Dear Steve, and others.
Measuring 'freshness'.
Your message raises very basic questions of measuring freshness. It would
take a long essay, almost a book, to do full justice, but let me express
briefly my views on this topic.
Fundamentally, only sensory methods can measure freshness of spoiling fish.
Visualise a situation in which a group of people with a passing acquaintance
with fish is presented with a very fresh fish, more or less straight out of
the water, and a fish that had been held in ice for some time, say, 2 weeks.
I think you would agree that the group would be unanimous in declaring one
fish fresh and the other stale. The would come to this conclusion based on
the sensory properties of the fish - appearance, odour, texture. If portions
of these fish were cooked and assessed by sensory properties - appearance,
odour, taste - again I think you would agree that the group would declare
one fish fresh and the other stale. The experiment can be extended by
inserting fish of different storage times between these extremes and it will
be observed that the changes from fresh to stale form a continuum of
freshness/staleness. This continuum is the basis of scalar methods of
sensory evaluation of freshness, or for grading fish on the basis of
freshness.
'Freshness' can be defined in a variety of ways, but the crucial point to
note in this discussion is that freshness as it is commonly understood can
be evaluated only by sensory methods.
In my view, when Steve writes 'Besides sensory evaluation ...', he is
approaching the topic from the wrong direction. Freshness can only properly
be measured by sensory method; there is not question of 'besides'. Chemical
methods of monitoring changes during storage of fish are surrogates for
sensory methods, that is, they produce a measure that can be used to
predict, to some extent, the results of a sensory evaluation of freshness. A
value of 35 mg TVB-N/100g of itself is not an index of freshness, but TBV's
values around this value are associated with fish that have been assessed by
sensory assessment as being very stale.
Steve asks if there are better or more modern ways of measuring freshness.
TVB, and the associated TMA, are very poor predictors of freshness. The
drawback is that they do not change during early days of storage when
sensory measures of spoilage are. TVB and TMA can predict, though poorly,
sensory measures of freshness only when the fish is spoiled. A company that
uses TVB or TMA in quality control is admitting that their products are
spoiled. A better predictor of freshness over a wider ranges of freshnesses
is measurement of ATP degradation products, for example, as k-value or
concentration of hypoxanthine. These measures, which date from the 1950's,
are more modern than TVB or TMA which go back to at least the 1920's.
K-value and hypoxanthine are better predictors of freshness than TVB or TMA,
but still not good enough for quality assurance in processing plants.
Correlation coefficients between k-value or hypoxanthine and scalar sensory
assessment is around 0.8-0.85; good, but not good enough. Now and again
papers appear in scientific journals describing 'improved' analytical
methods for measuring these chemical indices, but analytical simplicity and
accuracy are not the limiting factors for their use; even if a cheap,
instantaneous and accurate procedure for measuring TVB, or any other index,
became available it would still be a very poor indicator of freshness.
Other measures, chemical and physical, have been proposed for measuring
freshness - see Botta, J.R., 1995, Evaluation of seafood freshness quality,
VCH Publishers, Inc, New York, ISBN 1-56081-612-0, for a summary - but they
all fail because they do not measure freshness directly, and are not precise
enough predictors of freshness.
Visit any plant processing fish. In all, freshness will be assessed, or
quantitatively measured, by sensory methods, very few will use methods other
than sensory.
Peter Howgate
----- Original Message -----
From: <Steve.Saunders@highlinerfoods.com>
To: Seafood research and extension information exchange
<seafood@ucdavis.edu>
Sent: Tuesday, May 21, 2002 8:56 PM
Subject: Measuring "Freshness"
>
> Some organizations measure "freshness" of frozen fish fillets using TVBN.
> This is the long standing chemistry method of measuring fish quality.
> Besides sensory evaluation, are there any "better" or more modern ways of
> measuring freshness?
>
> Steve Saunders
> High Liner Foods Inc. Box 910, Lunenburg NS B0J 2C0 Canada
> Telephone 902-634-8811 Fax 902-634-4577
> http://www.highlinerfoods.com
>
>
>
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