howgate (phowgate@rsc.co.uk)
Sun, 23 Jan 2000 20:37:34 -0000
Richard
On 20 January you wrote:
'Does anyone know what causes a mackerel to become mushy whilst not being
badly spoiled in the conventional sense. The flesh of the product in the
round can almost be squeezed out of its skin like toothpaste. I have only
come across it once, many years ago and I think it was attributed it to
protozoa. Can anyone enlighten me?'
You raised the possibility of protozoan infection, that is, by protozoans
that secrete proteolytic enzymes. Chris Leftwich described the situation in
his message of the 21st. In particular he refers to the myxosporidium,
Chloromyxum thyristes, which is usually reported as the parasite
responsible for the condition of milky flesh in Pacific hake, and in some
other species. (I am not sure that this organism has been reclassified as
Kudoa thyristes, because more recent papers on milky condition in Pacific
hake and Pacific whiting refer to Kudoa species including thyristes). As
Chris mentions, several species of fish have been described as being
affected by these parasites and exhibiting the milky condition.
I note that most descriptions of milky condition relate to fish caught in
the Pacific. I assume you are referring to Atlantic mackerel. When, and
where were the fish caught? I have not seen the milky condition in Atlantic
mackerel landed in Britain, and I can not recall any processor, or quality
controller, of mackerel reporting milky condition to me. I would be
interested to know if Chris has come across any instances. A feature of the
milky defect is that the fish appear normal when first caught and the milky
condition is observed about a day or so on storage in ice. You write in
your message that fish became mushy. Do I infer from this that he fish
seemed to have a normal texture when first caught and developed the mushy
texture on storage? If that is the case, then you probably have an example
of protozoal proteolysis. What proportion of the catch is affected?
I have come across instances of mackerel with very soft, pasty, textures,
but these were associated with fish with extremely high fat contents, in
the order of 30%.
Peter Howgate
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