ammonia in crab stock

[David Barzelay]

I've encountered this problem before but can't find a satisfying answer.  Today I steamed two live dungeness crabs, then picked the meat out and used it in two dishes.  It was delicious.  The crabs were fresh and smelled great when raw, smelled even better after being steamed, and tasted delicious, clean, and fresh.  Then, after dinner, I attempted to make crab stock by simply throwing every discarded part of the crab (i.e. everything but the meat) into an electric pressure cooker for 30 minutes.  When it was done, I released the pressure and opened it up to find a stock that smelled strongly of ammonia.

I have three questions:

1) Where does the ammonia come from?  Results of a google search yield only talk of product that has spoiled ("Ur crab isn't fresh dude"), but in this case, the crab was fresh, handled well as far as I know, and was alive immediately prior to cooking.  Still, the ammonia thing happened.  It's also happened before, but doesn't happen every time I make crab stock.  It has definitely happened when making stock in a pot on the stove, so I don't think the pressure cooker has anything to do with it.
2) Is there a particular way I can avoid the ammonia when making crab stock?  For instance, if the ammonia came from the gills ("dead man's fingers"), I could simply leave the gills out of the stock.
3) Is there any way to rid the stock of ammonia and render it usable (i.e. safe and no remaining ammonia smell)?  For instance, could I neutralize with a strong acid?  Boiling for a long time or at a high temperature doesn't seem to have the desired effect, even though ammonia is much more volatile than water and has a much lower boiling point.

Thanks,
David Barzelay
www.eatfoo.com

[Martin Lersch]

> 1) Where does the ammonia come from?

Crabshell is mainly chitin (http://en.wikipedia.org/wiki/Chitin) which
is a polymer of N-acetylglucosamine. My guess is that the pressure
cooking liberates ammonia from chitin. Perhaps less though conditions
would leave the chitin more intact? (i.e. no pressure cooking, just
simple boiling)

> 3) Is there any way to rid the stock of ammonia

Acidification will drive the ammonia/ammonium quillibrium over to
ammonium which has no smell at all. This is similar to what happens
when we wash our fingers with lemon water after eating shrimps. The
amines which give rise to the characteristic fishy smell are
protonated, and on the ionic form they are no longer volatile and
loose their aroma.

Martin

--
Martin Lersch, Ph.D.

Website dedicated to molecular gastronomy and popular food science
http://blog.khymos.org

[pablo escolar]

David:

Did you see this link: http://www.ari1.com/id41.htm?  The author cites several reasons why this could happen  (DECOMPOSING MORTALITIES, THE METABOLIC PROCESS (OFF THE GILLS), SILT/MUD, FECES (POOP)

REGURGITATED FOOD), and goes into more detail.  They also provide some advice for managing the ammonia, but more from a supplier perspective than a chef perspective.  

Cheers, 

-p

ps: If you have had that happen with skate or shark, I have written about that over here: http://blog.medellitin.com/2008/11/cheap-skate-i8p.html.

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From: david.b...@gmail.com
Date: Fri, 26 Feb 2010 01:55:06 -0800
Subject: [MG] ammonia in crab stock
To: molecular-...@googlegroups.com

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