CN115678061A - Method for preparing antibacterial preservative film by utilizing litchi seed/shell extract and application - Google Patents
Method for preparing antibacterial preservative film by utilizing litchi seed/shell extract and application Download PDFInfo
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Abstract
The invention discloses a method for preparing an antibacterial preservative film by utilizing a lychee seed/shell extract, which adopts lychee seed/shell as a raw material source, and takes flavone and polysaccharide extracted from lychee shell and starch and flavone extracted from lychee seed as raw materials, and is matched with chitosan, gelatin and glycerol to prepare a composite film. The method is simple and easy to operate, the litchi waste is used for developing the composite membrane, the added value of litchi can be increased, the economic income of fruit growers can be increased, and compared with the existing composite membrane, the litchi composite membrane has the advantages that the used raw materials are natural substances extracted from litchi kernels/shells, the litchi composite membrane is safer and more reliable, the antibacterial property and the oxidation resistance are higher, and the fresh-keeping effect is better.
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 utilizing a lychee seed/shell extract and application thereof.
Background
At present, the pulp of the litchi is processed into dried fruit, juice, cans and other products more and more, and the litchi shell generated in the processing process is generally discarded as waste, so that the litchi shell does not exert sufficient utilization value, not only pollutes the environment, but also wastes resources. Secondly, the plastic type packaging material also has serious pollution to the land at present, and some plastic preservative films have the condition of abusing plasticizers, and the content of the plasticizers exceeding the standard is easy to enter the human body along with food, thereby generating the risk of carcinogenesis to the human body. In the prior art, a composite film is adopted to carry out fresh-keeping treatment on food, and the composite film is a porous network structure film formed by cross-linking of molecules of two or more film-forming materials. The composite film can improve the defects of a single film and make the performance of the film in all aspects better, but has limited compromise on the performance of all aspects.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a method for preparing an antibacterial preservative film by utilizing a litchi seed/shell extract, so as to solve the problems that the utilization value of litchi seed shells is not fully exerted, so that the resource waste is caused, and the composite film in the prior art is difficult to realize all aspects of performances.
In order to solve the technical problems, the invention adopts the following technical scheme:
a method for preparing an antibacterial preservative film by utilizing a lychee seed/shell extract comprises the following steps:
step 1: obtaining extracts from litchi shells and litchi seeds, respectively; wherein, lychee shell flavone extracting solution and lychee shell polysaccharide extracting solution are obtained from lychee shells; obtaining a lychee seed starch extract and a lychee seed flavone extract from lychee seeds;
step 2: mixing chitosan with an acetic acid solution with the mass fraction of 1% to prepare a chitosan solution with a certain concentration; wherein, when the lychee shell extract is used as the raw material, the addition amount of the chitosan is 1.5 to 3.5 percent according to the mass percentage; when the lychee seed extract is used as the raw material, the addition amount of the chitosan is 10-50 g/L;
and 3, step 3: when the lychee shell extract is used as a raw material, adding gelatin into a chitosan solution, stirring at 50 ℃ until the gelatin is completely dissolved, sequentially adding lychee shell flavone extracting solution and lychee shell polysaccharide extracting solution, stirring uniformly at 50 ℃ to obtain a mixed solution, adding glycerol accounting for 2% of the mass of the mixed solution, and stirring uniformly to obtain a composite membrane liquid I; wherein, the addition amount of the gelatin is 1.0-3.0 percent, the addition amount of the lychee shell flavone is 5-25 percent and the addition amount of the lychee shell polysaccharide is 5-25 percent according to the mass percentage;
when the lychee seed extract is used as a raw material, adding distilled water into the lychee seed starch extract, uniformly mixing, then pasting at a constant temperature of 90 ℃ for 40min to obtain a pasting liquid, cooling to 50 ℃, then uniformly mixing the chitosan solution obtained in the step (2) with the pasting liquid, then adding the lychee seed flavone extract and glycerol, and uniformly mixing to obtain a composite membrane liquid II; wherein, the addition amount of the lychee seed flavone is 1-5 percent by weight percent; the addition amount of the lychee seed starch is 40-80 g/L, and the addition amount of the glycerol is 5-25 g/L;
and 4, step 4: ultrasonically degassing the composite membrane liquid I at 50 ℃ and 100Hz for 30min, uniformly casting and standing, and drying to form a membrane to obtain the antibacterial preservative membrane; and filtering the composite membrane liquid II by using a 40-mesh sieve, uniformly casting, and drying to form a membrane to obtain the antibacterial preservative membrane.
Compared with the prior art, the invention has the following beneficial effects:
1. the invention adopts lychee seed/shell as a raw material source, and takes flavone and polysaccharide extracted from lychee shell and starch and flavone extracted from lychee seed as raw materials to prepare a composite film by matching with chitosan, gelatin and glycerol; the method is simple and easy to operate, the litchi waste is used for developing the composite membrane, the added value of litchi can be increased, the economic income of fruit growers can be increased, and compared with the existing composite membrane, the litchi composite membrane is safer and more reliable because the raw materials used in the method are natural substances extracted from litchi seeds/shells.
2. When the litchi shell extract is selected as the raw material, the thickness of the obtained composite film is 0.145mm, the light transmittance is 54.51%, the water solubility is 43.80%, and the water vapor transmission rate is 1.92 multiplied by 10 -6 g·m·(m 2 ·s·pa) -1 The tensile strength is 9.19MPa, and the elongation at break is 167.34%; compared with a single chitosan-gelatin blended film, the light transmittance is reduced by 13.47%, the water solubility is increased by 6.97%, and the water vapor transmission rate is reduced by 2.33 multiplied by 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 percent; when the semen litchi extract is selected as raw material, the tensile strength of the composite membrane is 2.83 + -0.21 MPa, the elongation at break is 96.13 + -4.66%, the membrane thickness is 0.18mm, the light transmittance is 2.14 + -0.11%, and the water vapor transmission rate is 7.1544 + -0.2319 × 10 -5 (g.mm -1 .d -1 ) The solubility is 43.32 +/-2.6%; compared with a single chitosan film, the tensile strength is improved by 0.93MPa, 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 indicates that the mechanical properties of the composite film can be significantly improved by the addition of the lychee seed/shell extract.
3. The composite membrane prepared by taking the lychee shell extract as the main raw material is applied to the fresh keeping of the mulberries, after the mulberries are stored for 5 days, the rotting rate is 38.2%, the weight loss rate is 3.1%, and the content of soluble solid matters is 8.6%.
4. According to the method, the cherry tomatoes are wrapped by the composite film prepared by taking the lychee seed extract as a main raw material for preservation, and the rotting rate and the weight loss rate of the cherry tomatoes wrapped by the composite film in 2-10 days after the cherry tomatoes are placed are lower than those of the cherry tomatoes wrapped by the composite film without flavone and the cherry tomatoes of a control group; the soluble solid content is higher than the soluble solid content of cherry tomatoes wrapped by the composite film without the flavone and cherry tomatoes of a control group; when the cherry tomatoes are placed for 10 days, the weight loss rate of the cherry tomatoes wrapped by the litchi seed extract composite membrane is respectively reduced by 1.06% and 3.2% compared with the weight loss rate of the cherry tomatoes wrapped by the composite membrane without the added flavone and the weight loss rate of the cherry tomatoes of a control group; the rotting rate is respectively 3.33 percent and 23.3 percent lower; the content of soluble solid is respectively 0.5 percent and 2.17 percent higher; has 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 composite film seeds without flavone and composite films without litchi seed extract.
Drawings
FIG. 1 is a graph showing the response surface and contour of chitosan and gelatin to tensile strength when litchi shell extract is used as a raw material.
FIG. 2 is a graph showing the response surface and contour of chitosan and flavone to tensile strength when litchi shell extract is used as the raw material.
FIG. 3 is a graph showing the response surface and contour of flavone and polysaccharide to tensile strength when litchi shell extract is used as a raw material.
FIG. 4 is a graph showing the response surface and contour of polysaccharide and gelatin to tensile strength when litchi shell extract is used as a raw material.
FIG. 5 is a graph showing the effect of different treatments on the rotting rate of mulberries.
Fig. 6 is a graph showing the effect of different treatment methods on the weight loss rate of mulberries.
FIG. 7 is a graph showing the effect of different treatments on soluble solids of Mori fructus.
FIG. 8 is a graph showing the interaction of starch addition and chitosan addition to a membrane when a lychee seed extract is used as a raw material.
FIG. 9 is a response surface and contour diagram of the addition amount of flavone and starch in semen litchi when semen litchi extract is used as raw material.
FIG. 10 is a response surface and a contour diagram of the addition amount of glycerol and the addition amount of lychee seed starch when lychee seed extract is used as a raw material.
FIG. 11 is a response surface and contour plot of the flavone addition and chitosan addition when lychee seed extract is used as raw material.
FIG. 12 is a response surface and contour plot of the added amount of flavone and the added amount of glycerol when litchi seed extract is used as raw material.
Fig. 13 shows the effect of different treatment methods on the weight loss ratio.
FIG. 14 shows the effect of different treatments on decay rate.
Fig. 15 shows the effect of different treatment regimes on soluble solids.
Detailed Description
The invention will be further explained with reference to the drawings and examples.
1. Method for preparing antibacterial preservative film by taking lychee shell extract as raw material
1. Extraction and content determination of total flavone in lychee shells
Fully drying litchi shells, crushing, sieving, weighing 5.0g of litchi shell powder, adding an 80% ethanol solution according to a material-liquid ratio of 1 (g: mL) to 20 (g: mL), extracting at the constant temperature of 78 ℃ for 4h, filtering, concentrating the filtrate at 35 ℃ in vacuum to dryness, and taking a 70% ethanol solution to fix the volume to 50mL. 1mL of the extract of lychee seed flavone is taken, the absorbance at 510nm is measured to be 0.87, and the flavone content is calculated to be 62.4 mu g/mL.
2. Extraction and content determination of polysaccharide in litchi shells
Completely drying litchi shells, crushing, sieving, weighing 5.0g of powder, adding water according to a material-liquid ratio of 1 (g/mL) to 20, leaching at constant temperature of 100 ℃ for 3h, performing suction filtration, concentrating filtrate in vacuum at 60 ℃, and adding water to a constant volume of 100mL. Sucking 10mL of polysaccharide extracting solution, adding 40mL of 95% ethanol solution, standing at 4 ℃ for 12h, centrifuging at the rotating speed of 4900r/min for 10min, washing the alcohol precipitate with 95% ethanol solution, and finally diluting to a constant volume of 25mL with distilled water. Diluting 1mL of lychee shell polysaccharide extract to 50mL, taking 1mL of the diluted solution, measuring the absorbance at 490nm to be 0.327, and obtaining the polysaccharide content of 257.1 mu g/mL by calculation.
3. Preparation of composite membrane based on lychee shell extract
(1) Dissolving: taking 100mL of 1.0% acetic acid solution, carrying out ultrasonic treatment for 1min at the ultrasonic power of 100W and the temperature of 45 ℃, adding chitosan accounting for a certain mass fraction of the solution after uniform dispersion, and stirring for 50min at 50 ℃. After the chitosan is dissolved, adding gelatin which accounts for a certain mass fraction of the solution, and stirring for 30min at 50 ℃ until the gelatin is completely dissolved.
(2) Mixing: and after the chitosan and the gelatin are completely dissolved, adding litchi chinensis flavone extract and litchi chinensis polysaccharide extract which account for a certain volume fraction of the solution, and stirring for 30min at 50 ℃ to fully and uniformly mix the membrane liquid.
(3) Stirring: and 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: placing the mixed composite membrane liquid at 50 ℃ and 100Hz, and carrying out ultrasonic degassing for 30min.
(5) Casting: 30mL of membrane liquid is weighed and poured into a glass culture dish for uniform casting, and standing and cooling are carried out for 10min.
(6) And (3) drying: and (3) placing the glass culture dish in a constant-temperature air-blast drying oven at 50 ℃, and drying for 3-4 hours to form a film.
3.1 optimization of the method for preparing composite Membrane from litchi Shell extract
Taking the chitosan addition amount, the gelatin addition amount, the litchi chinensis flavone addition amount and the litchi chinensis polysaccharide addition amount as single factors of research, designing 5 step levels for each factor, and performing a single-factor experiment, wherein the fixed chitosan addition amount is 2.0%, the gelatin addition amount is 1.5%, the litchi chinensis flavone addition amount is 10%, the litchi chinensis chitin addition amount is 10%, and the glycerol addition amount is 2%. Setting 3 single factors as fixed values, taking the other 1 as variable, measuring the thickness, light transmittance, water solubility, water vapor transmission rate, tensile strength and elongation at break of the composite film, and finally comprehensively judging to obtain the optimal level of each factor. The design of the single factor experiment is shown in table 3.
TABLE 3
| Level of | Chitosan/% | Gelatin/%) | Litchi shell flavone/%) | Lichee 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. Effect of Chitosan addition on composite membranes
TABLE 4
| Chitosan/% | Thickness/mm | Transmittance/% | Water vapor transmission rate 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 thickness of the composite film increased due to the increased amount of chitosan added and the increased viscosity of the composite film solution. Because the chitosan is light yellow, the addition amount of the chitosan is continuously increased, the influence of the color of the chitosan on the color of the composite film is more and more prominent, and the light transmittance of the composite film is reduced. The water vapor permeability of the composite membrane is reduced firstly, probably because the addition amount of the chitosan is increased, the molecular weight of the chitosan in unit volume in membrane liquid is increased, and the effect of hydrogen bonds among molecules is enhanced, so that the molecular interval in the composite membrane is shortened, and the membrane structure becomes more compact; when the addition amount of the chitosan is increased to2.5%, the lowest water vapor transmission rate is (1.56 +/-0.04) x 10 -6 g·m·(m 2 ·s·Pa) -1 The water vapor transmission rate is increased because the effect of hydrogen bonds among molecules tends to be saturated, and if chitosan is continuously added, a large amount of hydrophilic groups are added. The addition amount of the chitosan is increased, and the water solubility of the composite membrane is increased firstly due to the hydrophilicity of the chitosan; when the addition amount is increased to 2.5%, the water solubility is at most (52.90 ± 5.06)%, but then the water solubility is reduced, probably because the surface area of the film of equal mass in water is reduced due to the increase of the thickness of the composite film, so that the water solubility of the composite film is reduced. The molecular weight in the membrane liquid of unit volume is increased due to the increase of the addition amount of the chitosan, and free amino in the chitosan can generate hydrogen bond action with the hydroxyl of the chitosan and the hydroxyl contained in the polysaccharide in the solution after being protonated by acetic acid, so that the compactness of the composite membrane structure is increased, and the tensile strength and the elongation at break of the composite membrane are increased; when the addition of the chitosan is increased after reaching 2.5 percent, the excessive hydrogen bonds can increase the rigid structure of the composite film, so that the composite film becomes hard and brittle, the fluidity of the 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 the chitosan reaches 2.5 percent, the tensile strength of the composite film is the maximum (4.82 +/-0.70) MPa, and the elongation at break reaches the maximum (214.17 +/-58.81)%. In combination, the chitosan addition amount is preferably 2.5%.
b. Influence of gelatin addition on composite membranes
TABLE 5
As can be seen from Table 5, after gelatin and chitosan are mixed, gelatin absorbs water to swell, secondary bonds in the gelatin structure are broken, and free hydroxyl groups of gelatin and water molecules form hydrogen bonds;the amino of the chitosan can be connected with the free amino and hydroxyl of the gelatin molecule through hydrogen bonds; the free amino group of the chitosan and the carboxyl end of the peptide chain form an ionic bond, and a new viscosity phase is formed in the mixed membrane liquid through the electrostatic interaction between molecules. Therefore, the thickness of the composite film increases with the addition amount of gelatin. The gelatin is light yellow, and the influence of the color of the gelatin on the color of the composite film is more and more obvious due to the increase of the addition amount of the gelatin, so that the light transmittance of the composite film is reduced. In the blended film, gelatin can generate cross-linking effect with chitosan molecules through ionic bonds and hydrogen bonds, so that the network structure of the film is densified, and the water molecules are reduced from passing through the film. When the addition amount of gelatin is increased to 2.5%, the water vapor transmission rate of the composite membrane is at least (1.98 + -0.06) × 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 transmission rate begins to increase, and the water solubility of the composite film is increased; when the gelatin addition amount was increased to 2.5%, the water solubility of the film was at most (60.24 ± 1.46)%. Originally, the increase of the addition amount of the gelatin can continuously increase the water solubility of the composite film, or the increase range is not large, but after the increase, the water solubility of the composite film starts to decrease, and after deep analysis, the electrostatic interaction is probably generated between the amino group of the chitosan molecule and the carboxyl group of the gelatin molecule in the composite film, a polyelectrolyte complex is formed, a three-dimensional network structure is generated, the compactness of the composite film is increased, the structure of the composite film becomes stable, and the water solubility of the composite film starts to decrease. The gelatin has good plasticizing effect, and the tensile strength and the elongation at break of the composite film tend to increase along with the increase of the addition amount of the gelatin. When the addition amount of the gelatin reaches 2.5 percent, 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 percent). However, this is then reduced, possibly due to excessive plasticization of the gelatin. In combination, a gelatin addition of 2.5% is most preferred.
c. Influence of litchi rind flavone addition on composite membrane
TABLE 6
As can be seen from table 6, the flavone molecules have hydrophobicity, resulting in poor compatibility between them and hydrophilic chitosan, and the addition of flavone makes the cross-sectional structure of the composite membrane rough and uneven, so the thickness of the composite membrane increases with the increase of the addition amount of flavone, and the water solubility decreases. The flavone is darker in color, and the composite film turns yellow and darker after the flavone is added, so that the light transmittance is reduced. Originally, flavone only brings adverse effects to the performance of the composite membrane, because although the compatibility of the flavone and chitosan is poor, intermolecular hydrogen bonds can be formed between the chitosan and the flavone, so that the affinity of the composite membrane to water vapor can be reduced, and the network structure of the membrane is more compact due to the addition of the flavone, so that the water vapor is prevented from passing through; when flavone is added in 15%, the lowest water vapor transmission rate of the composite membrane is (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 flavone create smaller water vapor channels in the composite membrane matrix. After the lychee shell flavone is added, the tensile strength of the composite membrane is increased, probably because hydrogen bonds are formed between chitosan and flavone molecules, and the compactness of the composite membrane structure is improved. The elongation at break of the composite film tends to increase, probably because crystals are generated in flavone molecules, the molecular polymerization reaction is reduced, and the flexibility of the composite film is improved. When the flavone is added in 15%, the tensile strength of the composite film reaches the maximum value, namely (5.65 +/-0.48) MPa, and the elongation at break of the film is the highest and is (138.27 +/-13.03)%, which shows that the tensile property and ductility of the composite film are good under the conditions. The tensile strength and the elongation at break of the composite film are reduced, probably because the hydrophobicity of the flavone molecules can destroy the regularity of the chitosan molecules, weaken the hydrogen bond action among the chitosan molecules and hinder the interaction among the high molecular chains. Compared with the control group without flavone, the tensile strength is increased by 2.62MPa and the breaking elongation is increased after 15% of lychee shell flavone is addedThe length rate is increased by 62.1% at most, which shows that the mechanical property of the composite membrane is improved after litchi shell flavone is added. In combination, the optimum level was selected to be 15% of the flavone addition.
d. Influence of litchi chitin addition on composite membrane
TABLE 7
| Polysaccharide/%) | Thickness/mm | Transmittance/% | Water vapor transmission rate 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, hydroxyl groups and carboxyl groups in the polysaccharide molecules are bound to water molecules to form hydrogen bonds, and the water content of the composite film is reduced and the thickness thereof is increased due to an increase in the amount of the added polysaccharide. The polysaccharide is yellow brown under the condition of being heated, so that the transparency of the composite membrane is reduced; secondly, the fewer the components of the composite film, the more uniform and compact the structure of the film, the smoother the surface, and the fewer micropores and cracks exist, so that the light transmittance of the composite film is higher and the transparency is better. However, as the addition amount of the polysaccharide continues to increase, the particle sizes of different polymers are different, so that the sedimentation rates in the film forming process are different, the microstructure of the film is also changed, the surface of the film is rough, and the film is scattered or reflected when meeting light, so that the light transmittance of the composite film is reduced. The polysaccharide has a plurality of hydrophilic groups, and the hydrogen bond interaction between the polysaccharide and chitosan molecules causes the membrane structure to become compact, so that the water vapor can be effectively prevented from passing through; when the addition amount of the polysaccharide reaches 15%, the water vapor transmission rate 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 shows an increasing trend, probably because the hydrogen bonding between the polysaccharide and chitosan reaches a saturation state. 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 addition amount of the polysaccharide is 15%, the water solubility of the film is (43.80 +/-0.43)%, and when the addition amount of the polysaccharide is increased, the water solubility is reduced, and the chitosan is probablyThe force between the polysaccharide and the membrane is increased and the membrane structure becomes compact. The addition of the polysaccharide leads the tensile strength and the elongation at break of the composite film to show the trend of increasing firstly and then decreasing. The increase of the two is probably because strong hydrogen bonding action is formed between protonated amino groups of the chitosan and 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 addition amount of the polysaccharide is 25%, the tensile strength of the composite film reaches the maximum value, namely (5.56 +/-0.11) MPa, and the elongation at break is also the highest (133.89 +/-9.35)%, which shows that the composite film has good comprehensive performance under the condition. Then, the addition amount of the polysaccharide is increased, and the addition amount of the polysaccharide and the polysaccharide are reduced, probably because the water content of the composite film is reduced, the composite film becomes hard and thick, and chitosan and polysaccharide molecules are intertwined and interact with each other, so that the mobility of the molecules is weakened. Compared with a control group without the added polysaccharide, the tensile strength of the composite membrane is increased by 2.94MPa and the elongation at break is increased by 74.49% after 15% of lychee chitin is added, which indicates that the mechanical property of the composite membrane is favorably improved after the lychee chitin is added, so that the optimal level of the added polysaccharide is selected as 15%.
e. Response surface test design and results
Based on the result of a single-factor experiment, the original prediction shows that the combination of the addition amount of chitosan, the addition amount of gelatin, the addition amount of lychee shell flavone and the addition amount of lychee shell polysaccharide has a large influence on the performance of the composite membrane, but the verification of a response surface test is required. The method is characterized in that the chitosan addition amount, the gelatin addition amount, the lychee shell flavone addition amount and the lychee shell polysaccharide addition amount are selected as response factors, the main measurement index tensile strength influencing the use characteristics of the composite film is used as a response value, and the optimal process formula condition of the composite film is determined through a Design Expert 8.0 software Design response surface test. The level of response surface design, arrangement and results are shown in tables 8 and 9.
TABLE 8
| Level of | A chitosan/%) | B gelatin/%) | C litchi shell flavone/%) | D lichee chitin/%) |
| -1 | 2.0 | 2.0 | 10 | 10 |
| 0 | 2.5 | 2.5 | 15 | 15 |
| 1 | 3.0 | 3.0 | 20 | 20 |
TABLE 9
Note: * Very significant differences (p < 0.01); * The difference was significant (p < 0.05).
Analysis of table 10 by Design-Expert 8.0 software gave the tensile strength in comparison to a: the addition amount of chitosan B: gelatin addition amount C: and (3) adding amount of lychee shell flavone D: regression quadratic equation between the addition amounts of lychee 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 。
Significance test of the model p, as seen from ANOVA<0.0001, mismatching term p value 0.8115>0.05, is not significant, and shows that the regression equation of the formula optimization experiment has small error and high fitting degree; determining the coefficient R 2 =0.9924, and therefore, the influence of the addition amount of chitosan, gelatin, lychee shell flavone and lychee shell polysaccharide on the tensile strength of the composite film can be better explained. Comprehensively, the following steps are found: 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.
From the comparison of the F values, it is found that the four factors of the test all have extremely significant influence on the tensile strength of the composite film (p)<0.01 A) order of influence>B>C>D, namely: chitosan>Gelatin>Flavone>A polysaccharide. A. The 2 、B 2 、C 2 、D 2 The influence on the tensile strength of the composite film is extremely remarkable (p)<0.01 ); among the interactive items, the interactive item BD has very significant influence on the tensile strength of the composite film (p)<0.01 ); the influence of the interaction terms AB, AC and CD on the tensile strength of the composite film is obvious (p)<0.05 Other interactive items have no significant impact.
f. Analysis of tensile strength response surface and determination of optimal process conditions
In order to visually reflect the influence of the chitosan addition amount, the gelatin addition amount, the lychee shell flavone addition amount and the lychee shell polysaccharide addition amount on the response value tensile strength, two factors are fixed at zero level, a Model Graph program is used for a response surface diagram of the interaction of the chitosan addition amount, the gelatin addition amount, the lychee shell flavone addition amount and the lychee shell polysaccharide addition amount, and the result is shown in figures 1, 2, 3 and 4.
As can be seen from fig. 1, the AB term is significant. When the addition amount of the gelatin is fixed, the tensile strength of the composite film is increased and then reduced due to the increase of the addition amount of the chitosan, and when the addition amount of the chitosan is 2.75%, the tensile strength of the composite film is the maximum value. The addition amount of the chitosan is fixed, the tensile strength of the composite film is increased and then reduced due to the increase of the addition amount of the gelatin, and the tensile strength of the composite film is the maximum value when the addition amount of the gelatin is 2.71%.
As can be seen from fig. 2, the contour lines appear elliptical and the AC items interact significantly. When the addition amount of the flavone is fixed, the tensile strength of the composite film is increased and then reduced due to the increase of the addition amount of the chitosan. The addition amount of the chitosan is fixed, the tensile strength of the composite film is increased and then reduced due to the increase of the addition amount of the flavone, and when the addition amount of the flavone is 15.25%, the tensile strength of the composite film is the maximum value. Therefore, the tensile strength of the composite film can be improved by properly controlling the addition amount of the chitosan and the addition amount of the flavone.
As can be seen from fig. 3, the contour lines appear elliptical, illustrating that the BD items are interactive significantly. The addition amount of the fixed gelatin is unchanged, the tensile strength of the composite film is increased and then reduced due to the increase of the addition amount of the polysaccharide, and when the addition amount of the polysaccharide is 14.97%, the tensile strength of the composite film is the maximum value. When the addition amount of the polysaccharide is fixed, the tensile strength of the composite film is increased and then reduced due to the increase of the addition amount of the gelatin. Therefore, the tensile strength of the composite film can be improved by properly controlling the addition amount of the gelatin and the addition amount of the polysaccharide.
As can be seen from fig. 4, the contour lines appear as ovals, indicating that the CD items interact significantly. When the addition amount of the flavone is fixed, the tensile strength of the composite film is increased and then reduced due to the increase of the addition amount of the polysaccharide. When the addition amount of the polysaccharide is fixed, the tensile strength of the composite membrane is increased and then reduced due to the increase of the addition amount of the flavone. 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 membrane, the optimal theoretical process formula for obtaining the composite membrane is that the addition amount of chitosan is 2.75%, the addition amount of gelatin is 2.71%, the addition amount of lychee shell flavone is 15.25%, and the addition amount of lychee shell polysaccharide is 14.97%, and the theoretical tensile strength of the composite membrane under the process formula is 9.26MPa. The improved process formula according to the actual experimental conditions is as follows: the addition amount of chitosan is 2.8%, the addition amount of gelatin is 2.7%, the addition amount of lychee chitosan flavone is 15.2%, and the addition amount of lychee chitosan is 15.0%, experiments are carried out according to the process formula, the obtained tensile strength is 9.04MPa, 9.28MPa and 9.23MPa, and the average tensile strength is 9.19MPa, so that the data of the optimization experiment have certain reliability.
4. Preservation effect verification
The damaged epidermis of the mulberry makes the mulberry more susceptible to bacterial and fungal infections, resulting in rottenness of fruits. Fig. 5 shows that after the storage period of 5d, the rotting rates of all three treatment methods tend to increase, but the rotting rate of the mulberries which are not wrapped by the film is the highest and reaches (100 +/-0.00)%, and the rotting rate increases at a high rate. The rotting rate of the mulberries wrapped by the composite film without the litchi shell extract is (72 +/-0.45)%, the growing rate of the rotting rate is slower than that of the mulberries wrapped by the composite film without the litchi shell extract in the first 3 days, and mainly chitosan has certain antibacterial capacity, so that the mulberries can be protected from being infected by fungi, and the rotting of fruits can be reduced. After 5d of the mulberry wrapped by the composite film added with the lychee shell flavone and the lychee shell polysaccharide, the rotting rate is (38 +/-0.53)%, and the increasing rate of the rotting rate is slower than that of other two groups, which indicates that after the lychee shell flavone and the lychee shell polysaccharide are added into the composite film, the antibacterial capacity of the composite film is improved, and the rotting rate of the mulberry can be obviously reduced.
The weight loss of the mulberry is mainly caused by the respiration of the mulberry and the migration of water in the fruit. Fig. 6 shows that after the storage period of 5d, the fruits of the three treatment modes have continuous weight loss, but the weight loss rate of the mulberries which are not wrapped by the film is the highest and reaches (17.55 +/-0.70)%, and the weight loss rate is relatively high. The weight loss rate of the mulberries wrapped by the composite membrane without the litchi shell extract is (8.5 +/-0.11)%, after 5d, the increase rate of the weight loss rate is slower than that of a blank group, mainly because chitosan in the composite membrane has hydrophilicity and interacts with water molecules, so that the flow of water vapor in the mulberries is better maintained, and the water is prevented from being evaporated too fast. The mulberry wrapped by the composite film added with lychee shell flavone and lychee shell polysaccharide has the weight loss rate of (3.1 +/-0.08)% after 5d, and the increase rate of the weight loss rate is slower than that of other two groups, which indicates that after the lychee shell flavone and lychee shell polysaccharide are added into the composite film, the compactness of the structure of the composite film is improved through the interaction between the lychee shell flavone and lychee shell polysaccharide and chitosan and gelatin molecules, and oxygen, water and carbon dioxide among fruits and the film are better remained in the film, so that the weight loss rate of the mulberry is reduced.
Sweetness and mouthfeel of fruit is related to the soluble solids content of the fruit. The soluble solids content is reduced because the mulberry needs to maintain respiration by breaking down its own sucrose. Fig. 7 shows that the soluble solid content decreased with the increase of the storage time, and the soluble solid content of the blank mulberry without any treatment was (7.8 ± 0.15)% after 5 days, and the decrease rate was faster. The content of soluble solid matters of the mulberries wrapped by the composite membrane without the litchi shell extract is (8.2 +/-0.10)%, the decrease rate of the content of the soluble solid matters is slow compared with that of a blank group, the content of the soluble solid matters of the mulberries wrapped by the composite membrane with the litchi shell flavone and the litchi shell polysaccharide is (8.6 +/-0.06)%, and the decrease rate of the content of the soluble solid matters is slower compared with that of the other two groups, so that the soluble solid matters in the mulberries can be well maintained by adding the litchi shell flavone and the litchi shell polysaccharide.
Therefore, when the composite film prepared by using the lychee shell extract as the raw material is applied to the fresh keeping of the mulberries, after the mulberries are stored for 5 days, the rotting rate is 38.2%, the weight loss rate is 3.1%, and the content of soluble solid matters is 8.6%.
2. Method for preparing antibacterial preservative film by using lychee seed extract as raw material
1. Extraction and content determination of starch in lychee seeds
Weighing 10g of lychee seed powder, and mixing the following raw materials in a ratio of 1:20 (g: mL) adding water, extracting with ultrasonic assistance for 20min, sieving with a 120-mesh sieve, repeatedly extracting for 3 times, mixing the extractive solutions, centrifuging the filtrate (4000 r/min,15 min), discarding the supernatant, and oven drying the precipitate at 40 deg.C to obtain crude lychee seed starch. The starch content in the crude lychee seed starch is 61.2369% by acid hydrolysis.
2. Extraction and content determination of flavone in lychee seeds
Weighing about 20g of dried lychee seed powder with constant weight in a conical flask, and mixing the lychee seed powder with 50% ethanol solution according to the ratio of 1:10 (g: mL), setting the temperature of a 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 solution, transferring the concentrated solution into a 100mL volumetric flask, and fixing the volume to the scale to obtain the crude flavone extracting solution. Diluting the crude flavone extractive solution by 25 times, measuring absorbance value of the diluted solution at 510nm, and calculating out flavone concentration of the concentrated solution according to the absorbance value, wherein the calculation result is 879.96 μ g/mL.
3. Method for preparing composite membrane by using 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 weighing a certain amount of crude lychee seed starch, adding a certain amount of distilled water into a beaker, uniformly mixing, pasting in a constant-temperature water bath kettle at 90 ℃ for 40min, taking out the pasting liquid, cooling the pasting liquid to 50 ℃, taking 50mL of chitosan solution and 50mL of starch pasting 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 using a 40-mesh sieve, sucking 20mL of filtrate, adding the filtrate into a culture dish with the diameter of 90mm, drying in a 50-DEG C forced air drying box for 3h, and then uncovering the membrane.
3.1 optimization of the method for preparing composite membranes from semen litchi extract
Determining the basic formula of the lychee seed extract composite membrane. According to the proportion 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%, 5%; the addition amount of glycerol is 5g/L, 10g/L, 15g/L, 20g/L and 25g/L; designing 5 step levels for each factor, then carrying out single factor test, setting 3 factors as fixed values and the other 1 factor as variables, carrying out membrane performance measurement after membrane formation, and screening out the optimal membrane. The single factor test is shown in Table 11.
TABLE 11
| Level of | A litchi seed starch adding amount (g/L) | Chitosan addition amount (g/L) | C flavone addition amount (%) | Glycerol addition amount (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 litchi seed starch addition on film performance
The addition amount of the lychee seed starch is changed into: blanks (0 g/L), 40g/L, 50g/L, 60g/L, 70g/L and 80 g/L) are used for researching the influence of the lychee seed starch on the membrane performance. The results are shown in Table 12:
TABLE 12
In this case, the amount of starch added is used as an independent variable; the mechanical properties of the film can reflect the stretching of the food packaging film during handling, processing and storageThe ability to remain intact, and to reflect the quality of the film properties, therefore the mechanical properties of the film are chosen as the dependent variables, wherein the tensile strength and the elongation at break are part of the mechanical properties. Originally, the predicted larger the addition amount of lychee seed starch, the mechanical properties of the film should be increased, and the film should be balanced under a certain concentration, but the experimental data and the predicted results have differences, which can be known from table 12: with the increase of the addition amount of the lychee seed starch, the tensile strength of the film is in direct proportion to the addition amount of the starch; the elongation at break is increased and then reduced, the maximum value of the elongation at break is when the addition amount of starch is 60g/L, and when the addition amount of lychee seed starch exceeds 60g/L, the elongation at break is gradually reduced; when the addition amount of the starch is 80g/L, the tensile strength reaches the maximum, the value is 3.52 +/-0.33 MPa, but the elongation at break of the film is only 57.4 +/-4.95 percent at the lowest; when the addition amount of the starch is 40g/L, although the breaking elongation is more appropriate, the tensile strength is the lowest; this is probably because as the amount of starch added is increased gradually, the number of starch molecules also increases gradually, resulting in a more dense network structure and thus an increase in the tensile strength of the film, but this dense structure leads to a decrease in molecular mobility; the litchi seed starch is added and is a rigid polymer, the higher the content of the litchi seed starch is, the lower the toughness of the film is, the stronger the rigidity is, and therefore, the elongation at break is lower. Therefore, only when the addition amount of the starch is 60g/L, the tensile strength and the elongation at break of the film are proper, the mechanical property of the film is good, and the tensile strength and the elongation at break of the film are obvious compared with those of a blank group (p)<0.05 Higher than blank, a starch addition of 60g/L was chosen for the optimization experiment. Meanwhile, the addition amount of starch 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 keeping fruits and vegetables fresh is achieved; the water vapor transmission rate is the lowest and is (8.84 +/-0.31) multiplied by 10 -5 (g.mm -1 .d -1 ) Because the water vapor transmission rate is low, when the addition amount of the lychee seed starch is 60g/L, the barrier capability of the film to external water vapor is good, the growth and the propagation of bacteria can be slowed down, and the effect of prolonging the fresh-keeping period of fruits and vegetables is achieved; the solubility was highest at this time and was 41.66. + -. 1.9%, said to beObviously, the composite film is easy to dissolve and degrade when meeting water, can achieve the aim of keeping fruits and vegetables fresh and has less environmental pollution, so when the addition amount of the lychee seed starch is 60g/L, the composite film has better comprehensive performance.
b. Effect of Chitosan addition on Membrane Performance
The chitosan addition amount was changed to: blank (0 g/L), 10g/L, 20g/L, 30g/L, 40g/L and 50g/L, and the influence of the lychee seed starch on the membrane performance is researched. The results are shown in Table 13:
watch 13
As can be seen from table 13: the research is carried out by taking the addition amount of the chitosan as an independent variable, when the addition amount of the chitosan 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, probably because-OH contained in lychee seed starch and NH on a chitosan chain 3 + Due to the formation of hydrogen bonds between the two, charged amino groups can promote the polarization of other chains, so that the attractive force between the chains is increased, the tensile strength of the film is gradually increased, and the maximum amount is reached when the addition amount of chitosan is 30 g/L; when charged with NH 3 + When the addition amount of the chitosan is increased, the repulsion between the charges of the same species is increased, so that the arrangement between the molecules of the membrane is difficult, the acting force between hydrogen bonds is weakened, the compactness of the membrane is gradually reduced, a blending system is difficult to form, and the tensile strength of the membrane is gradually reduced. However, the chitosan also has a plasticizing effect, which is beneficial to improving the tensile property of the film, and the plasticizer and the polymer have interaction, so that the overall fluidity and flexibility of molecular chains are increased, the elongation at break of the film is gradually increased, when the addition amount of the chitosan reaches 30g/L, the elongation at break of the film reaches the maximum, and when the addition amount of the chitosan exceeds 30g/L, the elongation at break of the film is reduced along with the increase of the addition amount of the chitosan; the reduction of the elongation at break may be due to the enlargement of lychee seeds by excessive chitosanCrystallinity between starches. Therefore, when the addition amount of chitosan is 30g/L, the tensile strength of the film is the highest, the elongation at break is also the highest, and the film is remarkable compared with a blank group (p)<0.05 Higher than blank group), the optimization experiment was performed when the chitosan addition amount was 30g/L according to the mechanical properties of the membrane. Meanwhile, when the addition amount of the chitosan is 30g/L, the light transmittance of the film is 2.34 +/-0.12%, and although the light transmittance is higher than that when the addition amount of the chitosan is 50g/L, the light transmittance is obviously lower than that of a blank group, so that the light shading property of the film can be increased by adding a certain amount of chitosan into the starch-based film, and the oxidation of food can be delayed to achieve the fresh-keeping effect; the water vapor transmission rate is (8.45 +/-0.64) multiplied by 10 -5 (g.mm -1 .d -1 ) The water vapor transmission rate of the film is obviously lower than that of the film when the addition amount of the chitosan is 20g/L and 40g/L, which shows that the 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 that of the other experimental groups, and was significantly higher than that of the blank group, indicating that the solubility of the film was better. Therefore, when the addition amount of the chitosan is 30g/L, the comprehensive performance of the composite membrane is better.
c. Influence of lychee seed flavone addition amount on membrane performance
The addition amount of the lychee seed flavone is changed into: blank (0%), 1%, 2%, 3%, 4%, 5%, study the effect of lychee seed starch on film performance. The results are shown in Table 14:
TABLE 14
As can be seen from table 14: the research is carried out by taking the addition amount of flavone as an independent variable, the tensile strength is gradually reduced and 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 membrane is the highest and reaches 3.15 +/-0.22 MPa, but the elongation at break is the lowest and is only 96.95 +/-7.71%; when the addition amount of flavone is 5%, the elongation at break of the membrane can reach 125.93 +/-8.72% at most, but the tensile strength of the membrane is 1.43 +/-0.07 MPa at least, which is probably due to the flavoneThe addition destroys the compactness of the membrane, thereby causing the tensile strength of the membrane to be reduced; however, the elongation at break of the film is increased because the flavone can form crystals and reduce polymerization, thereby increasing the toughness of the film. When the flavone addition amount is 2%, the tensile strength and the elongation at break of the film are both proper and are significant compared with the blank group (p)<0.05 Higher than blank group) and therefore optimized experiments were performed with a flavone addition of 2%. Meanwhile, the light transmittance of the film is gradually reduced along with the increase of the addition amount of the flavone, probably because the light transmittance of the film is gradually reduced due to the color of the crude flavone extracting solution, and when the addition amount is 2%, the light transmittance of the film is lower and is 2.36 +/-0.1%, and is obviously lower than that of a blank group. When flavone is added in 2%, the water vapor permeability is the lowest and is (8.40 + -0.22) × 10 -5 (g.mm -1 .d -1 ) The flavone content is obviously lower than that of a blank group and other experimental groups, and the fact that the proper flavone adding amount can improve the water vapor transmission rate of the film and prevent the fruit and vegetable from being rotten and deteriorated is shown; the solubility is 41.81 +/-1.16% at the highest, which is obviously higher than that of the blank group and other experimental groups, and the solubility of the film is the highest. Therefore, when the addition amount of the flavone is 2%, the comprehensive performance of the membrane is better.
d. Effect of Glycerol addition on Membrane Performance
The glycerol addition was changed to: blank (0 g/L), 5g/L, 10g/L, 15g/L, 20g/L and 25g/L, and the influence of the lychee seed starch on the membrane performance is researched. The results are shown in Table 15:
watch 15
As can be seen from table 15: the glycerol addition amount is used as an independent variable for research, 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; when the addition amount of the glycerol is 5g/L, the tensile strength is the largest, the value is 3.29 +/-0.19 MPa, and the elongation at break is the lowest at the moment, and the value is 76 +/-5.26%; when the addition amount of the glycerol is 25g/L, the tensile strength is lowest, the value is 1.17 +/-0.07 MPa, the breaking elongation is highest, and the value is 120.3 +/-8.53 percent; this may be due to the glycerolAdding the starch to increase the number of hydrogen bond junctions among starch molecules, and gradually forming hydrogen bonds, so that the fluidity of the starch molecules is improved, and the elongation at break of the film is increased; however, the compactness of the network structure is affected by the decrease of the hydrogen bond bonding sites between the starch molecules, and the intermolecular force is deteriorated, so that the tensile strength of the film is lowered. When the addition amount of the glycerol 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 percent, and the tensile strength and the elongation at break are both proper and higher than those of a blank group. Therefore, the optimized experiment was carried out with the addition amount of 15g/L of glycerin. Meanwhile, when the addition amount of the glycerol is 15g/L, the light transmittance of the film is 2.35 +/-0.08%, which is obviously lower than that of a blank group, and the light transmittance of the film can be improved by adding the glycerol; the water vapor transmission rate is (8.44 +/-0.28) multiplied by 10 -5 (g.mm -1 .d -1 ) Although the addition amount of the glycerol is higher than that of the glycerol by 5g/L, the addition amount of the glycerol is obviously lower than that of the blank group, and the water vapor transmission rate of the membrane can be improved to a certain extent by adding the glycerol; the solubility is 42.58 + -2.61%, which is lower than that of glycerol added in an amount of 25g/L, but higher than that of blank group, and it is shown that glycerol has a certain effect on improving the solubility of the film. Therefore, when the addition amount of the glycerol is 15g/L, the comprehensive performance of the composite membrane is better.
e. Composite membrane response surface analysis and optimal process condition determination prepared from lychee seed extract
Based on single-factor experimental result analysis, the influence of the addition amount of lychee seed starch and the addition amount of chitosan may be the largest, while the influence of the addition amount of flavone and the addition amount of glycerol may be relatively small, but a response surface test is still required for verification. Selecting factors with larger influence as response factors: the litchi seed starch addition amount, the chitosan addition amount, the flavone addition amount and the glycerol addition amount are not easily damaged in the storage and transportation processes due to the fact that a packaging bag with high tensile strength is not easily damaged, and therefore the fruits and vegetables can be kept fresh for a long time under the condition that the transportation time is long, and response surface design experiments are conducted with the tensile strength as a response value. And (3) designing a response surface by adopting software, and determining the optimal formula of the lychee seed extract composite membrane.
Through experimental data analysis, the tensile strength and the fracture rate are obtained, and the ratio of A: adding amount of lychee seed starch, B: the addition amount of chitosan, 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, the influence of A, C and D on the lychee seed extract composite membrane is very obvious (p)<0.01 D) and the order of influence is D>A>C>B, namely: glycerol addition>Adding amount of semen litchi starch>Flavone adding amount>Chitosan is added in an amount of A 2 、B 2 、C 2 Has extremely remarkable tensile strength (p) to the film<0.01 Influence of D), D 2 Has obvious (p) tensile strength to the film<0.05 The influence of); the influence of the interaction items AB, AC, AD, BC and CD on the tensile strength of the lychee seed composite membrane is very obvious (p)<0.01 Other interactive items have no significant impact.
In order to visually reflect the influence of the four factors of the litchi seed starch addition amount, the chitosan addition amount, the flavone addition amount and the glycerol addition amount and the interaction on the tensile strength of the response value, two of the factors are fixed at a zero level, and a Model Graph program is used for a response surface diagram of the interaction of the litchi seed starch addition amount, the chitosan addition amount, the litchi seed starch addition amount, the flavone addition amount, the litchi seed starch and glycerol addition amount, the flavone addition amount and the chitosan addition amount and the flavone addition amount, as shown in fig. 8, 9, 10, 11 and 12.
As can be seen from fig. 8, the contour lines are elliptical, indicating that AB interaction is significant. The addition amount of the lychee seed starch and the chitosan is increased, and the tensile strength is increased and then reduced. When the addition amount of the chitosan is fixed, the tensile strength is gradually increased along with the increase of the addition amount of the lychee seed starch. When the addition amount of the lychee seed starch is fixed, the tensile strength is increased and then reduced along with the increase of the addition amount of the chitosan. Therefore, the tensile strength can be improved by properly adding the lychee seed starch and the chitosan.
As can be seen from fig. 9, the contour lines are elliptical, indicating significant AC interaction. The addition amount of the lychee seed starch and the addition amount of the flavone are increased, and the tensile strength is increased and then reduced. When the addition amount of the flavone is constant, the tensile strength is gradually increased along with the increase of the addition amount of the lychee seed starch. When the addition amount of the lychee seed starch is constant, the tensile strength is gradually reduced along with the increase of the addition amount of the flavone. Therefore, the tensile strength can be improved by properly adding the flavone and the lychee seed starch.
As can be seen from fig. 10, the contour lines are elliptical, indicating that AD interaction is significant. The addition amount of the glycerin and the addition amount of the lychee seed starch are increased, and the tensile strength is increased firstly and then reduced. When the addition amount of the glycerin is constant, the tensile strength is increased along with the increase of the addition amount of the lychee seed starch. When the addition amount of the lychee seed starch is constant, the tensile strength is reduced along with the increase of the addition amount of the glycerol. Therefore, the tensile strength can be improved by adding proper amount of glycerin and litchi seed starch.
As can be seen from fig. 11, the contour line is elliptical, indicating that BC interaction is significant. The addition amount of litchi flavone and the addition amount of chitosan are increased, and the tensile strength is increased and then reduced. When the addition amount of chitosan is fixed, the tensile strength is gradually reduced along with the increase of the addition amount of litchi flavone. When the addition amount of litchi flavone is fixed, the tensile strength is increased and then reduced along with the increase of the addition amount of chitosan. Therefore, the tensile strength can be improved by properly adding the litchi flavone and the chitosan.
As can be seen from fig. 12, CD interaction is illustrated as significant. The addition amount of litchi flavone and the addition amount of glycerin are increased, and the tensile strength is gradually reduced. When the addition amount of the glycerin is fixed, the tensile strength is gradually reduced along with the increase of the addition amount of the litchi flavone. When the addition amount of the lychee flavone is fixed, the tensile strength is gradually reduced along with the increase of the addition amount of the glycerol. Therefore, the tensile strength can be improved by properly adding the litchi flavone and the glycerol.
The optimal film formula is obtained according to the design response surface 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 the process condition is 3.45852MPa. The improved process parameters according to the experimental conditions are as follows: the addition amount of the lychee seed starch is 70g/L,the addition amount of chitosan is 29g/L, the addition amount of lychee flavone is 1.0 percent, the addition amount of glycerin is 10g/L, 3 times of parallel experiments are carried out according to the process, the tensile strength of the obtained film is 3.40 +/-0.28 MPa, the elongation at break is 86.13 +/-4.66 percent, the film thickness is 0.18 +/-0.00 mm, the light transmittance is 2.14 +/-0.11 percent, and the water vapor transmission rate is 7.1544 +/-0.2319 multiplied by 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, averagely dividing the cherry tomatoes into groups A, B, C and 3 and numbering the groups A1-A50, B1-B50 and C1-C50, wherein the group A is a control group, and the experimental group comprises the following components: and (3) performing experiments on all cherry tomatoes in the group B (using the composite membrane with the optimal formula) and the group C (using the composite membrane without litchi flavone and with the same addition amount of other substances as the group B), sampling at 3d intervals, and measuring the weight loss rate, the rotting rate and the soluble solids of the cherry tomatoes.
After the cherry tomatoes are picked, because the cherry tomatoes still have vigorous respiration and transpiration effects, the cherry tomatoes still lose weight due to water loss, and as can be seen from fig. 13: the weight loss rate of the cherry tomatoes is obviously increased along with the increase of the storage time. From the whole view, the weight loss rate of the control group is higher than that of the group for preserving freshness by using the composite membrane, wherein the composite membrane group using the optimal formula and the composite membrane group without the added lychee flavone are used. Compared with the preservation group, the weight loss rate of the cherry tomatoes which are not added with the litchi flavone and made into the composite film for preservation is higher than that of the cherry tomatoes which are made into the composite film with the optimal formula for preservation; the litchi flavone has an antioxidation effect and plays a role in inhibiting the respiration of cherry tomatoes, so that the speed of other vacuoles for water loss is reduced, and the fresh-keeping effect is better. The increase in the weight loss ratio of the control group was increased from the 4 th day, and the weight loss ratio of the control group was 17.69 ± 0.27% at the 10 th day. The weight loss ratio of the cherry tomatoes preserved by the optimal formula at the 10 th day is 14.49 +/-0.0026%, and is remarkably reduced (p is less than 0.05) compared with a control group, so that the composite film prepared by the optimal formula has a certain preservation effect on the cherry tomatoes; the cherry tomatoes are preserved by using a film without flavone, the weight loss ratio of the cherry tomatoes is 15.54 +/-0.0016%, and is lower than that of a control group and higher than that of the cherry tomatoes preserved by using the optimal formula composite film. The results show that from the weight loss rate of cherry tomatoes, the freshness of the composite film prepared by the formula without adding litchi flavone is lower than that of the composite film prepared by the optimal formula, and the freshness of the composite film prepared by the optimal formula is the best.
It is easily squeezed by machinery and infected by mold during transportation and storage, as can be seen from fig. 14: in 2-4d, the rotting rate of the cherry tomatoes in the control group and the experimental group is 0%, and when the rotting rate of the experimental group after 4d is lower than that of the control group, the rotting rate of the cherry tomatoes preserved by the composite film prepared by the optimal formula is lower than that of the cherry tomatoes in the control group, and the rotting rate of the cherry tomatoes preserved by the film prepared by not adding flavone is lower than that of the cherry tomatoes prepared by the film prepared by the control group. The fresh-keeping effect of the composite film prepared by using the optimal film formula is superior to the fresh-keeping effect of the composite film prepared without adding flavone from the aspect of the index of the rotten rate. When the storage time is 10 days, the rot rate of the cherry tomatoes in the control group is 0.57 +/-0.06%, the rot rate of the cherry tomatoes under the preservation of the composite film without flavone is 0.37 +/-0.06%, the rot rate of the cherry tomatoes under the preservation of the film using the optimal formula is 0.30 +/-0.00%, wherein the rot rate of the blank group is obviously higher (p < 0.05) than that of the experimental group. The composite film prepared by the optimal formula has a certain fresh-keeping effect on cherry tomatoes from the viewpoint of rotting rate.
The content of soluble solids can directly reflect the maturity, quality and the like of the fruits. As can be seen from fig. 15: according to the cherry tomatoes preserved in different modes, the soluble solid content of the cherry tomatoes is in a descending trend along with the increase of the storage time, and the reduction of the content of the soluble solid content is possibly related to the self-respiration and other physiological metabolism of the cherry tomatoes. In general, the content of soluble solids in the cherry tomatoes of the experimental group in 2-10 days is higher than that of the cherry tomatoes of the control group. When the cherry tomatoes are stored for 10 days, the content of soluble solids of the cherry tomatoes in a control group is 2.43%, the content of the soluble solids of the cherry tomatoes prepared by the composite film prepared by the optimal formula is 4.6%, the content of the soluble solids of the cherry tomatoes prepared by the composite film without litchi flavone is 4.1%, and the content of the soluble solids of the cherry tomatoes prepared by the composite film is remarkably higher (p is less than 0.05) than that of the cherry tomatoes in the control group, which shows that the composite film can effectively slow down the reduction of the soluble solids of the cherry tomatoes, so that the cherry tomatoes have a certain fresh-keeping effect.
Wrapping the lychee seed extract composite membrane and the composite membrane without lychee flavone on cherry tomatoes for fresh keeping, and measuring the rotting rate, the weight loss rate and the soluble solid content of the cherry tomatoes after 0d, 2d, 4d, 6d, 8d and 10d respectively. The results show that: on the whole, the rotting rate and the weight loss rate of the cherry tomatoes wrapped by the optimal film in 2-10 days after the cherry tomatoes are placed are lower than the rotting rate and the weight loss rate of the cherry tomatoes wrapped by the composite film without the flavone and the cherry tomatoes of a control group; the content of soluble solid is higher than that of the cherry tomatoes wrapped by the composite film without the flavone and the cherry tomatoes of the control group. After the cherry tomatoes are placed for 10 days, the weight loss rate of the cherry tomatoes wrapped by the litchi seed extract composite membrane 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 membrane without the added flavone and the weight loss rate of the cherry tomatoes of a control group; the rotting rate is respectively 3.33 percent and 23.3 percent lower; the content of soluble solid 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 superior to that of a composite film without flavone.
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and not for limiting the technical solutions, and those skilled in the art should understand that modifications or equivalent substitutions can be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all that should be covered by the claims of the present invention.
Claims (8)
1. A method for preparing an antibacterial preservative film by utilizing a lychee seed/shell extract is characterized by comprising the following steps:
step 1: obtaining extracts from litchi shells and litchi seeds, respectively; wherein, lychee shell flavone extracting solution and lychee shell polysaccharide extracting solution are obtained from lychee shells; obtaining a lychee seed starch extract and a lychee seed flavone extract from lychee seeds;
step 2: mixing chitosan with an acetic acid solution with the mass fraction of 1% to prepare a chitosan solution with a certain concentration; wherein when the litchi shell extract is used as a raw material, the addition amount of the chitosan is 1.5-3.5% in percentage by mass; when the lychee seed extract is used as a raw material, the addition amount of the chitosan is 10-50 g/L;
and step 3: when the lychee shell extract is used as a raw material, adding gelatin into a chitosan solution, stirring at 50 ℃ until the gelatin is completely dissolved, sequentially adding lychee shell flavone extracting solution and lychee shell polysaccharide extracting solution, stirring uniformly at 50 ℃ to obtain a mixed solution, adding glycerol accounting for 2% of the mass of the mixed solution, and stirring uniformly to obtain a composite membrane liquid I; wherein, the addition amount of the gelatin is 1.0-3.0%, the addition amount of the lychee shell flavone is 5-25%, and the addition amount of the lychee shell polysaccharide is 5-25% in percentage by mass;
when the lychee seed extract is used as a raw material, adding distilled water into the lychee seed starch extract, uniformly mixing, then pasting at a constant temperature of 90 ℃ for 40min to obtain a pasting liquid, cooling to 50 ℃, then uniformly mixing the chitosan solution obtained in the step (2) with the pasting liquid, then adding the lychee seed flavone extract and glycerol, and uniformly mixing to obtain a composite membrane liquid II; wherein, the addition amount of the lychee seed flavone is 1-5% by mass percent; the addition amount of the lychee seed starch is 40-80 g/L, and the addition amount of the glycerol is 5-25 g/L;
and 4, step 4: ultrasonically degassing the composite membrane liquid I at 50 ℃ and 100Hz for 30min, uniformly casting and standing, and drying to form a membrane to obtain the antibacterial preservative membrane; and filtering the composite membrane liquid II by using a 40-mesh sieve, uniformly casting, and drying to form a membrane to obtain the antibacterial preservative membrane.
2. The method for preparing the antibacterial preservative film by using the lychee seed/shell extract as claimed in claim 1, wherein in the step 1, the lychee shell flavone extract is obtained by the following method:
fully drying the lychee shells, crushing and sieving, and mixing the dried lychee shells with water according to the mass volume ratio of 1g:20mL, adding litchi shell powder into an 80% ethanol solution, leaching at the constant temperature of 78 ℃ for 4h, filtering, concentrating the filtrate in vacuum to dryness, and dissolving in a 70% ethanol solution; wherein the flavone content in the litchi rind flavone extract is 62.4 mug/mL.
3. The method for preparing the antibacterial preservative film by using the lychee seed/shell extract as claimed in claim 1, wherein in the step 1, the lychee chitin extract is obtained by the following method:
completely drying litchi shells, crushing, sieving, and mixing according to a mass volume ratio of 1g:20mL, adding litchi shell powder into water, leaching at constant temperature of 100 ℃ for 3h, concentrating the filtrate after suction filtration in vacuum to dryness, dissolving the filtrate in water, adding the filtrate into 95% ethanol solution, standing at 4 ℃ for 12h, performing centrifugal separation, washing sediments by using 95% ethanol solution, and performing constant volume by using distilled water to obtain a litchi shell polysaccharide extracting solution; wherein the polysaccharide content in the lychee chitin extracting solution is 257.1 mu g/mL.
4. The method for preparing the antibacterial preservative film by using the lychee seed/shell extract as claimed in claim 1, wherein in the step 1, the lychee seed starch extract is obtained by the following method:
crushing lychee seeds into powder, and mixing the powder according to the mass volume ratio of 1g:20mL, dissolving the lychee seed powder in water, performing ultrasonic extraction, sieving with a 120-mesh sieve, repeatedly extracting for 3 times, combining extracting solutions, centrifuging, taking precipitate, and drying at 40 ℃ to obtain a lychee seed starch extract; wherein the starch content is 61% -62%.
5. The method for preparing the antibacterial preservative film by using the lychee seed/shell extract as claimed in claim 1, wherein in the step 1, the lychee seed flavone extract is obtained by the following method:
crushing lychee seeds into powder, drying, and mixing the powder with the following components in a volume mass ratio of 1g:10mL, mixing with 50% ethanol solution, refluxing at 80 ℃ for 1h, extracting for 2 times, filtering, mixing the extracting solutions, and concentrating and fixing the volume of the extracting solution to obtain lychee seed flavone extracting solution; wherein the concentration of flavone in the lychee seed flavone extracting solution is 879.96 mu g/mL.
6. The method for preparing the antibacterial preservative film by using the lychee seed/shell extract as claimed in claim 1, wherein when the lychee seed extract is used as a raw material, the addition amount of the lychee seed starch is 70g/L, the addition amount of the chitosan is 29g/L, and the addition amount of the glycerol is 10 g/L; according to the mass percentage, the addition amount of the lychee seed flavone is 1.0%.
7. The method for preparing the antibacterial preservative film by using the lychee seed/shell extract as claimed in claim 1, wherein when the lychee shell extract is used as a raw material, the chitosan addition amount is 2.8%, the gelatin addition amount is 2.7%, the lychee shell flavone addition amount is 15.2%, and the lychee shell polysaccharide addition amount is 15.0% in percentage by mass.
8. The application of the litchi seed/shell extract in preparing 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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