Thawed shrimp floating vs. sinking

Question

I bought two 2-pound bags of frozen 41-50 count/pound shrimp. When I thawed them in cold water, one bag's shrimp all sank and the other bag's shrimp all floated.

           Bag 1          Bag 2
buoyancy   sink           float
color      more white     slightly brownish
rinsing    clear          frothy
marking    66H/15TT/976   668/21TT/0H354

I assume one of them is not nearly so fresh as the other, or possibly has been frozen and thawed multiple times. I did some searching but couldn't find any real help.

Assuming that one is not so fresh, I would like to return it to the store. But which one? I'm guessing the floating and darker color indicate a more advanced state of decomposition, but your input is appreciated. I'd like to have ammunition at the store in case they question me.

Answer 1

According to this study, frozen seafood has a density ranging from 972 to 1017 kg/m³. Thawed, raw seafood at 20°C has a density ranging from 1042 to 1093 kg/m³. Since the density of water at room temperature is about 998 kg/m³, it is certainly possible—and most likely normal—for one batch of frozen seafood to float while another sinks. Thawed seafood that has been brought to room temperature, on the other hand, should almost always sink since its minimum density (1042 kg/m³) is higher than water's. Therefore, I would only worry if the shrimp continue to float after they have been thawed. If they are shell-on, there is a chance that there could be air stuck between the shell and the body, though, which would of course be perfectly fine yet affect the density/floating.

Edit: What are the ingredients listed on the bags? Very often nowadays salt and salt-based preservatives are added to frozen shrimp in order to retard water loss. It could be the case that one bag has preservatives while the other does not. This could of course affect buoyancy by changing the density of the shrimp. (In general, one should try and only buy individually quick-frozen shrimp where "shrimp" is the only ingredient.)

Answer 2

You're jumping to conclusions here. While it's entirely possible that you got a bad batch of shrimp, there are plenty of variables that could explain differences in color, boyancy, and froth. Species, location, food source, processing method, and wild vs. farmed are all factors that could be involved. Indeed, many shimp varieties have names that relate to their coloration: there are black tiger shrimp, white shrimp, pink shrimp, brown shrimp...

The best way to tell if your shrimp are in good shape is to smell them. It'll be harder to detect any off odor if you thaw the shrimp under running water, so thaw them overnight in the fridge if you have time. A whiff of ammonia indicates a shrimp that has seen better days, but use your head: any scent that makes you think "Whew! That smells like really old shrimp!" is reason for concern.