Refreezing seafood; why is it bad?
May 24, 2013 9:11 AM   Subscribe

I'm trying to find technical information on why refreezing seafood is bad. I've found a pretty good one explaining the problem behind refreezing food in general at The Straight Dope, but I'm looking to dive deeper into it, specifically what effect it has on things like lipids, what the chemical changes are, what nutrients are lost or start to break down, etc . . . along with the spoilage risk which sounds like its much worse with shellfish even if kept at refrigerated temperatures. (At least that's what the internet tells me)

Bonus points if anyone can also point me to something that explains the problem with long term freezer storage of shellfish, again, specifically what nutrients break down and how. It is recommended only 3-6 months as opposed to a year for other meat.
posted by [insert clever name here] to Food & Drink (7 answers total) 3 users marked this as a favorite
In my experience, seafood is more delicate tissue, and more easily damaged by ice crystals. Any home refrigerator or freezer will have some temperature variation, allowing ice crystals to get bigger and smaller, ruining the texture. Plus, shellfish is really prone to bacterial contamination.
posted by theora55 at 9:19 AM on May 24, 2013 [1 favorite]

Part of the problem with shellfish and spoilage is that most of the time you have THE WHOLE ANIMAL, not a slice or chunk of the animal's muscle (and maybe bone) like you do with most larger animals. That includes whatever poop and stuff the animal had in it when it was captured/killed, so it's pretty much guaranteed to have a significant amount of bacteria in it.

I think a big part of the question you're asking is just "why is fish meat so much more delicate than mammal/bird meat?" I don't know the answer to that. But asking why you can't freeze fish for as long as you can freeze chicken is kind of like asking why you can't *cook* fish as long as you cook chicken. I suspect it probably it has to do with the different amounts and types of fats in fish and other meat. Like, a chicken thigh and a salmon fillet both get about half their calories from fat but it's not the same kind of fat.
posted by mskyle at 9:32 AM on May 24, 2013

Response by poster: Yup. Pretty sure it is be a use of the omega-3 fatty acids. Just trying to understand how those break down, is it the thawing that does it, or ice crystals forming again destroy something else.
posted by [insert clever name here] at 9:44 AM on May 24, 2013

Bonus points if anyone can also point me to something that explains the problem with long term freezer storage of shellfish, again, specifically what nutrients break down and how. It is recommended only 3-6 months as opposed to a year for other meat.

From McGee ch. 4 (which is worth owning if you often wonder about things like this): "The cold aquatic enviroment is also responsible for the notorious tendency of fish and shellfish to spoil faster than other meats. The cold has two different effects. First, it requires fish to rely on the highly unsaturated fatty acids that remain fluid at low temperatures: and these molecules are highly susceptible to being broken by oxygen into stale-smelling, carboardly fragments. More importantly, cold water requires fish to have enzymes that work well in the cold, and the bacteria that live in and on the fish also thrive at low temperatures. The enzymes and bacteria typical of our warm-blooded meat animals normally work at 100F/40C, and are slowed to a crawl in a refrigerator at 40F/5C. But the same refrigerator feels perfectly balmy to deep-water fish enzymes and spoilage bacteria. And among fishes, cold-water species, especially fatty ones, spoil faster than tropical ones..."

(note that "fish" in these quotes seems to be used as a cover term for fish and shellfish.) "The initial stages of inevitable deterioration are caused by fish enzymes and oxygen, which conspire to dull colors, turn flavor stale and flat, and soften the texture. This doesn't really make the fish inedible. That change is caused by microbes, especially bacteria, with which fish slime and gills come well stocked -- particularly Pseudomonas and its cold-tolerant ilk. They make fish inedible in a fraction of the time they take to spoil beef or pork, by consuming the savory free amino acids and then proteins and turning them into obnoxious nitrogen-containing substances (ammonia, trimethylamine, indole, skatole, putrescine, cadaverine) and sulfur compounds (hydrogen sulfide, skunky methanethiol)."


"the proteins in fish muscle (especially cod and its relatives) turn out to be unusually susceptible to "freeze denaturation," in which the loss of their normal environment of liquid water breaks some of the bonds holding the proteins in their intricately folded structure. The unfolded proteins are then free to bond to each other. The result is tough, spongy networks that can't hold onto its moisture when cooked, and in the mouth becomes a dry, fibrous wad of protein."

There's some discussion of shellfish in particular, I won't copy it all. But, on crustaceans: "the most important organ in the crustacean is what biologists call the midgut gland or hepatopancreas, and what the rest of us usually call the "liver". ...... But it's also what makes crustaceans spoil so readily. The gland is made up of tiny fragile tubes; and when the animal is killed, the tubules are readily attacked and damaged by their own enzymes, which then spread into the muscle tissue and break it down into mush." There's less detail on molluscs except that they begin spoiling immediately and quickly when killed, and that scallops are often shucked immediately leading to deterioration starting right away.
posted by advil at 11:30 AM on May 24, 2013 [3 favorites]

Bah! I was just about to reference On Food and Cooking but advil beat me to it. But I will give you the chapter title: Life in Water and The Special Nature of Fish, sections: The Paleness and Tenderness of Fish Flesh, and Storing Fresh Fish: Refrigeration and Freezing.

A great reference for all cooking and chemistry related questions.
posted by hydrobatidae at 11:36 AM on May 24, 2013 [1 favorite]

Response by poster: I think a big part of the question you're asking is just "why is fish meat so much more delicate than mammal/bird meat?"

That was just an example; I actually don't care that it does other than I want to know what's different and why does one go bad faster. What I really want to know is the chemical processes taking place. I know that highly unsaturated fatty acids like DHA and EPA break down; but I am hoping to find out the chemical process that causes this and other tissue break down, especially on refreezing. Oxidation seems to play a role, but I'm not sure what role. It also seems to have something to do with flash freezing, creating smaller ice crystals and then freezing at home which is slower, and creates larger, more destructive ice crystals.

Along with, hopefully, the nutrients lost.

Does On Food and Cooking delve any deeper into this subject in the areas I'm wondering about? If so, then that boom might be have the answers I'm looking for.
posted by [insert clever name here] at 1:13 AM on May 25, 2013

I think I got the highlights of most of the relevant sections for your particular question that I spotted, though I can't guarantee I found everything -- overall it does tend for breadth rather than depth (it covers many, many things about food). The bibliography should be useful for digging deeper, it looks like there are a solid batch of references on fish and shellfish. Independently, I was trying to find stuff about refreezing meat (in general) in that book a few weeks ago and didn't manage to dig up much beyond the very obvious things (it is bad, there are crystals), so refreezing in particular may be a small gap in the book. But I didn't follow this up in the bibliography. I still think it would be worth at least getting a library copy and reading the fish chapter, there's much interesting detail on individual fish / shellfish, and there isn't a better comprehensive source out there.

My guess from what I have found in there is it isn't that the processes involved in (re)freezing per se are all that different in (shell)fish, except maybe in terms of water content, but that the other processes leading to deterioration are different and proceed at lower temperatures, so the effects/results of refreezing are exaggerated.
posted by advil at 10:03 AM on May 25, 2013 [1 favorite]

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