Longan fresh-keeping agent and preparation method thereof

Abstract

The application discloses a longan preservative and a preparation method thereof, and belongs to the technical field of fruit and vegetable preservation. The preservative is composed of a pretreatment liquid and a composite coating preservative liquid, and the two are prepared and used in sequence. The pretreatment liquid is prepared from sodium pyrosulfite and water, and the composite coating preservative liquid is prepared from citric acid, chitosan, L-citrulline, L-proline, vitamin C and water, and can further contain hydroxytyrosol. In the preparation, the citric acid is dissolved in water first, then the chitosan is added and stirred and dispersed, and then the remaining components are added and uniformly mixed to obtain the composite coating preservative liquid. Compared with the prior art, the L-citrulline and the L-proline in the application are different in the action link and are connected with each other, and the composite coating preservative liquid has synergistic effect after compounding, and the surface antioxidant effect of hydroxytyrosol can significantly inhibit the browning of longan pericarp and the autolysis of longan pulp, the preservation effect is good, the raw materials are all food grade, the process is simple, and the application is suitable for large-scale application.

Description

Technical Field

[0001] This invention relates to the field of fruit and vegetable preservation technology, and in particular to a longan preservative and its preparation method. Background Technology

[0003] Chinese Patent Publication No. CN1415218A discloses a longan preservative and its preparation method, composed of potassium chlorate, oxalic acid, calcium chloride, sodium sulfite, and activated carbon. The components react slowly in a sealed environment to generate a bactericidal gas and alter its composition ratio, inhibiting longan respiration and achieving room-temperature preservation. However, its drawback lies in its reliance on chemical reactions to generate the gas, making the gas release and composition difficult to control, posing safety risks. Furthermore, it cannot regulate the physiological mechanisms of peel browning and pulp autolysis. Chinese Patent Publication No. CN101433235A discloses a chitosan longan preservative and its preparation method, composed of chitosan with a 95% degree of deacetylation, glacial acetic acid, vitamin C, zinc chloride, thiourea, and water. It extends the longan's shelf life through film formation and antioxidant effects. However, its drawback is that it relies solely on film formation and conventional antioxidants, contains non-food-grade components such as thiourea, and lacks regulation of post-harvest senescence metabolism. Chinese Patent Publication No. CN104920600A discloses a longan preservative and its preparation and preservation method. It uses extracts of various Chinese herbal medicines such as forsythia, houttuynia cordata, mulberry leaves, and cloves, combined with tea tree oil, eugenol, chitosan, etc., and is used in conjunction with a preservation bag. Its shortcomings are that the formula is complex, the extraction process is cumbersome, the aroma of Chinese herbal medicines affects the original flavor of the fruit, the stability of the components is poor, and it does not address the problems of longan browning and autolysis metabolic mechanisms.

[0004] In summary, existing technologies either rely on sulfur-based agents or chemical reagents, which pose issues related to residues, flavor, and safety; or they depend solely on external treatments such as film formation and antibacterial action, neglecting the regulation of the longan fruit's own senescence metabolism, resulting in limited control over peel browning and pulp autolysis. Furthermore, existing technologies do not address the issue from the perspective of amino acid metabolism regulation, nor do they utilize the synergistic effects of amino acid combinations at different stages of action to improve preservation. Therefore, there is an urgent need for a longan preservative with safe raw material food, capable of synergistically regulating post-harvest senescence of longan at the level of fruit physiological metabolism, and effectively inhibiting peel browning and pulp autolysis, along with its preparation method. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention aims to provide a longan preservative and its preparation method.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A longan preservative comprises a pretreatment solution and a composite coating preservative solution, which are prepared separately and used sequentially. By weight, the pretreatment solution consists of 80-120 parts water and 3-8 parts sodium metabisulfite; the composite coating preservative solution consists of 80-120 parts water, 1-3 parts citric acid, 0.5-2 parts chitosan, 0.02-0.1 parts amino acids, and 0.05-0.2 parts vitamin C; the amino acids are composed of L-citrulline and L-proline in a mass ratio of 0.5-2:0.5-2.

[0007] Preferably, by weight, the pretreatment solution consists of 90-110 parts water and 4-6 parts sodium metabisulfite; the composite coating preservation solution consists of 90-110 parts water, 1.5-2.5 parts citric acid, 0.8-1.2 parts chitosan, 0.04-0.06 parts amino acids, and 0.08-0.12 parts vitamin C.

[0008] The amino acid is composed of L-citrulline and L-proline in a mass ratio of 0.8-1.2:0.8-1.2.

[0009] Preferably, the composite coating preservative liquid also contains hydroxytyrosol, and the total amount of L-citrulline, L-proline and hydroxytyrosol is the same as the amount of the amino acids, and the mass ratio of L-citrulline, L-proline and hydroxytyrosol is 6-12:6-12:1-3.

[0010] More preferably, the mass ratio of L-citrulline, L-proline and hydroxytyrosol is 8-10:8-10:1.8-2.2.

[0011] The degree of deacetylation of the chitosan is not less than 90%, and the viscosity is 100-500 mPa•s.

[0012] The method of using the longan preservative is as follows: S1. Soak fresh longan in the pretreatment solution for 1-3 minutes, take it out and rinse it with water until the sulfur dioxide residue is no more than 0.05 g / kg, and dry the surface moisture of the fruit. S2. Soak the pretreated longan in the composite coating preservation solution for 10-20 minutes. The mass ratio of longan to composite coating preservation solution is 1:3-1:5. After taking it out, let it dry, pack it into a breathable packaging box, and store it under refrigeration.

[0013] The refrigeration temperature in S2 is 3-8℃, and the relative humidity is 85%-90%.

[0014] The preparation method of the longan preservative includes the following steps: Step 1: Preparation of pretreatment solution: Take water and sodium metabisulfite according to the weight parts, add sodium metabisulfite to water, stir until completely dissolved, and obtain the pretreatment solution; Step 2: Prepare the composite coating preservation solution: Take water, citric acid, chitosan, amino acids and vitamin C by weight. First, dissolve the citric acid in water, then add the chitosan and stir to disperse it. Then add the amino acids and vitamin C and continue stirring until the mixture is uniform to obtain the composite coating preservation solution.

[0015] The composite coating preservation liquid in step 2 also contains hydroxytyrosol. The hydroxytyrosol is added at the end of step 2 and stirred evenly to obtain the composite coating preservation liquid. The composite coating preservation liquid is stored away from light after preparation and should be used within 2-6 hours.

[0016] This invention discovered that existing chitosan-based longan preservative coatings rely solely on citric acid acidification, chitosan film-forming antibacterial action, and vitamin C antioxidant activity. These methods only block and inhibit bacteria from the fruit's exterior, lacking regulation of post-harvest senescence metabolism, resulting in rapid browning of the peel and autolysis of the pulp. To address this issue, this invention introduces amino acids that can be absorbed and utilized by the fruit into the composite coating preservative solution. These amino acids participate in the fruit's osmotic regulation and antioxidant metabolism, maintaining cell membrane structure integrity and thus delaying peel browning and pulp autolysis. Further screening revealed that L-citrulline, as a key intermediate in the nitric oxide cycle within the fruit, exhibits superior inhibitory effects on longan browning and autolysis among single amino acids.

[0017] Further research revealed that the action of a single amino acid is limited, resulting in a limited improvement in preservation effect. Furthermore, not any combination of two amino acids can enhance the effect. L-arginine and L-citrulline both belong to the nitric oxide cycle, and their functions highly overlap; combining them only increases the dosage of the same pathway without producing a synergistic effect. To address this issue, this invention combines L-citrulline and L-proline, which have different but interconnected action pathways. The former scavenges reactive oxygen species and stabilizes cell membranes through the nitric oxide pathway, while the latter maintains cell turgor pressure and activates the antioxidant enzyme system through osmotic regulation. Together, they block membrane lipid peroxidation from two levels: free radical scavenging and cell homeostasis maintenance, reducing the contact between phenolic substances and polyphenol oxidases. The combined effect of inhibiting browning and autolysis exceeds the sum of the effects of using either amino acid alone, achieving a synergistic effect.

[0018] Building upon the above, the inventors further discovered that amino acids primarily play a regulatory role at the physiological and metabolic level of fruit, their effectiveness depending on the fruit's absorption and transformation. They lack direct control over the oxidation reactions already occurring on the peel surface, leaving room for improvement in their anti-browning effect. To address this issue, the present invention further introduces hydroxytyrosol into the coating solution. Hydroxytyrosol directly scavenge free radicals on the peel surface and in the coating layer, blocking the oxidation chain reaction of phenolic substances, protecting phenolic substrates such as epicatechin that are easily oxidized by polyphenol oxidase, and reducing and regenerating them with vitamin C. This effect is independent of and complements the regulatory effects of the two amino acids at the physiological and metabolic level, further enhancing the inhibition of peel browning and pulp autolysis.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects: 1) This invention combines L-citrulline and L-proline. The two have different mechanisms of action but are interconnected, resulting in a synergistic effect. The inhibitory effect on browning of longan peel and autolysis of pulp is significantly better than that of preservatives containing single amino acids or no added amino acids.

[0020] 2) This invention further introduces hydroxytyrosol, which directly scavenges free radicals on the surface of the peel and in the coating layer. This complements and connects with the regulatory role of amino acids in the physiological metabolism of the fruit, further enhancing the anti-browning and anti-autolysis effects.

[0021] 3) This invention adopts a two-stage treatment process of pretreatment liquid and composite coating preservation liquid. All raw materials are food grade. The preparation process is simple and low cost. The sulfur dioxide residue is controllable and suitable for large-scale preservation of longan. Detailed Implementation

[0022] Some material sources or parameters: The sodium metabisulfite is food grade, with CAS number 7681-57-4, and its main content is not less than 96.5%.

[0023] The citric acid is food-grade anhydrous citric acid, with CAS number 77-92-9 and a purity of not less than 99.5%.

[0024] The chitosan is food grade, with CAS number 9012-76-4, a degree of deacetylation of not less than 90%, and a viscosity of 100-500 mPa·s.

[0025] The L-citrulline is food grade, with CAS number 372-75-8 and a purity of not less than 98%.

[0026] The L-proline is food grade, with CAS number 147-85-3 and a purity of not less than 98.5%.

[0027] The L-arginine is food grade, with CAS number 74-79-3 and a purity of not less than 98.5%.

[0028] The glycine in question is food grade, with CAS number 56-40-6 and a purity of not less than 98.5%.

[0029] The L-alanine in question is food grade, with CAS number 56-41-7 and a purity of not less than 98.5%.

[0030] The vitamin C mentioned is food-grade L-ascorbic acid, CAS number 50-81-7, with a purity of not less than 99%.

[0031] The hydroxytyrosol is food grade, with CAS number 10597-60-1 and a purity of not less than 98%.

[0032] All raw materials used in the embodiments of this invention are commercially available products.

[0033] Example 1

[0034] The longan preservative in this embodiment consists of a pretreatment solution and a composite coating preservative solution, which are prepared separately and used sequentially.

[0035] The preparation method of the longan preservative is as follows, in parts by weight: Step 1: Prepare the pretreatment solution: Take 100 parts of water and 5 parts of sodium metabisulfite; add sodium metabisulfite to water and stir until completely dissolved to obtain the pretreatment solution; Step 2: Prepare the composite coating preservation solution: Take 100 parts water, 2 parts citric acid, 1 part chitosan, 0.05 parts amino acids, and 0.1 parts vitamin C; first, dissolve the citric acid in water, then add the chitosan and stir to disperse it, then add the amino acids and vitamin C, and continue stirring until the mixture is uniform to obtain the composite coating preservation solution. After preparation, the composite coating preservation solution should be stored away from light and used within 2-6 hours.

[0036] The method of using the longan preservative is as follows: S1. Soak fresh longan in the pretreatment solution for 2 minutes, take it out and rinse it with water. The surface-treated fresh fruit has a sulfur dioxide (SO2) residue of ≤0.05 g / kg, and the fruit surface moisture is dried. S2. Soak the pretreated longan in the composite coating preservation solution for 15 minutes. The mass ratio of longan to preservation solution is 1:4. After taking it out, dry it, pack it into a breathable packaging box, and store it under refrigeration conditions. The refrigeration temperature is 4°C and the relative humidity is 85%.

[0037] The amino acid in question is L-citrulline.

[0038] Example 2

[0039] The preparation method of a longan preservative is basically the same as that in Example 1, except that the amino acid is L-proline.

[0040] Example 3

[0041] The preparation method of a longan preservative is basically the same as that in Example 1, except that the amino acid is L-arginine.

[0042] Example 4

[0043] The preparation method of a longan preservative is basically the same as that in Example 1, except that the amino acid is glycine.

[0044] Example 5

[0045] The preparation method of a longan preservative is basically the same as that in Example 1, except that the amino acid is L-alanine.

[0046] Example 6

[0047] The preparation method of a longan preservative is basically the same as that in Example 1, except that the amino acids are L-citrulline and L-proline in a mass ratio of 1:1.

[0048] Example 7

[0049] The preparation method of a longan preservative is basically the same as that in Example 1, except that the amino acids are L-arginine and L-citrulline in a mass ratio of 1:1.

[0050] Example 8

[0051] The preparation method of a longan preservative is basically the same as that in Example 6, except that the composite coating preservative liquid also contains hydroxytyrosol. The L-citrulline, L-proline and hydroxytyrosol are composed in a mass ratio of 9:9:2, and the total amount of the three is the same as the amount of amino acids in Example 6. The hydroxytyrosol is added at the end of step 2 and stirred evenly to obtain the composite coating preservative liquid.

[0052] Comparative Example 1 The preparation method of a longan preservative is basically the same as that in Example 1, except that the amino acid is not added.

[0053] Test Example 1 Determination of the browning index of fruit peel Fresh longan fruits of the same batch, uniform size, without mechanical damage or pests, and at about 80% maturity were randomly divided into 9 groups. Preservatives were prepared according to the methods described in Examples 1-8 and Comparative Example 1, and treated according to their usage. After treatment, the fruits were placed in breathable packaging boxes and refrigerated at 4±1℃ and 85%-90% relative humidity. Each group was replicated 3 times, with 30 fruits per replicate. On the 9th day of storage, the fruits were removed and graded according to the browning area grading standard: Grade 0: no browning; Grade 1: browning area ≤ 1 / 4 of total peel area; Grade 2: browning area > 1 / 4 and ≤ 1 / 2; Grade 3: browning area > 1 / 2 and ≤ 3 / 4; Grade 4: browning area > 3 / 4. The peel browning index was calculated using the following formula: Peel Browning Index = Σ(Browning Grade × Number of Fruits at that Grade) / (Highest Browning Grade × Total Number of Fruits). The result was the average of the 3 replicates. The results are shown in Table 1.

[0054] Table 1

[0055] Test Example 2 Determination of the autolysis index of fruit pulp: Fresh longan fruits of the same batch, uniform in size, without mechanical damage or pests, and at about 80% maturity were randomly divided into 9 groups. Preservatives were prepared according to the methods described in Examples 1-8 and Comparative Example 1, and treated according to their application methods. After treatment, the fruits were placed in breathable packaging boxes and refrigerated at 4±1℃ and 85%-90% relative humidity. Each group was replicated 3 times, with 30 fruits per replicate. On the 9th day of storage, the fruits were removed, the peel was removed, and the autolysis of the pulp was observed. Fruits were graded according to the autolysis area grading standard: Grade 0: intact flesh tissue, no autolysis; Grade 1: autolysis area ≤ 1 / 4 of flesh surface area; Grade 2: autolysis area > 1 / 4 and ≤ 1 / 2; Grade 3: autolysis area > 1 / 2 and ≤ 3 / 4; Grade 4: autolysis area > 3 / 4. The autolysis index was calculated using the following formula: Autolysis index = Σ (autolysis grade × number of fruits at that grade) / (highest autolysis grade × total number of fruits). The result was the average of three replicates. The results are shown in Table 2.

[0056] Table 2

[0057] The composite coating preservative solution in Comparative Example 1 relies solely on citric acid acidification, chitosan film-forming antibacterial action, and vitamin C antioxidant effects for preservation, lacking regulation of post-harvest senescence metabolism in longan fruit, resulting in rapid browning of the peel and autolysis of the pulp. In Examples 1-5, the introduction of amino acids allows them to be absorbed and utilized by the fruit, participating in osmotic regulation and antioxidant metabolism, maintaining cell membrane structural integrity, thereby delaying peel browning and pulp autolysis. L-citrulline is a key intermediate in the nitric oxide cycle within the fruit, promoting continuous nitric oxide production to remove excess reactive oxygen species and inhibit polyphenol oxidase activity, while also possessing free radical scavenging capabilities, thus providing the most direct protection to the cell membrane. L-arginine is easily diverted to the polyamine synthesis pathway, L-proline primarily functions in osmotic regulation, and glycine and L-alanine only have mild osmotic regulation and flavor-improving effects, with limited action pathways. Therefore, Example 1 demonstrates superior performance among single amino acids.

[0058] In Example 6, L-citrulline and L-proline have different but interconnected mechanisms of action. L-citrulline scavenge reactive oxygen species and stabilizes the cell membrane through the nitric oxide pathway, while L-proline maintains cell turgor pressure and membrane protein conformation through osmotic regulation, activates the antioxidant enzyme system, and promotes the regeneration of antioxidants. Both block membrane lipid peroxidation from the perspectives of free radical scavenging and maintaining cell homeostasis, reducing the contact between phenolic substances and polyphenol oxidase. Therefore, the combined effect of inhibiting browning and autolysis exceeds the sum of the effects of using either ingredient alone, demonstrating a synergistic effect. In Example 7, L-arginine and L-citrulline both belong to the nitric oxide cycle. L-arginine is metabolized to produce nitric oxide and L-citrulline, and L-citrulline can be recycled back to L-arginine. Their functions highly overlap, and their combined use is only equivalent to increasing the dosage of the same pathway; therefore, they do not have a synergistic effect.

[0059] Example 8 introduces hydroxytyrosol based on Example 6. Hydroxytyrosol is a catechol-based natural antioxidant that can directly scavenge free radicals on the surface of the peel and in the coating layer, block the oxidation chain reaction of phenolic substances, and protect phenolic substrates such as epicatechin in the longan peel that are easily oxidized by polyphenol oxidase. Its effect occurs at the chemical antioxidant level on the fruit surface, and its regulatory effects on the physiological metabolism of fruit do not overlap with those of L-citrulline and L-proline, but are interconnected. Furthermore, vitamin C can reduce and regenerate oxidized hydroxytyrosol, maintaining its antioxidant capacity. Therefore, the effect of Example 8 in inhibiting peel browning and pulp autolysis is further enhanced based on Example 6.

Claims

1. A longan preservative, characterized in that, It consists of a pretreatment solution and a composite coating preservation solution, which are prepared separately and used sequentially. By weight, the pretreatment solution consists of 80-120 parts water and 3-8 parts sodium metabisulfite; the composite coating preservation solution consists of 80-120 parts water, 1-3 parts citric acid, 0.5-2 parts chitosan, 0.02-0.1 parts amino acids, and 0.05-0.2 parts vitamin C; the amino acids are composed of L-citrulline and L-proline in a mass ratio of 0.5-2:0.5-2.

2. The longan preservative according to claim 1, characterized in that, By weight, the pretreatment solution consists of 90-110 parts water and 4-6 parts sodium metabisulfite; the composite coating preservation solution consists of 90-110 parts water, 1.5-2.5 parts citric acid, 0.8-1.2 parts chitosan, 0.04-0.06 parts amino acids, and 0.08-0.12 parts vitamin C.

3. The longan preservative according to claim 1, characterized in that, The amino acid is composed of L-citrulline and L-proline in a mass ratio of 0.8-1.2:0.8-1.

2.

4. The longan preservative according to claim 1, characterized in that, The composite coating preservation liquid also contains hydroxytyrosol. The total amount of L-citrulline, L-proline and hydroxytyrosol is the same as the amount of amino acids. The mass ratio of L-citrulline, L-proline and hydroxytyrosol is 6-12:6-12:1-3.

5. The longan preservative according to claim 4, characterized in that, The mass ratio of L-citrulline, L-proline, and hydroxytyrosol is 8-10:8-10:1.8-2.

2.

6. The longan preservative according to claim 1, characterized in that, The degree of deacetylation of the chitosan is not less than 90%, and the viscosity is 100-500 mPa·s.

7. The longan preservative according to claim 1, characterized in that, The method of using the longan preservative is as follows: S1. Soak fresh longan in the pretreatment solution for 1-3 minutes, take it out and rinse it with water until the sulfur dioxide residue is no more than 0.05 g / kg, and dry the surface moisture of the fruit. S2. Soak the pretreated longan in the composite coating preservation solution for 10-20 minutes. The mass ratio of longan to composite coating preservation solution is 1:3-1:

5. After taking it out, let it dry, pack it into a breathable packaging box, and store it under refrigeration.

8. The longan preservative according to claim 7, characterized in that, The refrigeration temperature described in S2 is 3-8 ℃, and the relative humidity is 85%-90%.

9. A method for preparing the longan preservative as described in any one of claims 1-8, characterized in that, Includes the following steps: Step 1: Preparation of pretreatment solution: Take water and sodium metabisulfite according to the weight parts, add sodium metabisulfite to water, stir until completely dissolved, and obtain the pretreatment solution; Step 2: Prepare the composite coating preservation solution: Take water, citric acid, chitosan, amino acids and vitamin C by weight. First, dissolve the citric acid in water, then add the chitosan and stir to disperse it. Then add the amino acids and vitamin C and continue stirring until the mixture is uniform to obtain the composite coating preservation solution.

10. The preparation method according to claim 9, characterized in that, The composite coating preservation liquid in step 2 also contains hydroxytyrosol. The hydroxytyrosol is added at the end of step 2 and stirred evenly to obtain the composite coating preservation liquid. The composite coating preservation liquid is stored away from light after preparation and should be used within 2-6 hours.

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