The selection of materials for frozen vegetable and fruit packaging bags should not be based solely on the product name. Frozen corn kernels and spinach strips on the same production line may have completely opposite material requirements due to differences in moisture content, shape, and processing speed. Before selecting materials, it is essential to first confirm the packaging line environment, then consider issues such as frosting and air discharge, and only then determine the structure. Below, we will explain in this order how frosting, air discharge, and packaging line environment affect various performance indicators.
Quick Overview
- Frosting is the result of moisture migration inside the bag and is directly related to hygroscopicity, but it is not necessarily better to have lower values.
- Displacement and dry loss are two factors that need to be balanced, and the perforation scheme is also restricted by the product shape.
- Low temperatures slow down the migration of the lubricant, so the film positioning performance must be confirmed under the actual temperature in the packaging area.
- Frosting caused by insufficient blanching or pre-cooling can only be alleviated by changing materials.
Why does frosting relate to the moisture permeability of the material?
Internal frosting in the bag is a result of moisture migration. Free water on the product surface, residual water from cleaning, and moist air trapped inside the bag during filling will migrate toward the coldest area when the temperature drops, typically the side facing the cold air. The moisture permeability of the material determines whether these moisture molecules can diffuse outward through the film. Films with low moisture permeability retain moisture inside the bag, leading to more pronounced frosting.
But lower water vapor permeability is not always better. Moisture remaining inside the bag can cause frosting and clumping, while excessive loss leads to drying and discoloration. The selection of materials should first clarify the objective: prioritize moisture retention or prioritize prevention of frosting, as the material orientation differs between the two. The method for determining water vapor transmission rate can be referenced as follows: ASTM F1249However, this data describes the film itself and cannot be directly equated to the performance of the finished bag.
How to set up vent holes and vent channels
The purpose of venting is to provide an outlet for moisture and air inside the bag. Common methods include drilling micro-holes in the film, using micro-porous film, or leaving an exhaust passage in the sealing area. All three methods allow dry, cold air to enter the bag, so the exhaust volume must be balanced with the dry loss.
Exhaust setting depends on product shape and line speed. Regular particles are less likely to clog holes, making the microporous solution feasible; however, leaf-like and fragmented materials tend to clog holes, leading to reduced exhaust efficiency. At high line speeds, insufficient exhaust volume results in incomplete air removal from the bag, causing it to swell and leading to uneven force on the sealing area. During sample testing, both frosting and weight loss should be measured simultaneously.
How to select the film for frozen fruit and vegetable packaging bags based on the packaging line speed
The higher the speed, the more important the smoothness and stiffness of the film. At lower speeds, a slightly higher friction coefficient is acceptable; however, at higher speeds, a higher friction coefficient can cause the film to shift on the forming wheel and rollers, leading to misalignment and wrinkling at the sealing area. When selecting a film, you should provide the actual speed, film width, and tension to the supplier, rather than just specifying a bag type.
Another often overlooked factor is the slickness at low temperatures. Slip agents migrate more slowly at low temperatures, so a film that performs normally at room temperature may become rough in cold storage or low-temperature packaging areas, making it difficult to pull the bag. This is one of the essential site conditions that must be confirmed during material selection.
Which products are not suitable for high barrier structures in frozen packaging
Products with still strong respiration are not suitable. Some quick-frozen fruits and vegetables continue to respire slowly after processing. If sealed completely with a high barrier structure, the gas inside the bag will change, leading to off-odors. Most quick-frozen fruits and vegetables have weak respiration after blanching, so high barrier packaging is usually acceptable. However, exceptions apply if blanching is insufficient or if the product is a composite item.
Products that require frequent opening and closing should be handled with caution. High barrier properties often mean a thicker and harder structure, which may result in a poorer zipper or opening experience. For home users, repeated opening and resealing can lead to ineffective sealing, resulting in lower-than-expected protection performance. Refer to this for guidance.Low Temperature Freezing Packaging Bag SelectionPractical usage-oriented instructions.
Should the sample be tested under simulated cold storage and transportation conditions?
Yes, and it must be simulated according to the actual path. Only sampling at room temperature and checking the appearance of the bags cannot reflect issues such as frosting, sealing, and stacking. The proper approach is to freeze the samples according to the actual freezing method, store them for the actual duration, and then perform a handling test according to the real stacking height and transportation method. The focus should be on inspecting the inner edge of the seal and the folded corner at the bottom.
Transport performance testing can refer to ASTM D4169 Follow the proposed approach, but ensure the test conditions correspond to your actual logistics method. If the product undergoes multiple transfers, the test should reflect multiple loading and unloading steps; otherwise, the conclusion will be overly optimistic. If domestic standards need to be referenced, you may proceed accordingly.National Standard Information Public Service PlatformCheck current version and applicability status.
Conditions to Confirm Before Material Selection
- Product Form and Moisture Condition: Granules, leaves, fragments, whether the surface contains free water.
- Packaging line conditions: machine model, actual line speed, film width, tension, and packaging area ambient temperature.
- Storage Conditions: Freezing Method, Storage Duration, Pallet Stack Height, Transportation Method, and Number of Turns.
- Usage: Use once or multiple times by opening the bag repeatedly. Is a zipper or easy-open notch required?
- Objective: Prioritize anti-frosting, prioritize anti-drying, or need to balance oxygen barrier.
Applicable Scenarios and Boundaries
The above material selection logic applies to conventional B2B frozen vegetables and fruits, including frozen vegetables, mixed vegetables, and some frozen fruits, packaged in flexible bags or roll film. It does not apply to products requiring high-temperature sterilization, fresh products requiring modified atmosphere packaging, or large-scale industrial packaging.
The boundary also lies in the product itself. If frosting mainly results from insufficient blanching or pre-cooling, changing materials can only provide temporary relief and not solve the root cause. Material selection and process should be evaluated separately to avoid using materials to compensate for process shortcomings.
Which structural directions are suitable for frozen vegetables and fruits packaging
From practical application, there are three common orientations: low-cost structures mainly using co-extruded PE, suitable for regular granules and high line speed; composite structures with nylon layers, suitable for products with sharp edges that require puncture resistance, such asFrozen Seafood Packaging BagsThis type of product features a structure with an oxygen barrier layer, suitable for items prone to oxidation or requiring a longer shelf life. The three options are potential directions; thickness and layer configuration will be confirmed after sample testing.
Conclusion
The selection order for materials used in frozen vegetable and fruit packaging bags should be: first confirm the packaging line environment and distribution conditions, then evaluate the target for frosting and moisture loss, and finally choose among several structural options. The air permeability, moisture permeability, and slip properties should be considered together; optimizing any single property alone may lead to new problems. All material recommendations should be regarded as potential options, and the final selection should be determined after testing with real products or on the packaging line.
Common Questions
Do frozen vegetable and fruit packaging bags need to have vent holes?
It’s not necessarily required. The vent hole addresses the issue of moisture and air inside the bag having no outlet. If the product has low moisture content and sufficient vacuum is drawn before sealing, the bag can still be used without vent holes. Vent holes are typically needed when the product surface is highly moist or when the packaging line speed is high, causing insufficient air removal from the bag. It is recommended to compare the frosting and weight loss data between vented and non-vented samples during sample testing, and make a decision on whether to retain the vent after covering the full storage period.
Does lower moisture permeability prevent frosting more effectively?
For frozen vegetable and fruit packaging bags, low moisture permeability retains moisture inside the bag, making frosting and clumping more pronounced. Excessive moisture permeability, on the other hand, leads to dryness and discoloration. The optimal balance depends on the target moisture content of the product and the storage duration. When evaluating, both internal frosting and weight loss should be measured simultaneously, rather than just comparing moisture transmission rate values, as actual performance at low temperatures is also influenced by sealing quality and internal space within the bag.
Why does the film become difficult to handle in low-temperature environments?
The main issue is that the lubricant migrates more slowly at low temperatures, causing an increase in the film’s friction coefficient. A film that performs normally at room temperature may become sticky in cold storage or low-temperature packaging areas, making it difficult to pull through the forming wheel and rollers, and leading to misalignment during sealing. When selecting film, inform the supplier of the actual temperature in the packaging area and test the machine at that temperature to confirm the compatibility between film width, tension, and line speed.
Can changing the material solve the problem of frosting in frozen fruits and vegetables?
It can only address part of the issue. If frosting results from insufficient draining after washing, inadequate blanching, or improper pre-cooling, changing materials offers limited improvement. If the cause is moisture retention inside the bag and mismatched moisture permeability of the film, adjusting the structure may be helpful. When troubleshooting, prioritize examining the process first, then the materials. For structural considerations, refer to the following guidelines.Freezing Food Packaging SolutionsClassification by product form.