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I think this is a clear cut case of melanosis, particularly if the
discolouration is around the cephalothorax and tail of the shell of the
prawn. I have observed melanosis appear in several shades from black
through green to a rusty coloured red/brown. The freezing treatment before
the sulfite may even be exacerbating the problem. The enzymic activity in
the hepatopancreas can be increased at prefreezing temperatures (around -1
to -5 degrees C). Also if melanin precusors are already created (during
sorting, time on the deck etc, the prawn will turn black when exposed to
oxygen. Tropical shrimp will have a slower freezing rate, subjected to a
longer period at the temperature ranges where enzymic activity is greatest.
Treatment with sulfites before freezing should decrease the amount of the
catch turning black. Making sure the prawns are frozen quickly should also
help the problem and improve quality.
Regards
Jacquie Edwards
Seafood Technologist
Centre for Food Technology
19 Hercules Street
Hamilton Q 4007
voice: 07 3406 8504
fax: 07 3406 8698
email: [log in to unmask]
> -----Original Message-----
> From: Mariana Horsten [SMTP:[log in to unmask]]
> Sent: Thursday, 17 August 2000 18:41
> To: [log in to unmask]
> Subject: Green post-freezing discoloration in tropical shrimp?
>
> I have received the following query, regarding shrimp fisheries in
> Cameroon:
>
>
> " Here (in Cameroon), 'Giant Sea Shrimps (Sea Bob)' are caught at sea, and
> frozen onboard. After about a week at sea, the boats return to port,
> where
> the catch is treated with 'sulfite' and prepared for sale. However, about
> 20% of the catch develops a green discoloration, and has to be discarded.
> What causes this? I suspect it's either a bacterial infection or
> 'enzymatic
> oxidation'. I've read that Ivory Coast has a similar problem, but there
> the
> shrimp turn black. "
>
> If anyone has information on this phenomenon, let us know!
>
> Regards,
> Mariana
>
> .
> ==========================================
> Mariana B. Horsten
> Department of Marine Fisheries
> Danish Institute for Fisheries Research (DIFRES)
> DK-2920 Charlottenlund
> DENMARK
>
> Email: [log in to unmask] --- Tel: (+45) 33 96 33 59
> Website: http://mbhorsten.tripod.com/ (updated: 21.06.2000)
> ==========================================
>
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