Chitosan slightly acidic electrolyzed water and preparation method and application thereof

Abstract

The present invention provides a chitosan slightly acidic electrolyzed water and a preparation method and application thereof, and relates to the technical field of food preservation. The chitosan slightly acidic electrolyzed water provided by the present invention comprises slightly acidic electrolyzed water and chitosan dissolved in the slightly acidic electrolyzed water; the pH value of the chitosan slightly acidic electrolyzed water is 5.85-6.15. Edible fungi are treated with the chitosan slightly acidic electrolyzed water of the present invention, and the acidic electrolyzed water can efficiently remove microorganisms on the surface of the edible fungi. At the same time, after electrolysis, chitosan will form a protective film on the surface of the edible fungi, blocking the invasion of other microorganisms and reducing the loss of the flavor of the edible fungi, extending the storage period of the edible fungi, and preventing them from oxidative deterioration, thereby overcoming the problems caused by using electrolyzed water alone.

Description

Technical Field

[0001] The invention relates to the technical field of food preservation, and in particular to chitosan slightly acidic electrolyzed water and a preparation method and application thereof. Background Art

[0002] China is a veritable producer of edible fungi. After being harvested, edible fungi still have high physiological activities and respiratory rates, which can lead to rapid aging and deterioration, such as water loss and browning, resulting in loss of edible value and overall decay. The short shelf life of fresh edible fungi is an obstacle to distribution and marketing.

[0003] Electrolyzed water (EW) is a solution produced by mixing an electrolyte containing chlorine (usually sodium chlorite (NaCl) and / or hydrochloric acid (HCl)) with tap water through an electrolytic cell. When the electrolyte is electrolyzed in an electrolytic cell without a diaphragm, slightly acidic electrolyzed water (SAEW) with a pH of 5.0 to 6.5 is produced. SAEW has been shown to have the potential to prevent spoilage and inhibit pathogens in fresh produce, but it can also damage the surface of vegetables, introduce or spread microorganisms (especially when the water is reused), and accelerate the deterioration of edible fungi.

[0004] How to extend the post-harvest storage time while ensuring the quality of edible fungi is a technical problem that needs to be solved urgently. Summary of the invention

[0005] The purpose of the present invention is to provide a chitosan slightly acidic electrolyzed water and a preparation method and application thereof, which can prolong the post-harvest storage time of edible fungi while ensuring their quality.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The invention provides a chitosan slightly acidic electrolyzed water, comprising slightly acidic electrolyzed water and chitosan dissolved in the slightly acidic electrolyzed water; the pH value of the chitosan slightly acidic electrolyzed water is 5.85-6.15.

[0008] Preferably, the concentration of chitosan in the chitosan slightly acidic electrolyzed water is 1 to 2.5 g / L.

[0009] The present invention provides a method for preparing the chitosan slightly acidic electrolyzed water described in the above scheme, comprising the following steps:

[0010] mixing chitosan, water and glacial acetic acid to obtain a chitosan solution;

[0011] Mixing sodium chloride, concentrated hydrochloric acid and water to obtain a chloride-containing electrolyte solution;

[0012] mixing the chitosan solution with a chloride-containing electrolyte solution to obtain an electrolyte;

[0013] The electrolyte is electrolyzed to obtain the chitosan slightly acidic electrolyzed water.

[0014] Preferably, the concentration of the glacial acetic acid is 17.5 mol / L; the volume ratio of water to glacial acetic acid is 200:(1-2).

[0015] Preferably, the concentration of the chitosan solution is 6-10 g / L.

[0016] Preferably, the concentration of sodium chloride in the chlorine-containing electrolyte solution is 2.5-4 g / L, the content of concentrated hydrochloric acid is 260-350 μL / L; and the concentration of the concentrated hydrochloric acid is 12 mol / L.

[0017] Preferably, the volume ratio of the chitosan solution to the chloride-containing electrolyte solution is 1:3-5.

[0018] Preferably, the voltage of the electrolysis is 30V.

[0019] Preferably, the electrolysis time is 20 to 30 minutes.

[0020] The present invention provides application of the slightly acidic electrolyzed chitosan water described in the above scheme or the slightly acidic electrolyzed chitosan water prepared by the preparation method described in the above scheme in the storage of edible fungi.

[0021] The invention provides a chitosan slightly acidic electrolyzed water, comprising slightly acidic electrolyzed water and chitosan dissolved in the slightly acidic electrolyzed water; the pH value of the chitosan slightly acidic electrolyzed water is 5.85-6.15.

[0022] The chitosan slightly acidic electrolyzed water of the present invention is used to treat edible fungi. The acidic electrolyzed water can efficiently remove microorganisms on the surface of the edible fungi. At the same time, after electrolysis, the chitosan forms a protective film on the surface of the edible fungi, which blocks the invasion of other microorganisms and reduces the loss of the flavor of the edible fungi, prolongs the storage period of the edible fungi, and prevents them from oxidative deterioration, thereby overcoming the problems caused by using electrolyzed water alone. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Transmission electron microscopy for CS-SAEW;

[0024] Figure 2 Effects of storage at 4°C for 15 days on (A) weight loss, (B) firmness, (C) color parameters brightness (L*), total color change (ΔE), and browning index (BI) of Flammulina velutipes;

[0025] Figure 3 The changes of microorganisms on the surface of Flammulina velutipes stored at 4℃ for 15 days;

[0026] Figure 4The changes of cellulose (A), chitin (B) and cellulase activity (C) in Flammulina velutipes stored at 4°C for 15 days. DETAILED DESCRIPTION

[0027] The invention provides a chitosan slightly acidic electrolyzed water, comprising slightly acidic electrolyzed water and chitosan dissolved in the slightly acidic electrolyzed water; the pH value of the chitosan slightly acidic electrolyzed water is 5.85-6.15.

[0028] In the present invention, the pH value of the chitosan slightly acidic electrolyzed water is preferably 5.9-6.1, more preferably 6.0. In the present invention, the concentration of chitosan in the chitosan slightly acidic electrolyzed water is preferably 1-2.5 g / L, more preferably 1.5-2 g / L.

[0029] The chitosan slightly acidic electrolyzed water of the present invention is used to treat edible fungi. The acidic electrolyzed water can efficiently remove microorganisms on the surface of the edible fungi. At the same time, after electrolysis, the chitosan forms a protective film on the surface of the edible fungi, which blocks the invasion of other microorganisms and reduces the loss of the flavor of the edible fungi, prolongs the storage period of the edible fungi, and prevents them from oxidative deterioration, thereby overcoming the problems caused by using electrolyzed water alone.

[0030] The present invention provides a method for preparing the chitosan slightly acidic electrolyzed water described in the above scheme, comprising the following steps:

[0031] mixing chitosan, water and glacial acetic acid to obtain a chitosan solution;

[0032] Mixing sodium chloride, concentrated hydrochloric acid and water to obtain a chloride-containing electrolyte solution;

[0033] mixing the chitosan solution with a chloride-containing electrolyte solution to obtain an electrolyte;

[0034] The electrolyte is electrolyzed to obtain the chitosan slightly acidic electrolyzed water.

[0035] In the present invention, unless otherwise specified, the raw materials used are commercially available products well known in the art.

[0036] The invention mixes chitosan, water and glacial acetic acid to obtain a chitosan solution.

[0037] In the present invention, the water is preferably deionized water; the concentration of the glacial acetic acid is preferably 17.5 mol / L, and the volume ratio of the water to the glacial acetic acid is preferably 100: 1. In the present invention, the amount of chitosan is preferably such that the concentration of the chitosan solution is 6 to 10 g / L, more preferably 7 to 8 g / L, and further preferably 7.5 g / L.

[0038] In the present invention, the mixing preferably includes: adding chitosan to water, maintaining the temperature at 37°C for 30 minutes and stirring continuously, and adding glacial acetic acid in 5 times during the stirring process to dissolve the chitosan to obtain a chitosan solution. In the present invention, the amount of glacial acetic acid added each time is preferably equal, and the interval between two adjacent additions of glacial acetic acid is 6 minutes. The present invention reduces experimental errors and prevents chitosan from being degraded due to temperature increase by controlling the mixing temperature to 37°C.

[0039] The invention mixes sodium chloride, concentrated hydrochloric acid and water to obtain a chlorine-containing electrolyte solution.

[0040] The present invention has no special requirements for the mixing, and any method known in the art that can uniformly mix sodium chloride, concentrated hydrochloric acid and water can be used.

[0041] In the present invention, the concentration of the concentrated hydrochloric acid is preferably 12 mol / L. In the present invention, the amount of the concentrated hydrochloric acid preferably satisfies that the content of the concentrated hydrochloric acid in the aluminum-containing electrolyte solution is 260 to 350 μL / L, more preferably 280 to 320 μL / L, and most preferably 290 μL / L; the amount of the sodium chloride preferably satisfies that the concentration of the sodium chloride in the chlorine-containing electrolyte solution is 2.5 to 4 g / L, more preferably 3 g / L.

[0042] After obtaining the chitosan solution and the chlorine-containing electrolyte solution, the present invention mixes the chitosan solution with the chlorine-containing electrolyte solution to obtain an electrolyte.

[0043] In the present invention, the volume ratio of the chitosan solution to the chloride-containing electrolyte solution is preferably 1:4.

[0044] After obtaining the electrolyte, the present invention electrolyzes the electrolyte to obtain the chitosan slightly acidic electrolyzed water.

[0045] In the present invention, the voltage of the electrolysis is preferably 30V, and the time is preferably 30min. In the present invention, the electrolysis is preferably performed in a membraneless generator. The present invention utilizes electrolysis to obtain acidic electrolyzed water, and chitosan is dissolved in the acidic electrolyzed water due to the presence of hydrochloric acid to form chitosan slightly acidic electrolyzed water.

[0046] The present invention provides the application of the slightly acidic electrolyzed chitosan water described in the above scheme or the slightly acidic electrolyzed chitosan water prepared by the preparation method described in the above scheme in the storage of edible fungi. The present invention has no special requirements for the application method, and the edible fungi to be stored can be directly immersed in the slightly acidic electrolyzed chitosan water at room temperature, or the slightly acidic electrolyzed chitosan water can be sprayed on the surface of the edible fungi.

[0047] The chitosan slightly acidic electrolyzed water provided by the present invention and its preparation method and application are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0048] Example 1

[0049] Preparation of chitosan solution: accurately weigh 1.5 g chitosan, add 200 mL deionized water, stir continuously at 37 °C for 30 min to fully disperse the chitosan powder, add 17.5 mol / L glacial acetic acid in 5 times to dissolve the chitosan, i.e., add 0.3 mL every 6 min to obtain a chitosan solution;

[0050] Mix NaCl, concentrated hydrochloric acid (concentration of 12 mol / L) and tap water to obtain a chlorine-containing electrolyte solution containing 3 g / L NaCl and 290 μL / L concentrated hydrochloric acid;

[0051] The chloride-containing electrolyte solution and the chitosan solution are mixed in a volume ratio of 4:1 to obtain an electrolyte solution;

[0052] The electrolyte was electrolyzed for 30 min at a voltage of 30 V using a laboratory-made membraneless generator to generate chitosan slightly acidic electrolyzed water with a pH value of 6.0±0.15, which was recorded as CS-SAEW.

[0053] Comparative Example 1

[0054] The chitosan solution of Example 1 is denoted as CS.

[0055] Comparative Example 2

[0056] The same laboratory-made membraneless generator was used. The addition amount of NaCl (3 g / L) and tap water (1 L) was fixed. By changing the addition amount of hydrochloric acid, SAEW with the same available chlorine focus (ACC) as CS-SAEW was generated under the conditions of 30 V voltage and 30 min of electrolysis.

[0057] Physicochemical property evaluation:

[0058] The pH and ORP values ​​of tap water, 7.5 g / LCS, SAEW, and CS-SAEW were measured using a dual-scale pH / ORP meter (PHS-3E; Shanghai Scientific Instruments, Shanghai, China). The ACC was determined using a digital chlorine detection system (YXL-1A, RC-2Z; Shanghai Electronic Technology Co., Ltd., Shanghai, China). The results are shown in Table 1.

[0059] Table 1 Physical and chemical parameters of CS-SAEW, SAEW and CS

[0060]

[0061] It can be seen from Table 1 that the present invention prepares slightly acidic electrolyzed water that meets the requirements.

[0062] Structural performance evaluation:

[0063] The CS-SAEW obtained in Example 1 was analyzed using a transmission electron microscope (TEM) (HT7700, HITACHI, Japan). The results showed that chitosan was uniformly dissolved in the electrolyzed water, with a sparse distribution but a uniform distribution ( Figure 1 ).

[0064] Edible mushroom storage

[0065] The enoki mushrooms used in this study were harvested from local farmers. After being picked, the enoki mushrooms were immediately transported to the laboratory and then stored in a dark environment at 4°C and 80% relative humidity. During the test, the enoki mushrooms were taken out of the dark environment and immersed in the chitosan slightly acidic electrolyzed water of Example 1, the chitosan solution of Comparative Example 1, and the SAEW of Comparative Example 2 for 3 minutes, respectively, and the following performance tests were performed:

[0066] (1) Quality Assessment

[0067] The changes in the hardness, weight loss rate and color of Flammulina velutipes reflect the preservation ability of CS-SAEW on Flammulina velutipes. The test method is as follows:

[0068] The penetration tester (TA-XT2, UK) was used to measure the penetration of a 2 mm diameter plunger at 2 mm·S -1 The force required to penetrate the surface of the mushroom cap at a rate of 1.5 Å was used to determine the hardness of the mushroom cap (N).

[0069] After storage at 4°C for 15 days, the weight loss was determined by dividing the difference between the initial and final weights by the initial weight.

[0070] The surface color of the edible mushroom cap was measured using digital imaging and software analysis (CS6 Photoshop) under controlled conditions (same lighting, distance between camera and sample, camera angle, and light source). The ideal mushroom color values ​​L*=97, a*=-2, and b*=0 were compared using ΔE, as shown in formula (1):

[0071] ΔE=[(L-97) 2 +(a-(-2)) 2 +b 2 ] 0.5 Formula (1)

[0072] Wherein, ΔE represents the degree of overall color change compared with the color value of the ideal mushroom; L* represents the (light / dark) value, a* represents the (red / green) value, and b* represents the (yellow / blue) value.

[0073] The browning index (BI) represents the purity of brown color and is calculated according to the following formulas (2) and (3):

[0074] x=(a+1.75L*) / 5.645L*+a*-3.012b*) Formula (2)

[0075] BI=[100(x-0.31)] / 0.172 Formula (3)

[0076] Test results see Figure 2 , Figure 2 The corresponding specific data are shown in Tables 2 to 6.

[0077] Table 2 Changes in weight loss rate with storage days (%)

[0078] 5d 10d 15d control (blank control group) 2.1 3.8 5.2 SAEW 1.8 3.6 5.9 CS 0.9 1.9 3.5 CS-SAEW 0.5 0.9 1.3

[0079] Table 3 Hardness changes with storage days (N)

[0080] 0d 5d 10d 15d control 18.056 17.56 15.896 14.598 SAEW 18.10 17.335 15.324 13.446 CS 18.078 17.452 16.551 15.606 CS-SAEW 18.065 17.669 17.054 16.596

[0081] Table 4 Changes of ΔE value with storage days

[0082]

[0083]

[0084] Table 5 Changes of BI value with storage days

[0085] 0d 5d 10d 15d control 1.2 9.8 21.9 32.6 SAEW 1.3 11.3 24.6 38.9 CS 1 8.4 12.6 25.7 CS-SAEW 1 5.9 12.7 18.4

[0086] Table 6 Changes of L* value with storage days

[0087] 0d 5d 10d 15d control 83 78 69 63 SAEW 83 77 65 60 CS 83 80 75 68 CS-SAEW 83 81 78 75

[0088] Depend on Figure 2 As shown in Figure A and Table 1, all samples lost weight during the 15 days of storage. The SAEW group showed a weight loss of 5.9%, which was the highest. The weight of the control, CS, and CS-SAEW groups lost 5.2%, 3.5%, and 1.3%, respectively. Obviously, the CS-SAEW group had the lowest weight loss. The higher weight loss was attributed to the unprotected thin epidermal structure of the enoki mushrooms, which led to rapid dehydration of the surface.

[0089] Depend on Figure 2 As shown in Figure B and Table 2, during storage at 4 °C for 15 days, the initial value of the control and treated samples was 18.0 ± 0.1 N, and a decrease in hardness was observed for all treatments during refrigeration. At the end of the storage period, the SAEW group had the fastest loss of hardness, about 25.3%, while the softening of the Flammulina velutipes in the blank control group, CS group, and CS-SAEW group was 18.9%, 13.3%, and 7.8%, respectively.

[0090] Figure 2 Tables C and 4 to 6 show the changes in the color parameters brightness (L*), total color change (ΔE), and browning index (BI) of the CS-SAEW group on the long-term preservation ability of Flammulina velutipes. The L* parameter is a sign of the blackening of mushrooms. There was no difference in the initial L* value of Flammulina velutipes, but the positive effect of CS-SAEW was observed after 16 days. The L* value of the control decreased from 83 to 63 within 15 days, while the L* value of the CS-SAEW group was the highest (about 75.3), that is, they generally looked whiter than other samples after 16 days. During storage, the ΔE value of the CS-SAEW group increased the least compared with the control group. The BI value of Flammulina velutipes on the 15th day in the SAEW group was the highest, which was 38.9±0.5, and the CS-SAEW group was the lowest, which was 18.4±0.2.

[0091] (2) Microbiological analysis

[0092] All samples were analyzed for Pseudomonas, yeast, and Candida albicans counts. 25 g of mushrooms were aseptically removed from each package and placed in 225 mL of 0.1% peptone water. The samples were homogenized at high speed for 2 minutes. 100 μL of the homogenate was taken and the number of viable bacteria was determined by the live bacteria culture counting method. To avoid too many colonies growing on the plate, a series of 10-fold dilutions were made and plates with colony counts between 30 and 300 were selected for counting. The results are shown in Figure 3 and Table 7.

[0093] Table 7 Changes in colony counts with storage days (log 10 cfu / g)

[0094]

[0095] Depend on Figure 3 As shown in Table 7, the initial bacterial colony counts of Pseudomonas, yeast, and Candida albicans on the surface of Flammulina velutipes were 5.40, 4.35, and 3.53 log, respectively. 10 cfu / g. After 15 days of refrigeration, the number of microorganisms in the CS-SAEW treatment was significantly reduced, and the reduction was more effective against yeast and mold counts. In contrast, an increase in the microbial population was observed in the control group and SAEW group of Flammulina velutipes. CS-SAEW has excellent microbial removal ability and can effectively reduce food spoilage microorganisms, foodborne pathogens, spoilage and mold-producing fungi.

[0096] (3) Cellulose content, chitin and cellulase activity

[0097] The cellulose content of edible fungi was determined by the acid washing method. Flammulina velutipes was frozen in liquid nitrogen, ground into fine powder in a ball mill and kept frozen (-80°C) until further use. 1 g of dry sample was added to 70 mL of pre-cooled 60% (v / v) H2SO4, digested in an ice bath for 30 min, then diluted with 30 mL of 60% H2SO4 solution, shaken and filtered. Take 2 mL of the diluted filtrate, add 2 mL of 2% (v / v) anthrone-ethyl acetate solution, slowly add 5 mL of 98% (v / v) H2SO4 along the wall, let stand in the dark for 10 min, and measure the absorbance at 530 nm. Use 60% (v / v) H2SO4 instead of the filtrate as a blank control;

[0098] Chitin content was determined by alcohol insoluble residue (AIR). 100 mg of frozen Flammulina velutipes powder was washed twice in anhydrous ethanol at 70 °C and then homogenized in 50 mM Tris–HCl (pH 7.2) containing 1% (W / V) SDS detergent for 30 min. The homogenate was collected, washed with a large amount of water, ethanol, and acetone in sequence, and freeze-dried. After digestion with α-amylase at room temperature, it was freeze-dried again to obtain a sample rich in chitin (alcohol insoluble residue, AIR). AIR (25 mg) was added to a 25 mL volumetric flask, mixed with 5 mL 2 mol / L HCl and 15 mL 98% (v / v) H2SO4, boiled for 30 min, cooled, and finally added with 25 mL deionized water to obtain a hydrolysis solution. Subsequently, 1 mL of the hydrolysis solution was added to a 10 mL volumetric flask and mixed with 1 mL dihydroxybenzene (2%) and 7.5 mL 75% (v / v) H2SO4. The supernatant was then boiled for 30 min, cooled and 10 mL of deionized water was added. Finally, the absorbance of the supernatant was measured at 500 nm using a Multiskan SkyHigh full-wavelength microplate reader (Thermo Fisher Scientific (China) Co., Ltd.).

[0099] Cellulase activity was determined by freezing Flammulina velutipes in liquid nitrogen, grinding it into fine powder in a ball mill and extracting the crude enzyme in 10 mL of a solution (1 mM EDTA-Na, 5% polyvinylpyrrolidone (w / v) and 0.2 M sodium acetate buffer (pH 5.0)) at 4°C and centrifugation at 10,000 g for 20 min. A solution containing 2.0 mL of 1% (w / v) carboxymethyl cellulose and 0.5 mL of crude enzyme was reacted at 50°C for 30 min, and the amount of reducing sugar released was determined by the dinitrosalicylic acid method.

[0100] Cellulose content is an important marker of aging. The cellulose content of each group first increased to a peak and then decreased, but the cellulose content was higher than the initial content. In the three groups, the cellulose content reached a peak on the 10th day. On the 15th day, the cellulose content of the CS-SAEW group was 2.18 mg / g, which was significantly lower than that of the other groups (P<0.05)( Figure 4 Middle A). The results showed that CS-SAEW reduced the accumulation and metabolic rate of cellulose in the cell wall of Flammulina velutipes, thereby delaying the senescence of Flammulina velutipes.

[0101] During storage, the chitosan content showed an increasing trend. After 15 days, the chitosan content in the control group, SAEW group, CS group, and CS-SAEW group was 15.6, 16.5, 13.4, and 8.9 mg / g, respectively. Therefore, the chitosan content in the CS-SAEW group was significantly lower than that in the other groups ( Figure 4 Middle B). CS-SAEW delayed postharvest senescence of edible fungi, reduced water loss during storage, and maintained low fibrosis in tissues.

[0102] During the entire storage period, the cellulase activity in the CS-SAEW group was significantly lower than that in the other groups (P<0.05) ( Figure 4 C). This indicates that CS-SAEW can effectively inhibit the aging of edible fungi by reducing cellulose activity, delaying the softening of edible fungi and maintaining the appearance quality.

[0103] In summary, the film produced by the CS-SAEW prepared in the present invention not only has the ability to resist microbial invasion, but also delays the aging and quality decline of Flammulina velutipes by reducing moisture, gas transfer and respiration. CS-SAEW solves the problem of food preservation and storage, and has revolutionary significance for the development of food industries such as edible fungi.

[0104] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. Application of slightly acidic electrolyzed chitosan water in edible fungus storage; the slightly acidic electrolyzed chitosan water comprises slightly acidic electrolyzed water and chitosan dissolved in the slightly acidic electrolyzed water; the pH value of the slightly acidic electrolyzed chitosan water is 5.85-6.15; The method for preparing the chitosan slightly acidic electrolyzed water comprises the following steps: mixing chitosan, water and glacial acetic acid to obtain a chitosan solution; Mixing sodium chloride, concentrated hydrochloric acid and water to obtain a chloride-containing electrolyte solution; mixing the chitosan solution with a chloride-containing electrolyte solution to obtain an electrolyte; Electrolyzing the electrolyte to obtain the chitosan slightly acidic electrolyzed water; The concentration of chitosan in the chitosan slightly acidic electrolyzed water is 1-2.5 g / L; The voltage of the electrolysis is 30V; The electrolysis time is 20 to 30 minutes.

2. The use according to claim 1, characterized in that: The concentration of the glacial acetic acid is 17.5 mol / L; the volume ratio of the water to the glacial acetic acid is 200:(1-2).

3. The use according to claim 1, characterized in that: The concentration of the chitosan solution is 6-10 g / L.

4. The use according to claim 1, characterized in that: The concentration of sodium chloride in the chlorine-containing electrolyte solution is 2.5-4 g / L, the content of concentrated hydrochloric acid is 260-350 μL / L; the concentration of the concentrated hydrochloric acid is 12 mol / L.

5. The use according to any one of claims 1 to 4, characterized in that: The volume ratio of the chitosan solution to the chloride-containing electrolyte solution is 1:3-5.

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