time) defined in the claims. Comparative Examples 4 and 5 show that when the key parameters are below the effective threshold, the advantages of this application cannot be realized.

[0107] In summary, the technical solution of this application, through innovative process combinations, successfully solves the technical problems of long cycle, easy browning, strong bitter taste, and unstable quality in the traditional preparation of candied young walnuts, and has achieved unexpected technical results.

[0108] To clearly illustrate the technical solutions of this application, the specification provides a detailed description of each embodiment. Given that some embodiments share common core steps, to avoid redundancy, the same or similar technical features may be described in detail or in a combination of brief description in different embodiments. Those skilled in the art can obtain a complete understanding by referring to the relevant parts of other embodiments when reading any embodiment.

[0109] It should be noted that the specific embodiments described above are merely illustrative examples, intended to fully disclose the technical details of this application so that those skilled in the art can practice it. The scope of protection of this application is defined by the appended claims and their legal equivalents, and is not limited to the above embodiments themselves.

Claims

1. A type of candied young walnut and its preparation method, characterized in that, The method includes the following steps in sequence: (1) Raw material pretreatment: Harvest the walnuts 30 to 60 days after flowering, and steam them under high pressure within 4 hours after harvesting to remove the green skin, so as to obtain tender walnuts; and perform surface slit treatment on the tender walnuts. (2) Cleaning and color protection: Soak the pretreated tender walnuts in a 0.1%~0.5% color protection solution for 10~30 minutes, and then clean them; (3) De-astringency treatment: Place the cleaned tender walnuts in a de-astringency solution at a temperature of 20~40℃ and use ultrasonic waves for assisted soaking treatment. After treatment, rinse and drain. (4) Ultrasonic-assisted sugaring: The tender walnuts after the astringency removal treatment are first immersed in a sugar solution with a sugar content of 35% to 45% at 90 to 95°C and cooked for 10 to 30 minutes; then, the sugar content of the sugar solution is adjusted to 55% to 70% and pre-cooled to 15 to 25°C, and ultrasonic-assisted sugaring is carried out in this sugar solution. (5) Ripening and drying: Take out the sugar-soaked tender walnuts and ripen them for 2 to 4 hours in an environment of 30 to 50°C and 50% to 60% relative humidity. Then put them into a forced-air drying oven and dry them at 50 to 70°C until the moisture content meets the standard. (6) Cooling and packaging: Cool the dried product and seal it under inert gas protection.

2. The method for preparing candied walnuts according to claim 1, characterized in that, The surface scribing treatment has a scribing length of 4 mm to 8 mm and a scribing depth of 1 mm to 2 mm.

3. The method for preparing candied young walnuts according to claim 1, characterized in that, The color-protecting agent is any one of ascorbic acid and citric acid, or a mixture of both.

4. The method for preparing candied young walnuts according to claim 1, characterized in that, The deastringent solution is a buffer solution containing 0.5% to 2% citric acid or malic acid and 0.5% to 2% food-grade tanninase.

5. The method for preparing candied young walnuts according to claim 1, characterized in that, In the sugaring step, the sugar solution used for ultrasonic-assisted sugaring is selected from any one or more of granulated sugar, trehalose, fructose syrup, and isomaltooligosaccharide.

6. The method for preparing candied young walnuts according to claim 1 or 5, characterized in that, The sugar solution also contains 0.1% to 0.5% by weight of a natural flavoring agent, which is selected from any one of honey, osmanthus extract, or rose extract.

7. The method for preparing candied young walnuts according to claim 1 or 5, characterized in that, The sugar solution also includes a preservative and a pH adjuster; the preservative is potassium sorbate, and its addition amount is 0.1% of the sugar solution mass; the pH adjuster is either citric acid or malic acid, and its addition amount is 0.1% to 0.3% of the sugar solution mass.

8. The method for preparing candied young walnuts according to claim 1, characterized in that, In the desiccation process, the ultrasonic power is 200W~350W and the processing time is 15~30 minutes.

9. The method for preparing candied young walnuts according to claim 1, characterized in that, In the ultrasonic-assisted sugar-soaking step, the power of the ultrasonic waves is 200W~350W, the processing time is 2~5 hours, and the sugar solution is kept circulating during the sugar-soaking process.

10. A type of candied walnut prepared by the method for preparing candied walnuts according to any one of claims 1 to 9, characterized in that, The moisture content of the tender walnut preserves is ≥25%.