Frozen food refers to foods that have passed the appropriate processing and processed, and are frozen at a temperature of -30 ° C, and stored and circulated at -18 ° C or lower after packaging. Due to the low temperature cold chain preservation throughout the process, frozen foods have the characteristics of long shelf life, non-corruption and convenient consumption, but they also pose greater challenges and higher requirements for packaging materials.
First, common frozen food packaging materials
At present, the following frozen material packaging bags are commonly used in the market:
1, PET / PE
This structure is more common in quick-frozen food packaging, and has good moisture-proof, cold-resistant, low-temperature heat-sealing performance and relatively low cost.
2, BOPP / PE, BOPP / CPP
Such structures are high in moisture resistance, cold resistance and low temperature heat sealing, and the cost is relatively economical. Among them, the appearance and feel of the bag of BOPP/PE structure is better than that of PET/PE structure, which can improve the product grade.
3, PET / VMPET / CPE, BOPP / VMPET / CPE
Such a structure is exquisitely printed due to the presence of an aluminum-plated layer, but the low-temperature heat-sealing property is slightly inferior and the cost is high, so the usage rate is relatively low.
4. NY/PE, PET/NY/LLDPE, PET/NY/AL/PE, NY/PE
The packaging of such a structure is resistant to freezing and impact. Due to the presence of the NY layer, its puncture resistance is good, but the cost is relatively high, and it is generally used for packaging angular or heavier products.
In addition, there is also a simple PE bag, which is generally used as an outer bag for vegetables and simple frozen food.
In addition to packaging bags, some frozen foods need to use plastic trays. The most commonly used tray material is PP. Food grade PP has good hygienic properties and can be used at low temperature of -30 °C. It also has PET and other materials. As a universal transportation package, corrugated boxes have the advantages of shockproof, compression resistance and cost, which are the first considerations for frozen food transportation packaging.
Second, the detection standard of frozen food packaging
Qualified products must have qualified packaging. In addition to testing the product itself, the product must be inspected and qualified before entering the circulation field.
At present, there is no special national standard for the detection of frozen food packaging, and industry experts are working together with frozen food production companies to actively promote the development of industry standards. Therefore, frozen food manufacturers must meet the general national standards for relevant packaging materials when purchasing packaging.
E.g:
GB 9685-2008 "Sanitary Standards for the Use of Additives for Food Containers and Packaging Materials" provides hygiene standards for additives for food containers and packaging materials;
GB/T 10004-2008 "Packaging plastic composite film, bag dry composite, extrusion compound" specifies a composite film, bag and dry composite, coextrusion composite process without plastic and aluminum foil , the appearance of the bag, physical indicators, and the amount of residual solvent in the composite bag, film;
GB 9688-1988 "Sanitary Standards for Polypropylene Moldings for Food Packaging" provides for the physical and chemical indicators of PP type packaging for food, and can be used as the basis for the designation of PP blister trays for frozen foods;
GB/T 4857.3-4 and GB/T 6545-1998 "Determination of the burst strength of corrugated board" respectively give the corrugated box stacking strength and burst strength requirements.
In addition, in actual operation, frozen food production enterprises will also formulate some enterprise standards that meet their own needs according to actual needs, such as the quantitative requirements for molded products such as plastic trays and foam barrels.
Third, the two major issues cannot be ignored
1. Dry consumption of food, "freezing and burning" phenomenon
Frozen storage can greatly limit the growth and reproduction of microorganisms and reduce the rate of food spoilage. However, for some frozen processes, the dry consumption and oxidation of foods will become more and more serious as the freezing time increases.
In the freezer compartment, there is a distribution of temperature and water vapor partial pressure: food surface > ambient air > cooler. This aspect is due to the fact that the heat on the surface of the food is transferred to the surrounding air, and the temperature is further lowered. On the other hand, the difference in the partial pressure of water vapor between the surface of the food and the surrounding air causes the moisture, ice crystals on the surface of the food to evaporate and sublimate. Water vapor is incorporated into the air.
At this point, the air containing more water vapor absorbs heat, the density decreases, and moves to the upper part of the freezer; when flowing through the cooler, the saturated water pressure at this temperature is also small because the cooler temperature is extremely low. While the air is being cooled, the water vapor contacts the surface of the cooler and condenses into a frost adhered thereto, and the density of the air after cooling becomes large, thereby sinking and coming into contact with the food again. This process is repeated and repeated, and the moisture on the surface of the food is continuously lost and the weight is reduced. This phenomenon is called “dry consumptionâ€.
During the continuous drying process, the surface of the food will gradually become porous, increasing the contact area with oxygen, accelerating oxidation of food fat and pigment, browning of the surface layer, and denaturation of the protein. "Freeze burning."
Since the transfer of water vapor and the oxidation of oxygen in the air are the root causes of the above phenomenon, as a barrier between frozen food and the outside world, the plastic packaging material used in the inner packaging should have good water vapor and oxygen barrier properties.
2. Influence of frozen environment on mechanical strength of packaging materials
It is well known that plastics become brittle, prone to cracking, and a sharp drop in physical properties when exposed to low temperatures for a long time, reflecting the weakness of plastic materials with poor cold resistance. Generally, the cold resistance of plastics is expressed by the embrittlement temperature. As the temperature decreases, the plastic becomes brittle and brittle due to the decrease of the activity of the polymer molecular chain. Under the specified impact strength, 50% of the plastic is brittle, and the temperature at this time is the embrittlement temperature. That is, the lower temperature limit for the normal use of plastic materials. If the packaging materials used in frozen foods have poor cold resistance, sharp projections of frozen foods can easily puncture the packaging during the later transportation loading and unloading process, causing leakage problems and accelerating food spoilage.
Frozen food is packaged in corrugated boxes during storage and transportation. The temperature of the cold storage is generally set at -24 ° C ~ -18 ° C. In the cold storage, corrugated boxes will gradually absorb the moisture in the environment, usually up to 4 days to achieve water balance. According to the relevant literature, when the corrugated box reaches the water balance, its water content will increase by 2% to 3% compared with the dry state. With the extension of refrigerating time, the edge pressure strength, compressive strength and bond strength of corrugated boxes will gradually decrease, and will decrease by 31%, 50% and 21% after 4 days, which means the mechanics of corrugated boxes after entering the cold storage. The intensity is affected to some extent, increasing the potential risk of post-cracking problems.
During the process of transporting frozen food from the cold storage to the place of sale, it will undergo multiple loading and unloading operations. The constant temperature difference will cause the water vapor in the air around the corrugated box to condense on the surface of the carton. The moisture content of the carton will rise rapidly to about 19%. The side pressure strength will drop by about 23% to 25%. At this time, the mechanical strength of the corrugated box is further damaged, increasing the probability of collapse. In addition, in the stacking process of the carton, the upper carton will exert a constant static pressure on the lower carton. When the carton has a pressure resistance decrease due to moisture absorption, the bottom carton will first deform and crush. According to statistics, the economic losses caused by the collapse of carton caused by moisture absorption and ultra-high stacking account for about 20% of the total loss in the circulation process.
Fourth, the solution
In order to minimize the frequency of occurrence of the above two major problems and ensure the safety of frozen foods, we can start from the following aspects.
1, choose high barrier, high strength inner packaging materials
There are many kinds of packaging materials and different performances. Only by understanding the physical properties of various packaging materials, can we choose reasonable materials according to the protection requirements of frozen foods, so that they can maintain the flavor and quality of the food, and reflect the value of the goods.
At present, plastic flexible packaging used in the field of frozen foods is mainly divided into three categories:
The first type is a single-layer packaging bag, such as a PE bag, which has a relatively poor barrier effect and is generally used for vegetable packaging;
The second type is a composite soft plastic packaging bag, which uses an adhesive to bond two or more layers of plastic film materials together, such as OPP/LLDPE, NY/LLDPE, etc., and has relatively good moisture resistance, cold resistance and puncture resistance;
The third type is a multi-layer co-extruded soft plastic packaging bag, which melts and extrudes different functional materials such as PA, PE, PP, PET, EVOH, etc., and joins in the total die, and is inflated and cooled together. This kind of material does not use adhesive, and has the characteristics of no pollution, high barrier, high strength, high and low temperature resistance.
The data show that in developed countries and regions, the use of the third type of packaging accounts for about 40% of the total frozen food packaging, while China only accounts for about 6%, which needs further promotion.
With the continuous advancement of technology, new materials are emerging, and edible packaging film is one of the representatives. It uses biodegradable polysaccharides, proteins or lipids as a matrix to form a protective film on the surface of frozen foods by using natural edible substances as raw materials and inter-molecular interactions by means of encapsulation, impregnation, coating or spraying. To control moisture transfer and oxygen permeation. The film has obvious water-resistance and strong resistance to gas permeation. The most important thing is that it can be eaten together with frozen food without any pollution, and has broad application prospects.
2. Improve the cold resistance and mechanical strength of the inner packaging materials
Method one, selecting a reasonable composite or co-extruded raw material.
Nylon, LLDPE and EVA have excellent low temperature resistance, tear resistance and impact resistance. The addition of such raw materials in the compounding or coextrusion process can effectively improve the waterproof gas barrier properties and mechanical strength of packaging materials.
Method 2, appropriately increase the proportion of plasticizer.
Plasticizers are mainly used to weaken the secondary bonds between polymer molecules, thereby increasing the mobility of polymer molecular chains and reducing crystallinity, which is manifested by the hardness of the polymer, the decrease in modulus embrittlement temperature, and the elongation, Increased flexibility.
3. Improve the compressive strength of corrugated boxes
At present, frozen food is basically transported in a slotted corrugated box on the market. The carton is surrounded by four corrugated cardboard, and the upper and lower sides are folded and sealed by four flaps. Through literature analysis and experimental verification, it can be found that the collapse of the carton occurs mostly in the four cardboard sections placed vertically in the box structure, so strengthening the compressive strength of the place can effectively improve the overall compressive strength of the carton. Specifically, firstly, a circular inner sleeve can be added around the inner wall of the carton. It is recommended to use A 楞 cardboard, which has good elasticity and shock absorption, and can prevent the frozen food from sharply piercing the damp cardboard. Secondly, a nested carton structure can be used. The box type is usually processed from a plurality of corrugated cardboard sheets, and the box body and the lid are separated and used by nesting. Tests have shown that under the same packaging conditions, the compressive strength of the ferrule structure carton is about twice that of the slotted structure carton.
4. Strengthen packaging testing efforts
Packaging is of great significance to frozen foods. Therefore, the State has formulated GB/T24617-2009 "Frozen Food Logistics Packaging, Marking, Transportation and Storage", SN/T0715-1997 "Exported Frozen Food Products Transportation Packaging Inspection Regulations" and other relevant standards and procedures. By setting the minimum requirements for the performance of packaging materials, the quality of the entire process from packaging raw material supply, packaging process to packaging effect is guaranteed. In this regard, the company should establish a complete packaging quality control laboratory, equipped with a three-cavity integrated block structure oxygen / water vapor transmission rate tester, intelligent electronic tensile testing machine, carton compression machine and other test equipment, on the frozen packaging materials Conduct a series of testing tests such as barrier properties, compression resistance, puncture resistance, tear resistance and impact resistance.
In summary, the packaging materials of frozen foods are faced with many new demands and new problems in the application process. Studying and solving these problems is of great benefit to improving the quality of frozen food storage and transportation. In addition, improving the packaging inspection process and establishing various test data systems for packaging materials will also provide a research basis for future material selection and quality control.
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