Plastic containers used for food storage and packaging are categorized using Resin Identification Codes (RIC), which are small numbers found inside a chasing arrows symbol, typically on the bottom of a container. Each number represents a different polymer with unique properties, particularly regarding its heat resistance and stability. The primary concern when exposing plastic to microwave energy is the potential for chemical compounds to migrate from the container into the food. Understanding the specific properties of each plastic type, especially number 5, is important for making informed choices about food safety.
How to Identify Plastic #5 (Polypropylene)
Plastic labeled with the number 5 is made from Polypropylene, commonly abbreviated as PP. This material is the second-most widely produced plastic resin globally due to its balance of flexibility, strength, and resistance to chemicals. You can identify it by looking for the number 5 placed inside the triangular recycling symbol, often accompanied by the letters “PP” underneath. Polypropylene is frequently used for items requiring durability and heat tolerance, such as food storage containers, yogurt tubs, margarine tubs, syrup bottles, and bottle caps. This plastic possesses a higher melting point compared to many other common polymers, which is why it is often considered for products that may encounter heat, like those intended for the top rack of a dishwasher.
Safety of Plastic #5 in the Microwave
Polypropylene (Plastic #5) is often considered the most heat-tolerant plastic for food use, but this heat stability does not automatically mean it is safe for microwaving. Its higher melting point ensures the container itself is unlikely to warp or melt under normal microwave conditions, distinguishing it from lower-grade plastics. However, the crucial distinction is that the “microwave safe” label is a certification that goes beyond simple heat resistance.
A container must be explicitly marked with a “microwave safe” symbol—often represented by wavy lines—to indicate it has been tested for food contact during heating. This certification shows that the plastic has been formulated and tested not to leach harmful compounds into food when heated by microwave energy. Nevertheless, some studies indicate that even Polypropylene labeled as “microwave safe” can still release microplastics and non-intentionally added substances (NIAS) when heated. Therefore, while the #5 code suggests heat tolerance, the safest practice is to only use those containers clearly bearing the “microwave safe” designation, and to exercise caution even then.
Risks of Microwaving Non-Approved Plastics
Heating any plastic container that lacks the specific “microwave safe” label introduces several risks, regardless of the Resin Identification Code. The primary concern is chemical migration, a process where heat causes chemical additives within the plastic to break down and leach into the food. These additives, such as Bisphenol A (BPA) or Phthalates, are often included to give the plastic specific qualities like rigidity or flexibility.
When a non-approved plastic is heated, the chemical bonds can degrade, leading to a higher release of these compounds into the meal. Many of these chemicals are known as endocrine disruptors, which can mimic or block the action of natural hormones in the body. This is particularly risky with plastics labeled #1 (PETE), #3 (PVC), and #6 (Polystyrene), or any single-use plastic not intended for repeated heating. Beyond chemical concerns, microwaving non-approved plastics also carries the risk of the container melting, warping, or cracking.
Safer Containers for Heating Food
To eliminate the risks associated with chemical migration from plastic, switching to non-plastic alternatives is the most reliable approach. Glass containers are the safest option for microwave use because they are chemically inert and do not contain organic chemical additives that can leach into food. Using glass cookware that is specifically designed for high temperatures, such as borosilicate glass, helps prevent cracking from temperature shock.
Ceramic containers are another excellent choice, offering similar heat stability to glass. When selecting ceramicware, ensure the item is labeled as microwave-safe, as decorative glazes or metallic trims can contain materials that may spark or break down when heated. If using silicone, confirm that it is food-grade and specifically rated for high-temperature use to ensure stability during the heating process.