The core objective of treating the inner wall of metal packaging mooncake jars is to form an inert insulating layer through physical or chemical means, thereby preventing direct contact between the mooncake ingredients and the metal substrate and avoiding reactions such as electrochemical corrosion, metal ion migration, or flavor adsorption. This process requires comprehensive consideration of the characteristics of the metal material, the complexity of the mooncake ingredients, and the compatibility with processing technologies.
The coating on the inner wall of the metal jar is a crucial barrier to prevent reactions. Traditional iron jars are prone to corrosion due to acidic components (such as organic acids) or sulfur compounds in mooncakes, leading to the dissolution of iron ions and altering the color and flavor of the mooncake. For example, sulfur-containing proteins decompose during high-temperature sterilization to produce hydrogen sulfide, which reacts with iron to form black iron sulfide, causing spots or off-odors on the mooncake. Therefore, the inner wall needs to be coated with epoxy phenolic, polyester, or acrylic resin coatings. These materials are chemically inert and effectively prevent contact between the mooncake ingredients and the metal. After the coating film forms, it must be non-toxic and odorless, and must not affect the sensory quality of the mooncake, such as avoiding plasticizer or solvent residues in the coating that could lead to odor migration.
The coating performance must adapt to the specific working conditions of mooncake processing. Mooncake production typically involves high-temperature sterilization (above 121℃) and long-term storage, requiring the coating to maintain stability under thermal stress. If the coating's heat resistance is insufficient, it may soften, peel off, or decompose at high temperatures, leading to the failure of the insulating layer. For example, an insufficiently cured coating may crack during sterilization due to the release of internal stress, exposing the metal to the mooncake components. Therefore, the coating must undergo rigorous thermal stability testing to ensure no physical or chemical changes occur under high-temperature environments. Furthermore, coating adhesion is crucial. Pretreatment processes such as phosphating are necessary to enhance the roughness of the metal surface, improve the bonding strength between the coating and the substrate, and prevent coating peeling due to mechanical friction or humidity changes during storage.
Given the characteristics of mooncake components, the coating must possess selective permeability. Mooncakes often contain oils, sugars, and proteins, which may penetrate the metal surface through the coating's micropores, causing corrosion. For example, free fatty acids in high-fat mooncakes may react with metal ions to form saponifications, affecting the mooncake's texture. Therefore, the coating needs to reduce porosity and form a dense barrier by adjusting the formula (such as increasing the filler ratio or using cross-linking technology). Simultaneously, for fruit-based mooncakes containing anthocyanins (such as strawberry and bayberry fillings), the coating needs to prevent the migration of metal ions (such as tin and iron) into the mooncake, preventing anthocyanin degradation caused by metal ion catalysis and avoiding discoloration.
Different types of mooncakes have different requirements for inner wall treatment. Meat-based mooncakes with high sulfur content (such as five-nut and pork floss fillings) require sulfur-resistant coatings. These coatings introduce inert metal elements such as zinc or aluminum to form a sacrificial anode protective layer that preferentially reacts with hydrogen sulfide, thereby protecting the iron substrate. For highly acidic fruit-based mooncakes (such as hawthorn and lemon fillings), acid-resistant coatings are required, improving corrosion resistance by increasing acid-resistant fillers (such as silica) in the coating. In addition, mooncakes with easily sticky fillings (such as red bean paste and lotus seed paste) need to be coated with an anti-stick coating. This type of coating reduces surface energy, decreasing friction between the mooncake and the can wall, facilitating demolding and maintaining the mooncake's shape.
The inner wall treatment of metal packaging mooncake cans must balance safety and functionality. The coating material must meet the safety standards for food contact materials, such as verifying through migration tests whether harmful substances (such as bisphenol A and heavy metals) in the coating are within safe limits. Simultaneously, the coating must be compatible with the mooncake processing technology, such as sterilization and vacuum packaging, to avoid coating failure due to process conflicts. Through comprehensive control of material selection, process optimization, and performance testing, metal packaging mooncake cans can effectively prevent reactions with mooncake ingredients, ensuring food safety and quality stability.
