In "How to Grill", the recipe for grilling oysters says to test for freshness by tapping them on the work surface to make sure they close their shells and to discard any that don't.
I recently purchased 6 oysters from my local supermarket which appeared to be closed when I got them. I stored them in the refrigerator for several hours before removing them for grilling. All appeared to be closed when I removed them from the refrigerator. I grilled them as indicated, but after 10 minutes, only one had opened. I did manually open another one, which smelled and looked OK, but was reluctant to eat it. I ended up throwing away 5 of the 6 I purchased and was extremely disappointed.
Any other tips on knowing if purchased oysters are fresh if they appear to be closed from purchase through the start of grilling? I hate to purchase twice as many as I need just to make sure that I get enough of them to open on the grill. Should I trust the supermarket counter person if they tell me the oysters are fresh?
Thanks for any advice you can provide.