How can I judge if vacuum-packed mussels are fresh, when they're invisible behind the seal?
I work in a small town in northern Canada that has only 1 supermarket, which sells only 1 form of mussels: see beneath. There are no other options for mussels. It feels impossible to judge freshness by the little aperture at the top. I already know that I should choose the farthest 'Best Before' date as possible.
Best Answer
Unfortunately I don't think you have a good option. You can try to determine what you can through the small tight section at the top. You should also be prepared to take them back and ask for your money back if you get them home and the freshness isn't acceptable.
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Quick Answer about "How can I judge if vacuum-packed mussels are fresh, when they're invisible behind the seal?"
if they are completely closed they should be fresh, after you buy them put them in cold water if they dont open it means they are bad. you will always have a few of them that wont open.Using a Vacuum Sealer for Sous Vide Cooking
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Answer 2
If you can see any of the mussel's tissue (the flesh) and it looks more grey, I wouldn't consider getting it! The raw flesh/meat should be somewhat translucent and almost fleshy in color. Like this: 
Once it is cooked it should turn almost orangy; assuming you haven't used spices that will alter its color; such as turmeric, cayenne, and etcetera.
Answer 3
if they are completely closed they should be fresh, after you buy them put them in cold water if they dont open it means they are bad. you will always have a few of them that wont open.
Answer 4
Don't have a solid answer for you other than some anecdotal experience. I've bought that specific brand from our local Costco numerous times and have generally never had an issue. They've worked out really well.The one time I did have an issue, by the time I went to cook them, I noticed the vacuum seal wasn't as tight as it usually is. As soon as I opened it, I noticed a stronger smell and a high number that did not move.
TLDR: I would look for a tight seal and as you said, find the furthest date you can.
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