Application of nicotinic acid in preservation of apple fruits

Abstract

The invention discloses application of nicotinic acid in apple fruit preservation, and relates to the technical field of fruit preservation. According to the method, the niacin aqueous solution is adopted to soak or spray the apple fruits, so that the appearance of the fruits can be kept, and the fruit hardness, protopectin content and soluble pectin content during the storage period of the fruits can be maintained; meanwhile, the activity of superoxide dismutase and catalase of the fruits is improved, and the oxidation resistance of the fruits is enhanced. Nicotinic acid is used for fruit preservation, the preservation effect is good, and safety and reliability are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of fruit preservation, in particular to the application of nicotinic acid in the preservation of apple fruits. Background Art

[0002] Apple (Malus domestica), a plant of the Rosaceae family, is rich in a variety of nutrients that are beneficial to the human body, such as polyphenols, flavonoids, vitamins, etc. The annual production of apples in my country reaches 45 million tons, accounting for about 55% of the global total production. However, with the extension of post-harvest storage time, the fruit quality and antioxidant capacity will decrease. Fruit quality mainly refers to the appearance, flavor and nutritional value of the fruit. During the storage of the fruit, due to changes in respiration and metabolism, the content of pectin substances changes, resulting in a decrease in the hardness and softening of the fruit, which in turn affects the edible quality and limits the storage life and transportation efficiency. At the same time, the reduction in the antioxidant capacity of apples accelerates the aging, softening and nutrient loss of the fruit, reducing the stress resistance and storage life of apples. Therefore, studying the storage technology of apples is crucial to extend its shelf life and maintain its nutritional value.

[0003] At present, apple preservation strategies include chemical preservation, low-temperature storage, coating preservation technology and microbial antagonistic preservation. Among them, chemical preservation uses chemical agents to inhibit the growth of microorganisms or kill microorganisms, thereby extending the shelf life of apples. Although chemical preservation can effectively inhibit the growth and reproduction of microorganisms and reduce the probability of apples rotting and deteriorating due to microbial invasion during storage, the use of chemical agents will remain on the surface or inside of apples, causing potential harm to human health. Low-temperature storage uses low temperature to reduce the respiration, metabolism and ethylene production of apples, thereby extending the shelf life of apples, but this preservation method relies on refrigeration equipment, consumes a lot of energy and has high requirements for humidity control. Microbial antagonistic preservation uses the antagonism between microorganisms. This preservation method has a good preservation effect, and some beneficial microorganisms may produce substances that improve the quality of apples, but the variety and storage environment of apples will affect the preservation effect. The activity of microorganisms is affected by temperature, humidity and pH, and this preservation method has high technical requirements and is not suitable for large-scale application. The smear preservation technology uses a smear on the surface of the apple to reduce the respiration and water loss of the apple. Therefore, it is particularly important to find a natural, safe, efficient and even beneficial method for preserving fruits and vegetables.

[0004] Nicotinic acid, also known as vitamin B3, is an important biologically active molecule with multiple physiological functions in the body, such as anti-oxidation, regulating fat and protein metabolism, promoting cell growth and repair, and building coenzymes. Nicotinic acid synthesizes NAD (nicotinamide adenine dinucleotide) through the Preiss-Handler pathway, which is a conservative metabolic pathway in terrestrial plants. However, there are no reports on the application of niacin in the field of fruit preservation. Summary of the invention

[0005] In view of the above prior art, the purpose of the present invention is to provide an application of nicotinic acid in the preservation of apple fruits. The present invention uses a nicotinic acid aqueous solution to soak or spray apple fruits, which can maintain the appearance of the fruit, maintain the fruit hardness, original pectin content and soluble pectin content during storage; at the same time, the superoxide dismutase and catalase activities of the fruit are increased, and the antioxidant capacity of the fruit is enhanced. The present invention uses nicotinic acid to preserve the fruit, which has a good effect and is safe and reliable.

[0006] To achieve the above object, the present invention adopts the following technical solution:

[0007] The first aspect of the present invention provides the use of nicotinic acid in fruit preservation.

[0008] Preferably, the fruit is apple fruit.

[0009] Further preferably, the apple fruits include Fuji apple fruits and Huaniu apple fruits.

[0010] Preferably, the nicotinic acid achieves fruit preservation through at least one of the following pathways (1)-(4):

[0011] (1) Maintain the appearance of the fruit during storage;

[0012] (2) Maintain fruit firmness during storage;

[0013] (3) Maintain the original pectin content and soluble pectin content of the fruit during storage;

[0014] (4) Improve the superoxide dismutase activity and catalase activity during fruit storage.

[0015] A second aspect of the present invention provides a method for preserving apple fruits, comprising the following steps:

[0016] The apple fruits can be kept fresh by spraying or soaking them with nicotinic acid aqueous solution and then drying them after the treatment.

[0017] Preferably, the concentration of the nicotinic acid aqueous solution is 1-100 mmol / L.

[0018] Further preferably, the concentration of the niacin aqueous solution is 10 mmol / L.

[0019] Preferably, the specific operation of the spraying treatment is: spraying the nicotinic acid aqueous solution evenly on the surface of the apple fruit, and repeating the spraying 2-4 times.

[0020] Further preferably, the total spraying amount per apple is 1.5-2.5 mL.

[0021] Preferably, the specific operation of the soaking treatment is: immersing the apple fruit in the niacin aqueous solution for 60-120 minutes.

[0022] Preferably, the dried fruits are stored at 20-25°C.

[0023] Beneficial effects of the present invention:

[0024] The present invention uses nicotinic acid aqueous solution to soak or spray apple fruits to achieve apple fruit preservation. The nicotinic acid aqueous solution is used to treat apple fruits, so as to maintain the appearance of the fruit, maintain the fruit hardness, original pectin content and soluble pectin content during storage; at the same time, the superoxide dismutase and catalase activities of the fruit are increased, and the antioxidant capacity of the fruit is enhanced. The present invention uses nicotinic acid for fruit preservation, which can ensure the preservation effect and the safety and reliability of the preservation method. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 :Effects of different concentrations of niacin aqueous solution on Fuji apples;

[0026] Figure 2 :Appearance changes of Fuji and Huaniu apples treated with 10mmol / L nicotinic acid aqueous solution and distilled water;

[0027] Figure 3 : Changes in firmness of Fuji and Huaniu apples treated with 10mmol / L nicotinic acid aqueous solution and distilled water;

[0028] Figure 4 :Changes in original pectin of Fuji and Huaniu apples treated with 10mmol / L niacin aqueous solution and distilled water;

[0029] Figure 5 :Changes in soluble pectin in Fuji and Huaniu apples treated with 10mmol / L niacin aqueous solution and distilled water;

[0030] Figure 6 :Changes in superoxide dismutase activity of Fuji and Huaniu apples treated with 10mmol / L niacin aqueous solution and distilled water;

[0031] Figure 7: Changes of catalase activity in Fuji and Huaniu apples treated with 10 mmol / L niacin aqueous solution and distilled water. DETAILED DESCRIPTION

[0032] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0033] In the prior art, as the storage time after harvesting increases, the fruit quality and antioxidant capacity will decrease. Therefore, apples need to be treated to extend their shelf life and maintain their nutritional value. At present, although chemical preservation can inhibit the reproduction of microorganisms, its safety is not high. Low-temperature storage requires equipment, and microbial antagonism has high technical requirements and is not suitable for large-scale application. Nicotinic acid, as a water-soluble vitamin, is one of the essential nutrients for organisms. It plays a key role in energy metabolism and oxidative phosphorylation and is essential for the metabolism of proteins, fats and carbohydrates. However, there is currently no relevant research on the application of nicotinic acid in the field of fruit preservation.

[0034] Previous studies have shown that the storage of apples is accompanied by a decrease in nicotinic acid and NAD content. Therefore, the inventors speculate that nicotinic acid may have a good characteristic of delaying the decline of fruit quality, and exogenous application of nicotinic acid may play a role in preserving apple fruits. Based on this, a systematic study was conducted on the role of nicotinic acid in fruit preservation. Specifically, the present invention selected the storage-resistant Fuji apple and the storage-intolerant Huaniu apple as test subjects, sprayed the nicotinic acid aqueous solution on the above apple fruits, and investigated the effects of nicotinic acid on physiological indicators such as fruit hardness, protopectin, soluble pectin, superoxide dismutase and catalase. The results of the study showed that nicotinic acid can significantly maintain the appearance of the fruit; effectively maintain the stability of fruit hardness, protopectin and soluble pectin during storage; at the same time, nicotinic acid can also significantly increase the activity of superoxide dismutase and catalase in the fruit, and enhance the antioxidant capacity of the fruit. In summary, nicotinic acid has shown broad application potential in delaying apple aging, extending storage period and shelf life.

[0035] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with specific embodiments.

[0036] The experimental materials used in the embodiments of the present invention are all conventional experimental materials in the art and can be purchased through commercial channels.

[0037] The nicotinic acid used in the present invention was purchased from Shanghai MacLean Biochemical Technology Co., Ltd. with the product number of N814565.

[0038] Embodiment 1: Apple fruit preservation method

[0039] Prepare a 10 mmol / L niacin aqueous solution, spray it evenly on the surface of apple fruit using the top-down spraying method, repeat the spraying 3 times to ensure that the total spraying amount for each apple is 2 mL. After the treatment, dry it in the air and store it at 20°C.

[0040] Embodiment 2:

[0041] Prepare a 1 mmol / L niacin aqueous solution, spray it evenly on the surface of apple fruit using the top-down spraying method, repeat the spraying 3 times to ensure that the total spraying amount for each apple is 1.5 mL. After the treatment, dry it in the air and store it at 20°C.

[0042] Embodiment 3:

[0043] Prepare a 100 mmol / L niacin aqueous solution, spray it evenly on the surface of apple fruit from top to bottom, repeat the spraying 3 times to ensure a total spraying amount of 2.5 mL for each apple. After the treatment, dry it in the air and store it at 20°C.

[0044] Embodiment 4: Apple fruit preservation method

[0045] A niacin aqueous solution with a concentration of 10 mmol / L was prepared, and apple fruits were immersed in the niacin aqueous solution for 100 minutes. After being taken out, they were air-dried and stored at 20°C.

[0046] Test Example 1:

[0047] 1. Test method:

[0048] 1.1 Experimental groups and treatments:

[0049] A total of 200 Fuji apple fruits (each weighing 206.4 g ± 41.2 g) from the same batch, with the same variety and specifications and no difference in freshness and other indicators were selected and randomly divided into 4 groups, with 50 fruits in each group.

[0050] Experimental group: apple fruits were preserved with niacin aqueous solutions of different concentrations;

[0051] Blank control group: distilled water was used to preserve apple fruits.

[0052] The apple fruit was sampled at 0, 15, 30, and 60 days after treatment to observe the phenotype and to test the fruit firmness, protopectin and soluble pectin content, superoxide dismutase activity, and catalase activity. Figure 1-Figure 7 shown.

[0053] At the same time, 100 Huaniu apple fruits (each fruit weighing 225.6g±36.6g) from the same batch, with the same variety and specifications and no difference in indicators such as freshness were selected and randomly divided into two groups, with 50 fruits in each group.

[0054] Experimental group: apple fruits were preserved with niacin aqueous solution at the optimal concentration;

[0055] Blank control group: distilled water was used to preserve apple fruits.

[0056] The apple phenotypes were observed and samples were taken at 0, 15, 30, and 45 days after treatment to detect changes in fruit firmness, protopectin content, soluble pectin content, superoxide dismutase activity, and catalase activity. The results are as follows: Figure 2-Figure 7 shown.

[0057] The specific operation of the fresh-keeping treatment is: use the top-down spraying method to evenly spray different concentrations of niacin aqueous solution or distilled water on the surface of the apple fruit, repeat the spraying 3 times, and ensure that the total spraying amount for each apple is 2mL. After the treatment is completed, dry it and store it at 20°C.

[0058] 1.2 Test method:

[0059] 1.2.1 Hardness determination method:

[0060] The firmness of apples was measured using a Stable Micro Systems TA.XT plus texture analyzer.

[0061] The measurement was performed using a P / 2 cylindrical probe (2 mm in diameter), with a speed of 2 mm / s before measurement, 1 mm / s for measurement, and 5 mm / s after measurement. The penetration depth was 10 mm and the minimum perception was 10 g. Ten apples of each concentration were randomly selected on each sampling day, and four points evenly dispersed and equidistant around the central equator of the apple were randomly selected for puncture measurement perpendicular to the fruit surface. The measured fruit hardness values ​​were analyzed using the TextureExponent 32 software, and the hardness results of each apple were taken as the average of the four points.

[0062] 1.2.2 Determination method of protopectin:

[0063] The original pectin content was determined using a kit (YGJ-2-G, Keming, Suzhou, China). The specific detection method is:

[0064] (1) Weigh 0.05 g of apple sample and put it into a mortar. Add 1 mL of extract solution 1, wherein the extract solution 1 is 80% ethanol by mass, and grind it quickly. Transfer all the slurry into a 2 mL centrifuge tube. Fasten the centrifuge tube, place it in a 90°C water bath for 30 min, cool it, and centrifuge it at 5000 × g and 25°C for 10 min. Aspirate and discard the supernatant and retain the precipitate.

[0065] (2) Pipette 1 mL of the extract and add it to the precipitate in step (1). Fasten the centrifuge tube, place in a 90°C water bath for 30 min, cool, and centrifuge at 5000 × g and 25°C for 10 min. Pipette and discard the supernatant and retain the precipitate.

[0066] (3) Take 1 mL of the second extract, wherein the second extract is 0.5 mol / L sulfuric acid, add it to the precipitate in step (2), incubate in a 90°C water bath for 1 h, cool, centrifuge at 8000×g, 25°C for 15 min, and retain the supernatant for testing.

[0067] (4) The supernatant obtained in step (3) was tested on a UV spectrophotometer to detect absorbance. The UV spectrophotometer was preheated for 30 min in advance, the wavelength was set to 530 nm, and zeroed with ddH2O;

[0068] The order and amount of sample addition in each EP tube before testing are shown in Table 1.

[0069] Table 1 Sample addition table

[0070]

[0071]

[0072] The liquids in the blank tube, standard tube, control tube and measuring tube were fully mixed and transferred into 1 mL glass cuvettes respectively. The absorbance values ​​of the blank tube, standard tube, control tube and measuring tube were read at 530 nm using an ultraviolet spectrophotometer, which were recorded as A1, A2, A3 and A4 respectively. The original pectin content was calculated using the above absorbance values. The calculation formula of the original pectin content is as follows:

[0073] Pectin content (mg g -1 )=0.25×(A4-A3)÷(A2-A1)÷W;

[0074] Wherein, W is the fresh weight of the sample (g); A1 is the absorbance of the blank tube, A2 is the absorbance of the standard tube, A3 is the absorbance of the control tube, and A41 is the absorbance of the measurement tube.

[0075] 1.2.3 Soluble pectin determination method:

[0076] The water-soluble pectin content was determined using a water-soluble pectin content determination kit (WSP-2-Y, Ke Ming, Suzhou, China). The specific detection method is:

[0077] (1) Weigh 0.3 g of apple sample, put it in a mortar, add 1 mL of 80% ethanol, grind quickly, transfer all the homogenate into a 2 mL centrifuge tube, fasten the centrifuge tube, place it in a 95°C water bath for 20 min, cool it, centrifuge it at 4000×g, 25°C for 10 min, remove the supernatant, and keep the precipitate. Wash the precipitate once with 1.5 mL of 80% ethanol and acetone (vortex for 2 min, centrifuge it at 4000×g, 25°C for 10 min, and keep the precipitate).

[0078] (2) Pipette 1 mL of reagent 1 and add it to the washed precipitate obtained in step (1), let it soak for 15 hours, centrifuge at 4000×g, 25°C for 10 minutes, discard the supernatant, and keep the precipitate. Dry the precipitate in an oven at 65°C and weigh it.

[0079] (3) Weigh 3 mg of the dried precipitate obtained in step (2), put it into a mortar, add 1 mL of reagent 2, and grind it quickly. Transfer the homogenate into a 1.5 mL centrifuge tube, centrifuge at 8000 × g, 4°C for 10 min, and take the supernatant for testing.

[0080] (4) The supernatant obtained in step (3) was tested on a UV spectrophotometer to detect absorbance. The UV spectrophotometer was preheated for 30 min in advance, the wavelength was set to 530 nm, and zeroed with ddH2O. Reagents 3 and 4 were placed in a 37°C water bath for more than 10 min before use.

[0081] The order and amount of sample addition in each EP tube before testing on the machine are shown in Table 2.

[0082] Table 2 Sample addition table

[0083]

[0084]

[0085] After mixing the solutions in the blank tube, standard tube, control tube and test tube, place them in a 95℃ water bath for 5 minutes, use a UV spectrophotometer to detect the absorbance at 530nm, and read the absorbance values ​​of the blank tube, standard tube, control tube and test tube respectively, and record them as A1, A2, A3 and A4. If A is greater than 2, the sample to be tested needs to be diluted with ddH2O (diluted 10 times or 20 times). Set up one blank tube and one standard tube, and set up one control tube for each test tube.

[0086] Then use the absorbance values ​​of the blank tube, standard tube, control tube and test tube to calculate the soluble pectin content. The calculation formula is as follows:

[0087] Soluble pectin content (mg g -1 )=0.05×(A4-A3)÷(A2-A1)÷W×dilution factor;

[0088] Where A1 is the absorbance value of the blank tube; A2 is the absorbance value of the standard tube; A3 is the absorbance value of the control tube; A4 is the absorbance value of the test tube; W is the dry weight of the sample, g.

[0089] 1.2.4 Superoxide dismutase assay method:

[0090] (1) Weigh 0.5 g of the mixed apple sample, add 1 mL of phosphate buffer (0.05 mol / L, pH = 7.8), grind in an ice bath, add another 1 mL of phosphate buffer, pour into a centrifuge tube, wash the mortar with 2 mL of phosphate buffer, pour into a centrifuge tube, balance, centrifuge at low temperature for 20 min, and take the supernatant after centrifugation and refrigerate.

[0091] (2) Take test tubes of the same model, add 50 μL of supernatant (add 50 μL of phosphate buffer to each of the 4 control test tubes), add 3 mL of reaction solution to each test tube, place 2 control test tubes in the dark, and react the remaining tubes under 4000 lx sunlight for 20-30 min (the light conditions of each tube must be consistent, the higher the temperature, the shorter the time, and the lower the temperature, the longer the time). After the reaction is completed, use the control test tube without light as a blank, and measure the absorbance value at 560 nm colorimetrically.

[0092] (3) 150 mL reaction solution composition: water 12.7 mL, phosphate buffer 76.5 mL, Met 15.25 mL, NBT 15.25 mL, EDTA-Na2 15.25 mL, riboflavin 15.25 mL.

[0093] (4) Calculation formula: The amount required to inhibit the photochemical reduction of NBT by 50% is the enzyme activity unit (U);

[0094] SOD activity (U g -1 FW)=(A ck -A s )×V / (A ck ×0.5×FW×V1).

[0095] In the formula, A ck A is the absorbance of the illumination control tube; s is the absorbance of the sample tube; V is the total volume of the sample (mL); V1 is the amount of sample used in the measurement (ml); FW is the mass of the apple (g).

[0096] The superoxide dismutase activity was calculated once at each sampling time, and the result of each test group was the average of 6 apples.

[0097] 1.2.5 Hydrogen peroxide determination method:

[0098] The peroxidase (POD) kit (POD-2-Y, Ke Ming, Suzhou, China) was used for determination. The specific detection process is as follows:

[0099] (1) Weigh 0.1 g of apple sample, put it into a mortar, add 1 mL of acetone as the extract, place it on ice, and homogenize it in an ice bath. Transfer the entire homogenate into a 1.5 mL centrifuge tube, centrifuge it at 8000 × g, 4°C for 10 min, take the supernatant, and store it at 4°C for testing.

[0100] (2) Detect absorbance:

[0101] Preheat the UV spectrophotometer for 30 minutes, set the wavelength to 470 nm, and adjust to zero with ddH2O. Mix reagent 1, reagent 2, and reagent 3 in a ratio of 2.6 (mL): 1.5 (μL): 1 (μL) to prepare a working solution; before use, place in a 25°C water bath for 10 minutes and use it immediately after preparation.

[0102] Mix 0.05 mL of supernatant and 0.95 mL of working solution, transfer into a 1 mL glass cuvette, and use a UV spectrophotometer to detect the absorbance value A1 at 470 nm at 1 min and the absorbance value A2 after 2 min.

[0103] (5) Calculation formula: Peroxidase activity (△OD470 min -1 g -1 FW) = 2000 × (A2-A1) ÷ W;

[0104] In the formula, A1 is the absorbance value at 1 minute; A2 is the absorbance value after 2 minutes;

[0105] Activity unit definition: A change of 0.01 in A470 per gram of tissue per mL of reaction system per minute is one enzyme activity unit.

[0106] 1.3 Test results:

[0107] Figure 1 The effects of different concentrations of niacin on Fuji apples when stored at room temperature for 60 days. Figure 1 It can be seen that the fruit skin of the control group showed obvious shrinkage at 60 days. The 1mmol / L niacin treatment had a slight improvement, and the 10mmol / L and 100mmol / L niacin treatments significantly improved the fruit shrinkage of apple fruits, effectively maintaining the appearance of the fruit. In order to save costs under the same effect, 10mmol / L was used as the optimal concentration for subsequent experiments.

[0108] Figure 2The appearance changes of Fuji and Huaniu apples after treatment with 10 mmol / L niacin aqueous solution and distilled water. Figure 2 It can be seen that on the 60th day, Fuji apples showed significant phenotypic changes, and the control group showed more severe skin shrinkage compared with the niacin treatment group. Huaniu apples showed the same changes on the 45th day of storage. This shows that niacin can significantly maintain the appearance of the fruit, and the time when the effect appears varies depending on the storage performance of the apple.

[0109] Figure 3 The hardness changes of Fuji and Huaniu apples under treatment with 10 mmol / L niacin aqueous solution and distilled water. Figure 3 It can be seen that on the 60th day of storage, the firmness of Fuji apples treated with niacin was significantly higher than that of the control group. The firmness of Huaniu apples treated with niacin was also significantly higher than that of the control group on the 45th day of storage, indicating that niacin can effectively maintain the firmness of fruits during storage.

[0110] Figure 4 and Figure 5 The changes of protopectin and soluble pectin in Fuji and Huaniu apples treated with 10 mmol / L niacin aqueous solution and distilled water. Figure 4 and Figure 5 It can be seen that on the 60th and 45th days of storage, the protopectin of Fuji and Huaniu apples treated with niacin was significantly higher than that of the control group, and the soluble pectin was significantly lower than that of the control group. This indicates that niacin effectively inhibits the conversion of protopectin to soluble pectin and maintains the content of protopectin and soluble pectin during storage.

[0111] Figure 6 and Figure 7 The changes of superoxide dismutase activity and catalase activity of Fuji and Huaniu apples under the treatment of 10mmol / L niacin aqueous solution and distilled water. Figure 6 and Figure 7 It can be seen that niacin treatment increased the superoxide dismutase activity and catalase activity in Fuji and Huaniu apples, and enhanced the antioxidant capacity of the fruit. Based on the above experiments, it can be clearly seen that 10mmol / L niacin aqueous solution treated apples has a good preservation effect.

[0112] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. Application of niacin in fruit preservation.

2. The use according to claim 1, characterized in that The fruit is an apple fruit.

3. The use according to claim 1, characterized in that The nicotinic acid achieves fruit preservation through at least one of the following pathways (1)-(4): (1) Maintain the appearance of the fruit during storage; (2) Maintain fruit firmness during storage; (3) Maintain the original pectin content and soluble pectin content of the fruit during storage; (4) Improve the activity of superoxide dismutase and catalase during fruit storage.

4. A method for preserving apple fruit, characterized in that: The following steps are involved: The apple fruits can be kept fresh by spraying or soaking them with nicotinic acid aqueous solution and then drying them after the treatment.

5. The method for preserving apple fruits as claimed in claim 4, characterized in that: The concentration of the nicotinic acid aqueous solution is 1-100 mmol / L.

6. The method for preserving apple fruits as claimed in claim 4, characterized in that: The specific operation of the spraying treatment is: spray the nicotinic acid aqueous solution evenly on the surface of the apple fruit, and repeat the spraying 2-4 times.

7. The method for preserving apple fruits as claimed in claim 6, characterized in that: Repeat spraying 2-4 times, with a total spraying amount of 1.5-2.5 mL per apple fruit.

8. The method for preserving apple fruits as claimed in claim 4, characterized in that: The specific operation of the soaking treatment is: immersing the apple fruit in the niacin aqueous solution for 60-120 minutes.

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