Plastic and safety
Polypropylene, BPA, and what a microwave does and does not do to it.
Which plastic, and how to tell
Reusable food boxes of this kind are almost always polypropylene, marked with a 5 inside the recycling triangle. It is chosen for food storage because it has a high enough melting point to survive a microwave and a dishwasher, it does not absorb fat and odour the way polyethylene does, and it is not made using bisphenol A.
The number on the base is worth knowing how to read in general. Code 5 is polypropylene. Code 7 is the catch all category that used to include polycarbonate, the material BPA is associated with. Code 1 is PET, fine for a bottle, not intended for repeated reheating.
What BPA free does and does not mean
Bisphenol A is a building block of polycarbonate plastics and some epoxy linings. Concern about it centres on its behaviour as an endocrine disruptor, and it has been removed from most food contact plastic sold for domestic use. Polypropylene was never made with it, so a polypropylene box being BPA free is a statement of fact rather than a reformulation.
What the phrase does not mean is that nothing migrates from plastic into food ever. Any polymer in contact with hot, fatty or acidic food will transfer a small amount of something. The regulatory position, both in the US under the FDA's food contact rules and in the EU, is that these amounts are held below thresholds set with a safety margin. If you want to reduce it further, the lever that actually works is temperature: reheat in glass and store in plastic.
Microwaves
Polypropylene is microwave safe in the sense that it does not soften or deform at the temperatures food reaches in a domestic microwave. Two caveats are worth taking seriously.
- Fat gets much hotter than water. A cheese topping or an oily sauce can exceed the temperature the rest of the dish reaches by a wide margin, and it is the one situation where a box can distort.
- Lids are not the dish. Snap lids trap steam and build pressure. Take the lid off, or set it on loosely so steam can leave. A lid that has been forced off by pressure will not seal properly afterwards.
Staining, and why black bases exist
Tomato, turmeric and paprika stain polypropylene. It is a surface effect rather than a sign the plastic is degrading, and it is largely cosmetic, but it does not come out. Rubbing with a little oil before adding a tomato sauce reduces it. A black base hides it entirely, which is the practical reason half the range is black rather than clear.
When to retire a box
A box with a cracked rim, a warped base that no longer sits flat, or a lid that has stopped clipping is finished as a food container. Deep scratches are worth taking seriously too: they hold residue that a dishwasher does not reach. Buying fifty at a time is partly about being able to throw one away without hesitating.