The question of whether aluminum cans leach substances into the beverages and foods they contain is a common public health concern. The cylindrical container found on store shelves is not a monolithic piece of metal but rather a complex, multi-layered packaging system engineered for preservation. While the can’s structure is predominantly aluminum, its ability to safely store contents relies on multiple components designed to prevent chemical interaction with the liquid or food inside. Addressing the query about leaching requires an understanding of the can’s physical anatomy and the protective barriers in place.
The Structure of Aluminum Cans
The modern aluminum can is crafted primarily from an aluminum alloy, selected for its lightweight nature, durability, and high recyclability. The body is typically made from a series 3004 alloy, which contains small amounts of manganese and magnesium to enhance its strength and formability. The lid, or “end,” often uses a different alloy, such as a series 5052 or 5182, which has a higher magnesium content to make it stiffer and stronger to withstand the sealing process and internal pressure.
The can is formed through a process called “drawing and ironing,” which transforms a flat disk into the familiar shape with very thin walls. This structure requires a specialized interior surface treatment to ensure the contents remain stable. Without this inner coating, the aluminum metal would react with the contents, especially acidic foods or carbonated drinks, quickly leading to corrosion and product spoilage.
Aluminum Metal Leaching Explained
The leaching of the aluminum metal itself into the contents is a concern directly addressed by the can’s design. Aluminum, when exposed to air, naturally forms a thin, dense, and inert layer of aluminum oxide on its surface, which provides inherent corrosion resistance. However, this natural oxide layer is not sufficient to withstand prolonged contact with the acidity of many canned products.
Under highly specific conditions, such as extreme pH levels or high temperatures, the aluminum oxide layer can be dissolved, allowing aluminum ions to migrate into the food or drink. Studies show that very acidic liquids can theoretically cause the can’s material to corrode if the protective coating is compromised. In modern, properly manufactured cans, the internal barrier makes direct metal leaching extremely rare. Research has concluded that the aluminum content that does migrate does not present a significant risk for individuals with healthy kidney function.
The Importance of the Protective Liner
The interior of every aluminum can is coated with a thin, food-grade polymer layer, known as the protective liner. This liner acts as the true barrier between the contents and the metal. The primary function of this liner is to prevent the metal from corroding and to ensure the product’s taste remains unaffected by metallic flavors. This coating is a complex epoxy resin or polymer that must be stable under the high heat and pressure used during the sterilization process.
Historically, this protective coating was an epoxy resin made using Bisphenol A (BPA), a compound that came under intense scrutiny due to its potential to act as an endocrine disruptor. The concern centered on the migration of trace amounts of unreacted BPA from the polymer matrix into the food or beverage. This process was influenced by factors like temperature, contact time, and the fat content of the food. Consumer pressure and scientific concern drove the packaging industry to transition to “BPA-free” alternatives.
Today, the vast majority of food and beverage cans utilize alternative linings made from materials like acrylics, polyester resins, or oleoresins. While these new materials eliminate BPA migration, they introduce other compounds, such as Bisphenol S (BPS) or Bisphenol F (BPF). These are structurally similar to BPA and may also have endocrine-disrupting properties. Manufacturers continue to work on next-generation coatings, including new epoxy monomers like TMBPF, which have shown promising results.
Regulatory Oversight and Consumer Safety
The safety of can linings and the potential for chemical migration are rigorously overseen by regulatory bodies globally, such as the U.S. Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA). These agencies establish safety standards for all Food Contact Materials (FCMs). This ensures that substances do not transfer into food at levels harmful to human health or alter the food’s composition.
Regulators conduct toxicological risk assessments for all substances used in can liners and set migration limits. These include a Specific Migration Limit (SML) for individual substances and an Overall Migration Limit (OML). The OML is a total maximum of 60 milligrams of all substances combined per kilogram of food.
In the European Union, testing protocols often involve the use of food simulants under worst-case conditions to measure migration. The FDA uses a different approach but also relies on data to ensure the safety of food additives. The current scientific consensus, backed by these regulatory systems, is that modern aluminum cans, with their protective polymer liners, remain a safe and effective means of packaging for the general public.