Pamela Tom (pdtom@ucdavis.edu)
Thu, 3 Jun 1999 15:20:32 -0700 (PDT)
To: Seafood HACCP Mailing List
I am posting this message on behalf of Donald Gibson who is subscribed to
this list. (There are some technical difficulties which are preventing
his post to circulate via the mailing list.)
From: "Donald Gibson" <dmgibson@sol.co.uk>
Date: Thu, 03 Jun 99 20:40:37 PDT
Last week, Michel Desbiens posed a question about the browning of
marinated herring in a recipe including raw onions.
He is correct that the browning has been ascribed to the formation of
2,5-diketogluconic acid. The original reference is Ruiter, 1963,
Lebensmitt. vol 59 110 -111. The bacteria involved are now classified as
Erwinia herbicola, which helps to explain their origin, and are deposited
in the NCIMB , the UK equivalent of ATTC, under accession numbers NCIMB
9744, 9747, 9748, 10845, and 10846. The mechanism of formation of 2,5
diketo-gluconate is quite straight forward, being a series of
dehydrogenases acting first on glucose to form gluconate, then another to
form 2-keto gluconate and then the product which can cyclize to form a
brown precipitate.
While fish have little free glucose, there may be more in the onions and
in other components of the marinade. So, no glucose, no browning.
How do I know?
Well, I looked at these organisms during my PhD under Dr JM Shewan many
years ago. We thought that the organism might be related to the
'Achromobacter- Alcaligenes' group found on spoiling fish. In fact these
bacteria are now classified as Acinetobacter or Psychrobacter or Moraxella
if non-motile and Alcaligenes, Deleya etc if motile. This is why the
taxonomy and identification of fish spoilage bacteria is confusing as many
names have been used to classify these rather unreactive microbes which
are not usually regarded as active spoilers except under special
conditions. Anyway, it brought back memories of the days when biochemistry
could solve problems without the need for molecular genetics and PCR
reaction!
Donald Gibson
Biodon International
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