Why do some freshwater fish taste muddy?

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So after Graz detailed his quest to eat carp and the (surprisingly delicious) results (Part I and Part II), I thought I’d delve into what causes the biggest complaint about eating carp and freshwater fish in general, that they taste muddy. Talking to Graz while he was in Vietnam recently illustrated the point. While his Canberra carp was delicious, he recited a story about eating a carp a few days earlier that was rendered almost inedible by a strong muddy taste and odour. It would seem not all fresh water fish are created equal.

So without further ado, the main culprits for the muddy taste according to science are two chemicals, geosmin and 2-methylisoborneol. These two compounds are made by a variety of actinobacteria and cyanobacteria. While both compounds are harmless to humans, they don’t taste all that good. When the micro organisms that create these compounds die, the compounds are released into the surrounding water or soil the bacteria are growing in and can accumulate in quite high concentrations in some water courses. As a quick aside, not only does geosmin give fish a muddy taste, its also responsible for the earthy smell after a rain shower. In cooler climates, the presence of these two compounds is seasonal, dropping off during the colder months and peaking during the warmer months, especially after a rainfall events. In warmer climates the bacteria grow happily year round. These two compounds are absorbed through the gills of the fish and accumulate in the flesh of the fish quickly (within 24 hours) once they are present in a watercourse. Geosmin is also highly soluble in lipids, so it likely to accumulate in higher levels in the fattier parts of the fish, e.g. the skin. So, there it is, why some freshwater fish taste muddy. 

So how can you avid the muddy taste of not just carp, but other fresh water fish? Firstly, the easiest way to avoid muddy fish is to catch fish from clean waterways that don’t accumulate high levels of these compounds. If the compounds aren’t present, the fish won’t taste muddy. Easy! As Graz found out, carp from clean waterways are likely to taste good, carp from dirty waterways are likely to be muddy. So if there has been a recent rain event, or you know a certain waterway often produces fish with a “muddy” taste its probably easiest to avoid eating fish from those waterways.

Now if you want to keep fish from waterways where there is a chance the fish will taste “muddy”, there are a few things you can do to reduce the “muddy” flavour. The common “fixes” are putting the fish straight on ice once they are caught, deep skinning and removing the fattiest parts of the fish, keeping only “lean” fish from flowing water and “flushing” the fish by keeping them in fresh water for a week or so. Given we now know what causes the taste, we can see that there is a rationale behind each of these methods. Removing the fat or eating lean fish from flowing water achieves the same objective, that is, reducing the fat content in the fish. Given geosmin accumulates in fatty tissues, these techniques are likely to significantly reduce any muddiness in the fish. Flushing fish by keeping them in clean, gesomin free water long enough for them to get rid of these compounds is also likely to work, although its a lot of effort. Putting fish on ice immediately once caught is a no brainer. While it may not reduce geosmin and  2-methylisoborneol in the flesh, it will stop the build up of histamines in the flesh and generally make the fish you eat fresher and better to eat. Another option in Southern climates would be to only eat fish from lakes and watercourses that often accumulate these two compounds in winter when the concentrations are likely to be far lower. 

The last strategy, a strategy of last resort, is to take a leaf out of the book of the countries that commonly eat fish that are likely to be contaminated by these two compounds. Mask that **** with spices! Chilli, ginger garlic, soy, sichuan pepper and the like. For inspiration look to the cuisines of Thailand, Vietnam, China and the like. If you can’t get rid of the muddy taste, cover it up with a load of other delicious flavours 🙂

Good luck on the water and happy carp eating 😉

Cheers

Hamish

flickandflyjournal.com

Hamish Webb, Dan Firth, Graham Fifield and Lee Georgeson have been fishing the south-east Australian region since 1987. Since then they’ve become avid sportfishermen who are constantly looking for new ways to challenge themselves. They are all scientists and conservationists who are passionate about the long-term sustainability of the ecosystem in which they live. They promote understanding and appreciation of the complex socio-political, economic and environmental issues surrounding fish, fishing and fisheries, while never losing sight of the various motivations that keep them coming back. In English, that means they love all things fishing and have a damn good time on the water, and that’s all that really counts in the end!

3 thoughts on “Why do some freshwater fish taste muddy?

  • July 28, 2014 at 2:22 pm
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    Nice post Ham. Interesting that it’s a bacterial product and not mud itself.
    The Teochew Chinese (my peeps) who are considered the kings of steamed fish use a bit of acid from tomato and preserved plum to cut through the muddy taste of river fish. Whether this is just a matter if masking the flavour or whether there’s a chemical inactivation I’m not sure but might be worth a punt.

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  • July 28, 2014 at 2:38 pm
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    Well geosmin is broken down far faster under acidic conditions, so adding acids would speed up breakdown and definitely reduce muddiness 🙂 The Teochew are smart.

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