I would ask the hardware store what kind of rubber they are, and if they are food grade or FDA food contact compliant. They most likely won't have a clue, but they can ask their supplier the questions, or better yet give you the contact information for the supplier so you can ask.
Chances are the washers are nitrile rubber, also called BUNA N rubber. Chances are they aren't food grade, but you never know. Sometimes the maker of the washer makes everything food safe just to make life simpler for manufacturing - not having to keep track of what is food safe and what isn't in their plant and in their inventory.