CN115181350B

CN115181350B - Fruit and vegetable fresh-keeping base film material and preparation method and application thereof

Info

Publication number: CN115181350B
Application number: CN202210889843.6A
Authority: CN
Inventors: 肖乃玉; 肖更生; 罗文翰; 仲芸芸; 陈国健; 王红蕾; 王奕丹; 王青; 钟乐; 翟万京; 张雪琴; 刘卫玲
Original assignee: Guangdong Danqing Flexible Packaging & Printing Co ltd; Zhongkai University of Agriculture and Engineering
Current assignee: Guangdong Danqing Flexible Packaging & Printing Co ltd; Zhongkai University of Agriculture and Engineering
Priority date: 2022-07-27
Filing date: 2022-07-27
Publication date: 2024-01-16
Anticipated expiration: 2042-07-27
Also published as: CN115181350A

Abstract

The invention discloses a fruit and vegetable fresh-keeping base film material which is a composite film mainly prepared from inorganic particles, low Density Polyethylene (LDPE) and ethylene-octene copolymer elastomer (POE), wherein the inorganic particles are silica gel or calcium chloride. Further, micropores are also arranged on the surface of the composite membrane. The fruit and vegetable fresh-keeping base film material is also disclosed, and the application of the fruit and vegetable fresh-keeping base film material in the aspects of fruit and vegetable storage and fresh keeping after picking. The preservative film material has better hydrophilicity and water absorption, light transmittance and mechanical property, and higher CO ₂ /O ₂ The permeability ratio is improved by arranging micropores on the surface of the base film material, so that the fruit and vegetable preservative film has better preservation and fresh-keeping effects on fruits and vegetables, especially strawberries, and can also relieve the phenomenon of condensation in the base film material and prolong the shelf life of the fruits and vegetables, especially strawberries.

Description

Fruit and vegetable fresh-keeping base film material and preparation method and application thereof

Technical Field

The invention belongs to the technical field of fruit and vegetable fresh-keeping materials, and particularly relates to a fruit and vegetable fresh-keeping base film material, and a preparation method and application thereof.

Background

In the process of transporting and storing the picked fruits and vegetables, the fruits and vegetables can not supplement water from plant bodies and simultaneously continuously disperse the water by the transpiration, and the water is gradually discharged out of the fruits and vegetables to form water vapor. For the packaging bag for storing and preserving fruits and vegetables, the gas transmittance and the water vapor transmittance of the packaging bag are generally poor, and the water vapor in the packaging bag cannot be timely discharged out of the packaging bag, so that the humidity in the packaging bag can rise rapidly. As the external temperature decreases during refrigerated transportation, the saturated water vapor pressure in the bag decreases, and the cooled air in the bag cannot contain as much water vapor, so that the water vapor in the bag cannot be discharged out of the bag in time, and condensation of the water vapor in the bag occurs. The water drops condensed in the packaging bag contact CO generated by fruits and vegetables due to respiration ₂ After that, weak acid liquid is formed, and microorganisms on the surfaces of fruits and vegetables can be promoted to reproduce by contacting or attaching the surfaces of the fruits and vegetables, so that the putrefaction of the fruits and vegetables is accelerated.

For conventional packaging materials, the CO of the packaging material ₂ /O ₂ The transmittance is preferably between 4 and 6. For fruit and vegetable packaging material, CO of the packaging material ₂ /O ₂ The transmittance is 8-10, which is suitable for fresh-keeping of fruits and vegetables. For fruit and vegetable packaging materials, if O is contained in the packaging bag ₂ The excessive concentration can promote the respiration intensity of fruits and vegetables in the bag and continuously consume nutrient substances in the fruits and vegetables; if packetBagging CO ₂ When the concentration is too high, the fruits and vegetables can generate anaerobic respiration, toxic gases such as ethanol and acetaldehyde are generated, and glycolysis reaction is generated in the fruits and vegetables. Therefore, the air permeability adjustment of the fruit and vegetable packaging material has important significance for fruit and vegetable fresh-keeping.

In recent decades, the development of functional polymer membrane materials, membrane technologies and membrane industries is gradually changed, and the functional polymer membrane materials, the membrane technologies and the membrane industries become one of the most important high-tech technologies at present. The Low Density Polyethylene (LDPE) is a common packaging bag material and has the advantages of rich sources, low price, good chemical stability, easy processing, market demand and the like. But the poor softness, elasticity, cohesiveness, air permeability and water vapor permeability of the LDPE do not meet the requirements of fruit and vegetable storage, and the application range of the LDPE in fruit and vegetable fresh-keeping is limited. Ethylene-octene co-elastomers (POE) are thermoplastic elastomers formed from ethylene and octene by in situ polymerization. When LDPE and POE are blended, the addition of POE can increase the elasticity of LDPE and the air permeability and the water vapor permeability, but the addition of POE leads to the CO of LDPE/POE base film ₂ /O ₂ The transmittance is limited.

Disclosure of Invention

The invention aims to provide a fruit and vegetable fresh-keeping base film material which has better hydrophilicity and water absorption, light transmittance and mechanical property and higher CO ₂ /O ₂ The permeability ratio is improved by arranging micropores on the surface of the base film material, so that the fruit and vegetable preservative film has better preservation and fresh-keeping effects on fruits and vegetables, especially strawberries, and can also relieve the phenomenon of condensation in the base film material and prolong the shelf life of the fruits and vegetables, especially strawberries.

The invention also aims to provide a preparation method of the fruit and vegetable fresh-keeping base film material, which has simple process and low cost.

The final object of the invention is to provide the application of the fruit and vegetable fresh-keeping base film material in the aspects of fruit and vegetable storage and fresh keeping after picking.

The first object of the present invention is achieved by the following technical scheme: the fruit and vegetable fresh-keeping base film material is a composite film mainly prepared from inorganic particles, low-density polyethylene (LDPE) and ethylene-octene copolymer elastomer (POE), wherein the inorganic particles are silica gel or calcium chloride.

Preferably, in the base film material, the mass ratio of the Low Density Polyethylene (LDPE) to the ethylene-octene copolymer elastomer (POE) is 70-90: 10 to 30, wherein the mass of the inorganic particles is 0.5 to 2.5 percent of the total mass of the Low Density Polyethylene (LDPE) and the ethylene-octene copolymer elastomer (POE).

More preferably, in the base film material, the mass ratio of the Low Density Polyethylene (LDPE) to the ethylene-octene copolymer elastomer (POE) is 80:20, wherein the mass of the silica gel or calcium chloride inorganic particles is 2% of the total mass of the Low Density Polyethylene (LDPE) and the ethylene-octene copolymer elastomer (POE).

Further, micropores are formed on the surface of the composite membrane.

Preferably, a plurality of micropores are arranged on the surface of the composite film by adopting laser, and the radius of each micropore is 140-160 mu m.

More preferably, the number of the micropores is two, and the radius of the micropores is 150 μm.

The second object of the present invention is achieved by the following technical scheme: the preparation method of the fruit and vegetable fresh-keeping base film material comprises the following steps: selecting low-density polyethylene (LDPE) particles and ethylene-octene copolymer elastomer (POE) particles, adding inorganic particles and a coupling agent, uniformly mixing in a material barrel of a double-screw extrusion casting machine, melting and extruding in the double-screw extrusion casting machine, and then carrying out subsequent conventional treatment to obtain the composite film.

Preferably, the temperature of the twin-screw extrusion casting machine in the process of melting and extruding is set to be 165-175 ℃ for a first material cylinder, the rotating speed of a melt pump is 10-12 rpm, and the linear speed of a casting rod is 1.5-2.5 m/min.

The last object of the invention is achieved by the following technical scheme: the application of the fruit and vegetable fresh-keeping base film material in the aspects of fruit and vegetable storage and fresh keeping after picking.

Preferably, the fruit is strawberry.

Compared with the prior art, the invention has the following advantages:

(1) The composite film containing 2wt.% of calcium chloride has better hydrophilicity and water absorption, excellent light transmittance and mechanical property and proper gas transmittance, so that the composite film is more suitable for preparing anti-dewing fruit and vegetable fresh-keeping packaging bags;

(2) Comprehensively comparing the storage conditions of the packaging bags of all the groups, the fresh-keeping effect of the packaging bags of all the groups is ordered from good to poor into 2 micropores, 4 micropores, 6 micropores, and unpunched common fresh-keeping bags, the composite film packaging bag with the micropores of 2 stores strawberries (about 100+/-5 g) at the temperature of 5 ℃ so as to be more beneficial to maintaining various nutrition indexes of the strawberries, and the composite film packaging bag with the micropores of 2 can effectively relieve the occurrence of dewing phenomenon in the packaging bag and prolong the shelf life of the strawberries in the packaging bag.

Drawings

FIG. 1 shows the oxygen and carbon dioxide permeation coefficients of the composite membrane at various amounts of hygroscopic inorganic particles added in example 2, wherein (a) the oxygen permeation coefficient of the Silica gel (Silica gel) composite membrane was added, (b) the oxygen permeation coefficient of the calcium chloride composite membrane was added, (c) the carbon dioxide permeation coefficient of the Silica gel composite membrane was added, and (d) the carbon dioxide permeation coefficient of the calcium chloride composite membrane was added;

FIG. 2 is a graph showing the carbon dioxide/oxygen transmission ratio of the composite film at various amounts of hygroscopic inorganic particles in example 2, wherein (a) silica gel, (b) calcium chloride;

FIG. 3 is an SEM image of a composite film at various amounts of added silica gel of example 2, wherein (a) 0wt.% silica gel, (b) 0.5wt.% silica gel, (c) 1wt.% silica gel, and (d) 2wt.% silica gel;

FIG. 4 is an SEM image of a composite film of example 2 with varying amounts of calcium chloride added, wherein (a) 0wt.% CaCl ₂ ，(b)0.5wt.％CaCl ₂ ，(c)1wt.％CaCl ₂ ，(d)2wt.％CaCl ₂ ；

FIG. 5 is a stress-strain plot of the composite film at various amounts of hygroscopic inorganic particles added in example 2, wherein (a) silica gel, (b) calcium chloride;

FIG. 6 shows the water vapor transmission coefficients (WVP) of composite membranes with different amounts of hygroscopic inorganic particles added in example 2, (a) silica gel, (b) calcium chloride;

FIG. 7 is the effect of different amounts of added silica gel on the contact angle of the composite film surface in example 2, wherein (a) 0wt.% silica gel, (b) 0.5wt.% silica gel, (c) 1wt.% silica gel, (d) 2wt.% silica gel;

FIG. 8 is a graph of various CaCl's in example 2 ₂ Effect of additive amount on composite film surface contact angle, wherein (a) 0wt.% CaCl ₂ ，(b)0.5wt.％CaCl ₂ ，(c)1wt.％CaCl ₂ ，(d)2wt.％CaCl ₂ ；

FIG. 9 is the effect of the addition of different hygroscopic inorganic particles on the water absorption (Water absorpition) of the composite film in example 2, wherein (a) silica gel, (b) calcium chloride;

FIG. 10 is the transmittance (transmissibility) of the composite film at various addition amounts of hygroscopic inorganic particles in example 2, wherein (a) silica gel, (b) calcium chloride;

FIG. 11 is an optical microscopy image of the effect of varying amounts of added silica gel on LDPE/POE based film in example 2, (a) 0wt.% silica gel, (b) 0.5wt.% silica gel, (c) 1wt.% silica gel, (d) 2wt.% silica gel;

FIG. 12 is an optical micrograph of the effect of varying amounts of calcium chloride added to LDPE/POE based films of example 2, (a) 0wt.% CaCl ₂ ，(b)0.5wt.％CaCl ₂ ，(c)1wt.％CaCl ₂ ，(d)2wt.％CaCl ₂ ；

FIG. 13 is a DTG plot of the composite membrane at varying amounts of hygroscopic inorganic particles added in example 2, wherein (a) silica gel, (b) calcium chloride;

FIG. 14 is a TGA graph of composite films at various hygroscopic inorganic particle loadings in example 2, wherein (a) silica gel, (b) calcium chloride;

FIG. 15 is a schematic view of strawberry packages with different numbers of holes in the package of example 3;

FIG. 16 is the O in each group of the packages during storage in example 3 ₂ And CO ₂ Content, wherein (a) is O ₂ (b) the graph is CO ₂ ；

FIG. 17 is a plot of the change in sensory evaluation score (Sensory evaluation score) of strawberries in each group of packages during storage in example 3;

FIG. 18 is a graph showing the variation of the strawberries in each group of bags during storage in example 3;

fig. 19 shows the color change of the strawberries in each group of bags during the storage period in example 3, wherein (a) shows the change of a-value (redness) of the strawberry skin in the bag, (b) shows the change of b-value (yellowness) of the strawberry skin in the bag, (c) shows the change of L-value (luminosity) of the strawberry skin in the bag, and (d) shows the change of h-value (chromaticity angle) of the strawberry skin in the bag;

FIG. 20 is a graph showing the change in Weight Loss (Weight Loss) of the strawberries in each group of bags during storage in example 3;

FIG. 21 is a graph showing the variation in hardness (hardness) of the strawberries in each group of bags during storage in example 3;

FIG. 22 is a graph showing the change in Decay rate (Decay) of the strawberries in each group of bags during storage in example 3;

FIG. 23 is a graph showing the change in Soluble solids (Soluble solids) content of strawberries in each group of bags during storage in example 3;

fig. 24 is a graph showing the change in vitamin C (Vitamin C content) content of strawberries in each group of packages during storage in example 3;

FIG. 25 is a graph showing the change in Titratable acid (Titratable acid) content of strawberries in each group of bags during storage in example 3;

fig. 26 shows the variation of the total number of strawberry colonies (Total number of colonies) in each group of bags during storage in example 3.

Detailed Description

EXAMPLE 1 Effect of LDPE/POE base film preparation Process on packaging Properties

The preparation process parameters are optimized by adopting a response surface method by taking low-density polyethylene (LDPE) and ethylene-octene copolymer elastomer (POE) materials as base film materials, taking the temperature of a charging barrel, the rotating speed of a melt pump and the linear speed of a casting rod as experimental variables and taking the reduction of the thickness of an LDPE/POE film as an optimization target. On the basis, the gas permeability, the water vapor permeability and the CO of the LDPE/POE base film are further explored in the range of 0 to 50 parts by weight of the POE additive amount in the LDPE/POE base film ₂ /O ₂ The effect of the permeation ratio and the compatibility of LDPE/POE systems.

TestThe result shows that the mass part ratio of the low-density polyethylene (LDPE) to the ethylene-octene copolymer elastomer (POE) is 70-90: 10 to 30, the LDPE/POE base film has air permeability, water vapor permeability and CO ₂ /O ₂ The permeability ratio and the LDPE/POE system compatibility are good; the mass part ratio of the Low Density Polyethylene (LDPE) to the ethylene-octene copolymer elastomer (POE) is 80:20, permeability to gases, permeability to water vapor, CO of LDPE/POE based films ₂ /O ₂ The transmission ratio and the LDPE/POE system are the best in compatibility.

EXAMPLE 2 Effect of hygroscopic inorganic particles on the Performance of LDPE/POE based films

LDPE: POE weight ratio = 80 parts: 20 parts of a base film material, and respectively melting and blending silica gel particles with certain hygroscopicity, calcium chloride particles and the base film material to prepare a silica gel/LDPE/POE composite film and a calcium chloride/LDPE/POE composite film with certain hygroscopicity.

By testing the gas permeability, CO, of the composite membrane ₂ /O ₂ The effect of the addition of hygroscopic inorganic particles on the properties of the LDPE/POE base film was determined by the transmission ratio, water vapor permeability, light transmittance, mechanical properties, and thermal stability. The static contact angle test and the water absorption test of the composite film are measured, and the hydrophilic performance and the water absorption capacity of the composite film are characterized. In addition, the dispersion and combination of inorganic particles in the LDPE/POE base film are further analyzed by the optical microscope and scanning electron microscope results of the composite film.

1. Experimental materials and instruments

The chemicals and instrumentation used in the experiments included, but are not limited to, the following:

low Density Polyethylene (LDPE): model 2426H (food contact), the company plasticised by the billows; ethylene-octene copolymer (POE): 6102FL (food contact), vistamaxx company, usa; silica gel: 800-1000 mesh (micron-sized, food contact), shanghai microphone Biochemical technology company; calcium chloride dihydrate: superfine powder (micron-sized food can be contacted), and chemical industry of Shandong Rui Chen; titanate coupling agent: TC-114, tianchang city, tianchen chemical industry; silane coupling agent, a800523, shanghai microphone Biochemical technology company.

Twin-screw extrusion casting machine host: MEDI-20/40, a common laboratory analytical instrument Co., ltd., guangzhou; casting tensile test line: FDHU-35, guangzhou City Co-operation laboratory analytical instruments Co., ltd; an oil temperature circulation controller, AOS-05A, shenzhen God mechanical Co., ltd; oil-free air compressor: JYK30, zhejiang permanent source machine electric manufacturing Co., ltd; an electronic balance: YP2002; FA2004C: shanghai, instrument and meter Inc.; digital display electrothermal blowing drying box: TOM-9240A, TOMOS life sciences group; digital display micrometer thickness gauge: heng Zhouai general metering instruments Co., ltd; scanning electron microscope: JSM-6330F, japan electronics company;

Digital biological microscope: BK600, chongqing Orte optical instruments Limited; surface tension tester: DCAT20, dataPhysics, germany; single beam uv-vis spectrophotometer: UV-7504, shanghai Xinmao instruments Co., ltd; gas permeability tester: n500, guangzhou standard packaging equipment limited; moisture transmittance tester: w301, guangzhou standard packaging equipment Co., ltd; thermogravimetric analyzer: mettler tga 2, meltrele-tolidol, switzerland; electronic tensile testing machine: GBU-1, guangzhou standard packaging equipment Co., ltd.

2. Experimental method

2.1 preparation of silica gel/LDPE/POE composite Membrane

The LDPE particles and POE particles are mixed in advance according to the weight ratio of 80 parts: 20 parts are dried in a forced air drying oven for 12 hours to reduce the moisture of the polymer particles themselves. Mixing silica gel powder into four equal parts according to 0wt.%, 0.5wt.%, 1wt.%, 2wt.% and a silane coupling agent (the addition amount is 3wt.% of the addition amount of the silica gel powder), LDPE and POE of the total mass of polymer particles respectively, uniformly mixing the four equal parts in a material barrel of a casting-extruding machine, and carrying out melt extrusion in a twin-screw extrusion casting machine, wherein the temperature of the twin-screw extruding machine is set to be the component 1 temperature (the temperature of a material barrel is set to be the temperature of the material barrel), the rotating speed of a melt pump is set to be 11rpm, and the linear speed of a casting rod is set to be 2m/min.

2.2 preparation of calcium chloride/LDPE/POE composite Membrane

The weight ratio of LDPE particles to POE particles is 80 parts: 20 parts are dried in a forced air drying oven for 12 hours to reduce the moisture of the polymer particles themselves. Mixing calcium chloride powder according to 0wt.%, 0.5wt.%, 1wt.%,2wt.% of the total mass of the polymer particles respectively, and titanate coupling agent (the addition amount is 3wt.% of the addition amount of the calcium chloride powder), LDPE and POE into four equal parts, and respectively placing into a material barrel of a casting-extruding machine for uniform mixing. And (3) carrying out melt extrusion in a twin-screw extrusion casting machine, wherein the temperature of the twin-screw extruder is set to be the component 1 temperature (the temperature of a charging barrel I is 170 ℃), the rotating speed of a melt pump is 11rpm, and the linear speed of a casting rod is 2m/min.

3. Measurement of film Properties

3.1 testing the gas permeability coefficient performance of the film;

3.2 testing mechanical properties of the film: the mechanical property test of the film is carried out by adopting a GBU-1 electronic universal material tester according to the national standard ISO1184-1983 method.

3.3 testing the water vapor permeability coefficient performance of the film;

3.4 static contact angle test of film: the surface of the film was subjected to static contact angle measurement using a DCAT20 surface tension tester from DataPhysics, germany.

3.5 Water absorption test of film: the film Water Absorption (WA) WAs tested according to GB/T-1034-2008 test method.

3.6 film transmittance test: the film was tested for light transmittance using a UV-7504 type ultraviolet-visible spectrophotometer.

3.7 film thermal stability test: the films were tested for thermal stability using a Mettler tga 2 thermogravimetric analyzer.

3.8 thin film optical microscopy test: observing the surface morphology of the film by adopting an optical microscope, placing the sample film on a glass sheet, covering the sample film by using a glass slide, and placing the sample film on a microscope sample stage for observation.

3.9 thin film SEM test: the cross-sectional morphology of the film was observed by using a JSM-6330F scanning electron microscope from Japanese electronics company.

4. Results and analysis

4.1 analysis of air permeability of composite films

In the fruit and vegetable packaging film, the packaging materialThe oxygen and carbon dioxide permeability of the material plays a decisive role in the atmosphere in the packaging belt, and the proper CO in the packaging bag ₂ /O ₂ The transmittance is favorable for weakening the respiration of fruits and vegetables in the packaging bag and prolonging the shelf life of the fruits and vegetables. FIG. 1 shows the effect of different amounts of hygroscopic inorganic particles on the oxygen and carbon dioxide permeation coefficients of the composite membrane.

When no hygroscopic inorganic particles are added, the oxygen permeability coefficient and the carbon dioxide permeability coefficient of the LDPE/POE base film are 2736cm respectively ³ *cm/cm ² * s.times.Pa and 4160cm ³ *cm/cm ² * s.Pa. When 0.5wt.% of inorganic particles was added, the air permeability coefficient of the composite film was significantly reduced, as shown in fig. 1. The reason for this is probably that the moisture absorbent is tightly adhered with the LDPE/POE base film after being treated by the coupling agent, and the addition of the inorganic particles fills the gap between the LDPE and the POE chain segment and the chain segment, so that the air permeability of the composite film is greatly reduced. As the amount of hygroscopic inorganic particles added increases, the gas permeability coefficient of the composite film gradually increases. The reason for this is probably that the addition of inorganic particles causes the LDPE/POE base film to generate tiny micropores in the tape casting and stretching process, so that the air permeability of the composite film is improved to a certain extent.

As can be seen from fig. 1 (c) and (d), the carbon dioxide permeation coefficient of the composite film increases more than the oxygen permeation coefficient as the hygroscopic inorganic particles increase. This is because the gas permeability coefficient of the composite membrane is inversely related to the internal friction coefficient of the gas, wherein the internal friction coefficient of oxygen is 1.908 ×10 ^-5 Pa.s, internal friction coefficient of carbon dioxide of 1.803 ×10 ^-5 Pa·s. When the addition amount of the hygroscopic inorganic particles is fixed, the internal friction coefficient of the carbon dioxide is smaller, which is favorable for reducing friction force between the carbon dioxide and the composite membrane, so that the carbon dioxide gas can permeate the composite membrane more.

Is suitable for CO ₂ /O ₂ The permeation ratio plays an important role in prolonging the shelf life of fruits and vegetables, and is generally considered to be CO ₂ /O ₂ The fruit and vegetable fresh-keeping is facilitated when the transmittance is 8-10. As can be seen from FIG. 2, the CO of LDPE/POE based films ₂ /O ₂ The transmittance is 1.52, and is not suitable for fresh-keeping of fruits and vegetables. By adding moisture absorptionThe agent inorganic particles can effectively improve the CO of the composite membrane ₂ /O ₂ Transmittance ratio. CO with LDPE/POE based films ₂ /O ₂ Composite film CO with addition of 2wt.% silica gel particles compared to transmission ratio ₂ /O ₂ The transmittance is obviously improved to 31.59, but the distance between the transmittance and the transmittance suitable for fruit and vegetable fresh-keeping is larger. When 2wt.% CaCl is added ₂ After that, CO of the composite film ₂ /O ₂ The transmission ratio was 19.58. 2wt.% CaCl was added compared to a composite film with 2wt.% silica gel added ₂ Is more similar to the gas transmission ratio suitable for fruit and vegetable fresh-keeping.

4.2 SEM analysis of composite films

SEM image shows the cross-sectional morphology of the composite film and the combination of the inorganic particles of the moisture absorbent and the LDPE/POE base film. FIGS. 3 and 4 show SEM pictures of the cross-section of inorganic particles in LDPE/POE base films.

As can be seen from fig. 3 (a) and fig. 4 (a), the LDPE/POE base film is cut by scissors to form a thin film section before inorganic particles of the moisture absorbent are not added, and the thin film is not pulled to form a thread due to ductile fracture, so that the thin film is cut by scissors to form a smooth and flat section, and the bonding condition of the inorganic particles on the cross section of the thin film is more easily observed. FIG. 3 is a topography of silica gel particles on a LDPE/POE basement membrane cross section. The silica gel has good thermal stability and is not easy to deform under the high temperature condition of 200-300 ℃. As can be seen from fig. 3 (c) and 3 (d), the cross section of the composite film clearly shows the protrusion of the silica gel particles when the content of the silica gel particles is 2 wt.%. The reason for this is probably that when the silica gel/LDPE/POE composite film receives external force impact, the interfacial binding force between silica gel particles and the matrix is smaller than the external force impact, so that obvious particle bulge phenomenon exists on the section of the film. Notably, as can be seen from the graph (d) in fig. 3, the particle size on the cross section of the composite film to which 2wt.% of the silica gel particles were added was significantly increased compared to the cross section of the composite film to which 1wt.% of the silica gel particles were added, whereby it was seen that the cross section of the composite film to which 2wt.% of the silica gel particles were added exhibited a significant particle aggregation phenomenon.

FIG. 4 is CaCl ₂ SEM pictures of particles on LDPE/POE base film cross section. As can be seen from FIG. 4 (c) and FIG. 4 (d), when CaCl ₂ At an addition level of 0.5wt.% of particles, a small amount of fine particles appear to be present in the polymer film cross section in a physically cross-linked manner; when CaCl ₂ CaCl was added at 1wt.% of the particles ₂ The particles are enlarged, embedded in the base film and blurred at the interface with the base film, showing CaCl ₂ The particles and the base film have good interfacial bonding force. When CaCl ₂ When the addition amount of the particles is 2 wt%, the particles are partially embedded in the section of the base film and exist in the polymer network, and CaCl is used for preparing the polymer film ₂ Fine micro-pores appear between the particles and the matrix bond. It can be seen that CaCl ₂ The particles are well combined in the polymer, no obvious stripping phenomenon exists with the polymer, and further, the addition of the inorganic particles is verified to be beneficial to increasing the micropore number of the composite film, so that the air permeability of the composite film is increased. Furthermore, it can be observed from the cross section of the composite film that 2wt.% CaCl is added compared to silica gel particles ₂ The aggregation phenomenon of particles in the section of the composite membrane is relatively less.

4.3 mechanical Property testing of composite films

TABLE 1 tensile Properties of composite films at different silica gel loadings

Sample name	Tensile strength (MPa)	Elongation at break (%)	Young's modulus (MPa)
				Silica gel 0wt.%	19.5±0.1	316.2±1.1	6.2±0.1
Silica gel 0.5wt.%	15.2±0.5	578.7±8.7	2.6±0.1
				Silica gel 1wt.%	16.0±0.9	595.1±7.8	2.7±0.1
Silica gel 2wt.%	16.2±0.9	624.2±5.6	2.6±0.1

TABLE 2 tensile Properties of composite films at different calcium chloride addition levels

Sample name	Tensile strength (MPa)	Elongation at break (%)	Young's modulus (MPa)
				CaCl ₂ 0wt.％	19.5±0.1	316.2±1.1	6.2±0.1
CaCl ₂ 0.5wt.％	28.4±0.7	823.6±17.2	3.4±0.1
				CaCl ₂ 1wt.％	25.5±0.4	705.2±4.0	3.6±0.1
CaCl ₂ 2wt.％	22.5±0.3	687.9±8.0	3.3±0.1

The composite film added with the inorganic particles of the moisture absorbent is used for fresh-keeping packaging of fruits and vegetables, and the mechanical property of the composite film is one of factors to be considered in the packaging and transportation process of the fruits and vegetables. In the transportation process, the fruit and vegetable packaging material is influenced by factors such as falling impact received in the transportation process and the like when bearing the weight of a packaging object. Therefore, it is required that the packaging material have a certain ductility and mechanical strength. Fig. 5 is a stress-strain diagram of the composite film at various additive amounts of inorganic particles of moisture absorbent. Tables 1 and 2 show silica gel and CaCl, respectively ₂ Specific parameters of tensile strength, elongation at break and Young modulus of the composite film are added in different amounts.

As can be seen from the graph (a) in FIG. 5 and the graph in Table 3, the LDPE/POE base film has a tensile strength of 19.5.+ -. 0.1MPa, an elongation at break of 316.2.+ -. 1.1MPa and a Young's modulus of 6.2.+ -. 0.1MPa. Compared with LDPE/POE base film, the tensile strength of the composite film is reduced by 22% and the Young's modulus is reduced by 58% after adding silica gel particles. With the increase of the added amount of silica gel, the tensile strength and Young's modulus of the composite film are basically unchanged. Generally, the tensile strength of a film containing inorganic particles is related to the binding force of the particles in the base film. From this, it is further proved that the interface bonding force between the silica gel particles and the base film is weaker in the silica gel/LDPE/POE composite film, when the composite film is impacted by an external force, the silica gel cannot disperse the impact of the external force on the composite film in the base film, so that the tensile strength of the composite film is obviously weakened after the silica gel particles are added. The tensile strength result of the composite film is consistent with the result observed by a scanning electron microscope. After the silica gel particles are added, the elongation at break of the composite film is obviously increased, and the elongation at break of the composite film is increased along with the increase of the content of the silica gel. The reason for this may be that the silica gel particles in the composite film partially absorb moisture in the air in the process of pulling up to form gel, so that the toughness of the particles in the composite film is enhanced, and the elongation at break of the composite film as a whole is obviously increased. This result is attributable to the increased viscoelasticity of the silica gel particles at the matrix interface and the matrix yielding (Wu et al, 2002).

As can be seen from the graph (b) in FIG. 5 and the graph in Table 4, caCl was not added ₂ In contrast, caCl was added ₂ The tensile strength and elongation at break of the composite film are significantly improved, but the Young's modulus of the composite film is reduced. With CaCl ₂ The tensile strength and the elongation at break of the composite film tend to increase and then decrease when the addition amount is increased, and the Young's modulus of the composite film is not greatly changed. The reason for this may be CaCl ₂ And the molecular chain of the basal membrane forms a network structure through the cross-linking action of the interface layer, caCl ₂ Wherein it acts as a cross-linking point. When the composite membrane is damaged by external stress, the crosslinking points can play a role in dispersing the external stress, so that the overall damage to the composite membrane is reduced. However, with CaCl ₂ Increased amount of CaCl ₂ The aggregation phenomenon occurs to a certain extent in the composite film, so that the interface defects of the composite film are increased, and the tensile strength and the elongation at break of the composite film are reduced along with the increase of the addition amount. However, when compared to LDPE/POE based films, caCl ₂ When the addition amount reaches 2wt.%, the tensile strength of the composite film is increased by 17%, and the elongation at break is increased by 115%.

From the mechanical property data of the added silica gel and calcium chloride particles, it can be further seen that CaCl ₂ Dispersibility of particles in LDPE/POE base film and base filmThe binding force is superior to that of silica gel particles, so that 2wt.% CaCl is added ₂ The tensile strength and the elongation at break of the composite film are superior to those of the composite film added with silica gel particles. According to the requirement that the tensile strength of the packaging material cannot be lower than 17MPa, 2wt.% of CaCl is added ₂ The tensile strength of the composite film is 22.8+/-0.3 MPa, and the mechanical property requirement of the packaging material in the transportation process can be met.

4.4 analysis of Water vapor Transmission of composite films

The fruit and vegetable packaging material has proper water vapor permeability, which is beneficial to prolonging the fresh-keeping period of fruits and vegetables. If the water vapor permeability of the packaging material is too high, a great deal of water loss of fruits and vegetables in the packaging bag can be caused, and the fruits and vegetables are withered. If the water vapor permeability of the packaging material is too low, a large amount of moisture in the packaging tape is accumulated, and the dew condensation phenomenon in the bag is serious.

Fig. 6 shows the water vapor transmission coefficient of the composite film at different amounts of hygroscopic inorganic particles added. As can be seen from fig. 6, the addition of the hygroscopic inorganic particles significantly decreases the water vapor transmission coefficient of the composite film. On one hand, the addition of the moisture absorbent fills the gaps between the chain segments in the LDPE/POE base film, so that the water vapor property of the composite film is reduced. On the other hand, because the moisture absorbent has the characteristic of absorbing moisture, when the water vapor permeates the composite membrane, one part of the water vapor can be absorbed by the electrodeless particles in the composite membrane, and the other part of the water vapor permeates the composite membrane. Therefore, when the content of the composite film moisture absorbent increases, the water vapor transmission amount of the composite film tends to be weakly reduced. When the amount of the hygroscopic inorganic particles to be added is necessarily such that the water vapor permeability of the composite film to which calcium chloride is added is relatively low. The water vapor transmission rate of the composite film with 2wt.% calcium chloride particles added was 6X 10 compared to LDPE/POE based film ^-13 g·m/m ² s.Pa, the water vapor permeability is lowered.

4.5 analysis of surface hydrophilicity and Water absorption of composite Membrane

The effect of the addition of the moisture absorbent on the hydrophilicity of the composite film was measured by the contact angle test. FIGS. 7 and 8 are silica gel and CaCl, respectively ₂ The effect of the amount of addition on the hydrophilicity of the composite film. As can be seen from fig. 7 and 8, the moisture absorbentBefore addition, the contact angle of the LDPE/POE base film is 101.7 degrees (more than 90 degrees) and shows obvious hydrophobicity. When the electrodeless particles are added into the LDPE/POE base film, the contact angle of the composite film is close to or smaller than 90 degrees, and the composite film has hydrophilicity. In general, a decrease in contact angle indicates an increase in the wettability of the film surface, thereby making the composite film more conducive to moisture absorption.

As can be seen from fig. 7, the hydrophilic increase of the composite membrane was small with the increase of the silica gel content. Referring to fig. 9 (a), the addition amount of silica gel has a small increase in water absorption rate of the composite film, and when the addition amount of silica gel is 2wt.%, the contact angle of the composite film is 87 °, and the water absorption rate of the composite film is 1.9%. As can be seen from FIG. 8, caCl ₂ The hydrophobicity of the LDPE/POE compound die is obviously improved. When CaCl ₂ At an addition level of 0.5wt.%, the composite film already exhibits hydrophilicity. As can be seen from the view (b) in FIG. 9, when CaCl ₂ When the amount of the additive was 2wt.%, the contact angle of the composite film was 85.2 °, and the water absorption was 3.5%. It can be seen that CaCl ₂ The increment effect of the addition of the silica gel composite membrane on the hydrophilicity and the water absorption of the composite membrane is larger than that of silica gel particles. The reason for this may be that external moisture may fill in the interstices of the framework formed by the bonding of the silicon oxygen tetrahedra within the silica gel. The hydrophilicity and water absorption data of the composite film at different addition amounts of the moisture absorbent particles further indicate the reason why the water vapor permeability of the composite film gradually decreases with the increase of the addition amount of the moisture absorbent particles.

4.6 analysis of light transmittance Property of composite film

The light transmittance of the packaging material reflects the transparency of the material under the condition of visible light, and has direct influence on the appearance display of fresh-keeping objects in the fresh-keeping package. Fig. 10 shows the change in transmittance of the composite film at different moisture absorbent contents.

From FIG. 10 (a), it is clear that when no silica gel particles were added to the LDPE/POE base film, the transmittance of the LDPE/POE base film reached 80% in the visible light region (500-900 nm). When 0.5wt.% of the silica gel particles were added, the transmittance of the composite film in the visible region was similar to that before the addition of the silica gel particles. With the increase of the addition amount of the silica gel particles, the transmittance of the composite film in the visible light region is obviously reduced. When 1wt.% and 2wt.% of silica gel particles were added, the light transmittance of the composite film was 70% and 50%, respectively. The reason for this may be that, when the addition amount of the silica gel particles is increased, the aggregation phenomenon of the silica gel particles in the composite film causes uneven distribution of the silica gel particles in the composite film, thereby causing a significant decrease in the transmittance of the composite film.

FIG. 10 (b) shows CaCl ₂ The effect of the amount of added LDPE/POE base film on light transmittance. When 0.5wt.% CaCl was added ₂ After that, the transmittance of the composite film is slightly increased to 82%. With CaCl ₂ The light transmittance of the composite film is slightly reduced by increasing the amount of the additive. At the addition of 2wt.% CaCl ₂ After that, the transparency of the composite film can still reach 75%, thus the CaCl can be seen ₂ The amount of (2) added has little effect on the transparency of the composite film.

Thus, the CaCl addition was compared with the silica gel addition ₂ The particles can enable CaCl ₂ And more evenly distributed in the LDPE/POE base film. And when 2wt.% CaCl is added to the LDPE/POE base film ₂ After that, the composite film still has good transparency in the visible light region, and has little influence on the displaceability of the packaging object.

4.7 optical microscopy of composite films

To further analyze the relationship between the dispersibility of the inorganic particles of the moisture absorbent in the LDPE/POE base film and the light transmittance, the dispersion of the inorganic particles in the base film was observed by an optical microscope. The better the transmittance of the composite film, the better the dispersibility of the moisture absorbent in the base film.

As can be seen from fig. 11 and 12, as the addition amount of the moisture absorbent increases, the density of the moisture absorbent on the surface of the base film increases significantly and the moisture absorbent is uniformly distributed in the surface of the base film. It is noted that, in fig. 11 (d) and 12 (d), when the addition amount of the moisture absorbent was increased to 2wt.%, the aggregation of the silica gel particles in the surface of the LDPE/POE base film was significantly higher than that of the calcium chloride particles, so that the silica gel particles appeared to have a larger particle size on the surface of the base film, which further verified the reason why the light transmittance of the composite film was significantly reduced after the addition of the silica gel particles.

4.8 analysis of thermal stability of composite films

The DTG/TGA plots of fig. 13 and 14 investigate the thermal stability of the composite films at different moisture absorber addition levels. As can be seen from fig. 13 and 14, after the moisture absorbent particles are added to the LDPE/POE base film, the thermal degradation peak of the composite film is not significantly changed, and thermal degradation is performed at 470 ℃. And with the increase of the addition amount of the moisture absorbent particles, the thermal degradation peak of the composite film has no obvious deviation, which indicates that the addition amount of the moisture absorbent has no obvious change on the thermal stability of the composite film. Therefore, the addition of hygroscopic particles has little effect on the thermal stability of the LDPE/POE base film.

5. The knot of this embodiment

(1) The gas permeability analysis of the composite membrane shows that with the addition of the absorbent silica gel particles and the calcium chloride particles, the composite membrane CO ₂ /O ₂ The transmittance ratio is obviously improved, and the CO of the LDPE/POE base film ₂ /O ₂ The transmittance was 1.52, and when the calcium chloride was added in an amount of 2wt.% of CO of the composite film ₂ /O ₂ A transmittance of 19.58, and a CO content of 2wt.% calcium chloride particle composite membrane compared with 2wt.% silica gel particles ₂ /O ₂ The transmittance is closer to that of the fruit and vegetable packaging gas CO ₂ /O ₂ The transmittance (8-10) is required;

(2) According to SEM test and mechanical property analysis of the composite membrane, compared with a composite membrane containing silica gel particles, the combination degree of the calcium chloride particles and the LDPE/POE base membrane is better, and the mechanical property analysis of the composite membrane shows that the tensile strength and the elongation at break of the composite membrane are gradually reduced along with the increase of the addition amount of the silica gel particles; with the increase of the addition amount of calcium chloride, the tensile strength and the elongation at break of the composite film tend to increase and then decrease. Compared with the tensile strength and the elongation at break of the LDPE/POE base film, the tensile strength of the composite film containing 2wt.% of calcium chloride is increased by 17 percent, and the elongation at break is increased by 115 percent; the tensile strength of the composite film containing 2wt.% of silica gel is reduced by 20%, and the elongation at break is increased by 97%. The mechanical property analysis of the composite membrane further proves that the binding force between the calcium chloride particles and the LDPE/POE base membrane is better.

(3) As is apparent from the analysis of the water absorption, surface hydrophilicity and light transmittance data of the composite film, the water absorption and film surface hydrophilicity of the composite film containing calcium chloride particles are better, the LDPE/POE base film has no hydrophilicity and almost 0 water absorption, and when the addition amount of calcium chloride particles is 2wt.%, the water absorption of the composite film is 3.5%, while the water absorption of the composite film containing 2wt.% silica gel is only 1.9%, and in addition, the light transmittance data of the composite film shows that after the silica gel particles are added, the light transmittance of the composite film is obviously reduced, compared with the LDPE/POE base film, the light transmittance of the composite film containing 2wt.% silica gel particles is only 50%, the light transmittance of the composite film is reduced by 30%, the light transmittance of the composite film containing 2wt.% calcium chloride particles is less affected, and the change of the light transmittance of the composite film further indicates that the dispersibility of calcium chloride in the LDPE/POE base film is better.

The composite film containing 2wt.% of calcium chloride has better hydrophilicity and water absorption, light transmittance and mechanical property and proper gas transmittance, thus being more suitable for preparing the anti-dewing fruit and vegetable fresh-keeping packaging bag.

Example 3 preparation of composite packaging film and application thereof in fresh-keeping and storage of fruits and vegetables, especially strawberry

The study data in example 1 show that when the LDPE: POE weight ratio is 80 parts: 20 parts of LDPE/POE base film has an oxygen permeability coefficient and a carbon dioxide permeability coefficient of 2736cm respectively ³ *cm/cm ² * s.times.Pa and 4160cm ³ *cm/cm ² *s*Pa，CO ₂ /O ₂ The transmittance was 1.5, and the water vapor permeability was 101.4975 ×10 ^-15 g*m/m ² * s Pa, but still fails to meet the requirement of fruit and vegetable packaging material CO ₂ /O ₂ The water vapor in the bag cannot be removed from the bag in time due to the requirement of the transmission ratio (8-10). After water drops are formed by water vapor in the bag, the water drops are contacted with carbon dioxide generated by respiration of fruits and vegetables to form weak acidic liquid, so that corrosion is easily generated on the surface of the fruits and vegetables, microorganism propagation on the surface of the fruits and vegetables is promoted, and fruit and vegetable spoilage is accelerated.

In example 2, the gas permeability and CO of the LDPE/POE composite film were adjusted by adding inorganic particles having hygroscopicity to give a LDPE/POE film a certain hygroscopicity ₂ /O ₂ Transmittance ratio. As a result of the study, it was found that, after inorganic particles were added, LDPE/POE was compoundedHygroscopicity, gas permeability and CO of the membrane ₂ /O ₂ The adjustment of the transmittance has a certain improvement effect, but still cannot completely meet the proper CO of the vegetable packaging material ₂ /O ₂ The transmittance and the high moisture permeability are required.

To realize the automatic adjustment of CO in the fruit and vegetable packaging bag ₂ 、O ₂ Gas content and water vapor content. In the embodiment, the fruit and vegetable fresh-keeping film is processed by laser perforation to form the microporous fruit and vegetable fresh-keeping packaging bag. Microporous membrane refers to microporous membranes formed in a specified number or size during the manufacturing process. The microporous fruit and vegetable packaging bag is beneficial to independently regulating the gas components of fruits and vegetables in the bag and increasing the water vapor permeability of the fruit and vegetable packaging bag. It is worth noting that the number of micropores in the packaging bag plays an important role in the fruit and vegetable fresh-keeping effect. If the number of the micropores is too large, the gas components in the bag are consistent with the external components, and the gas components in the bag cannot be regulated independently through the micropores; if the number of micropores is too small, the gas components suitable for fruit and vegetable fresh-keeping cannot be automatically regulated.

The edible part of the strawberry reaches more than 97%, the moisture content accounts for 90-95% of the weight of the fruit body, and the moisture content is extremely high. In addition, the surface of the strawberry is extremely thin, and the pulp is tender, so that the strawberry is extremely easy to be damaged by external force or infected by microorganisms in the storage and transportation processes, thereby causing deterioration and decay of the strawberry. In the embodiment, the moisture absorption calcium chloride particles are added and the laser perforation technology is combined to prepare the anti-condensation packaging film. The dewing phenomenon of the dewing prevention packaging bag under the low temperature (5 ℃) condition and the change condition of the strawberry nutrition index in the fresh-keeping process are researched, so that the optimal micropore number of the strawberry under the low temperature refrigeration condition is determined, and theoretical basis and practical application reference data are provided for the strawberry in the low temperature storage by utilizing the dewing prevention packaging bag.

1. Experimental method

1.1 preparation of calcium chloride/LDPE/POE composite Membrane

The weight ratio of LDPE particles to POE particles is 80 parts: 20 parts are dried in a forced air drying oven for 12 hours to reduce the moisture of the polymer particles themselves. Mixing 2wt.% of calcium chloride powder with titanate coupling agent (the addition amount is 3wt.% of the addition amount of the calcium chloride powder), LDPE and POE according to the total mass of polymer particles, placing the mixture into a material barrel of a casting-extruding machine, uniformly mixing, and carrying out melt extrusion in a twin-screw extrusion casting machine, wherein the temperature of the twin-screw extruder is set to be the component 1 temperature (the temperature of a material barrel is 170 ℃) in the melt extrusion process, the rotation speed of a melt pump is 11rpm, and the linear speed of a casting rod is 2m/min.

1.2 preparation of anti-dewing packaging bag and application thereof in strawberry preservation

Firstly, selecting an LDPE/POE composite film with the thickness of 33-37 mu m and containing 2wt.% of calcium chloride, and preparing a fruit and vegetable fresh-keeping packaging bag with the length of 20cm and the width of 15 cm. The fresh-keeping package bags (the sizes of the bags are the same as those of the fresh-keeping bags prepared above) sold in the market and the fresh-keeping bags which are not subjected to laser perforation treatment are used as comparison. The laser drilling speed is set to be 100m/s, the minimum power is set to be 13, the maximum power is set to be 13, and the dotting time is 0.01s. Laser punching treatment is carried out above the packaging bag filled with the strawberries by using a laser puncher, and anti-condensation packaging films with the micropore numbers of 2, 4 and 6 (the radius of the micropore is about 150 μm) are prepared for strawberry preservation experiments (the schematic diagram of the packaging bag is shown in figure 15). Strawberry was picked at the 12 th year 2020 strawberry planting base, and all strawberries were from the same variety. And (3) putting about 100+/-5 g of strawberries into a dewing-prevention packaging bag, sealing the strawberries in a hot sealing way, and storing the strawberries in a constant temperature refrigerator with preset temperature of 5 ℃ and 80% -90% RH. And extracting three strawberries in the same package for each test, and taking the average value of the three tests as the final value.

1.3 Performance measurement of strawberry storage quality

1.3.1 packaging bag internal O ₂ And CO ₂ Determination of the content: by hand-held O ₂ /CO ₂ O in the packaging bag of the detector ₂ And CO ₂ The content was tested.

1.3.2 sensory evaluation of strawberries in a bag: the ratio of male and female is 1 in a food college: 10 students of 1 were rated as panelists and sensory evaluation was performed on strawberries in the package every two days.

1.3.3 strawberry colorimetric determination: the colorimetry is carried out on the epidermis of three points selected equidistantly from the equatorial position of the strawberry in the packaging bag by using a colorimeter, three packaging bags are randomly extracted from each experimental group every two days, and 5 strawberries are randomly extracted from each packaging bag for colorimetry.

1.3.4 strawberry weight loss rate test: weight loss percentage of strawberries in the package was measured using a weighing method.

1.3.5 strawberry hardness test: the hardness of the strawberries in the package was measured using a hand-held durometer.

1.3.6 strawberry decay Rate determination: three bags of packaging bags are randomly extracted from each experimental group in each test, and the rotten fruits are judged to be rotten fruits when the number of the strawberry fruits exceeds 1/4. The decay rate of the strawberries was counted every two days.

1.3.7 strawberry soluble solids content test: three bags were randomly drawn from each experimental group every two days, 3 strawberries were randomly drawn from each bag, and the refractive index value of the solution was measured using a hand-held refractometer after the strawberry juice was filtered with gauze. The final test results are expressed as averages in%.

1.3.8 strawberry vitamin C content determination: three bags of packaging bags are randomly extracted from each experimental group every two days, 3 strawberries are randomly extracted from each packaging bag, and the vitamin C content of the strawberries is determined by using a 2, 6-dichloro sodium indophenol titration method in GB 5009.86-2016.

1.3.9 strawberry titratable acid content determination: three bags of bags were randomly drawn from each experimental group every two days, 3 strawberries were randomly drawn from each bag, and the titratable acid content of strawberries was determined by acid-base titration (Xu Huijin blue, etc., 2018).

1.3.10 strawberry colony count: three bags of bags were randomly drawn from each experimental group every two days, 3 strawberries were randomly drawn from each bag, and the total number of colonies of strawberries in the bags was measured according to "food microbiology test total number of colonies measurement" GB 4789.2-2016.

1.4 data processing: experimental data were processed using origin8.5 software and data are expressed as mean ± standard deviation. Significance analysis was performed using the Duncan multiple comparison test. If the difference in results is significant, p <0.05; if the difference in results is not significant, p >0.05.

1.5 results and discussion

1.5.1 variation of gas content in anti-dewing packaging bag during storage

FIG. 16 shows the commercial fresh-keeping bag films, the number of the composite film and the micropores (the micropore radius is about 150 μm) which are not subjected to laser perforation treatment are 2, 4 and 6, respectively, and the composite films are packaged in the package bag O during storage ₂ And CO ₂ Variation of content.

As can be seen from fig. 16, O during storage of the bag film and the non-laser perforated composite film ₂ And CO ₂ The variation of the content is similar. During the period of 0-12 days of storage, the content of carbon dioxide in the packaging bag obviously rises along with the extremely rapid decrease of the oxygen content in the packaging bag. The oxygen and carbon dioxide contents in the package were gradually equilibrated during days 12-18, at about 1% -2% and 16% -17%, respectively. The gas in the composite film bag with the composite film hole number of 2 gradually reaches balance at 15 days, and the balance concentration of oxygen and carbon dioxide is about 9.5 percent and 10.5 percent. When the number of pores of the composite membrane was increased to 4, the gas in the bag reached equilibrium at day 12, the oxygen concentration in the bag at the time of equilibrium was 11%, and the carbon dioxide concentration was 8.5%. When the number of pores of the composite film was increased to 6, the oxygen content and the carbon dioxide content in the bag were equilibrated after the next day of storage, wherein the equilibrium concentration of oxygen was 19%, and the equilibrium concentration of carbon dioxide was 0.2%. The number of micropores of the fresh-keeping bag is 6, and the gas components in the bag are similar to the gas components in the outside, so that the micropores have no obvious regulating effect on the gas components in the bag.

Therefore, the composite film packaging bag with the micropore number of 2 can maintain the balance concentration of oxygen and carbon dioxide in the bag to be about 9.5 percent and 10.5 percent in the later period of storage, and can delay the reduction of the sensory quality of the strawberries in the packaging bag, so that the quality of the strawberries in the packaging bag can keep better sensory quality within 18 days.

1.5.2 sensory evaluation changes in strawberry during storage

The influence of the packaging bags with different micropore numbers on the sensory quality of the strawberries can be intuitively seen through sensory evaluation of the strawberries in the packaging bags during storage. The change in sensory evaluation of strawberries in the package during storage by an evaluator at the food institute is shown in fig. 17.

To more intuitively understand the storage condition of the strawberries in each packaging bag and the condensation condition in the packaging bag. Fig. 18 shows the sensory changes of strawberries in various packages and dew condensation in the packages during storage. The sensory quality of the strawberries of the individual component packages did not change significantly during the storage period from day 0 to day 12.

On day 12 of storage, dew phenomenon appears in both the unperforated composite film package and the common fresh keeping bag, and the strawberry skin in the package begins to break. For the composite film fresh-keeping bag which is not perforated, the water absorbing agent on the composite film can only partially absorb the water vapor generated in the bag, so that the water absorbing agent also has dew phenomenon in the packaging bag at the later stage of storage. The composite film packaging bag with the micropore number of 2,4 and 6 has no dew condensation phenomenon temporarily when stored at 12 th day. The strawberries in the composite film packaging bag with 6 micropores begin to form condensation on the 15 th day of storage, and the condensation in the bag is obviously observed on the 18 th day of storage. On day 18 of storage, severe condensation of the unperforated composite film bags and conventional freshness bags occurred and the strawberries in the bags had deteriorated severely. Obvious dew condensation phenomenon does not appear in the compound film fresh-keeping bag with 2 micropores and 4 micropores, which reflects the condition that water vapor in the packaging bag is condensed in the bag under the synergistic effect of water absorbent in the packaging bag and micropore treatment.

1.5.3 color change of strawberry during storage

During the storage of the strawberries, the color of the strawberry skin changes with the change of the state of the strawberries. The color change of the strawberries in the package was evaluated by measuring the a (redness), b (yellowness), L (luminosity) and h (chromaticity angle) of the strawberry skin in the package stored at 5 ℃ for 18 days. The color difference value of the strawberry skin in each package is shown in fig. 19.

As can be seen from the data of fig. 19 (a), the value of the skin chromaticity a increases slightly as the strawberries in the respective bags mature gradually at the beginning of the storage period. The a values of the strawberry skins of each group reached a maximum value at day 6 of storage. After storage day 6, there was a significant drop in the a-value of the strawberry skin in each pack (P < 0.05), indicating that the strawberries in the pack began to age and the pigments in the strawberries began to degrade. The decrease in a-value of strawberry epidermis in the composite film package with 2 micropores was significantly smaller than that of the other four groups (P < 0.05). At the end of the storage period, the values of the strawberry epidermis a in the common fresh-keeping bag and the non-perforated composite film fresh-keeping bag are minimum, about 17.50+/-0.52. For the strawberry skin a in the bags with 2, 4 and 6 micropores, the values gradually decrease with the increase of the number of micropores in the bags, and the values are respectively: 25.60.+ -. 0.35, 23.34.+ -. 0.32 and 20.56.+ -. 0.45. As shown in fig. 19 (b), the b-value of the strawberry skin in each group of bags gradually decreases with the progress of the storage period. When the storage is finished, the b-value of the strawberry skin in the composite film packaging bag with the micropore number of 2 is obviously higher than that of the rest four groups (P < 0.05), and the b-value of the strawberry skin in the common composite film and the non-perforated composite film packaging bag is minimum.

The values of h (chromaticity angle) and L (luminosity) of the strawberry skin during storage can be combined to reflect the overall change in strawberry status and color. As can be seen from fig. 19 (c), the strawberry chromaticity angle values in the respective packages gradually decrease during storage. During the storage period of 0-9 days, except for the composite film packaging bag with 2 micropores, the h and L of the strawberry components of the packaging bag have no significant difference (P > 0.05). After the ninth day of storage, the h and L values of the unperforated composite film bags and the normal bags were significantly reduced (P < 0.05). At the end of storage, the h and L of the compound film fresh-keeping bags with the micropore number of 2 are obviously higher than the rest components (P < 0.05), which indicates that the compound film fresh-keeping bags with the micropore number of 2 are beneficial to maintaining the overall chromaticity and glossiness of strawberries and slowing down the chromaticity decomposition of fruits caused by aging.

Variation of weight loss rate of strawberries during 1.5.4 storage

The weight loss rate of fruits and vegetables is an important index for reflecting the freshness of fruits and vegetables during storage. During the storage of fruits and vegetables, the combined effect of the transpiration and respiration of the fruits can cause the evaporation of water and the consumption of nutrients in the fruits, thereby reducing the overall weight of the fruits.

Fig. 20 shows the change in weight loss rate of strawberries in each group of bags during storage. As can be seen from the data in the figures, during storage, weight loss occurred in all component strawberries. When the composite film fresh-keeping bag is stored for the ninth day, the integral weight loss rate of the composite film fresh-keeping bag and the common fresh-keeping bag which are not subjected to perforation treatment is obviously higher than that of other components (P is less than 0.05), and the weight loss rate is close to 6 percent. The overall weight loss rate of the two groups increases significantly with increasing storage time. At the end of the storage period, the weight loss rate of both reaches 11%. According to related researches, when the weight loss rate of the fruit body exceeds 6%, the fruit body is shrunken, and the fruit body has no commodity value. For the anti-dewing packaging bag, the weight loss rate of the strawberries in the compound film fresh-keeping bag with 2 micropores is not more than 3% at the end of the storage period, which indicates that the packaging bag can effectively delay the self-generated water loss of the strawberries, thereby delaying the shrinkage phenomenon of the strawberries.

1.5.5 hardness variation of strawberry during storage

Hardness of fruits and vegetables is one of the important factors affecting storage performance and commodity value. Meanwhile, the hardness of fruits and vegetables is one of important indexes for reflecting the storage quality and the maturity of fruits and vegetables. Fig. 21 is a graph showing the change in hardness of the strawberries in each group of packages during storage.

As can be seen from the data in fig. 21, the hardness of the strawberries in the package of each component decreased with the increase in storage time. The composite film package having 2 micropores maintains optimum hardness for the strawberries in the package compared to other package components. The reason for this is probably that the gas component in the composite film freshness protection package with the micropore number of 2 is beneficial to reducing the respiration rate of the strawberries in the package, thereby relieving the hardness loss of the strawberries.

Change in decay rate of strawberry during 1.5.6 storage

During low-temperature storage, the dewing phenomenon in the packaging bag is beneficial to colony propagation of the strawberry epidermis, so that decay of the strawberry is accelerated. Fig. 22 shows the change in the rate of decay of the strawberries in each group of bags during storage.

The data in fig. 22 shows that during 18 days of storage, there was a decay of the strawberry from each component. However, the decay rate of the nonporous composite film packaging bag and the common fresh-keeping bag is obviously increased after the ninth day of storage. The dew forming phenomenon occurs in the packaging bag, so that the aggregation and propagation of the strawberry epidermis are promoted, and the decay rate is obviously increased. For the strawberries in the anti-dewing packaging bag, the in-bag dewing phenomenon occurs in the packaging bag with 6 composite film with micropores on the 15 th day of storage, and after the storage is finished, the rotting rate of the strawberries in the packaging bag reaches 50 percent. After the storage is finished for 18 days, the decay rate of the strawberries in the composite film packaging bag with 2 micropores is only 12.5%, which indicates that the packaging bag of the type is beneficial to relieving the occurrence of the dewing phenomenon in the packaging bag, thereby reducing the decay rate of the strawberries.

Variation of soluble solids content of strawberry during 1.5.7 storage

The soluble solid in the fruits and vegetables mainly refers to soluble sugar, protein and the like which are soluble in water in the fruits and vegetables, and is an important index for measuring the maturity and quality of the fruits and vegetables in the storage period. Fig. 23 shows the change in soluble solids content of strawberries in each group of packages during storage.

As can be seen from the data in fig. 23, the soluble solids of the strawberries in each group of packages tended to decrease as the storage period progressed. On the third day of storage, the soluble solids of the strawberries in the packaging bags of each component are obviously increased due to further maturity of the strawberries in the packaging bags, but the difference of the soluble solids content of each component is not obvious. The reason for this change may be that during further ripening of the strawberry, further hydrolysis of the strawberry's own proteins and carbohydrates is promoted, such that the soluble sugar and protein content in the fruit body increases, resulting in an increase of the soluble solids in the fruit body. The soluble solids of the strawberry in the composite film packaging bag with 2 micropores reach the maximum value in the sixth day of storage, and the soluble solids content of the rest components gradually decreases after the third day of storage. After 9 days of storage, the soluble solids content of strawberries in the unperforated composite film pouches and the conventional pouches was significantly lower than that of the other components (P < 0.05). The reason is that the gas components in the two groups of packaging bags are in a state of high carbon dioxide content in the later storage period, so that the respiration of the strawberries is accelerated, and the soluble sugar of the fruits is continuously consumed. During storage, the soluble solids content of the strawberries in the composite film packaging bag with the micropore number of 2 is higher than that of other components, which indicates the respiration of the gas components in the packaging bag, so that the consumption of the soluble solids of the strawberries is reduced.

Variation of vitamin C content of strawberry during 1.5.8 storage

Vitamin C (ascorbic acid) content is an important indicator for measuring freshness of strawberries, and the vitamin C content of strawberries has an important influence on quality during storage. Fig. 24 shows the change in vitamin C content of strawberries in each group of packages during storage.

From the data in fig. 24, it can be seen that the vitamin C content of the strawberries of each component tended to increase and then decrease during storage. Three days before storage, the strawberries were further ripened in the package, resulting in an increase in vitamin C content of the strawberries themselves with increasing maturity of the strawberries. The vitamin C content of the strawberries in the individual component packages did not vary significantly (P > 0.05) during days 0-6 of storage. As the storage time increases, the vitamin C content of the strawberries in the component packaging bags tends to decrease. The vitamin C content of the strawberries in the unperforated composite film package and the conventional package was significantly reduced (P < 0.05) after day 9 of storage, indicating that the strawberries in the package had aged and spoiled. After the storage is finished, the vitamin C content of the strawberry in the packaging bag with the micropore number of 2 still is 35.96+/-1.58 mg/100g, which indicates that the strawberry in the packaging bag still keeps better quality.

Change in titratable acid content of strawberry during 1.5.9 storage

The total sugar and total acid in the strawberry fruit body are important substrates representing the metabolic activity of the strawberry fruit body, and the content change of the total sugar and the total acid in the strawberry fruit body reflects the consumption condition of the nutrient substances in the strawberry fruit body during storage. 90% of the titratable acid content in the strawberry fruit body is citric acid, and the change in content is mainly related to respiration and color change of the strawberries. Fig. 25 shows the change in titratable acid content of strawberries in each group of packages during storage.

As can be seen from fig. 25, the titratable acid content of the strawberries in each group of packages was reduced during storage. The titratable acid content of the strawberries in the composite film package with 2 micropores decreased more slowly during storage than the strawberries in the other component packages. The result proves that the gas component in the packaging bag can effectively reduce the respiration of the strawberries and reduce the utilization of titratable acid of the strawberry fruits.

Variation in total strawberry colony count during 1.5.10 storage

The surface of the strawberry is thinner and rough, and colonies are easier to propagate on the surface of the strawberry during storage, so that the strawberry is aged and rotted, and the shelf life of the strawberry is greatly influenced. Fig. 26 is a plot showing the variation in colony count of strawberries in each group of bags during storage.

The total number of the strawberry epidermis bacterial colony in the composite film packaging bag after perforation treatment is 10 days before storage ² -10 ³ In order of magnitude, in contrast, the total number of the strawberry epidermis in the composite film packaging bag without punching treatment and the common fresh-keeping bag is 10 ³ -10 ⁴ Between orders of magnitude. But at the beginning of the 9 th day of storage, the total number of strawberry skin colonies in the composite film packaging bag and the common fresh-keeping bag which are not subjected to perforation treatment is obviously increased. The total aggregation number of the strawberry epidermis in the common fresh-keeping bag reaches 3 multiplied by 10 at 12 days of storage ⁵ The strawberry skin in the composite film packaging bag is 8 multiplied by 10 ⁴ On the order of magnitude. In sensory evaluation, the occurrence of decay of the strawberry skin can be obviously observed in both groups of packaging bags. After the storage period is over, the total number of strawberry colonies in the two groups of packaging bags is 5×10 respectively ⁶ Order of magnitude and 7 x 10 ⁵ On the order of magnitude, a pronounced rot and mildew condition has been observed on the surface of strawberries. The reason is mainly that the concentration of carbon dioxide gas in the packaging bag is high in the later period of storage, so that the aging and decay of the strawberries are accelerated, and the propagation of the putrefying bacteria of the strawberries is caused. For the strawberry in the composite film packaging bag with 6 micropores, the total aggregation number of the strawberry surface after the storage is 5×10 ⁵ On the order of magnitude, and the phenomena of rot and mildew are clearly seen on the surface of the strawberries. The reason for this may be that when the number of micropores in the package is increased, the gas composition in the package is similar to the external gas composition, but is unfavorable for the strawberry to indicate the proliferation of microorganisms. Composite film fresh-keeping method with 2 and 4 microporesThe total number of bags in the bag was in a slowly rising state during storage, indicating that the gas components in the two sets of packaging films inhibited to some extent the colonisation of the strawberry epidermis.

1.6 knots in this embodiment

(1) The storage conditions of the packaging bags of all the groups are comprehensively compared, and the fresh-keeping effect of the packaging bags of all the groups is ordered from good to poor into 2 micropores, 4 micropores, 6 micropores and unpunched common fresh-keeping bags. The composite film packaging bag with the micropore number of 2 is more beneficial to maintaining various nutrition indexes of the strawberries when the strawberries (about 100+/-5 g) are stored at the temperature of 5 ℃. The gas components reaching an equilibrium state in the later stage of storage are 9.5% of oxygen and 10.5% of carbon dioxide, no obvious dew condensation phenomenon is observed in the package, the appearance of strawberries in the package only appears in a dehydrated state, no obvious aging and rot phenomenon is observed, and the composite film package bag with the micropore number of 2 can effectively relieve the dew condensation phenomenon in the package bag and prolong the shelf life of strawberries in the package bag. The strawberry in the composite film packaging bag with the micropore number of 4 has a slightly worse fresh-keeping effect in the storage period than the strawberry in the composite film packaging bag with the micropore number of 2. For the composite film packaging bag with 6 micropores, as the gas components in the packaging bag are equivalent to the external gas components, the microporous treatment cannot perform spontaneous regulation and control on the gas components in the packaging bag, so that the strawberry preservation effect in the packaging bag is only better than that of a packaging preservation bag without holes.

(2) For the composite film fresh-keeping bag and the common fresh-keeping bag which are not subjected to laser perforation treatment, the dew condensation phenomenon of fresh-keeping of fruits and vegetables starts to appear after the 12 th day of storage. In the later storage period, the gas components of the two are the gas combination of high carbon dioxide and low oxygen, so that the strawberries in the two groups of packaging bags are obviously aged and rotten in the later storage period.

Example 4

The fruit and vegetable fresh-keeping base film material provided by the embodiment is a composite film mainly prepared from inorganic particles, low Density Polyethylene (LDPE) and ethylene-octene copolymer elastomer (POE), wherein the mass ratio of the inorganic particles to the Low Density Polyethylene (LDPE) to the ethylene-octene copolymer elastomer (POE) is 90:10, wherein the mass of the inorganic granular calcium chloride is 0.5% of the total mass of the inorganic granular calcium chloride and the inorganic granular calcium chloride. Two micropores with a radius of 150 μm were provided on the surface of the packaging bag made of the composite film having a size as in example 3 using a laser. The preparation method of the fruit and vegetable preservative film material refers to example 2-example 3.

Example 5

The fruit and vegetable fresh-keeping base film material provided by the embodiment is a composite film mainly prepared from inorganic particles, low Density Polyethylene (LDPE) and ethylene-octene copolymer elastomer (POE), wherein the mass ratio of the inorganic particles to the Low Density Polyethylene (LDPE) to the ethylene-octene copolymer elastomer (POE) is 70:30, wherein the mass of the inorganic granular calcium chloride is 1.0% of the total mass of the inorganic granular calcium chloride and the inorganic granular calcium chloride. Two micropores with a radius of 150 μm were provided on the surface of the packaging bag made of the composite film having a size as in example 3 using a laser. The preparation method of the fruit and vegetable preservative film material refers to example 2-example 3.

Example 6

Unlike example 1, the inorganic particles were silica gel in an amount of 1.5% of the total mass of Low Density Polyethylene (LDPE) and ethylene-octene copolymer elastomer (POE).

Example 7

Unlike example 1, the inorganic particles were silica gel in an amount of 2.5% of the total mass of Low Density Polyethylene (LDPE) and ethylene-octene copolymer elastomer (POE).

While there have been described what are presently considered to be preferred embodiments of the present invention, it will be understood by those skilled in the art that the present invention is susceptible to various modifications, adaptations, combinations, alternatives and the like, all of which are within the spirit of the invention.

Claims

1. A fruit and vegetable fresh-keeping base film material is characterized in that: the base film material is a composite film mainly prepared from inorganic particles, low-density polyethylene, ethylene-octene copolymer elastomer and a coupling agent, wherein the inorganic particles are calcium chloride;

in the base film material, the mass ratio of the low-density polyethylene to the ethylene-octene copolymer elastomer is 80:20, wherein the mass of the calcium chloride inorganic particles is 2% of the total mass of the low-density polyethylene and the ethylene-octene copolymer elastomer;

micropores are also arranged on the surface of the composite membrane;

The number of the micropores is two, and the radius of the micropores is 140-160 mu m.

2. The preparation method of the fruit and vegetable fresh-keeping base film material as claimed in claim 1, which is characterized by comprising the following steps: selecting low-density polyethylene particles and ethylene-octene copolymer elastomer particles, adding inorganic particles and a coupling agent, uniformly mixing in a material barrel of a double-screw extrusion casting machine, melting and extruding in the double-screw extrusion casting machine, and performing subsequent conventional treatment to obtain the composite film.

3. The preparation method of the fruit and vegetable fresh-keeping base film material according to claim 2, which is characterized in that: in the melt extrusion process, the temperature of the twin-screw extrusion casting machine is set to be 165-175 ℃ for a first material cylinder, the rotating speed of a melt pump is 10-12 rpm, and the linear speed of a casting rod is 1.5-2.5 m/min.

4. The use of the fruit and vegetable preservative film material according to claim 1 in the preservation and fresh-keeping of picked fruits and vegetables.

5. The use according to claim 4, characterized in that; the fruit is strawberry.