[Abstract] This article, based on the main quality-influencing factors in the preservation of dairy products, details the application of barrier properties testing in dairy packaging and discusses the relationship between the barrier properties of packaging materials and the shelf life of products. In recent years, with the sustained and stable development of China's economy and the continuous improvement of people's living standards, the dairy industry has entered a golden period of development, becoming one of the fastest-growing and most promising industries in China's food industry. Today, milk and dairy products have become a symbol of healthy living, and various dairy products have extensive sales networks. However, due to different sales environments, it is difficult to guarantee that dairy products will be stored in a consistently stable environment (even for fresh milk, maintaining a consistently low-temperature refrigerated environment is quite difficult). Therefore, dairy packaging has a significant impact on dairy product quality. The quality of packaging directly affects the shelf life of dairy products, and thus relates to consumer safety. 1. Main Factors Affecting Dairy Product Quality and Corresponding Packaging Forms Dairy packaging, as a component of dairy products, profoundly influences the development of the dairy industry. Because milk is perishable, both ultra-high-temperature (UHT) milk and fresh milk have strict quality requirements for their packaging. This section mainly introduces three categories: fresh milk, fermented milk, and milk powder. 1.1 Fresh Milk Fresh milk can be processed into different types of dairy products, and each product has different packaging requirements based on its own storage characteristics. The most common are pasteurized milk and ultra-high temperature (UHT) milk. Pasteurization is the most widely used method for processing fresh milk, typically using high-temperature instantaneous sterilization, that is, heating raw milk to 70℃～72℃ and maintaining it for 10～20 seconds. Pasteurization mainly kills bacteria in raw milk to ensure quality and safety for consumption; it also eliminates other microorganisms and enzymes that can cause flavor changes, thus extending the shelf life of fresh milk. However, pasteurization generally only kills 90%～99% of the microorganisms in milk, so its shelf life is not very long, and a low-temperature refrigerated storage environment is required. Light is a major cause of vitamin loss in pasteurized milk, and other nutrients will also decompose due to photochemical reactions; therefore, dairy product packaging needs to be protected from light. Oxygen is also a crucial factor that cannot be ignored. Excessive oxygen in the headspace of the packaging, or excessive oxygen permeability of the container, will intensify the oxidation reaction of the dairy products within the packaging, leading to a rapid decline in quality. Commonly used packaging materials include glass bottles, composite cardboard boxes, and plastic bags. Ultra-high temperature (UHT) sterilization allows fresh milk to continuously pass through a heating process, undergoing a very short time (1-2 seconds) at a high temperature (at least 135°C), followed by aseptic packaging. The loss of nutrients in UHT milk during storage depends on the storage temperature, the initial oxygen content in the milk, and the properties of the packaging material (oxygen permeability and light transmittance). Oxidation reactions and off-flavors from rancidity can also enter UHT milk; the extent of these reactions depends on the oxygen content in the milk and the storage temperature. Various types of packaging can be used for UHT milk, with aluminum foil-lined composite packaging being the most common. The aluminum foil layer serves as the primary barrier layer, effectively preventing oxygen penetration and preserving the flavor components of the fresh milk. 1.2 Fermented Milk Fermented dairy products are made from fresh milk or its derivatives through homogenization, sterilization, and other processes, followed by microbial fermentation (e.g., the very popular yogurt, fermented milk drinks, etc.). Fermented milk is a typical fermented dairy product; adding fruits, fruit flavorings, or sugars can create different types of products. Based on their form, they can be divided into: set-type (hard), stirred-type (pasty), and liquid type. Unlike fresh milk, the carbon dioxide content in fermented milk has a significant impact on flavor changes and microbial growth. Currently, fermented milk is packaged in glass bottles, composite cardboard boxes, and plastic bags (plastic containers, plastic cups). 1.3 Milk Powder Milk powder is also a widely consumed dairy product, made from fresh milk or dairy products through spray drying. The oxidation reaction is one of the main factors determining the shelf life of milk powder. To ensure or extend the shelf life of milk powder, it must be sealed in packaging to reduce the impact of oxygen on its quality. However, it is important to note that, unlike liquid dairy products, the moisture-proof and oxygen-proof properties of milk powder packaging are equally important. Commonly used packaging forms include metal cans, aluminum foil composite flexible packaging bags, and paper-based composite cans. 2. Important Testing Items for Dairy Packaging As summarized in the previous section, light exposure and the penetration of oxygen and water vapor are the main factors affecting dairy product quality. Light protection for dairy products is entirely achievable; currently used dairy packaging materials generally provide excellent light blocking. However, achieving barrier properties is not so simple. Firstly, it is directly related to the high cost of barrier materials. Achieving ideal packaging barrier properties inevitably leads to higher packaging costs, ultimately increasing product production costs. Secondly, multi-layer composite methods, whether for composite paper boxes, composite plastic flexible packaging bags, or composite plastic cups, are a very suitable and effective way to improve barrier properties. However, the corresponding multi-layer composite production equipment and raw materials are not yet sufficiently developed domestically, mainly relying on imports. This is particularly evident in composite paper box packaging. Therefore, although the high cost of dairy packaging has attracted widespread attention, replacing these high-cost packaging materials remains very difficult. Furthermore, a decrease in the barrier properties of packaging materials leads to an increase in the amount of oxygen and water vapor penetrating the packaging, or a faster rate at which the filling gas leaks out of the packaging. This ultimately results in increased oxygen content and humidity within the packaging, which are the main causes of spoilage in dairy products. Therefore, it can be said that, to a certain extent, for the same type of dairy product (with consistent processing methods), the barrier properties of the packaging material are a major determinant of its shelf life. Thus, testing the barrier properties of packaging materials is crucial for dairy products. How much does the barrier property of packaging materials affect the shelf life of dairy products? Here are some representative barrier property test data for dairy packaging materials, and the differences are immediately apparent. The testing equipment used was a Labthink VAC-V1 gas permeability meter (for air permeability testing) and a TSY-T3 moisture permeability tester. Note: The unit for oxygen permeability is: m1/m2•24h•0.1MPa. The unit for moisture permeability is g/m²•24h. The samples listed in the table basically cover all the packaging materials required for dairy products and dairy product packaging forms. For example, the paper-plastic-aluminum composite material used in roof-gable and Tetra Pak cartons, the coated/composite packaging film used in aseptic packaging, the PE black and white film most commonly used in mass-market packaging, and the AL composite plastic flexible packaging film used for milk powder packaging. The data in the table shows that the paper-plastic-aluminum composite material used in composite cartons and the AL composite plastic flexible packaging film used in milk powder packaging have the best barrier properties among various dairy product packaging materials, with air and moisture permeability data in the high barrier range. Of course, the corresponding cost is also high; according to relevant information, the cost of one Tetra Pak carton is no less than 0.5 yuan. The coated/composite packaging film used in aseptic packaging is the next best, falling into the medium barrier range, with a moderate cost. The most common packaging materials, PE black and white film and milk film, have the worst barrier properties. Their air permeability alone is thousands of times higher than paper-plastic-aluminum composite materials and AL composite plastic flexible packaging films, and hundreds of times higher than coated/composite packaging films. Of course, they are also the cheapest, often costing less than 0.1 yuan. The choice of packaging materials, as well as their structure and barrier properties, is entirely determined by the product's intended shelf life. For example, dairy products packaged with high-barrier paper-plastic-aluminum composite materials and AL composite plastic flexible packaging films often have a shelf life of several months and do not require high-quality storage conditions, generally not needing refrigeration. Using materials with low barrier properties could lead to premature spoilage of large quantities of product or create greater difficulties in transportation and storage. On the other hand, dairy products packaged with black and white film only have a shelf life of a few days. These products are often consumed in large quantities and are fast-moving. Using packaging materials with excessively high barrier properties (such as medium-barrier materials) will not significantly extend the shelf life but will instead increase packaging costs. Therefore, packaging materials must be selected based on the product's characteristics and shelf life. Meanwhile, the barrier properties of packaging materials can also be "customized" according to product needs, that is, the material structure is designed and appropriate materials are selected according to the required barrier properties. The data in the table shows that, although both are paper-plastic-aluminum composite materials, sample #2 has lower gas barrier properties than sample #1, but better moisture barrier properties; similarly, for PE films, some have a barrier value of over 900, while others reach over 1300. Increasing barrier properties is justified by increasing costs; therefore, as long as the packaging material can guarantee the quality of the product within its shelf life, it is sufficient, and unilaterally pursuing higher barrier properties does not offer much benefit. 3. Conclusion Currently, domestically produced dairy packaging materials in China can meet general shelf-life requirements (considering the material's barrier properties, light-blocking properties, mechanical properties, and sealing performance), but high-end dairy packaging still relies on imports. However, with the continuous introduction of new products and the promotion of new processing technologies in the dairy industry, the research and development of new packaging forms and the use of packaging materials have entered a period of rapid development. However, the requirements for material selection are actually higher. Only by conducting comprehensive testing of all material properties can the quality of dairy products and the safety of consumers be better guaranteed.