Determining if a plastic container is safe for microwave use often confuses consumers. Many look at the triangular recycling symbol with a number inside, but this number is not the complete answer. The number, known as the Resin Identification Code (RIC), is primarily a tool for sorting materials in the recycling industry. To determine if a plastic item can be safely heated, a separate safety designation is required.
Decoding the #5 Symbol
The number 5 inside the symbol identifies the plastic polymer Polypropylene (PP). This plastic is one of the most common materials used in food packaging due to its strength, flexibility, and high heat tolerance. You will frequently find the #5 symbol on containers designed for products like yogurt, margarine, and reusable food storage tubs.
The Resin Identification Code tells recyclers the plastic type, not whether the item was manufactured for thermal stress. While PP is considered one of the safer plastics for food contact, the #5 code alone does not guarantee microwave safety. A container’s fitness for heating depends on its specific formulation and thickness, which the recycling code does not indicate.
Is Polypropylene Microwave Safe?
Polypropylene is often used in microwave-safe products because it has a high melting point, typically withstanding temperatures up to approximately 230°F (110°C) without physical deformation. This heat resistance means a #5 container is unlikely to physically melt or warp under normal microwave conditions.
However, the primary concern when heating any plastic is chemical migration, or leaching, into the food. Although Polypropylene is naturally free of Bisphenol A (BPA), heating the plastic can increase the release of other chemical additives used during manufacturing, such as stabilizers or non-BPA bisphenols like BPS or BPF. This chemical transfer is more likely when the plastic is exposed to high temperatures, especially when the food contains fats or oils. Fats and oils can reach temperatures well above the boiling point of water, stressing the polymer and accelerating the breakdown of chemical bonds.
Manufacturers must explicitly test and formulate a container to minimize this chemical risk during heating. Therefore, the only way to be certain a #5 plastic container is safe for the microwave is if it bears the separate, dedicated “microwave safe” symbol. This label, often depicting a microwave oven with wavy lines, confirms the container’s specific formulation has been tested and approved for heating food. If the container lacks this specific designation, the physical stability of the material is not a guarantee of chemical safety.
General Rules for Heating Food Safely
To completely eliminate any risk of chemical transfer, the safest practice is to transfer food from plastic containers into glass or ceramic dishes before heating. These alternative materials are completely non-porous and do not contain chemical additives that can migrate into your food, even under high temperatures. They provide a simple, risk-free solution for reheating leftovers.
If you must use plastic, you should avoid heating high-fat foods, such as oily sauces, cheese, or meats, in any plastic container, even if it is labeled as microwave-safe. The high heat generated by fats causes localized hot spots that can easily exceed the plastic’s temperature threshold and accelerate the leaching process.
Furthermore, always ensure that lids are vented or loosely placed on the container when heating food. This allows steam to escape, which prevents excessive pressure and heat buildup that could otherwise degrade the plastic and cause warping.
You should also never reuse single-use plastic containers, like cottage cheese or yogurt tubs, for microwaving, even if they carry the #5 symbol. These items are designed for cold storage and are not engineered to withstand the repeated thermal stress of heating cycles. Using containers specifically designed for repeated use, such as those explicitly labeled as “microwave-safe” storage containers, is the best way to mitigate risk if you choose to heat food in plastic.