dv said:
Some 18 years ago I reheated a rice dish in the microwave in a non-microwave safe thin plastic container. When I was eating the rice I noticed that it tasted very weird, but mainly very spicy in the back of the mouth (whereas the initial rice dish was not itself spicy). On closer inspection I found that part of the based of the plastic container had melted and burnt and affected the food.
Any ideas on why this might have tasted spicy?
Not off hand, I’ve only ever eaten raw plastic, and never noticed any spiciness (except possibly urethane?). Can you look at the recycling label to see what sort of plastic it was?
Plastic is mostly made from the same elements as food: carbon, hydrogen, oxygen, nitrogen, with some chlorine thrown in.
(Checks web).
Spices are made from the same elements, capsaicin has a benzene ring, inline nitrogen and three oxygen atoms. Below are some other spicy chemicals.

14 to 21 carbon atoms. Mostly but not necessarily containing a benzene ring.
The chemical structure of polyurethane is remarkably similar to that of spices. A urethane monomer contains two benzene rings, two inline nitrogens, and 17 carbon atoms. A breakdown of polyurethane with heat could yield spice-flavoured chemicals.
Want to try it out by experiment?