Air Leakage Detection for Bag-Packed Foods: Technology, Methods, and Industry Practice Guide

1. The Hazards of Air Leakage in Bag-Packed Foods and the Importance of Detection

   
   

Air leakage in bag-packed food is a common and serious problem in the food industry. When the seal of a packaging bag is compromised, it allows gas to exchange between the inside and outside of the bag, causing oxidation, moisture, mold, and even spoilage, seriously impacting product quality and safety. For consumers, leaking food can pose a health risk; for manufacturers, it can lead to product recalls, damage to brand reputation, and financial losses.

Air leakage in food packaging occurs primarily due to two factors: poor barrier properties of the packaging material, which cannot effectively block external oxygen and moisture; and poor sealing during the production process of the finished packaging, particularly defects in the heat seal. In actual production, excessive or low heat seal strength, as well as uneven heat seal strength, can easily lead to air leakage. This is especially true for air-filled packaged foods, such as potato chips and puffed foods, where variations in the gas content within the bag can directly impact the shelf life and taste of the product.

Carrying out airtightness testing is crucial for food companies. It is not only a key component of quality control but also an effective means of preventing costly leaks. Through scientific testing, companies can promptly identify sealing defects, providing a reference for improving packaging processes and ultimately enhancing overall product quality and market competitiveness. Modern testing technology now enables 100% monitoring of packaging quality, minimizing the risk of leaking food entering the market.

2 Mainstream Testing Methods and Their Principles

2.1 Water Decompression Method (Vacuum Method)

The water decompression method is currently the most widely used traditional testing method, primarily based on the national standard "GB/T 15171 Test Method for Sealing Performance of Flexible Packages." The basic principle of this method is to immerse the sample under test in a liquid. By evacuating the vacuum chamber, a pressure differential is created between the inside and outside of the sample. If the sample leaks, the internal gas will escape through the leak hole, forming a continuous stream of visible bubbles, which can be used to determine whether the sample is leaking and locate the leak point. The specific operation process includes: injecting an appropriate amount of distilled water into the vacuum chamber, securing the sample in a fixture and completely immersing it in water; closing the sealing cap, closing the exhaust valve, and starting the vacuum pump to evacuate the chamber; typically adjusting the vacuum level to a preset value (such as 20, 30, 50, or 90 kPa) within 30-60 seconds; maintaining the vacuum level for 30 seconds after stopping the evacuation, observing for the generation of continuous bubbles; finally, opening the inlet valve to restore the pressure to normal, removing the sample and inspecting for water infiltration.

The key to this method is distinguishing between continuous and isolated bubbles. Only continuously generated bubbles are considered a sign of a leak, while isolated bubbles may be gas attached to the packaging surface and do not constitute a leak. The choice of vacuum level should be determined based on the characteristics of the sample. Excessively high pressure differentials may cause the sample to rupture or crack the seal, affecting the accuracy of the test.

Although the underwater decompression method is simple and inexpensive, it is a destructive test. Products tested are generally unsalable. Its sensitivity to minor leaks is limited, making it primarily used for sampling rather than comprehensive inspections. 2.2 Vacuum Decay Method

The vacuum decay method is an advanced non-destructive micro-leak detection technology based on ASTM F2338, which has been recognized by the US FDA as a consensus standard for package integrity testing. Its principle is to place a specimen in a specially designed test chamber, evacuate it to create a negative pressure environment, and monitor changes in the vacuum level within the chamber using a high-precision pressure sensor. If a package leaks, gas will enter the chamber through the leak, causing a pressure change within the chamber. By measuring the vacuum decay value, it is possible to determine whether the specimen is leaking and even quantify the leak rate.

Compared to the water decompression method, the vacuum decay method offers significant advantages: Qualified products can continue to be sold after non-destructive testing; high sensitivity, capable of detecting leaks as small as 1 micron; high automation, allowing integration into production lines for in-line testing; and quantitative output, capable of not only determining leak presence but also calculating leak size. These features make it particularly suitable for the demanding food packaging and pharmaceutical industries.

A vacuum decay testing system typically consists of a vacuum pump, a high-precision sensor, a dedicated test chamber, and data processing software. During the test, the instrument records vacuum changes over a predetermined period of time and uses an algorithm to analyze and determine package integrity. The entire testing process requires no sample preparation and doesn't rely on subjective operator judgment, resulting in more objective and reliable results.

2.3 Differential Pressure Leak Detection Technology

The differential pressure leak detector is another precision testing method, based on the national standard "GB/T25752-2010 Differential Pressure Leak Detector." This method uses a differential pressure sensor as a pressure-sensing element to determine the presence and rate of a leak by comparing the pressure difference between the object under test and a reference object.

During testing, the reference object (a reference object with the same shape, material, and volume as the object under test and with negligible leak rate) and the object under test are simultaneously connected to the testing system. Both are initially filled with gas at the same pressure, and after the pressure is equalized by a balancing valve, the test phase begins. The differential pressure sensor continuously monitors the pressure difference between the object under test and the reference object. If a leak is detected, the pressure difference will change, and this change is calculated to determine the leak status.

Differential pressure leak detectors can be categorized into five types based on test pressure: vacuum leak detectors (below atmospheric pressure), micropressure leak detectors (0-50 kPa), low-pressure leak detectors (50-200 kPa), medium-pressure leak detectors (200-700 kPa), and high-pressure leak detectors (above 700 kPa). This categorization allows for the selection of equipment based on the test pressure requirements of different packaging, improving detection suitability and accuracy.

This technology offers high sensitivity and stable and reliable test results, making it particularly suitable for testing high-value-added food packaging with extremely stringent sealing requirements, such as modified atmosphere packaging (MAP) testing for high-end milk powder and coffee products.

2.4 Tracer Gas Methods and New Technologies

For packaging with special requirements, tracer gas methods offer highly sensitive detection solutions. Among them, the carbon dioxide tracer gas method is widely used in the modified atmosphere packaging field. This method places a package containing CO2 gas in a test chamber, creating a vacuum and a pressure differential. If the package leaks, a CO2 sensor within the chamber will sensitively detect the leaking gas and trigger an alarm. This method does not require destroying the packaging, and the inspected product remains dry and intact, allowing it to continue to be sold. It is particularly suitable for in-line testing of modified atmosphere packaged foods.

In addition, ultrasonic testing technology utilizes the characteristics of ultrasound combined with imaging technology to perform non-contact testing of the sealability of flexible packaging bags. When a package leaks, the propagation characteristics of the ultrasound wave change, and analysis of these changes can be used to determine the seal status. This method is fast and easy to automate, but requires specialized equipment and technical support.

In recent years, with technological advances, in-line automatic leak detection systems manufactured by Jinan Maotong have become a growing trend. These systems typically combine multiple technologies, such as machine vision and pressure testing, to achieve 100% inspection of bagged foods on high-speed production lines. For example, some advanced leak and air leakage detection equipment can achieve inspection speeds of 200-400 bags per minute, with a leak rejection rate of up to 99.9%, significantly improving production efficiency and product quality. 

Table: Comparison of Major Bag Food Leak Detection Methods

3 Common Testing Equipment and Applications

Bagged food leak detection equipment is categorized into various types based on their principles and applicable scenarios. The MFY-CM seal tester is a typical underwater decompression tester consisting of a vacuum chamber, sealing cover, specimen holder, vacuum pump, and control system. The device features a 7-inch touchscreen interface and can achieve a vacuum level of -90 kPa. It is suitable for industries such as food, plastic flexible packaging, wet wipes, pharmaceuticals, and daily chemicals. Users can set the vacuum level and hold time based on product characteristics, achieving standardized testing.

For high-precision testing needs, the MFY-02A Micro-Leakage Leak Tester utilizes vacuum decay technology and is equipped with a high-precision pressure sensor (resolution up to 1Pa), capable of detecting leaks ≥1μm. This device is suitable for micro-leak testing in various packaging types, including vials, prefilled syringes, bags, cans, and bottles. With a test time of just 5-30 seconds, it is highly efficient and accurate. The device utilizes a 10-inch industrial-grade touchscreen tablet computer, enabling intelligent data statistics and multi-level permission management, meeting the data traceability requirements of the pharmaceutical and food industries.

The WM-230 Air Leak Tester is a multifunctional device suitable for soft bags (such as milk powder, cheese, and coffee), semi-rigid packaging (such as chilled meat, fruit and vegetable salads, and yogurt), and rigid packaging (such as beverage bottles, oil drums, and cans). Equipped with a high-precision digital vacuum gauge, the device allows for setting the target negative pressure and hold time, and automatically triggers a pressure alarm upon reaching the set pressure. The vacuum chamber is constructed of high-quality transparent acrylic sheet, facilitating observation of test conditions within the chamber and customizable to non-standard sizes.

As for the online inspection system, advanced leak and air leak detection equipment can be directly integrated into the production line, enabling high-speed automated inspection. These devices typically include an inspection conveyor, a squeeze detection mechanism, and a defective product rejection device, operating at speeds of 200-400 packages per minute. They effectively detect and reject defective products such as leaking and bloated packages, significantly improving production efficiency. The equipment offers wide compatibility, handling packages ranging in size from 40×40mm to 100×100mm to meet diverse production needs.

4 Testing Process and Standards

4.1 Testing Operational Procedures

A standardized testing process is crucial for ensuring accurate results. Taking the most commonly used underwater decompression method as an example, the standard testing steps are as follows:

Sample Preparation: Randomly select the packaging bags to be tested and inspect the surface for cleanliness and obvious defects. If there are wrinkles or foreign matter on the packaging surface, gently wipe it clean to avoid obstructing observation. Equipment Preparation: Fill the vacuum chamber with an appropriate amount of distilled water, ensuring the water level is slightly above the sample placement height. Check the vacuum chamber seal for integrity and that the lid is fully sealed.

Sample Placement: Secure the sample in a dedicated fixture and completely immerse it in water. During this time, the distance between the top of the sample and the water surface should be at least 25 mm to ensure sufficient water pressure and facilitate bubble observation.

Evacuation: Close the vacuum chamber lid, close the exhaust valve, and open the vacuum valve to begin vacuuming. The vacuuming process typically takes 30-60 seconds, stopping when the desired vacuum level (e.g., -30 kPa to -90 kPa) is reached.

Pressure Observation: Maintain the desired vacuum level for 30 seconds, closely observing the sample surface for the formation of continuous bubbles. Note that isolated bubbles may be caused by gas adhering to the packaging surface and should not be considered leaks.

Restoring Pressure: Open the air inlet valve to vent the vacuum chamber to atmosphere. Once the pressure returns to normal, open the lid. Result Assessment: Remove the sample, wipe off any surface moisture, and inspect the interior of the package for water infiltration. If no continuous bubbles are generated during the vacuum and pressure-holding phases, and no water seeps into the package, the sample is considered sealed.

4.2 Testing Standards and Specifications

Air leak testing for packaged food must comply with relevant national and industry standards to ensure consistent and comparable test results. GB/T 15171, "Test Method for Sealing Performance of Flexible Packages," is the primary standard in China for testing the sealing performance of flexible packages. It details the test apparatus, test conditions, and result assessment methods for the water decompression method. This standard is applicable to sealed flexible packages made of various materials and is the most commonly used testing method by food companies.

ASTM D3078, "Standard Test Method for Leakage of Flexible Packages by the Bubble Method," is an international standard developed by the American Society for Testing and Materials. It also uses the bubble method, providing a unified testing method for international trade. For high-precision testing, ASTM F2338, the vacuum decay method, has been recognized by the US FDA as a consensus standard for package integrity testing. In addition to test method standards, relevant instrument standards are also crucial. GB/T25752, "Differential Pressure Leak Detector," specifies the technical requirements, test methods, and inspection rules for differential pressure leak detectors to ensure reliable performance. GB/T31473, "Halogen Gas Leak Detector," specifies the technical requirements for halogen gas leak detectors. Although this testing method is less commonly used in the food industry, it is still used for certain specialized packaging.

Companies should select appropriate testing standards based on product characteristics and customer requirements and establish internal quality control procedures, including sampling frequency, test parameter settings, and acceptance criteria, to form a systematic leak monitoring system.

5. Testing Method Selection and Industry Outlook

5.1 How to Select the Appropriate Testing Method

When selecting a leak detection method for bagged foods, multiple factors should be considered:

Product Characteristics and Packaging Type: For inflatable packaging such as puffed foods, the underwater decompression method is more cost-effective. For high-value-added products or modified atmosphere packaging, the vacuum decay method or differential pressure method is more suitable. Testing Purpose: For rapid, full inspections of a production line, non-destructive methods (such as vacuum decay) should be selected. For quality control sampling only, the underwater decompression method may be more economical.

Sensitivity Requirements: For products with stringent requirements for trace leaks (such as medical device packaging), highly sensitive methods (such as vacuum decay or tracer gas methods) are required. For general food packaging, traditional underwater decompression methods may be sufficient.

Productivity and Efficiency: High-speed production lines require online inspection systems, such as automatic extrusion tracers or online vacuum decay systems. Small-batch production or laboratory environments can use benchtop inspection equipment.

Cost Budget: Underwater decompression equipment is relatively low cost, but labor costs are high. Advanced inspection equipment requires a high initial investment, but offers low long-term operating costs and high efficiency.

5.2 Industry Development Trends and Outlook

Bagged food leak detection technology is developing towards intelligent, automated, and high-precision features. With the deepening of the Industry 4.0 concept, online automated leak detection systems will become increasingly popular, enabling 100% inspection rather than random sampling. The seamless integration of test data with enterprise quality systems will enable real-time monitoring and preventative quality control.

In the future, testing technology will place greater emphasis on non-destructive testing, reducing waste while improving product quality control. The application scope of non-destructive testing methods such as vacuum decay will further expand and may become the mainstream technology for food packaging testing. Furthermore, with the advancement of sensor technology and artificial intelligence, testing accuracy and efficiency will continue to improve, and the ability to identify small leaks will continue to increase.

In addition, standardization and regularization are also important trends in industry development. With the country's increasing emphasis on food safety, relevant testing standards will continue to be improved, providing companies with clear technical guidance. The development of third-party testing services will also help small and medium-sized food companies improve their packaging quality control and promote overall quality improvement in the industry.

In summary, air leakage detection for packaged food is a critical step in ensuring food safety and quality. Companies should scientifically select testing methods based on their product characteristics and production needs, and establish a systematic quality control system to gain an advantage in market competition and provide consumers with safe and reliable food products.