After a week of being stored in it, our freshly ground five-spice powder melted a plastic container. Here's why.
When we developed a recipe for freshly ground five-spice powder, we blitzed whole star anise, cloves, white pepper, cinnamon, and fennel seeds in a spice grinder and then stored the powder in a plastic deli-style container. A week later, we were surprised to find that the plastic appeared to have melted where it made contact with the spices.
It turns out that cloves and star anise contain high concentrations of two chemically similar oils called eugenol and anethole. The chemical structures of these oils are similar to that of the plastic called polystyrene (stamped PS or plastic #6 on the bottom of containers) and are therefore able to soften or dissolve the plastic.
BOTTOM LINE: We recommend storing ground cloves or star anise (whole spices were not problematic) in glass jars.