I used to live in a coastal town and never worried about getting fresh mussels, but now I live in inland Ecuador and today bought a bag of frozen clams and a bag of frozen mussels. They all seem to be slightly open, but just a crack, around 1-2mm. Is that normal for frozen mussels/clams? The bags don't smell bad, but now I'm worried as they are just in a plastic packet with no name and no description. How will I know if they are good to go? The shop is about two hours away so I can't go back and check. I do not know if they have been partially pre-cooked or if they have just been frozen raw.
You are right to be concerned. The main reasons for frozen bivalves to be slightly open:
- They were steamed before freezing. This is common, and fine. You should be able to verify this by thawing 1-2 of them and checking if they are cooked.
- They were frozen very slowly. This is bad, because the bivalves can die and go bad during the freezing process.
- They've thawed and then been re-frozen. This is also bad, because it also makes contamination much more likely.
If you check them, and it turns out that they are not precooked, then you are taking a substantial risk by eating them. I wouldn't do it.
Sorry that I have no references for the above; I've searched the relevant literature, and the only thing I can find is some Reddit questions verifying that it's normal for bivalves to open on thawing.