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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.

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2 Answers 2

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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 by individually quick-frozen shrimp where "shrimp" is the only ingredient.)

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I believe the only ingredient was shrimp (I returned the floaters to the store). Even after fully thawing to cold tap water temperature, the shrimp still floated and still were more frothy than the others with more rinsing. Maybe the froth does suggest an additive. –  ErikE Aug 1 '11 at 21:45

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.

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Most of the variables you mentioned seem dubious because the shrimp were the same manufacturer, same packaging. –  ErikE Aug 3 '11 at 21:54
    
@ErikE: Even so, the catch is bound to change from day to day and certainly from week to week. I don't mean to say that you shouldn't return food if you think that it's bad, but only that some differences between bags doesn't necessarily mean that one is bad. I think your nose would know if your shrimp were so far gone that they'd changed color. Anyway, you'd be smart to call the manufacturer and ask. The last thing they'll want is for you to eat bad shrimp with their name on it, and they should be able to tell you what differences are okay or not. –  Caleb Aug 3 '11 at 22:26

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