CN115678061B - Method for preparing antibacterial preservative film by using lychee seed/shell extract and application of antibacterial preservative film - Google Patents
Method for preparing antibacterial preservative film by using lychee seed/shell extract and application of antibacterial preservative film Download PDFInfo
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Classifications
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
- Y02W90/00—Enabling technologies or technologies with a potential or indirect contribution to greenhouse gas [GHG] emissions mitigation
- Y02W90/10—Bio-packaging, e.g. packing containers made from renewable resources or bio-plastics
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- Manufacture Of Macromolecular Shaped Articles (AREA)
Abstract
The invention discloses a method for preparing an antibacterial preservative film by using lychee seed/shell extracts, which adopts lychee seed/shell as a raw material source, and respectively adopts flavone and polysaccharide extracted from lychee seed and starch and flavone extracted from lychee seed as raw materials, and is matched with chitosan, gelatin and glycerin to prepare a composite film. The method is simple and easy to operate, the litchi waste is utilized to develop the composite film, the added value of litchi can be increased, the economic income of fruit farmers can be increased, and compared with the existing composite film, the raw materials used by the method are natural substances extracted from litchi seeds/shells, so that the method is safer and more reliable, has stronger antibacterial property and oxidation resistance, and has better fresh-keeping effect.
Description
Technical Field
The invention relates to the technical field of preservation and corrosion prevention, in particular to a method for preparing an antibacterial preservative film by using a lychee seed/shell extract and application thereof.
Background
At present, the pulp of litchi is processed into products such as dried fruits, fruit juice, cans and the like, and the litchi shells generated in the processing process are also generally thrown away as waste, so that the full utilization value is not exerted, the environment is polluted, and the resources are wasted. Secondly, the pollution condition of plastic type packaging materials to the land is serious at present, and some plastic preservative films have the condition of abusing plasticizers, and the content of the plasticizers exceeds the standard and easily enters the human body along with food, so that the risk of carcinogenesis to the human body is generated. In the prior art, the composite film is used for carrying out fresh-keeping treatment on foods, and the composite film is a porous network structure film formed by crosslinking between two or more film forming material molecules. The composite film can improve the defect of a single film and make the performance of the film in all aspects better, but has limited performance in all aspects.
Disclosure of Invention
Aiming at the defects existing in the prior art, the invention aims to provide a method for preparing an antibacterial preservative film by using a lychee seed/shell extract, so as to solve the problems that the utilization value of lychee seed shells is not fully exerted in the prior art, the resource waste is caused, and the performance of the composite film in the prior art is difficult to achieve.
In order to solve the technical problems, the invention adopts the following technical scheme:
a method for preparing antibacterial preservative film by using lychee seed/shell extract comprises the following steps:
step 1: respectively obtaining extracts from litchi shells and litchi seeds; wherein, the flavone extract and the polysaccharide extract of the litchi shells are obtained from the litchi shells; obtaining lychee seed starch extract and lychee seed flavone extract from lychee seeds;
step 2: mixing chitosan with acetic acid solution with mass fraction of 1% to prepare chitosan solution with certain concentration; wherein, when the litchi shell extract is used as the raw material, the addition amount of chitosan is 1.5 to 3.5 percent according to the mass percent; when the lychee seed extract is used as a raw material, the addition amount of the chitosan is 10 g/L-50 g/L;
step 3: when the litchi shell extract is used as a raw material, gelatin is added into chitosan solution, the gelatin is stirred at 50 ℃ until the gelatin is completely dissolved, then the litchi shell flavone extract and the litchi shell polysaccharide extract are sequentially added, the mixture is stirred uniformly at 50 ℃ to obtain a mixed solution, and glycerin accounting for 2% of the mass of the mixed solution is added into the mixed solution, and the mixed solution is stirred uniformly to obtain a composite membrane solution I; wherein, the addition amount of gelatin is 1.0 to 3.0 percent, the addition amount of litchi shell flavone is 5 to 25 percent, and the addition amount of litchi shell polysaccharide is 5 to 25 percent according to the mass percent;
When the lychee seed extract is used as a raw material, distilled water is added into the lychee seed starch extract, the lychee seed starch extract is uniformly mixed and then is subjected to constant-temperature gelatinization at 90 ℃ for 40min to obtain gelatinized liquid, the gelatinized liquid is cooled to 50 ℃, then the chitosan solution obtained in the step 2 is uniformly mixed with the gelatinized liquid, and the lychee seed flavone extract and glycerol are added and uniformly mixed to obtain a composite membrane liquid II; wherein, the addition amount of the flavone of the lychee seed is 1 to 5 percent according to the mass percent; the adding amount of the lychee seed starch is 40 g/L-80 g/L, and the adding amount of the glycerol is 5 g/L-25 g/L;
step 4: carrying out ultrasonic degassing on the composite film liquid I at 50 ℃ and 100Hz for 30min, carrying out uniform casting and standing, and drying to form a film to obtain the antibacterial preservative film; and (3) filtering the composite film liquid II by a 40-mesh sieve, carrying out uniform casting, and drying to form a film to obtain the antibacterial preservative film.
Compared with the prior art, the invention has the following beneficial effects:
1. the invention adopts lychee seeds/shells as raw material sources, and respectively adopts flavone and polysaccharide extracted from lychee shells and starch and flavone extracted from lychee seeds as raw materials, and is matched with chitosan, gelatin and glycerin to prepare a composite film; the method is simple and easy to operate, the litchi waste is utilized to develop the composite film, the added value of litchi can be increased, the economic income of fruit farmers can be increased, and compared with the existing composite film, the raw materials used by the method are derived from natural substances extracted from litchi seeds/shells, and the method is safer and more reliable.
2. The composite film prepared by the method of the invention has the thickness of 0.145mm, the light transmittance of 54.51 percent, the water solubility of 43.80 percent and the water vapor transmittance of 1.92 multiplied by 10 when the lychee shell extract is selected as the raw material -6 g·m·(m 2 ·s·pa) -1 Tensile strength 9.19MPa and elongation at break 167.34%; compared with a single chitosan-gelatin blend film, the light transmittance is reduced by 13.47%, the water solubility is increased by 6.97%, and the water vapor transmittance is reduced by 2.33×10 -6 g·m·(m 2 ·s·pa) -1 The tensile strength is improved by 6.36MPa, and the elongation at break is improved by 103.77%; when semen litchi extract is selected as raw material, the tensile strength of the composite film is 2.83+ -0.21 MPa, the elongation at break is 96.13+ -4.66%, the film thickness is 0.18mm, the light transmittance is 2.14+ -0.11%, and the water vapor transmittance is 7.1544 + -0.2319 ×10 -5 (g.mm -1 .d -1 ) The solubility is 43.32+/-2.6%; the tensile strength is improved by 0.93MPa compared with a single chitosan film, the breaking tensile rate is improved by about 11%, the light transmittance is reduced by 58%, the water vapor transmittance is reduced by 1.8%, and the solubility is improved by 15%. This shows that the addition of lychee seed/shell extract can significantly improve the mechanical properties of the composite film.
3. The compound film prepared by taking the litchi rind extract as the main raw material is applied to the fresh-keeping of mulberries, the decay rate of the mulberries is 38.2 percent, the weight loss rate of the mulberries is 3.1 percent, and the content of soluble solids is 8.6 percent, compared with the chitosan-gelatin compound film without adding the litchi rind flavone and the litchi rind polysaccharide, the decay rate of the compound film is reduced by 61.8 percent, the weight loss rate is reduced by 14.5 percent, and the content of the soluble solids is increased by 0.8 percent, so that the litchi rind extract compound film can effectively prolong the shelf life of the mulberries.
4. According to the method, the composite film prepared by taking the lychee seed extract as a main raw material is wrapped on the cherry tomatoes for fresh keeping, and the rotting rate and the weight loss rate of the cherry tomatoes wrapped by the composite film within 2-10d of the placement of the cherry tomatoes are lower than those of the cherry tomatoes wrapped by the composite film without adding flavone and the cherry tomatoes of a control group; the content of the soluble solids is higher than that of cherry tomatoes wrapped by the composite film without flavone and cherry tomatoes in a control group; when the cherry tomatoes are placed for 10d, the weight loss rate of the cherry tomatoes wrapped by the lychee seed extract composite film is respectively reduced by 1.06 percent and 3.2 percent compared with the weight loss rate of the cherry tomatoes wrapped by the composite film without flavone and the weight loss rate of the cherry tomatoes in a control group; the decay rate is respectively 3.33 percent and 23.3 percent lower; the content of soluble solids is respectively 0.5 percent and 2.17 percent higher; with significant differences; the litchi seed extract composite film has a fresh-keeping effect on cherry tomatoes, and the fresh-keeping effect is superior to that of a composite film core without flavone and a composite film without litchi seed extract.
Drawings
FIG. 1 is a response surface and contour plot of chitosan and gelatin to tensile strength when litchi rind extract is used as a raw material.
FIG. 2 is a graph of response surface and contour plot of chitosan and flavone to tensile strength when litchi rind extract is used as raw material.
FIG. 3 is a graph of response surface and contour plot of flavone and polysaccharide to tensile strength for litchi rind extract as a raw material.
FIG. 4 is a response surface and contour plot of polysaccharide and gelatin to tensile strength for litchi rind extract as a starting material.
Fig. 5 is a graph showing the effect of different treatment methods on the decay rate of mulberries.
Fig. 6 is a graph showing the effect of different treatment modes on the weight loss rate of mulberries.
Fig. 7 is a graph showing the effect of different treatments on soluble solids of mulberries.
FIG. 8 is a graph showing interaction of the starch addition amount of lychee seed and the chitosan addition amount with the film when the lychee seed extract is used as a raw material.
FIG. 9 is a response surface and contour plot of the amount of flavone added and the amount of lychee seed starch added when lychee seed extract is used as a raw material.
FIG. 10 is a response surface and contour plot of glycerol addition and lychee seed starch addition when lychee seed extract is used as a starting material.
FIG. 11 is a response surface and contour plot of the amount of flavone and the amount of chitosan added when litchi seed extract was used as the starting material.
FIG. 12 is a response surface and contour plot of the amount of flavone and the amount of glycerol added when litchi seed extract was used as the starting material.
Fig. 13 is a graph showing the effect of different treatments on the weight loss rate.
FIG. 14 shows the effect of different treatments on decay rate.
FIG. 15 shows the effect of different treatments on soluble solids.
Detailed Description
The invention will be further described with reference to the drawings and examples.
1. Method for preparing antibacterial preservative film by taking litchi shell extract as raw material
1. Extraction and content determination of total flavonoids in litchi shells
Fully drying litchi shells, crushing, sieving, weighing 5.0g of litchi shell powder, adding 80% ethanol solution according to a feed-liquid ratio of 1:20 (g: mL), leaching at a constant temperature of 78 ℃ for 4 hours, filtering, concentrating filtrate in vacuum at 35 ℃ until the filtrate is dry, and taking 70% ethanol solution to constant volume of 50mL. 1mL of extract of the flavonoids in litchi shells is taken, the absorbance is measured at 510nm to be 0.87, and the flavonoids content is calculated to be 62.4 mug/mL.
2. Extraction and content determination of polysaccharide in litchi shells
The litchi shells are completely dried, crushed and sieved, 5.0g of powder is weighed, water is added according to the feed-liquid ratio of 1:20 (g/mL), the mixture is leached for 3 hours at the constant temperature of 100 ℃, the leaching is carried out, the filtrate is concentrated in vacuum at the temperature of 60 ℃, and the volume is fixed to 100mL by water. Sucking 10mL of polysaccharide extract, adding 40mL of 95% ethanol solution, standing at 4deg.C for 12h, centrifuging at 4900r/min for 10min, washing the precipitate with 95% ethanol solution, and fixing volume with distilled water to 25mL. 1mL of the lychee chitin extract is diluted to 50mL, 1mL of the diluted solution is taken, the absorbance at 490nm is measured to be 0.327, and the polysaccharide content is 257.1 mug/mL through calculation.
3. Preparation of composite membrane based on litchi shell extract
(1) Dissolving: taking 100mL of acetic acid solution with the concentration of 1.0%, carrying out ultrasonic treatment at the temperature of 45 ℃ for 1min at the ultrasonic power of 100W, adding chitosan accounting for a certain mass fraction of the solution after uniform dispersion, and stirring for 50min at the temperature of 50 ℃. After the chitosan is dissolved, adding gelatin accounting for a certain mass fraction of the solution, and stirring for 30min at 50 ℃ until the gelatin is completely dissolved.
(2) Mixing: after the chitosan and the gelatin are completely dissolved, adding the lychee shell flavone extracting solution and the lychee shell polysaccharide extracting solution which account for a certain volume fraction of the solution, and stirring for 30min at 50 ℃ to fully and uniformly mix the membrane solution.
(3) Stirring: after the composite membrane liquid is uniformly mixed, adding glycerol accounting for 2% of the mass fraction of the solution, stirring for 10min, and uniformly mixing.
(4) Degassing: and (3) placing the uniformly mixed composite membrane liquid in 50 ℃ and 100Hz for ultrasonic degassing for 30min.
(5) Casting: 30mL of the membrane solution was measured and poured into a glass petri dish, and the solution was uniformly cast, allowed to stand and cool for 10min.
(6) And (3) drying: and (3) placing the glass culture dish in a constant-temperature blast drying oven at 50 ℃ and drying for 3-4 hours to form a film.
3.1 optimization of method for preparing composite film from litchi rind extract
The chitosan addition amount, the gelatin addition amount, the litchi rind flavone addition amount and the litchi rind polysaccharide addition amount are taken as single factors for research, 5 step levels are designed for each factor, and a single factor experiment is carried out, wherein the fixed chitosan addition amount is 2.0%, the gelatin addition amount is 1.5%, the litchi rind flavone addition amount is 10%, and the glycerin addition amount is 2%. The thickness, light transmittance, water solubility and water vapor transmittance, tensile strength and elongation at break of the composite film were measured by setting 3 single factors as fixed values and the other 1 as variables, and finally comprehensively judging, thereby obtaining the optimal level of each factor. The single-factor experimental design is shown in table 3.
TABLE 3 Table 3
| Horizontal level | Chitosan/% | Gelatin/% | Litchi rind flavone/% | Litchi chitin/% |
| 1 | 1.5 | 1.0 | 5 | 5 |
| 2 | 2.0 | 1.5 | 10 | 10 |
| 3 | 2.5 | 2.0 | 15 | 15 |
| 4 | 3.0 | 2.5 | 20 | 20 |
| 5 | 3.5 | 3.0 | 25 | 25 |
3.2 results and analysis
a. Influence of the added amount of chitosan on the composite film
TABLE 4 Table 4
| Chitosan/% | Thickness/mm | Transmittance/% | Water vapor transmission x 10 -6 /g·m·(m 2 ·s·Pa) -1 | Water solubility/% | Tensile Strength/MPa | Elongation at break/% |
| 0 | 0.055±0.00 f | 85.25±0.77 a | 2.95±0.01 a | 45.07±1.24 b | 1.89±0.06 c | 106.12±1.46 c |
| 1.5 | 0.099±0.00 e | 74.36±0.56 b | 2.53±0.01 b | 46.05±0.40 b | 3.44±0.87 b | 184.28±22.60 ab |
| 2.0 | 0.110±0.00 d | 63.08±0.80 c | 2.30±0.06 c | 48.03±1.18 b | 3.82±0.90 b | 186.41±49.78 ab |
| 2.5 | 0.120±0.00 c | 61.47±0.18 d | 1.56±0.04 e | 52.90±5.06 a | 4.82±0.70 a | 214.17±58.81 a |
| 3.0 | 0.135±0.00 b | 57.24±0.42 e | 2.18±0.04 d | 43.30±3.33 b | 3.69±0.65 b | 184.43±13.65 b |
| 3.5 | 0.144±0.00 a | 40.61±1.22 f | 2.16±0.08 d | 34.02±1.35 c | 3.25±0.66 b | 146.11±23.04 bc |
As is clear from table 4, the addition amount of chitosan was increased, and the viscosity of the composite membrane solution was increased, so that the thickness of the composite membrane was also increased. Since chitosan itself presents pale yellow, the addition amount of chitosan is continuously increased, and the influence of the color of chitosan on the color of the composite film is more and more prominent, so that the light transmittance of the composite film is reduced. The water vapor transmission rate of the composite membrane is reduced, probably because the added amount of chitosan is increased, the molecular weight of chitosan in unit volume in the membrane liquid is increased, and the effect of intermolecular hydrogen bonds is enhanced, so that the molecular interval in the composite membrane is shortened, and the membrane structure is more compact; when the chitosan addition amount is increased to 2.5%, the water vapor transmittance is the lowest (1.56+/-0.04) multiplied by 10 -6 g·m·(m 2 ·s·Pa) -1 The water vapor transmission rate increases again after that because the intermolecular hydrogen bonding effect tends to be saturated, and if chitosan is continuously added, a large amount of hydrophilic groups are added. The added amount of chitosan is increased, and the water solubility of the composite membrane is increased firstly due to the hydrophilicity of the chitosan; when the amount of the additive was increased to 2.5%, the water solubility was (52.90.+ -. 5.06)%, but the water solubility was reduced after that, probably because the surface area of the film of equal mass in water was reduced due to the increase of the film thickness of the composite film, so that the water solubility of the composite film was reduced. The increase of the addition amount of chitosan leads to the increase of the molecular weight in the unit volume of membrane liquid, and the free amino group in the chitosan can be protonated with acetic acid to be combined with the hydroxyl group of the chitosan and the polysaccharide in the solution Hydrogen bonding is performed between the hydroxyl groups, which increases the compactness of the composite membrane structure and increases the tensile strength and the elongation at break of the composite membrane structure; when the chitosan addition amount reaches 2.5%, the excessive hydrogen bond can increase the rigid structure of the composite film, so that the composite film becomes hard and brittle, the fluidity of film liquid is reduced, and the surface of the film is uneven, so that the tensile strength and the elongation at break of the composite film are obviously reduced. When the addition amount of chitosan reaches 2.5%, the tensile strength of the composite film is maximum, namely (4.82+/-0.70) MPa, and the elongation at break reaches the maximum value, namely (214.17 +/-58.81). In combination, the chitosan addition amount of 2.5% is preferably selected.
b. Influence of the gelatin addition on the composite film
TABLE 5
As can be seen from table 5, after gelatin and chitosan are mixed, gelatin absorbs water and swells, secondary bonds in the gelatin structure are broken, and free hydroxyl groups of gelatin form hydrogen bonds with water molecules; the amino group of the chitosan can be connected with the free amino group and hydroxyl group of the gelatin molecule through hydrogen bonds; the free amino group of chitosan forms an ionic bond with the carboxyl terminal of the peptide chain, and then a new viscosity phase is formed in the mixed membrane liquid through the electrostatic interaction between molecules. The thickness of the composite film increases as the amount of gelatin added increases. The gelatin is light yellow, and the increase of the gelatin addition amount can make the influence of the color of the gelatin on the color of the composite film more and more obvious, so that the light transmittance of the composite film is reduced. In the blend membrane, gelatin can crosslink with chitosan molecules through ionic bonds and hydrogen bonds, so that the network structure of the membrane is densified, and water molecules are reduced from passing through the membrane. When the gelatin addition amount was increased to 2.5%, the water vapor transmission rate of the composite film was at least (1.98.+ -. 0.06). Times.10 -6 g·m·(m 2 ·s·Pa) -1 However, gelatin contains a large amount of hydrophilic groups such as amino groups, carboxyl groups and hydroxyl groups, so that the water vapor permeability starts to increase, and the water solubility of the composite membrane is increased; when the gelatin addition amount was increased to 2.5%, the water solubility of the film was (60.24.+ -. 1.46)% at the maximum. The increase of the gelatin addition amount is originally predicted to continuously increase the water solubility of the composite membrane or not to increase the water solubility greatly, but after that, the water solubility of the composite membrane is naturally reduced, and after deep analysis, the fact that the polyelectrolyte complex is formed due to the electrostatic interaction between the amino groups of chitosan molecules and the carboxyl groups of gelatin molecules in the composite membrane is found, a three-dimensional network structure is generated, the compactness of the composite membrane is increased, the composite membrane structure becomes stable, and the water solubility of the composite membrane is reduced. Gelatin has good plasticizing effect, and the tensile strength and the elongation at break of the composite film all tend to increase along with the increase of the added amount of gelatin. When the gelatin content reaches 2.5%, the tensile strength of the composite film is the maximum (5.82+ -0.53) MPa, and the elongation at break is the maximum (245.98 + -28.62)%. However, this later decrease, possibly due to excessive plasticization of the gelatin. In combination, the gelatin addition amount is preferably 2.5%.
c. Influence of addition of flavone in litchi shells on composite film
TABLE 6
As is clear from table 6, the flavone molecules had hydrophobicity, resulting in poor compatibility with hydrophilic chitosan, and the addition of flavone made the cross-sectional structure of the composite film rough and uneven, so as the amount of flavone added increased, the thickness of the composite film increased and the water solubility decreased. The color of the flavone is darker, and the composite film turns yellow and darker after the flavone is added, so that the light transmittance is reduced. The flavone is only detrimental to the performance of the composite membrane, because the flavone and the chitosan have poor compatibility, but intermolecular hydrogen bonds are formed between the chitosan and the flavone, which can reduce the water resistance of the composite membraneThe affinity of the steam, and the addition of the flavone can make the network structure of the film more compact, thereby preventing the water steam from passing through; when the addition amount of flavone is 15%, the water vapor transmittance of the composite membrane is the lowest (1.86+/-0.05) multiplied by 10 -6 g·m·(m 2 ·s·Pa) -1 The water vapor transmission rate then increases again, probably because the crystals of the flavones again create smaller water vapor channels in the composite membrane matrix. After the litchi rind flavone is added, the tensile strength of the composite film tends to be increased firstly, probably because a hydrogen bond is formed between chitosan and flavone molecules, and the compactness of the composite film structure is improved. The elongation at break of the composite membrane shows an increasing trend, probably because crystals are generated in the flavone molecules, molecular polymerization reaction is reduced, and the flexibility of the composite membrane is improved. When the amount of flavone added was 15%, the tensile strength of the composite film reached a maximum value of (5.65.+ -. 0.48) MPa, and at this time, the elongation at break of the film was the highest, which was (138.27.+ -. 13.03)%, indicating that the tensile strength and ductility of the composite film were good under this condition. The tensile strength and elongation at break of the composite film are reduced, probably because the hydrophobicity of the flavone molecules breaks the regularity of the chitosan molecules, weakens the hydrogen bonding action among the chitosan molecules, and hinders the interaction among the polymer chains. Compared with a control group without flavone, after 15% of the flavone in the litchi shells is added, the tensile strength is increased by 2.62MPa, and the elongation at break is increased by 62.1% to the maximum, which indicates that the mechanical property of the composite film is improved after the flavone in the litchi shells is added. In combination, the optimum level of addition of 15% of flavone was selected.
d. Influence of addition amount of litchi chitin on composite film
TABLE 7
| Polysaccharide/% | Thickness/mm | Transmittance/% | Water vapor transmission x 10 -6 /g·m·(m 2 ·s·Pa) -1 | Water solubility/% | Tensile Strength/MPa | Elongation at break/% |
| 0 | 0.117±0.00 f | 68.89±0.46 a | 3.85±0.05 a | 26.18±0.48 b | 2.62±0.33 e | 59.40±1.90 f |
| 5 | 0.126±0.00 e | 65.78±1.34 b | 3.18±0.05 b | 31.60±0.47 ab | 3.25±0.80 d | 107.64±2.32 c |
| 10 | 0.133±0.00 d | 60.17±0.70 c | 2.64±0.04 c | 32.27±1.83 ab | 3.56±0.69 cd | 119.77±3.14 b |
| 15 | 0.145±0.00 c | 54.51±0.41 d | 1.92±0.04 e | 43.80±0.43 a | 5.56±0.11 a | 133.89±9.35 a |
| 20 | 0.155±0.00 b | 47.72±0.55 e | 2.15±0.05 d | 38.27±0.11 ab | 4.81±0.61 b | 96.15±5.06 d |
| 25 | 0.164±0.00 a | 41.09±0.31 f | 2.07±0.06 d | 36.20±1.16 ab | 4.05±0.26 c | 83.32±6.41 e |
As is clear from Table 7, the hydroxyl groups and carboxyl groups in the polysaccharide molecule are bonded with water molecules to form hydrogen bonds, and the polysaccharideAn increase in the amount of addition results in a decrease in the moisture content of the composite film and an increase in the thickness. The polysaccharide turns yellow brown under the condition of heat, so that the transparency of the composite film can be reduced; secondly, the less the components of the composite film are, the more uniform and compact the film structure is, the smoother the surface is, and the fewer micropores and cracks exist, so that the composite film has higher light transmittance and better transparency. However, as the addition amount of the polysaccharide is continuously increased, the particle sizes of different macromolecules are different, so that the sedimentation rates in the film forming process are different, the microstructure of the film is changed, the surface of the film is roughened, and the film scatters or reflects when meeting light, so that the light transmittance of the composite film is reduced. The polysaccharide has a plurality of hydrophilic groups and has hydrogen bond action with chitosan molecules, so that the membrane structure becomes compact, and the water vapor can be effectively prevented from passing through; when the polysaccharide addition amount reaches 15%, the water vapor transmittance of the composite membrane is the lowest, and is (1.92+/-0.04) multiplied by 10 -6 /g·m·(m 2 ·s·Pa) -1 The water vapor transmission rate then again tends to increase, probably because the hydrogen bonding between polysaccharide and chitosan reaches saturation. The increase of the addition amount of the polysaccharide can introduce more hydrophilic groups into the membrane liquid, so that the water solubility of the composite membrane is increased; when the amount of the polysaccharide added was 15%, the water solubility of the membrane was at most (43.80.+ -. 0.43)%, and when the amount of the polysaccharide added was further increased, the water solubility was decreased, and it was possible that the force between chitosan and polysaccharide was increased, and the membrane structure became compact. The addition of the polysaccharide leads the tensile strength and the elongation at break of the composite membrane to be increased and then decreased. The increase of the two is probably due to the strong hydrogen bonding action formed between the protonated amino groups of the chitosan and the hydroxyl groups of the polysaccharide and the hydroxyl groups of the chitosan, and the continuity and compactness of the internal structure of the membrane are improved. When the amount of the polysaccharide added was 25%, the tensile strength of the composite film reached a maximum value of (5.56.+ -. 0.11) MPa, and at this time, the elongation at break was also the highest, which was (133.89.+ -. 9.35)%, indicating that the composite film had good overall properties under the conditions. Then, as the amount of the polysaccharide added increases, the water content of the composite film decreases, and the composite film becomes hard and thick, and chitosan and polysaccharide molecules are intertwined with each other and interact with each other, so that the fluidity of the molecules is weakened. And is combined with Compared with a control group without adding polysaccharide, the tensile strength of the composite film is increased by 2.94MPa, and the elongation at break is increased by 74.49% after adding 15% of the lychee chitin, which indicates that the mechanical property of the composite film is improved after adding the lychee chitin, so that the adding amount of the polysaccharide is selected to be 15% at the optimal level.
e. Response surface test design and results
Based on the result of a single factor experiment, the original prediction of the addition amount of chitosan and the addition amount of gelatin, the addition amount of flavone in litchi shells and the addition amount of litchi shells has great influence on the performance of the composite film, but the verification of a response surface experiment is needed. The method comprises the steps of selecting chitosan addition amount, gelatin addition amount, litchi shell flavone addition amount and litchi shell polysaccharide addition amount as response factors, taking tensile strength as a response value, which is a main measurement index affecting the use characteristics of the composite film, and determining the optimal process formula condition of the composite film through a Design Expert 8.0 software Design response surface test. The response surface design level, arrangement and results are shown in tables 8 and 9.
TABLE 8
| Horizontal level | A chitosan/% | B gelatin/% | C litchi rind flavone/% | D lychee chitin/% |
| -1 | 2.0 | 2.0 | 10 | 10 |
| 0 | 2.5 | 2.5 | 15 | 15 |
| 1 | 3.0 | 3.0 | 20 | 20 |
TABLE 9
Table 10
Note that: * The differences were very significant (p < 0.01); * The difference was significant (p < 0.05).
Analysis of table 10 by Design-Expert 8.0 software gave tensile strength vs. A: chitosan addition amount B: gelatin addition amount C: litchi rind flavone addition amount D: regression quadratic equation between the addition of litchi chitin:
tensile strength=8.56+1.39a+1.34b+0.40c+0.30d+0.28ab-0.3AC-0.24AD+0.018BC-0.35BD-0.34CD-1.35A 2 -1.55B 2 -2.06C 2 -1.90D 2 。
From the analysis of variance table, the significance test p of the model<0.0001, mismatching term p value 0.8115>0.05, insignificant, the regression equation of the formula optimization experiment has small error and high fitting degree; determining the coefficient R 2 Since = 0.9924, the effect of the added amount of chitosan, gelatin, litchi rind flavone, and litchi rind polysaccharide on the tensile strength of the composite film can be better illustrated. The comprehensive steps are as follows: the model has good fitting degree with the actual situation, and can be used for predicting the change situation of the tensile strength of the composite film.
As can be seen from comparison of the values of F, the four factors of the test all have a very significant effect on the tensile strength of the composite film (p<0.01 With an order of influence A>B>C>D, namely: chitosan>Gelatin>Flavone>Polysaccharide. A is that 2 、B 2 、C 2 、D 2 The effect on the tensile strength of the composite film was all very pronounced (p<0.01 A) is provided; in the interaction terms, the interaction terms BD were all very remarkable in the effect on the tensile strength of the composite film (p <0.01 A) is provided; the influence of the interaction terms AB, AC, CD on the tensile strength of the composite film was all significant (p<0.05 Other interactive items have no significant effect.
f. Tensile strength response surface analysis and determination of optimal process conditions
In order to intuitively reflect the influence of the four factors of chitosan addition, gelatin addition, litchi rinflavone addition and interaction on the tensile strength of the response value, two factors are fixed at a zero level, and a Model Graph program is applied to serve as a response surface diagram of interaction of the chitosan addition, gelatin addition, litchi rinflavone addition and litchi rinflavone addition, and the results are shown in figures 1, 2, 3 and 4.
As can be seen from fig. 1, the AB term interaction is significant. When the gelatin addition amount is fixed, the tensile strength of the composite film is increased and then reduced by increasing the chitosan addition amount, and when the chitosan addition amount is 2.75%, the tensile strength of the composite film is the maximum. The addition amount of chitosan is fixed, the increase of the addition amount of gelatin leads the tensile strength of the composite film to be increased and then reduced, and when the addition amount of gelatin is 2.71%, the tensile strength of the composite film is the maximum.
As can be seen from fig. 2, the contour lines appear elliptical, and AC item interactions are significant. When the addition amount of the flavone is fixed, the increase of the addition amount of the chitosan leads the tensile strength of the composite film to be increased and then decreased. The addition amount of chitosan is fixed, the increase of the addition amount of flavone leads the tensile strength of the composite film to be increased and then reduced, and when the addition amount of flavone is 15.25%, the tensile strength of the composite film is the maximum. Therefore, the tensile strength of the composite film can be improved by properly controlling the addition amount of chitosan and the addition amount of flavone.
As can be seen from fig. 3, the contour lines are elliptical, indicating significant BD item interaction. The addition amount of the fixed gelatin is unchanged, the increase of the addition amount of the polysaccharide leads the tensile strength of the composite film to be increased and then reduced, and when the addition amount of the polysaccharide is 14.97%, the tensile strength of the composite film is the maximum. When the addition amount of the polysaccharide is fixed, the increase of the addition amount of the gelatin leads to the increase and then decrease of the tensile strength of the composite film. The tensile strength of the composite film can be improved by appropriately controlling the addition amount of gelatin and the addition amount of polysaccharide.
As can be seen from fig. 4, the contour lines appear elliptical, indicating significant CD item interactions. When the addition amount of the flavone is fixed, the increase of the addition amount of the polysaccharide leads to the increase and then decrease of the tensile strength of the composite film. When the addition amount of the polysaccharide is fixed, the increase of the addition amount of the flavone leads to the increase and then decrease of the tensile strength of the composite film. Therefore, the tensile strength of the composite film can be improved by properly controlling the addition amount of the flavone and the addition amount of the polysaccharide.
According to the tensile strength of the composite film, the optimal theoretical process formula for obtaining the composite film is 2.75% of chitosan, 2.71% of gelatin, 15.25% of litchi rind flavone, 14.97% of litchi rind polysaccharide, and the theoretical tensile strength of the composite film is 9.26MPa under the process formula. The improved process formula according to the actual experimental conditions comprises the following steps: the addition amount of chitosan is 2.8%, the addition amount of gelatin is 2.7%, the addition amount of flavone in litchi shells is 15.2%, and the addition amount of chitosan in litchi shells is 15.0%, and experiments are carried out according to the process formula, so that the obtained tensile strength is 9.04Mpa, 9.28Mpa and 9.23Mpa, and the average tensile strength is 9.19Mpa, and therefore, the data of the optimization experiment are proved to have certain reliability.
4. Verification of fresh-keeping effect
The skin of the mulberry is damaged, so that the mulberry is more susceptible to bacterial and fungal infection, and fruit rot occurs. According to FIG. 5, the decay rate of the three treatments showed an increasing trend after the storage period of 5d, but the decay rate of the mulberries without film wrapping was highest, reaching (100.+ -. 0.00)%, and the decay rate increased faster. The decay rate of the mulberries wrapped by the composite film without the litchi shell extract is (72+/-0.45)%, and the decay rate is increased slowly compared with that of the blank group in the previous 3d, mainly the chitosan has a certain antibacterial capability, so that the mulberries can be protected from fungal infection, and the fruit decay is reduced. The decay rate of the mulberries wrapped by the composite film added with the litchi rind flavone and the litchi rind polysaccharide is (38+/-0.53)%, and the decay rate is increased more slowly than that of the other two groups, which shows that after the litchi rind flavone and the litchi rind polysaccharide are added into the composite film, the antibacterial capability of the composite film is improved, and the decay rate of the mulberries can be obviously reduced.
The weight loss of the mulberries is mainly caused by the respiration of the mulberries and the migration of water in the fruits. According to FIG. 6, after the storage period of 5d, the fruits of the three treatments showed continuous weight loss, but the weight loss rate of mulberries without film wrapping was highest, reaching (17.55.+ -. 0.70)%, and the weight loss rate was faster. The weight loss rate of the mulberries wrapped by the composite film without the litchi shell extract is (8.5+/-0.11)% after 5 days, and the increase rate of the weight loss rate is slower than that of a blank group, mainly because chitosan in the composite film has hydrophilicity and interacts with water molecules, the flow of water vapor inside the mulberries is better maintained, and the water evaporation is prevented from being too fast. The weight loss rate of the mulberries wrapped by the composite film added with the litchi rind flavone and the litchi rind polysaccharide is (3.1+/-0.08)%, and the increase rate of the weight loss rate is slower than that of the other two groups, which indicates that after the litchi rind flavone and the litchi rind polysaccharide are added into the composite film, the compactness of the composite film structure is improved through the interaction between the litchi rind flavone and the chitosan and gelatin molecules, and oxygen, moisture and carbon dioxide between fruits and the film are better remained in the film, so that the weight loss rate of the mulberries is reduced.
The sweetness and mouthfeel of fruit are related to the soluble solids content of the fruit. The soluble solids content is reduced because mulberry needs to maintain respiration by decomposing its own sucrose. According to fig. 7, it is shown that the soluble solids content decreases with the increase of the storage time, the soluble solids content of the blank group mulberries without any treatment after 5d is (7.8.+ -. 0.15)%, and the decrease rate is faster. The soluble solid content of the mulberry wrapped by the composite film without adding the lychee shell extract is (8.2+/-0.10)%, the descending rate of the soluble solid content is slower than that of a blank group, the soluble solid content of the mulberry wrapped by the composite film with adding the lychee shell flavone and the lychee shell polysaccharide is (8.6+/-0.06)%, and the descending rate of the soluble solid content is slower than that of other two groups, so that the addition of the lychee shell flavone and the lychee shell polysaccharide can better maintain the soluble solid content in the mulberry.
Therefore, the compound film prepared by taking the litchi rind extract as the raw material is applied to the fresh-keeping of mulberries, the decay rate of the mulberries is 38.2 percent, the weight loss rate of the mulberries is 3.1 percent, and the content of soluble solids is 8.6 percent, compared with the chitosan-gelatin compound film without adding the litchi rind flavone and the litchi rind polysaccharide, the decay rate is reduced by 61.8 percent, the weight loss rate is reduced by 14.5 percent, and the content of the soluble solids is increased by 0.8 percent, which indicates that the shelf life of the mulberries can be prolonged.
2. Method for preparing antibacterial preservative film by taking lychee seed extract as raw material
1. Extraction and content determination of starch in lychee seeds
Weighing about 10g of lychee seed powder according to a feed liquid ratio of 1:20 (g: mL) adding water, extracting with ultrasonic assistance for 20min, sieving with 120 mesh sieve, repeating extraction for 3 times, and extracting the solution, centrifuging the filtrate (4000 r/min,15 min), discarding supernatant, and oven drying the precipitate at 40deg.C to obtain semen litchi crude starch. The starch content in the lychee seed crude starch was 61.2369% as measured by acid hydrolysis.
2. Extraction and content determination of flavone in lychee seed
Weighing about 20g of dried and constant-weight lychee seed powder in an conical flask, and mixing the lychee seed powder with 50% ethanol in a liquid ratio of 1:10 (g: mL) adding, setting the temperature of the water bath kettle to 80 ℃, heating and refluxing for 1h at the temperature, extracting for 2 times, filtering, combining the extracting solutions, concentrating the extracting solutions, transferring the concentrated solution into a 100mL volumetric flask, and fixing the volume to a scale to obtain a coarse extracting solution of the flavonoid. The crude extract of flavone was diluted 25-fold, the absorbance value of the diluted solution was measured at 510nm, and then the concentration of flavone in the concentrate was calculated from the absorbance value, with the calculation result being 879.96. Mu.g/mL.
3. Method for preparing composite film from lychee seed extract
Mixing a certain amount of chitosan with 1% acetic acid solution, stirring with a constant temperature magnetic stirrer until the chitosan is dissolved, and preparing a chitosan solution with a certain concentration for later use; then adding a certain amount of distilled water into a beaker, uniformly mixing, placing the mixture into a constant-temperature water bath kettle with the temperature of 90 ℃ for gelatinization for 40min, taking out gelatinized liquid, cooling the gelatinized liquid to 50 ℃, taking 50mL of chitosan solution and starch gelatinized liquid respectively, uniformly mixing, adding a certain amount of lychee seed flavone extract and glycerol, stirring the mixed liquid on a constant-temperature magnetic stirrer for 1h, filtering by a 40-mesh sieve, sucking 20mL of filtrate, adding into a culture dish with the diameter of 90mm, drying in a blast drying oven with the temperature of 50 ℃ for 3h, and uncovering the membrane.
3.1 optimization of method for preparing composite film from semen litchi extract
Determining the basic formula of the lychee seed extract composite film. According to the weight ratio of 40g/L, 50g/L, 60g/L, 70g/L and 80g/L of lychee seed starch; the addition amount of chitosan is 10g/L, 20g/L, 30g/L, 40g/L and 50g/L; the addition amount of flavone is 1%, 2%, 3%, 4% and 5%; glycerol is added into the mixture at 5g/L, 10g/L, 15g/L, 20g/L and 25g/L; and designing 5 step levels for each factor, then performing a single factor test, wherein 3 factors are set to be fixed values, and the other 1 factors are set to be variables, performing film performance measurement after film formation, and screening out the optimal film. The single factor test is shown in Table 11.
TABLE 11
| Horizontal level | A lychee seed starch addition (g/L) | Chitosan addition (g/L) | C flavone addition (%) | Glycerol addition (g/L) |
| 1 | 40 | 10 | 1 | 5 |
| 2 | 50 | 20 | 2 | 10 |
| 3 | 60 | 30 | 3 | 15 |
| 4 | 70 | 40 | 4 | 20 |
| 5 | 80 | 50 | 5 | 25 |
3.2 results and analysis
a. Influence of the starch addition of lychee seed on film Properties
The addition amount of the lychee seed starch is changed into: blank (0 g/L), 40g/L, 50g/L, 60g/L, 70g/L, 80 g/L) and researching the influence of lychee seed starch on film performance. The results are shown in Table 12:
table 12
In this case, the starch addition is used as an independent variable; since the mechanical properties of the film reflect the ability of the food packaging film to remain intact when stretched during handling, processing and storage and to reflect the quality of the film properties, the mechanical properties of the film are chosen as a function of which the tensile strength and elongation at break are a part. Originally, the larger the addition amount of lychee seed starch is, the mechanical property of the film should be increased, and the film reaches equilibrium under a certain concentration, but experimental data and the predicted result are different, as can be seen from table 12: with the increase of the starch addition amount of the lychee seeds, the tensile strength of the film is in direct proportion to the starch addition amount; the breaking elongation is increased and then reduced, when the addition amount of starch is 60g/L, the maximum value of the breaking elongation, and when the addition amount of lychee seed starch exceeds 60g/L, the breaking elongation is gradually reduced; the tensile strength reaches the maximum at the starch addition of 80g/L and the value is 3.52 plus or minus 0.33MPa, but the elongation at break of the film is only 57.4 plus or minus 4.9 at the lowest 5%; the elongation at break is more proper when the starch addition amount is 40g/L, but the tensile strength is the lowest; this is probably because as the amount of starch added gradually increases, the number of starch molecules also gradually increases, resulting in a denser network structure, resulting in an increase in tensile strength of the film, but such a dense structure results in a decrease in molecular mobility; the litchi seed starch is added, and meanwhile, the higher the content of the litchi seed starch is, the poorer the toughness of the film is, the stronger the rigidity is, so the lower the elongation at break is. Therefore, the tensile strength and the elongation at break of the film are suitable only when the starch addition amount is 60g/L, the mechanical property of the film is good, and the tensile strength and the elongation at break of the film are obvious compared with the blank (p<0.05 Higher than the blank, the starch addition was chosen to be 60g/L for the optimization experiment. Meanwhile, when the starch addition amount is 60g/L, the light transmittance of the film is lower and is 2.33+/-1%, and the film can better prevent food oxidation caused by ultraviolet and visible light, so that the effect of preserving fruits and vegetables is achieved; the water vapor permeability is the lowest and is (8.84+/-0.31) multiplied by 10 -5 (g.mm -1 .d -1 ) Because the water vapor transmittance is low, when the adding amount of the lychee seed starch is 60g/L, the barrier capability of the film to outside water vapor is good, and the growth and propagation of bacteria can be slowed down, so that the effect of prolonging the fresh-keeping period of fruits and vegetables is achieved; the solubility is highest at the moment, and the value is 41.66+/-1.9%, which shows that the composite film is easy to dissolve and degrade when meeting water, and can achieve the purposes of keeping fruits and vegetables fresh and less environmental pollution, so that the composite film has better comprehensive performance when the adding amount of lychee seed starch is 60 g/L.
b. Influence of the Chitosan addition on the Membrane Performance
The addition amount of chitosan is changed as follows: blank (0 g/L), 10g/L, 20g/L, 30g/L, 40g/L, 50g/L, and study on influence of lychee seed starch on film performance. The results are shown in Table 13:
TABLE 13
As can be seen from table 13: added by chitosanWhen the amount of chitosan added is increased, the tensile strength of the film tends to increase and then decrease, and the elongation at break of the film also tends to increase and then decrease, which is probably due to-OH contained in lychee seed starch and NH on chitosan chains 3 + The charged amino groups can promote the polarization of other chains caused by the formation of hydrogen bonds, so that the attractive force between the chains is increased, the tensile strength of the film is gradually increased, and the maximum adding amount of chitosan is 30 g/L; when charged NH 3 + When the added amount of chitosan increases, the repulsive force between the same kind of charges increases, so that the arrangement between membrane molecules becomes difficult, the acting force between hydrogen bonds weakens, the compactness of the thin film gradually decreases, a blending system is difficult to form, and the tensile strength of the membrane gradually decreases. However, since chitosan also has a plasticizing effect, the tensile property of the film is improved, and interaction exists between the plasticizer and the polymer, so that the overall fluidity and flexibility of a molecular chain are increased, the elongation at break of the film is gradually increased, the elongation at break of the film is maximized when the addition amount of chitosan reaches 30g/L, and the elongation at break of the film is reduced with the increase of the addition amount of chitosan when the addition amount of chitosan exceeds 30 g/L; the reduced elongation at break may be caused by the fact that too much chitosan increases the crystallinity between lychee seed starches. Therefore, when the chitosan addition amount was 30g/L, the tensile strength of the film was the highest, the elongation at break was the highest, and the tensile strength was significant as compared with the blank (p <0.05 Higher than the blank group, the optimization experiment was performed when the chitosan addition amount was selected to be 30g/L according to the mechanical properties of the film. Meanwhile, when the addition amount of chitosan is 30g/L, the light transmittance of the film is 2.34+/-0.12%, and the light transmittance at the moment is higher than that when the addition amount of chitosan is 50g/L, but is obviously lower than that of a blank group, so that the light-shading property of the film can be improved by adding a certain amount of chitosan into the starch-based film, and the oxidation of foods can be delayed to achieve a fresh-keeping effect; the water vapor permeability was (8.45.+ -. 0.64). Times.10 -5 (g.mm -1 .d -1 ) The water vapor transmittance of the film was significantly lower than that of the film when the chitosan addition amount was 20g/L and 40g/L, indicating that the film was formed at this timeThe film has stronger barrier capability to water vapor, and can prolong the shelf life of fruits and vegetables; the solubility was 41.43.+ -. 3.11% higher than the other experimental groups and significantly higher than the blank group, indicating better film solubility at this time. Therefore, when the addition amount of the chitosan is 30g/L, the composite membrane has better comprehensive performance.
c. Influence of the addition of the flavone to the film properties
The addition amount of the flavone in the lychee seeds is changed into: blank (0%), 1%, 2%, 3%, 4%, 5%, and study of the influence of lychee seed starch on film properties. The results are shown in Table 14:
TABLE 14
As can be seen from table 14: the study is carried out by taking the addition amount of the flavone as an independent variable, the tensile strength is gradually reduced, the elongation at break is gradually increased along with the continuous increase of the addition amount of the flavone, when the addition amount of the flavone is 1%, the tensile strength of the film is highest and reaches 3.15+/-0.22 MPa, but the elongation at break is lowest and only 96.95+/-7.71%; when the addition amount of the flavone is 5%, the elongation at break of the film is up to 125.93 +/-8.72%, but the tensile strength of the film is lowest and is only 1.43+/-0.07 MPa, which is probably because the addition of the flavone damages the compactness of the film, so that the tensile strength of the film is reduced; however, since the flavone can form crystals and reduce polymerization reaction, toughness of the film is increased, resulting in an increase in elongation at break of the film. When the amount of flavone added was 2%, the tensile strength and elongation at break of the film were both suitable, and both were significant compared with the blank (p<0.05 Higher than the blank group, so that the addition of flavone was 2% was selected for the optimization experiment. Meanwhile, the light transmittance of the film gradually decreases along with the increase of the addition amount of the flavone, probably because the coarse extract of the flavone has color, the light transmittance of the film gradually decreases, and the light transmittance of the film is lower than that of a blank group when the addition amount is 2 percent, namely 2.36+/-0.1 percent, and is obviously lower than that of the blank group. The water vapor permeability was the lowest at 2% of the added flavone amount and was (8.40.+ -. 0.22). Times.10 -5 (g.mm -1 .d -1 ) Display deviceCompared with blank groups and other experimental groups, the proper addition amount of the flavone can improve the water vapor transmittance of the film and prevent the spoilage of fruits and vegetables; the solubility was 41.81.+ -. 1.16% at the highest, significantly higher than in the blank and other experimental groups, where the film solubility was the highest. Therefore, when the addition amount of the flavone is 2%, the film has better comprehensive performance.
d. Effect of glycerol addition on film Properties
The glycerol addition was changed to: blank (0 g/L), 5g/L, 10g/L, 15g/L, 20g/L, 25 g/L) and study on influence of lychee seed starch on film performance. The results are shown in Table 15:
TABLE 15
As can be seen from table 15: the study is carried out by taking the glycerol addition amount as an independent variable, and the tensile strength of the film is gradually reduced and the elongation at break is gradually increased along with the increase of the glycerol addition amount; the tensile strength is maximum when the glycerol addition amount is 5g/L, the value is 3.29+/-0.19 MPa, the elongation at break is minimum, and the value is 76+/-5.26%; the tensile strength is lowest when the glycerol addition amount is 25g/L, the value is 1.17+/-0.07 MPa, the elongation at break is highest, and the value is 120.3+/-8.53%; this is probably because the addition of glycerol increases the hydrogen bond bonding sites between starch molecules, gradually starts to form hydrogen bonds, thereby improving the fluidity of starch molecules and increasing the elongation at break of the film; however, the compactness of the network structure is affected due to the reduction of the hydrogen bond bonding points among starch molecules, and the acting force among the molecules is poor, so that the tensile strength of the film is low. When the glycerol addition amount is 15g/L, the tensile strength of the film is 2.57+/-0.15 MPa, the elongation at break of the film is 104.09 +/-6.59%, and the tensile strength and the elongation at break are both proper and higher than those of a blank group. Therefore, the amount of glycerol added was 15g/L for the optimization experiment. Meanwhile, when the glycerol addition amount is 15g/L, the light transmittance of the film is 2.35+/-0.08 percent, which is obviously lower than that of a blank group, and the addition of the glycerol can improve the light transmittance of the film; the water vapor permeability was (8.44.+ -. 0.28). Times.10 -5 (g.mm -1 .d -1 ) Although the addition amount of the glycerol is 5g/L higher than that of the glycerol, the glycerol is obviously lower than that of a blank group, which indicates that the glycerol can play a certain role in improving the water vapor transmittance of the membrane; the solubility was 42.58.+ -. 2.61%, which was lower than the case where the amount of glycerol added was 25g/L, but higher than that of the blank group, indicating that glycerol had a certain effect in improving the solubility of the film. Therefore, the composite membrane has better comprehensive performance when the glycerol addition amount is 15 g/L.
e. Composite membrane response surface analysis and optimal process condition determination for litchi seed extract preparation
Based on the analysis of the single-factor experimental results, the addition amount of lychee seed starch and the addition amount of chitosan may have the greatest influence, while the influence of the addition amount of flavone and the addition amount of glycerol may be relatively small, but still the response surface test is required to verify. And selecting factors with larger influence as response factors: the packing bags with high tensile strength are not easy to damage in the storage and transportation processes, so that fruits and vegetables can be kept fresh for a long time under the condition of long transportation time, and the response surface design experiment is carried out by taking the tensile strength as a response value. And (3) adopting software to design a response surface, and determining the optimal formula of the lychee seed extract composite film.
Through experimental data analysis, the tensile strength, the breaking rate and A: lychee seed starch addition, B: chitosan addition amount, C: flavone addition amount, D: regression quadratic equation between glycerol addition:
tensile strength = 2.42+0.21a-0.041B-0.11C-0.30D-0.16AB-0.23AC-0.12AD-0.13bc +0.002.002bd +0.09cd-0.074A 2 -0.39B 2 +0.034C 2 +0.022D 2 ；
Thus, A, C, D has extremely remarkable effect on the lychee seed extract composite film (p<0.01 With influencing order D>A>C>B, namely: glycerol addition>Litchi seed starch addition>Addition amount of flavone>Chitosan addition amount, wherein A 2 、B 2 、C 2 Has extremely remarkable tensile strength (p<0.01 D) influence of D 2 Has remarkable tensile strength to film(p<0.05 A) the influence of the (b); the interaction item AB, AC, AD, BC, CD has extremely remarkable influence on the tensile strength of the lychee seed composite film (p<0.01 Other interactive items have no significant effect.
In order to intuitively reflect the influence of four factors of the addition of lychee seed starch, the addition of chitosan, the addition of flavone and the addition of glycerol and interaction on the tensile strength of a response value, two factors are fixed at a zero level, and a Model Graph program is applied to a response surface diagram of interaction of the addition of lychee seed starch and the addition of chitosan, the addition of lychee seed starch and the addition of flavone, the addition of lychee seed starch and glycerol, the addition of flavone and the addition of chitosan, the addition of flavone and the addition of glycerol, as shown in figures 8, 9, 10, 11 and 12.
As can be seen from fig. 8, the contour lines are elliptical, indicating significant AB interactions. The addition amount of lychee seed starch and the addition amount of chitosan are increased, and the tensile strength is increased and then decreased. When the addition amount of chitosan is fixed, the tensile strength gradually rises with the increase of the addition amount of lychee seed starch. When the adding amount of the lychee seed starch is fixed, the tensile strength is firstly increased and then decreased along with the increase of the adding amount of the chitosan. Therefore, the proper addition of the lychee seed starch and the chitosan can improve the tensile strength.
As can be seen from fig. 9, the contour is elliptical, indicating significant AC interactions. The addition amount of lychee seed starch and the addition amount of flavone are increased, and the tensile strength is increased and then decreased. When the addition amount of flavone is fixed, the tensile strength gradually rises with the increase of the addition amount of lychee seed starch. When the adding amount of the lychee seed starch is fixed, the tensile strength gradually decreases along with the increase of the adding amount of the flavone. Therefore, the addition of proper flavone and litchi seed starch can improve the tensile strength.
As can be seen from fig. 10, the contour lines are elliptical, indicating significant AD interactions. The glycerol addition amount and the lychee seed starch addition amount are increased, and the tensile strength is increased and then decreased. When the glycerol addition amount is fixed, the tensile strength is increased along with the increase of the lychee seed starch addition amount. When the adding amount of the lychee seed starch is fixed, the tensile strength is reduced along with the increase of the adding amount of the glycerol. Therefore, the proper glycerol addition amount and the lychee seed starch addition amount can improve the tensile strength.
As can be seen from fig. 11, the contour lines are elliptical, indicating significant BC interactions. The addition amount of the litchi flavone and the addition amount of the chitosan are increased, and the tensile strength is increased first and then decreased. When the addition amount of chitosan is fixed, the tensile strength gradually decreases with the increase of the addition amount of the litchi flavone. When the addition amount of the litchi flavone is fixed, the tensile strength is firstly increased and then decreased along with the increase of the addition amount of the chitosan. So that the addition of the flavone and the chitosan of the litchi can improve the tensile strength.
As can be seen from fig. 12, the CD interaction is illustrated to be remarkable. The addition amount of the litchi flavone and the addition amount of the glycerol are increased, and the tensile strength is gradually reduced. When the glycerol addition amount is fixed, the tensile strength gradually decreases with the increase of the litchi flavone addition amount. When the addition amount of the litchi flavone is fixed, the tensile strength gradually decreases along with the increase of the addition amount of the glycerol. Therefore, the addition of the litchi flavone and the glycerol can improve the tensile strength.
The optimal film formula obtained according to the design response surface is as follows: the addition amount of lychee seed starch is 70g/L, the addition amount of chitosan is 29.08g/L, the addition amount of flavone is 1.0%, and the addition amount of glycerol is 10g/L. The theoretical tensile strength of the composite film under this process condition was 3.45852MPa. The improved technological parameters according to the experimental conditions are as follows: the addition amount of lychee seed starch is 70g/L, the addition amount of chitosan is 29g/L, the addition amount of lychee flavone is 1.0%, the addition amount of glycerin is 10g/L, 3 parallel experiments are carried out according to the process, the tensile strength of the obtained film is 3.40+/-0.28 MPa, the breaking elongation is 86.13 +/-4.66%, the film thickness is 0.18+/-0.00 mm, the light transmittance is 2.14+/-0.11%, and the water vapor transmittance is 7.1544 +/-0.2319 ×10 -5 (g.mm -1 .d -1 ) The solubility was 43.32.+ -. 2.6%, and the reliability of the optimized data was found.
4. Fresh-keeping effect
Selecting a plurality of fresh and mature cherry tomatoes, dividing the cherry tomatoes into A, B, C and 3 groups on average, numbering the cherry tomatoes, wherein the numbers are A1-A50, B1-B50 and C1-C50, the A group is a control group, and the experimental group comprises: group B (using the best formula composite film), group C (using the composite film without adding litchi flavone and with the same addition amount of other substances as that of group B) tests all cherry tomatoes at room temperature, samples are taken every 3d, and the weight loss rate, the rotting rate and the soluble solids of the cherry tomatoes are measured.
After cherry tomato picking, because there is still vigorous respiration and transpiration, it still causes loss of water and weight, as can be seen from fig. 13: the weight loss rate of cherry tomatoes increases along with the increase of storage time, and the cherry tomatoes can show a remarkable rising trend. In the whole, the weight loss rate of the control group is higher than that of the group which uses the composite film for fresh-keeping, wherein the control group comprises the composite film group which uses the optimal formula and the composite film group which uses the lychee flavone without adding. Compared with the fresh-keeping group, the weight loss rate of the cherry tomatoes which are fresh-keeping by using the composite film without adding the litchi flavone is higher than that of the cherry tomatoes which are fresh-keeping by using the composite film with the optimal formula; the litchi flavone has an antioxidation effect and plays a role in inhibiting the respiration of cherry tomatoes, so that the dehydration speed of other vacuoles is slowed down, and the fresh-keeping effect is better. The control group showed an increase in the weight loss rate from 4d, and the control group showed a weight loss rate of 17.69.+ -. 0.27% at 10 d. The weight loss rate of cherry tomatoes preserved by using the optimal formula at the 10 th d is 14.49 +/-0.0026 percent, compared with a control group, the weight loss rate is obviously reduced (p is less than 0.05), which indicates that the composite film prepared by the optimal formula has a certain preservation effect on cherry tomatoes; the cherry tomatoes are preserved by using the film which is not added with flavone, the weight loss rate of the cherry tomatoes is 15.54+/-0.0016%, which is lower than that of a control group and higher than that of the cherry tomatoes preserved by using the composite film with the optimal formula. The method shows that the antistaling property of the composite film prepared by not adding the litchi flavone is lower than that of the composite film prepared by the optimal formula from the aspect of the weight loss rate of cherry tomatoes, and the antistaling property of the composite film prepared by the optimal film formula is the best.
Is susceptible to mechanical compression and mold infection during transportation and storage, as can be seen in fig. 14: the rotting rate of the cherry tomatoes in the 2-4d is 0% in both the control group and the experimental group, and the rotting rate of the cherry tomatoes which are preserved by the composite film prepared by the optimal formula is lower than that of the cherry tomatoes in the control group and lower than that of the cherry tomatoes which are preserved by the film without flavone. The fresh-keeping effect of the composite film prepared by using the optimal film formula is better than that of the composite film prepared without adding flavone from the aspect of the decay rate index. When the storage time is 10d, the rotting rate of the cherry tomatoes in the control group is 0.57+/-0.06%, the rotting rate of the cherry tomatoes under the preservation of the composite film which is prepared by not adding flavone is 0.37+/-0.06%, and the rotting rate of the cherry tomatoes under the preservation of the film which uses the optimal formula is 0.30+/-0.00%, wherein the rotting rate of the blank group is obviously (p < 0.05) higher than that of the experimental group. From the perspective of decay rate, the composite film prepared by the optimal formula has a certain fresh-keeping effect on cherry tomatoes.
The content of the soluble solids can directly reflect the maturity, quality and the like of the fruits. As can be seen from fig. 15: cherry tomatoes which are preserved in different ways have a decreasing trend of soluble solids with increasing storage time, and the decrease of the content of the soluble solids can be related to the self respiration and other physiological metabolism of the cherry tomatoes. The content of soluble solids in cherry tomatoes in the experimental group in 2-10d is higher than that in the control group as a whole. When the cherry tomatoes are stored for 10d, the content of the soluble solids of the cherry tomatoes in the control group is 2.43%, the soluble solids of the cherry tomatoes preserved by the composite film prepared by using the optimal formula is 4.6%, the soluble solids of the cherry tomatoes preserved by using the composite film prepared without adding the litchi flavone is 4.1%, and the soluble solids of the cherry tomatoes preserved by using the composite film are all obviously (p < 0.05) higher than those of the control group, so that the cherry tomatoes can be effectively slowed down by using the composite film, and the cherry tomatoes have a certain preservation effect.
The litchi seed extract composite film and the composite film without litchi flavone are wrapped on cherry tomatoes for fresh keeping, and the rotting rate, the weightlessness rate and the soluble solid content of the cherry tomatoes are measured after 0d, 2d, 4d, 6d, 8d and 10d respectively. The results show that: in the whole, the rotting rate and the weight loss rate of the cherry tomatoes wrapped by the optimal film within 2-10d of the cherry tomatoes are lower than those of the cherry tomatoes wrapped by the composite film without flavone and the cherry tomatoes in the control group; the content of the soluble solids is higher than that of cherry tomatoes wrapped by the composite film without flavone and cherry tomatoes in a control group. When the cherry tomatoes are placed for 10d, the weight loss rate of the cherry tomatoes wrapped by the lychee seed extract composite film is respectively reduced by 1.06 percent and 3.2 percent compared with the weight loss rate of the cherry tomatoes wrapped by the composite film without flavone and the weight loss rate of the cherry tomatoes in a control group; the decay rate is respectively 3.33 percent and 23.3 percent lower; the content of soluble solids is respectively 0.5 percent and 2.17 percent higher; with significant differences. Therefore, the litchi seed extract composite film has a fresh-keeping effect on cherry tomatoes, and the fresh-keeping effect is better than that of the composite film without flavone.
Finally, it should be noted that the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the technical solution, and those skilled in the art should understand that modifications and equivalents may be made to the technical solution of the present invention without departing from the spirit and scope of the present invention, and all such modifications and equivalents are included in the scope of the claims.
Claims (6)
1. The method for preparing the antibacterial preservative film by using the lychee seed/shell extract is characterized by comprising the following steps of:
step 1: respectively obtaining extracts from litchi shells and litchi seeds; wherein, the flavone extract and the polysaccharide extract of the litchi shells are obtained from the litchi shells; obtaining lychee seed starch extract and lychee seed flavone extract from lychee seeds;
step 2: mixing chitosan with acetic acid solution with mass fraction of 1% to prepare chitosan solution with certain concentration;
step 3: when the litchi shell extract is used as a raw material, gelatin is added into chitosan solution, the gelatin is stirred at 50 ℃ until the gelatin is completely dissolved, then the litchi shell flavone extract and the litchi shell polysaccharide extract are sequentially added, the mixture is stirred uniformly at 50 ℃ to obtain a mixed solution, and glycerin accounting for 2% of the mass of the mixed solution is added into the mixed solution, and the mixed solution is stirred uniformly to obtain a composite membrane solution I; wherein, the addition amount of chitosan is 2.8 percent, the addition amount of gelatin is 2.7 percent, the addition amount of flavone in litchi shells is 15.2 percent, and the addition amount of chitosan is 15.0 percent according to the mass percentage; the influence sequence of each component on the tensile strength of the preservative film of the litchi shell extract is that the addition of chitosan is greater than that of gelatin, and the addition of litchi shell flavone is greater than that of litchi shell polysaccharide;
When the lychee seed extract is used as a raw material, distilled water is added into the lychee seed starch extract, the lychee seed starch extract is uniformly mixed and then is subjected to constant-temperature gelatinization at 90 ℃ for 40min to obtain gelatinized liquid, the gelatinized liquid is cooled to 50 ℃, then the chitosan solution obtained in the step 2 is uniformly mixed with the gelatinized liquid, and the lychee seed flavone extract and glycerol are added and uniformly mixed to obtain a composite membrane liquid II; wherein, the adding amount of lychee seed starch is 70 g/L, the adding amount of chitosan is 29 g/L, and the adding amount of glycerol is 10 g/L; the addition amount of the flavone in the lychee seeds is 1.0 percent according to the mass percentage; the effect sequence of each component on the tensile strength of the preservative film of the lychee seed extract is that the glycerol addition amount is greater than the lychee seed starch addition amount, the lychee seed flavone addition amount is greater than the chitosan addition amount;
step 4: carrying out ultrasonic degassing on the composite film liquid I at 50 ℃ and 100Hz for 30min, carrying out uniform casting and standing, and drying to form a film to obtain the antibacterial preservative film; and (3) filtering the composite film liquid II by a 40-mesh sieve, carrying out uniform casting, and drying to form a film to obtain the antibacterial preservative film.
2. The method for preparing an antibacterial preservative film by using lychee seed/shell extracts according to claim 1, wherein in step 1, the lychee seed flavone extract is obtained by the following method:
Fully drying litchi shells, crushing and sieving, wherein the mass volume ratio is 1g:20mL, adding litchi rind powder into 80% ethanol solution, leaching at a constant temperature of 78 ℃ for 4h, filtering, concentrating the filtrate in vacuum until the filtrate is dry, and dissolving the filtrate in 70% ethanol solution; wherein the flavone content in the flavone extract of the litchi shells is 62.4 mug/mL.
3. The method for preparing an antibacterial preservative film by using lychee seed/shell extracts according to claim 1, wherein in step 1, lychee chitin extract is obtained by the following method:
completely drying litchi shells, crushing, sieving, and mixing 1g of litchi shells according to the mass to volume ratio: 20mL of litchi rind powder is added into water, leaching is carried out for 3h at the constant temperature of 100 ℃, the filtrate after suction filtration is concentrated to dryness in vacuum and then is dissolved in water, the solution is added into ethanol solution with the concentration of 95 percent, the solution is stood for 12h at the temperature of 4 ℃, centrifugal separation is carried out, sediment is washed by ethanol solution with the concentration of 95 percent, and distilled water is used for constant volume to obtain litchi rind polysaccharide extract; wherein the polysaccharide content in the lychee chitin extract is 257.1 mug/mL.
4. The method for preparing an antibacterial preservative film using lychee seed/shell extract according to claim 1, wherein in step 1, the lychee seed starch extract is obtained by the following method:
Pulverizing lychee seeds into powder according to the mass-volume ratio of 1g: dissolving semen litchi in water for 20mL, performing ultrasonic extraction, sieving with 120 mesh sieve, repeating extraction for 3 times, mixing extractive solutions, centrifuging, collecting precipitate, and oven drying at 40deg.C to obtain semen litchi starch extract; wherein the starch content is 61% -62%.
5. The method for preparing an antibacterial preservative film by using lychee seed/shell extracts according to claim 1, wherein in step 1, the lychee seed flavone extract is obtained by the following method:
pulverizing lychee seeds into powder, and drying the powder according to the volume-mass ratio of 1g:10mL of the extract is mixed with 50% ethanol solution, the mixture is refluxed for 1h at a constant temperature of 80 ℃ for 2 times of extraction, the extract is combined after filtration, and the extract is concentrated and the volume is fixed to obtain the extract of the flavone of the lychee seed; wherein, the concentration of flavone in the lychee seed flavone extract is 879.96 mug/mL.
6. The application of the litchi seed/shell extract to preparation of the antibacterial preservative film is characterized in that the antibacterial preservative film prepared by the method of claim 1 is used for preserving fruits.
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