Plastic composite film is a commonly used packaging material for retort-resistant packaging. Retort and heat sterilization is an important process for packaging high-temperature retort food. However, the physical properties of plastic composite films are prone to thermal decay after being heated, resulting in unqualified packaging materials. This article analyzes common problems after cooking of high-temperature retort bags, and introduces their physical performance testing methods, hoping to have guiding significance for actual production.
High-temperature-resistant retort packaging pouches is a packaging form commonly used for meat, soy products and other ready meal food product. It is generally vacuum packed and can be stored at room temperature after being heated and sterilized at high temperature (100~135°C). Retort-resistant packaged food is easy to carry, ready to eat after opening the bag, hygienic and convenient, and can well maintain the flavor of the food, so it is deeply loved by consumers. Depending on the sterilization process and packaging materials, the shelf life of retort-resistant packaging products ranges from half a year to two years.
The packaging process of retort food is bag making, bagging, vacuuming, heat sealing, inspection, cooking and heating sterilization, drying and cooling, and packaging. Cooking and heating sterilization is the core process of the entire process. However, when packaging bags made of polymer materials – plastics, the molecular chain movement intensifies after being heated, and the physical properties of the material are prone to thermal attenuation. This article analyzes common problems after cooking of high-temperature retort bags, and introduces their physical performance testing methods.
1. Analysis of common problems with retort-resistant packaging bags
High-temperature retort food is packaged and then heated and sterilized together with the packaging materials. In order to achieve high physical properties and good barrier properties, retort-resistant packaging is made of a variety of base materials. Commonly used materials include PA, PET, AL and CPP. Commonly used structures have two layers of composite films, with the following examples (BOPA/CPP , PET/CPP), three-layer composite film (such as PA/AL/CPP, PET/PA/CPP) and four-layer composite film (such as PET/PA/AL/CPP). In actual production, the most common quality problems are wrinkles, broken bags, air leakage and odor after cooking:
1). There are generally three forms of wrinkling in packaging bags: horizontal or vertical or irregular wrinkles on the packaging base material; wrinkles and cracks on each composite layer and poor flatness; shrinkage of the packaging base material, and the shrinkage of the composite layer and other composite layers Separate, striped. The broken bags are divided into two types: direct bursting and wrinkling and then bursting.
2).Delamination refers to the phenomenon that the composite layers of packaging materials are separated from each other. Slight delamination is manifested as stripe-like bulges in the stressed parts of the packaging, and the peeling strength is reduced, and can even be gently torn apart by hand. In severe cases, the packaging composite layer is separated in a large area after cooking. If delamination occurs, the synergistic strengthening of the physical properties between the composite layers of the packaging material will disappear, and the physical properties and barrier properties will drop significantly, making it impossible to meet the shelf life requirements, often causing greater losses to the enterprise.
3).Slight air leakage generally has a relatively long incubation period and is not easy to detect during cooking. During the product circulation and storage period, the vacuum degree of the product decreases and obvious air appears in the packaging. Therefore, this quality problem often involves a large number of products. products have a greater impact. The occurrence of air leakage is closely related to the weak heat sealing and poor puncture resistance of the retort bag.
4). Odor after cooking is also a common quality problem. The peculiar smell that appears after cooking is related to excessive solvent residues in packaging materials or improper material selection. If PE film is used as the inner sealing layer of high-temperature cooking bags above 120°, the PE film is prone to odor at high temperatures. Therefore, RCPP is generally selected as the inner layer of high-temperature cooking bags.
2. Testing methods for physical properties of retort-resistant packaging
The factors leading to the quality problems of retort-resistant packaging are relatively complex and involve many aspects such as composite layer raw materials, adhesives, inks, composite and bag making process control, and retort processes. In order to ensure packaging quality and food shelf life, it is necessary to conduct cooking resistance tests on packaging materials.
The national standard applicable to retort-resistant packaging bags is GB/T10004-2008 “Plastic Composite Film for Packaging, Bag Dry Lamination, Extrusion Lamination”, which is based on JIS Z 1707-1997 “General Principles of Plastic Films for Food Packaging” Formulated to replace GB/T 10004-1998 “Retort Resistant Composite Films and Bags” and GB/T10005-1998 “Biaxially Oriented Polypropylene Film/Low Density Polyethylene Composite Films and Bags”. GB/T 10004-2008 includes various physical properties and solvent residue indicators for retort-resistant packaging films and bags, and requires that retort-resistant packaging bags be tested for high-temperature media resistance. The method is to fill the retort-resistant packaging bags with 4 % acetic acid, 1% sodium sulfide, 5% sodium chloride and vegetable oil, then exhaust and seal, heat and pressurize in a high-pressure cooking pot at 121°C for 40 minutes, and cool while the pressure remains unchanged. Then its appearance, tensile strength, elongation, peeling force and heat sealing strength are tested, and the decline rate is used to evaluate it. The formula is as follows:
R=（A-B）/A×100
In the formula, R is the decline rate (%) of the tested items, A is the average value of the tested items before the high-temperature resistant medium test; B is the average value of the tested items after the high-temperature resistant medium test. The performance requirements are: “After the high-temperature dielectric resistance test, products with a service temperature of 80°C or above should have no delamination, damage, obvious deformation inside or outside the bag, and a decrease in peeling force, pull-off force, nominal strain at break, and heat sealing strength. The rate should be ≤30%”.
3. Testing of physical properties of retort-resistant packaging bags
The actual test on the machine can most truly detect the overall performance of the retort-resistant packaging. However, this method is not only time-consuming, but also limited by the production plan and the number of tests. It has poor operability, large waste, and high cost. Through the retort test to detect physical properties such as tensile properties, peel strength, heat seal strength before and after retort, the retort resistance quality of the retort bag can be comprehensively judged. Cooking tests generally use two types of actual contents and simulated materials. The cooking test using actual contents can be as close as possible to the actual production situation and can effectively prevent unqualified packaging from entering the production line in batches. For packaging material factories, simulants are used to test the resistance of packaging materials during the production process and before storage. Testing cooking performance is more practical and operable. The author introduces the physical performance testing method of retort-resistant packaging bags by filling them with food simulation liquids from three different manufacturers and conducting steaming and boiling tests respectively. The test process is as follows:
1). Cooking test
Instruments: Safe and intelligent back-pressure high-temperature cooking pot, HST-H3 heat seal tester
Test steps: Carefully put 4% acetic acid into the retort bag to two-thirds of the volume. Be careful not to contaminate the seal, so as not to affect the sealing fastness. After filling, seal the cooking bags with HST-H3, and prepare a total of 12 samples. When sealing, the air in the bag should be exhausted as much as possible to prevent air expansion during cooking from affecting the test results.
Place the sealed sample into the cooking pot to start the test. Set the cooking temperature to 121°C, the cooking time to 40 minutes, steam 6 samples, and boil 6 samples. During the cooking test, pay close attention to the changes in air pressure and temperature in the cooking pot to ensure that the temperature and pressure are maintained within the set range.
After the test is completed, cool to room temperature, take it out and observe whether there are broken bags, wrinkles, delamination, etc. After the test, the surfaces of the 1# and 2# samples were smooth after cooking and there was no delamination. The surface of the 3# sample was not very smooth after cooking, and the edges were warped to varying degrees.
2). Comparison of tensile properties
Take the packaging bags before and after cooking, cut out 5 rectangular samples of 15mm×150mm in the transverse direction and 150mm in the longitudinal direction, and condition them for 4 hours in an environment of 23±2℃ and 50±10%RH. The XLW (PC) intelligent electronic tensile testing machine was used to test the breaking force and elongation at break under the condition of 200mm/min.
3). Peel test
According to method A of GB 8808-1988 “Peel Test Method for Soft Composite Plastic Materials”, cut a sample with a width of 15±0.1mm and a length of 150mm. Take 5 samples each in the horizontal and vertical directions. Pre-peel the composite layer along the length direction of the sample, load it into the XLW (PC) intelligent electronic tensile testing machine, and test the peeling force at 300mm/min.
4). Heat sealing strength test
According to GB/T 2358-1998 “Test Method for Heat Sealing Strength of Plastic Film Packaging Bags”, cut a 15mm wide sample at the heat sealing part of the sample, open it at 180°, and clamp both ends of the sample on the XLW (PC) intelligent On an electronic tensile testing machine , the maximum load is tested at a speed of 300mm/min, and the drop rate is calculated using the high temperature resistance dielectric formula in GB/T 10004-2008.
Summarize
Retort-resistant packaged foods are increasingly favored by consumers because of their convenience in eating and storage. In order to effectively maintain the quality of the contents and prevent food from deteriorating, every step of the high-temperature retort bag production process needs to be strictly monitored and reasonably controlled.
1. High temperature resistant cooking bags should be made of appropriate materials based on the content and production process. For example, CPP is generally selected as the inner sealing layer of high-temperature-resistant cooking bags; when packaging bags containing AL layers are used to package acid and alkaline contents, a PA composite layer should be added between AL and CPP to increase resistance to acid and alkali permeability; each composite layer The heat shrinkability should be consistent or similar to avoid warping or even delamination of the material after cooking due to poor matching of heat shrinkage properties.
2. Reasonably control the composite process. High temperature resistant retort bags mostly use dry compounding method. In the production process of retort film, it is necessary to select the appropriate adhesive and good gluing process, and reasonably control the curing conditions to ensure that the main agent of the adhesive and the curing agent react fully.
3. High-temperature medium resistance is the most severe process in the packaging process of high-temperature retort bags. In order to reduce the occurrence of batch quality problems, high-temperature retort bags must be retort tested and inspected based on actual production conditions before use and during production. Check whether the appearance of the package after cooking is flat, wrinkled, blistered, deformed, whether there is delamination or leakage, whether the decline rate of physical properties (tensile properties, peel strength, heat sealing strength) meets the requirements, etc.
Post time: Jan-18-2024
