Fish Sterilization Temps for Tapeworm Larvae

[JohnV]

HOST'S NOTE: Split off from the Sous Vide topic.

In light of these recent articles about tapeworms/Diphyllobothrium, (e.g. Gourmet, SF Chronicle), I was wondering if anyone had thoughts on sterilization temps for tapeworm larvae in fish, and salmon in particular.

There have been many posts about "mi cuit" salmon on this board and others, but it appears to be quite risky if not using deep-frozen fish. I don't have a blast freezer, so there is really no way I can achieve the required temps for sterilization at home. And freezing and holding in the comparatively warm home freezer would likely lead to significant decrease in quality during the slow-freezing process.

I did some brief searching for specific info on tapeworm sterilization temps, but could only find the consumer-oriented 140F cooking recommendations for fish. Anyone who's more familiar with the food safety scientific literature come across this info before? Most information we've been discussing has been about bacteria rather than parasites.

Edited August 24, 2008 by Chris Hennes (log)

[DouglasBaldwin]

In light of these recent articles about tapeworms/Diphyllobothrium, (e.g. Gourmet, SF Chronicle), I was wondering if anyone had thoughts on sterilization temps for tapeworm larvae in fish, and salmon in particular.

There have been many posts about "mi cuit" salmon on this board and others, but it appears to be quite risky if not using deep-frozen fish. I don't have a blast freezer, so there is really no way I can achieve the required temps for sterilization at home. And freezing and holding in the comparatively warm home freezer would likely lead to significant decrease in quality during the slow-freezing process.

I did some brief searching for specific info on tapeworm sterilization temps, but could only find the consumer-oriented 140F cooking recommendations for fish. Anyone who's more familiar with the food safety scientific literature come across this info before? Most information we've been discussing has been about bacteria rather than parasites.

←

Parasites consumed in unfrozen seafood that is uncooked or undercooked present a human health hazard, although one that is much less significant than the risk of illness from bacteria and viruses.  The process of heating raw fish sufficiently to kill bacterial pathogens is also sufficient to kill parasites. . . . Parasites in finfish are an emerging issue in industrialized countries and must be considered because of the increase in popularity of fish products that are typically eaten raw. . . . Wild-caught Pacific  salmon (Oncorhynchus spp.) should be considered to have A. simplex larvae present (67), and prevalence may exceed 75% in various  types of fresh U.S. commercial wild salmon (27).

So, from a food safety perspective, it really isn't advisable to cook salmon `mi-cuit'. While it may not be as tasty, I would recommend pasteurizing all fish at the temperatures and times I posted up thread (in post #2011).

Edited August 23, 2008 by DouglasBaldwin (log)

[jackal10]

This is a fine example of the confusion of threat and risk. Because something might happen, it doesn't mean it will.

While it is possible that some wild salmon is infested with tapeworm, the incidence is low, and very rare indeed for farmed salmon.

Generations have eaten raw fish as sushi, tatare, gravlax, lox and the rest without apparent harm.

I think you need to make the case that infections of tape worm have increased, and even then it is likely that the infected fish is from a few specific locations. For fish only from those locations preacutions would need to be taken until the outbreak is contained.

Edited August 23, 2008 by jackal10 (log)

[nathanm]

I agree with Jack about risk versus thinking that it is certain. In addition there are some other factors.

First, "tapeworms" is the wrong name - at least for pratical concerns. The primary risk with parasites in fish are Anisakid nematodes. These occur in wild fish caught near shore - rock fish, sea bass, salmon.

These are quite different than the tapeworms that infest humans. Those are a totally different creature (different genus), and you can't get them from fish - you get them from contamination with human feces. There are very very rare tapeworms that can come from fish - I will dicuss that in the next post.

The Anisakids normally parasitize seals that eat fish. They are unable to parasitive humans, - when a human ingests Anisakid nematodes you get a bad stomache ache.

Blue water fish, such as tuna, do not get anisakids.

Farmed salmon, or other farmed fish do NOT carry anisakids. In order to get infested with anisakids the fish have to eat anisakid contaminated seal feces.

Anisakids are pretty common. For my cookbook project we wanted to photograph some so we went to the local branch of a large organic supermarket chain and took a careful look at the halibut fillets on sale. We spotted one with anisakids and bought it. Got some great photos.

Despite the fact that they are common, there are very few cases of anisakid related illness per year in the US - about 10 per year. Japan has many more, but still only about 2000 cases a year. Mostly this is from home prepared sushi. But even 2000 cases a year is quite rare considering that the population is 127 million people, many of whom eat raw fish on a daily basis.

Here is a popular article. Here is the FDA report on Anisakids, and another FDA report on smoked fish.

Anisakids are killed two ways - by heat, and by freezing.

There are very few good studies on heat - one study says 60C/140F for 1 minute, but does not give other time and temperature combinations. I do not trust information given with just one data point like that - I am looking for better data.

The other approach is freezing. A standard freezer is good enough - you do NOT need a blast freezer.

The US FDA recommends -4F / - 20C for 4 days. Or -31F/-35C for 15 hours.

EU regulation is -4F / - 20C for 24 hours.

Any household freezer can reach -4F/-20C.

I eat salmon mi-cuit even without freezing it. The risk is very low, but it is possible. However, if I was concerned I would simply freeze the fish overnight.

Edited August 23, 2008 by nathanm (log)

[nathanm]

I should add that Diphillobothrium tapeworms can infect humans through salmon, as discussed in the Gourmet article but this is MUCH more rare than anisakids.

Worrying about Diphillobothrium is pretty futile, it is extremely rare in comparison with anksakids.

Freezing to kill anisakids will almost certainly kill Diphillobothrium, but the condition is so rare that the FDA does not even know this for sure - here is the FDA report on Diphyllobothrium.

Edited August 23, 2008 by nathanm (log)

[nathanm]

I don't have a blast freezer, so there is really no way I can achieve the required temps for sterilization at home. And freezing and holding in the comparatively warm home freezer would likely lead to significant decrease in quality during the slow-freezing process.

Have you tried it?

Frozen fish gets a bad name for many reasons. It is true that fast freezing is better than slow freezing.

However, it is very unclear to me that fish frozen in a home freezer, held for 24 hours to 4 days, then thawed and cooked sous vide would actually suffer much in quality. I will be doing some tests for my cookbook project but this is not done yet.

So, try it and find out.

The fair comparison would be buy some fish, divide in half, vacuum pack each half. Keep one half in the fridge, freeze the other half for 24 hours, then defrost the frozen one by putting in the fridge. Then cook both and see if you can tell the difference.

[JohnV]

Thanks Nathan, that was exactly the sort of quantitative info I was looking for.

To answer the other part of my question (about the effect of freezing rate on fish quality if you're using a home freezer to kill parasites) I found a discussion that says that rate does not generally affect quality, but storage temperature does. The specific numbers suggested are to "freeze" within 8 hours or so, and I'm presuming that "freeze" just means get to 32F/0C. Thus, it seems likely that a home freezer would be adequate for both quality and safety with the parasite problem, haven't tested it myself though. See here.

If anyone has contrary info, let me know.

John

Edited August 23, 2008 by JohnV (log)

[JohnV]

I don't have a blast freezer, so there is really no way I can achieve the required temps for sterilization at home. And freezing and holding in the comparatively warm home freezer would likely lead to significant decrease in quality during the slow-freezing process.

Have you tried it?

←

Haven't tried it yet, but see post above for what appears to be thoughts from a fish processing scientist.

John

[slkinsey]

I should add that Diphillobothrium tapeworms can infect humans through salmon, as discussed in the Gourmet article but this is MUCH more rare than anisakids. 

Worrying about Diphillobothrium is pretty futile, it is extremely rare in comparison with anksakids.

Freezing to kill anisakids will almost certainly kill Diphillobothrium, but the condition is so rare that the FDA does not even know this for sure - here is the FDA report on Diphyllobothrium.

Mmmmm. Anisakis.

[Mikels]

When I want to freeze food quickly I use dry ice. With a temperature of -109.3F (-78.5C ), things freeze rather quickly. If you want things to freeze faster, use liquid nitrogen −321F (−196C), but I doubt if the cost is worth it.

[DouglasBaldwin]

This is a fine example of the confusion of threat and risk. Because something might happen, it doesn't mean it will.

While it is possible that some wild salmon is infested with tapeworm, the incidence is low, and very rare indeed for farmed salmon.

Generations have eaten raw fish as sushi, tatare, gravlax, lox and the rest without apparent harm.

I agree with Jack about risk versus thinking that it is certain. In addition there are some other factors.

You are quite right that there is little risk in consuming unpasteurized fish, so long as you have a healthy immune system. The problem, is that many people reading these threads may think it is also safe to serve salmon mi cuit to highly susceptible or immune compromised individuals.

Just because it has been safe for millennia, does not necessarily mean that it is still safe. Our oceans are not as healthy as they were then --- they are much more polluted and many areas have been severely over fished. They also knew the provenance of their fish --- something that we rarely are privy to now. Therefore, we can no longer assume that the interior flesh of our fish is sterile and parasite free (as we have been able to do for millennia).

[nathanm]

You are quite right that there is little risk in consuming unpasteurized fish, so long as you have a healthy immune system.  The problem, is that many people reading these threads may think it is also safe to serve salmon mi cuit to highly susceptible or immune compromised individuals. 

This is a very interesting point.

It is very valid for bacterial or viral pathogens. People with compromised immune systems need an extra level of caution. This includes people who are:

- HIV positive

- In the middle of chemotherapy

- Transplant recipients on transplant anti-rejection drugs

- Infants

- Other conditions that compromise the immune system

In addition to virsuses and bacteria, it may be valid for some microscopic parasites.

However, the immune system is basically irrelevant for anisakid nemotodes (or for dyphllobothrium tapeworms, for that matter). These are large scale parasites that are in your gut and thus not subject to the immune system at all.

Watch the fascinating (and disgusting) video in slkinsley's post. The anisakid is about 1 mm wide and probably 25mm long. I recently found one in halibut bought at an organic supermarket chain that was even longer. It lives in the gut, which puts it beyond the reach of the immune system.

It is basically like swallowing a bone. The worm acts as a physical irritant because it tries to poke its way through your stomach wall. It can't do that successfully in humans (its built for seal guts), but it hurts when it pokes you. The immune system isn't going to stop that, anymore than the immune system could stop a swallowed bone from poking your gut.

I am sure that somebody with a compromised immune system would find these worms annoying. And if you are on death's door anyway, you don't need one extra problem. However, they shouldn't be any more medically dangerous for them than other people.

While we are on the topic, the immune system state is also pretty irrelevant for food poisoning - i.e. eating food that has bacterial toxins in it. If you eat food that has botulism toxin, or toxin from Bacillis cereus, or Staphlococcus aureus, then you will get sick regardless of your immune system, because the problem is a chemical toxin. Basically you NEVER want to consume botulism toxin! This is unlikely to be a surprise...

Edited August 27, 2008 by nathanm (log)

[MikeJ]

First, "tapeworms" is the wrong name - at least for pratical concerns.  The primary risk with parasites in fish are Anisakid nematodes.  These occur in wild fish caught near shore - rock fish, sea bass, salmon. 

These are quite different than the tapeworms that infest humans.  Those are a totally different creature (different genus), and you can't get them from fish - you get them from contamination with human feces. 

Very different indeed - nematodes and tapeworms are actually different phyla (nematoda and platyhelminthes, respectively). There are plenty of tapeworms you can get from fish: Diphyllobothrium latum would be one nasty example.