Method for removing astringency of walnut kernels

Abstract

The invention relates to a walnut kernel deastringency method which comprises the following steps: step 1, soaking walnut kernels in distilled water for 3-24 hours; step 2, taking out the soaked walnut kernels, and absorbing moisture on the surfaces of the walnut kernels; and step 3, drying the walnut kernels, wherein the drying treatment is drying in the sun or freeze-drying. According to the deastringency method for the walnut kernels, the deastringency effect equivalent to that of a chemical method can be achieved only by combining distilled water soaking with natural drying (drying in the sun or freeze-drying), the links of alkali liquor preparation, equipment investment and repeated washing in a traditional process are thoroughly avoided, and the comprehensive cost is reduced by 90% or above.

Description

Technical Field

[0001] The invention belongs to the technical field of walnut processing, and particularly relates to a method for removing astringency from walnut kernels. Background Art

[0002] Walnut (Juglans regia L.), one of the four major dried fruits in the world, is rich in polyphenol nutrients (such as tannins). A moderate amount of astringent substances can increase the refreshing taste of the fruit, enhance the sour taste, and improve the flavor of the fruit. Studies have shown that polyphenols have antioxidant, liver protection, anti-cancer, cardiovascular protection, cognitive function improvement, antibacterial, antiviral, melanin inhibition, immunotoxicity relief, and diabetes treatment effects. Appropriate intake is beneficial to health. However, excessive polyphenols can cause a strong astringency, which seriously affects the taste. The existing technology requires the removal of the endocarp, which is the core enrichment site of polyphenols, resulting in a serious loss of physiologically active components in walnuts.

[0003] In the published patent application documents, for example, Chinese invention patent application number CN201510090304.6 discloses a method for peeling and removing the astringency of walnut kernels, which comprises placing the walnut kernels in a peeling tank and soaking them in clean water for 0-12 hours, then adding a peeling agent to the peeling tank and stirring evenly to make the mass concentration of the peeling agent 0.6% to 0.9%, then passing steam into the peeling tank and heating it to 65°C-90°C, keeping it warm for 5 to 8 minutes, opening the peeling tank to discharge the peeling agent solution, and then moving the walnut kernels to a rinsing container and rinsing them clean to obtain the peeled and removed walnut kernels; the peeling agent is composed of NaOH and disodium ethylenediaminetetraacetic acid in a mass ratio of 8-14:1. The present invention can quickly remove the outer shell of walnut kernels in a low-concentration alkaline solution at a temperature below 90°C, with a peeling rate of more than 95% and substantially no kernel skin. The peeled walnut kernels are white in color, have no obvious alkaline taste after rinsing, and have a good taste. The prepared walnut milk product is white in color, has no astringency, and has a nutrient loss of less than 2%.

[0004] For another example, Chinese invention patent application number CN202010083790.X discloses a method for peeling and removing astringency of walnut kernels, which belongs to the field of walnut processing technology. The present invention comprises the following steps: (1) pretreatment of walnut kernels: soaking the walnut kernels in clean water for 0-1h, draining the water for standby use; or performing plasma activation treatment; (2) graded ultrasonic-assisted peeling; (3) taking out the ultrasonically treated walnut kernels, rinsing and peeling with water to obtain peeled walnut kernels, and soaking the peeled walnut kernels in a citric acid solution for 5-20 minutes. The peeling and astringency-removing method of the present invention has the characteristics of low temperature, rapidity, good peeling effect, and low cost, and the peeling and astringency-removing agent can be reused; the peeled walnut kernels are white in color, with little loss of nutrients, no obvious alkaline taste after rinsing, and good taste.

[0005] For example, China Invention Patent Authorization Announcement No. CN108967834B discloses a walnut kernel extraction and de-astringency process and walnut kernel, wherein the de-astringency process requires the preparation of an alkaline solution and extraction, and then washing, which is prone to chemical residues and the operation is relatively complicated; China Invention Patent Application No. CN202411634768.4 discloses a preparation method and application of de-astringent walnut kernel, wherein the de-astringency process requires alkaline solution soaking, steam treatment, cleaning, drying and other steps, which is relatively cumbersome; the de-astringency device is as shown in China Utility Model Patent Authorization Announcement No. The "Walnut de-astringency device" disclosed in China Utility Model Patent No. CN203776073U, the "American pecan de-astringency device" disclosed in China Utility Model Patent Authorization Announcement No. CN206659031U, the "A walnut kernel skinning and de-astringency device" disclosed in China Utility Model Patent Authorization Announcement No. CN210695846U, and the "A walnut microwave tempering, flavoring and de-astringency pretreatment device" disclosed in China Utility Model Patent Authorization Announcement No. CN220831795U are relatively expensive, require heating, and partially require removal of the inner seed coat to achieve the de-astringency effect.

[0006] At its root, existing solutions fail to address the fundamental conflict between efficient astringency removal and nutrient retention. On the one hand, they prioritize endocarp removal as a prerequisite for astringency removal, ignoring the water-solubility of polyphenols. On the other hand, they rely excessively on chemical reagents or complex equipment, making it difficult to achieve low-cost, universal application. This has led to a long-standing lack of a simple astringency removal method that eliminates astringency while maximizing nutrient retention.

[0007] In view of the above-mentioned defects or problems in the prior art, it is urgent to develop a method for removing astringency from walnut kernels. Summary of the Invention

[0008] The invention aims to provide a method for removing astringency from walnut kernels in view of the deficiencies in the prior art.

[0009] The method for removing astringency from walnut kernels of the present invention comprises the following steps:

[0010] Step 1: soaking walnut kernels in distilled water for 3-24 hours;

[0011] Step 2: Take out the soaked walnut kernels and dry the surface moisture;

[0012] Step 3: drying the walnut kernels, wherein the drying process is sun-drying or freeze-drying.

[0013] Furthermore, in step 1, the soaking time is 9 hours.

[0014] Furthermore, in step 1, when the walnut kernels are soaked in distilled water, the mass volume ratio of the walnut kernels to distilled water is 10g:100ml.

[0015] Furthermore, in step 3, the freeze-drying temperature is -40°C to -60°C.

[0016] Furthermore, in step 3, the ambient temperature of the sun-drying is 30°C to 50°C.

[0017] Furthermore, in step 1, the soaking process is carried out under light-proof conditions.

[0018] Furthermore, the walnut kernel's endocarp is not removed during steps 1 to 3 of the walnut kernel de-astringency method.

[0019] Furthermore, in step 1, the soaking time is determined by the following steps:

[0020] Soak the walnut kernels in distilled water in batches, setting multiple gradient soaking times;

[0021] The total phenol content of the soaking solution with different gradient soaking time was determined;

[0022] Manually evaluate the astringency of the walnut kernels after soaking for different gradient soaking times;

[0023] The optimal soaking time for removing astringency is determined based on the total phenol content of the soaking liquid and the result of the manual evaluation.

[0024] Furthermore, the total phenol content is determined by the Folin-phenol method.

[0025] The superior technical effects of the present invention are:

[0026] 1. The walnut kernel de-astringency method of the present invention only requires distilled water soaking combined with natural drying (sun drying or freeze drying) to achieve a de-astringency effect comparable to that of chemical methods, completely avoiding the alkali solution preparation, equipment investment, and repeated washing steps in traditional processes, thereby significantly reducing overall costs.

[0027] 2. The walnut kernel de-astringency method of the present invention adopts a "preserving the endocarp" de-astringency pathway, selectively dissolving astringent substances through the principle of osmotic pressure, while completely retaining active ingredients such as polyphenols and flavonoids in the endocarp of the walnut kernel, thereby solving the nutrient loss problem caused by peeling in the prior art.

[0028] 3. The walnut kernel de-astringency method of the present invention uses zero chemical additives throughout the entire process, eliminating the risk of residual residues. The drying process produces no wastewater (freeze-drying) or requires only natural energy (sun-drying), significantly reducing the environmental burden. The process product is a naturally degradable soaking liquid, eliminating secondary pollution.

[0029] 4. The walnut kernel de-astringency method of the present invention has a fixed soaking time (no need for segmented regulation) and a drying method that is flexible to adapt to different production conditions (sun drying is suitable for high-temperature dry areas, and freeze drying is suitable for high-value-added products), which greatly improves processing efficiency;

[0030] 5. The walnut kernel de-astringency method of the present invention effectively removes astringency while maintaining the natural flavor and crisp texture of the walnut kernel, significantly improving sensory acceptance, and retaining much more polyphenols than the peeling process, thereby ensuring the core nutritional function. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the process for removing astringency from walnut kernels according to the present invention;

[0032] Figure 2 This is a schematic diagram of the standard curve of tannic acid in the walnut kernel de-astringency method of the present invention;

[0033] Figure 3 This is a schematic diagram of the soaking liquid corresponding to different time gradients after treatment of the walnut kernel de-astringency method of the present invention. DETAILED DESCRIPTION

[0034] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods. It should be noted that, unless there is a conflict, the embodiments of the present application and the features in the embodiments can be combined with each other.

[0035] like Figures 1 to 3 As shown, the present invention provides a method for removing astringency from walnut kernels.

[0036] Embodiment: walnut astringency removal processing method, as Figure 1 shown.

[0037] 1. Material handling

[0038] Select walnut germplasm resources with high astringency, select complete and mold-free walnut kernels, retain the inner seed coat, weigh 10.00 g (accurate to 0.01 g) of walnut kernel sample, add 100 mL of distilled water to soak and remove the astringency.

[0039] Set the immersion time gradient: 3h, 6h, 9h, 12h, 15h, 18h, 21h, 24h (±5min), and repeat each time point 3 times independently;

[0040] After soaking, take out the walnut kernels, use sterile filter paper to absorb the surface moisture until there is no water dripping, and dry them separately.

[0041] Sun-dried group: Place in ventilated sunlight until constant weight, moisture ≤ 8%, avoid direct sunlight between 12:00-15:00 to prevent the walnut kernel from cracking;

[0042] Lyophilized group: spread on a lyophilization tray, pre-freeze at -50°C for 4 hours, and then freeze-dried at -45°C and 0.1 mbar vacuum for 96 hours;

[0043] Simultaneously, the immersion solution at each time point was quantitatively collected using a pipette and stored at 4°C in the dark until use.

[0044] 2. Determination of total phenol content

[0045] Standard curve preparation:

[0046] Accurately weigh 20.00 mg of tannic acid standard, dissolve it in 2 mL of 80% methanol, quantitatively transfer it to a 10 mL volumetric flask, dilute to the mark with distilled water, and mix by inverting the volumetric flask 10 times to prepare a 2000 mg / L stock solution.

[0047] Use a pipette to accurately draw the mother solution: 0mL, 0.2mL, 0.4mL, 0.6mL, 0.8mL, 1.0mL, 1.5mL, 2.0mL, respectively, and inject them into a 10mL volumetric flask. Add distilled water to the scale line and turn it upside down 10 times to obtain 0mg / L, 40mg / L, 80mg / L, 120mg / L, 160mg / L, 200mg / L, 300mg / L, and 400mg / L standard solutions.

[0048] Take 0.2 mL of standard solution of each concentration into a 10 mL centrifuge tube and add:

[0049] Use a pipette to slowly add 0.5 mL of Folin phenol reagent along the tube wall;

[0050] Use a pipette to quantitatively add 7.5 mL of distilled water;

[0051] 1.0 mL of saturated Na2CO3 solution, vortex mixed for 5 seconds;

[0052] 0.8 mL distilled water;

[0053] Seal the tube cap, mix by inverting 3-4 times, and react at room temperature at 24-26℃ in the dark for 60 minutes, inverting once every 15 minutes.

[0054] The reaction solution was colorimetrically analyzed at a wavelength of 725 nm using an ultraviolet spectrophotometer, and the absorbance was recorded to establish a standard curve equation for tannic acid. The standard curve equation for tannic acid is as follows: Figure 1 As shown, y = 0.0022x, R 2 =0.9995.

[0055] 3. Sample testing:

[0056] Soaking solution: Figure 3 As shown, the following treatments were performed on the corresponding soaking solutions of different time gradients;

[0057] Dilute the soaking solution 10 times;

[0058] Use a pipette to draw 0.2 mL of the diluted immersion solution into a 10 mL centrifuge tube;

[0059] Use a pipette to slowly add 0.5 mL of Folin phenol reagent along the tube wall;

[0060] Use a pipette to quantitatively add 7.5 mL of distilled water;

[0061] Add 1.0 mL of saturated Na2CO3 solution and vortex mix for 5 seconds;

[0062] Add 0.8 mL of distilled water;

[0063] Close the tube cap and mix by inverting 3 to 4 times;

[0064] Incubate at room temperature (24-26°C) in the dark for 60 minutes, turning the mixture upside down every 15 minutes. After the reaction is complete, measure the absorbance at 725 nm.

[0065] Calculate the total phenol concentration: Substitute the absorbance into the equation x = y / 0.0022 to obtain the total phenol concentration (mg / L) of the soaking solution;

[0066] Walnut kernels: The following treatments are performed on the corresponding walnut kernels stored at different time gradients:

[0067] Accurately weigh 1.99-2.01 g of dry sample, add 5 mL of 80% methanol and grind on ice for 3 min to form a homogenate;

[0068] Rinse the mortar with 15 mL of 80% methanol three times and transfer all of it to a 50 mL centrifuge tube;

[0069] Close the tube cap, invert 20 times to mix, and centrifuge at 8000g for 10 min;

[0070] Aspirate the supernatant, transfer to a 50 mL volumetric flask, dilute to the mark with distilled water, and mix thoroughly by inverting 15 times.

[0071] Pipette 0.2 mL of supernatant into a 10 mL centrifuge tube and add:

[0072] Use a pipette to slowly add 0.5 mL of Folin phenol reagent along the tube wall;

[0073] Use a pipette to quantitatively add 7.5 mL of distilled water;

[0074] 1.0 mL of saturated Na2CO3 solution, vortex mixed for 5 seconds;

[0075] 0.8 mL distilled water;

[0076] Seal the tube cap, mix by inverting 3-4 times, and react at room temperature at 24-26℃ in the dark for 60 minutes, inverting once every 15 minutes.

[0077] After the reaction was completed, the absorbance was measured at a wavelength of 725 nm.

[0078] Calculate the total phenol concentration: Substitute the absorbance into the equation x = y / 0.0022 to obtain the total phenol concentration in the walnut kernel extract (mg / L), which is then converted to the content in the walnut kernel dry basis (mg / g).

[0079] The results are shown in Table 1 below:

[0080] Table 1: Comparison of total phenol content under different treatments

[0081]

[0082]

[0083] Note: The data in the table are mean ± standard deviation.

[0084] 4. Verification of the astringency removal effect

[0085] Total phenols in the soaking solution reached a peak of 2144.85 mg / L at 12 h, an increase of 78.0% compared with 3 h.

[0086] In the group dried for 12 hours, the total phenol content of walnut kernels decreased to 2.02 mg / g, and the astringency reduction rate was 89.5%;

[0087] The astringency reduction rate of the freeze-dried group for 12 hours was 87.1%.

[0088] Sensory evaluation:

[0089] A 10-person blind panel evaluated the astringency of freeze-dried or sun-dried walnut kernels soaked for different times, with 10 being the strongest astringency and 0 being no astringency.

[0090] After 9 hours of soaking, the walnut kernels were confirmed by a 10-person blind evaluation panel to have basically no astringency, with a score of 0.5 points. After 15 hours of soaking, the walnut kernels were confirmed by a 10-person blind evaluation panel to have a slight astringency, indicating that as the soaking time continues to extend, the polyphenols in the soaking solution are reabsorbed by the walnut kernels.

[0091] Conclusion: After soaking for 9 hours, the astringency is basically gone, and relatively more polyphenols can be retained in the walnut kernels. In view of the health benefits of polyphenols to the human body, 9 hours is the best processing time.

[0092] Key innovations:

[0093] Endotephane retained;

[0094] During the soaking process, polyphenols are dissolved only through osmotic pressure difference. The structure of the inner seed coat is intact and no damage is found under microscopic examination.

[0095] Zero chemical additions;

[0096] No alkali / surfactant is used, the soaking liquid consists of pure water + dissolved polyphenols, which can be naturally degraded.

[0097] The present invention is not limited to the above-described embodiments. The above-described embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims.

Claims

1. A method for removing astringency from walnut kernels, characterized in that: The following steps are involved: Step 1: soaking walnut kernels in distilled water for 3-24 hours; Step 2: Take out the soaked walnut kernels and dry the surface moisture; Step 3: drying the walnut kernels, wherein the drying process is sun-drying or freeze-drying.

2. the walnut kernel de-astringency method according to claim 1, is characterized in that, In step 1, the soaking time is 9 hours.

3. the walnut kernel de-astringency method according to claim 1, is characterized in that, In step 1, during the soaking of the walnut kernels in distilled water, the mass volume ratio of the walnut kernels to the distilled water is 10 g:100 ml.

4. the walnut kernel de-astringency method according to claim 1, is characterized in that, In step 3, the freeze-drying temperature is -40°C to -60°C.

5. The walnut kernel de-astringency method according to claim 1, wherein In step 3, the drying environment temperature is 30°C to 50°C.

6. The walnut kernel de-astringency method according to claim 1, wherein During steps 1 to 3 of the walnut kernel de-astringency method, the inner seed coat of the walnut kernel is not removed.

7. The walnut kernel de-astringency method according to claim 1, wherein In step 1, the soaking process is carried out under light-proof conditions.

8. The walnut kernel de-astringency method according to claim 1, wherein In step 1, the soaking time is determined by the following steps: Soak the walnut kernels in distilled water in batches, setting multiple gradient soaking times; The total phenol content of the soaking solution with different gradient soaking time was determined; Manually evaluate the astringency of the walnut kernels after soaking for different gradient soaking times; The optimal soaking time for removing astringency is determined based on the total phenol content of the soaking liquid and the result of the manual evaluation.

9. The walnut kernel de-astringency method according to claim 8, wherein The total phenol content is determined by the Folin-phenol method.

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