Dear Tony, and others.
You asked about reference samples for use in validation of analytical
laboratories. I am not aware of any, and I believe the situation with regard
to tests you refer to is not amenable at present to validation. So far there
are no agreed methodologies for the analysis of TVB, TMA and TMAO. There
might be an AOAC method for pH in meat and fish, and there is one for water
content by drying which gives you dried weight. There are recommended
methods for TVB, TMA and TMAO, for example Canadian Technical Report of
Fisheries and Aquatic Sciences, No. 1448, cited by Andrew, but I don't think
this can be considered as 'official'. There is also the report of the
Analytical Methods Committee, 1979, Recommended general methods for the
examination of fish and fish products, Analyst, 104, 434-450. The nearest to
an 'official' method I am aware of in this field is the EU Commission
Decision of 8 March 1195 (95/149/EC) fixing the total volatile basic
nitrogen (TVB-N) limit values for certain categories of fishery products and
specifying the analysis methods to be used, Official Journal of the European
Communities, L 97/84-87 29.4.95. It specifies distillation of a perchloric
acid extract made alkaline with sodium hydroxide as the reference procedure,
and allows for the microdiffusion Conway, direct distillation with
magnesium oxide, and distillation of a TCA extract as 'routine' methods.
It is well known that the results for TVB determination differ
systematically with the procedure used to measure it. The West European Fish
Technologists Association (WEFTA) have carried out collaborative trials
across laboratories on determination of TVB (Antonacopoulos, N. & Vyncke, W.
1989. Determination of volatile basic nitrogen: a third collaborative study
by the West European Fish Technologists ' Association (WEFTA), Zeitschrift
für Lebensmittel-Untersuchung und -Forschung, 189, 309-316; Vyncke, W.,
Luten, J., Brünner, K. & Moermans, R. 1987. Determination of total volatile
bases in fish: a collaborative study by the West European Fish Technologists
' Association (WEFTA),
Zeitschrift für Lebensmittel-Untersuchung und -Forschung, 184, 110-114), and
the EC procedure has been compared with the routine methods (Vyncke, W.
1996. Comparison of the official EC method for the determination of total
volatile bases in fish with routine methods, Archiv für Lebensmittelhygiene,
47, 110-112). Interested readers should read the originals, but, in the
context of this discussion, these reports reveal the systematic differences
between procedures and the poor replication across laboratories.
If TVB measurement is to be a component of validation of laboratories, then
the analytical procedure would have to be stipulated.
I am not aware of formal reports on comparison of TMA procedures, though
some papers of development of procedures might compare a new procedure with
another. It would take a search of the literature to reveal these. In my
experience accuracy and precision of TMA analysis by either the Dyer picrate
procedure or the GLC procedure are good starting from the extract. I am less
sure about the effects of variations in preparing the extracts for analysis.
I see no problems in principle with regard to preparing and distributing
samples. A bulk of material can be finely minced and mixed, dispensed in
cans with snap-on lids, and frozen. I would query whether validation is
justified. Both TVB and TMA contents are poor indicators of quality of fish
products. It does not matter how accurate or precise the analytical results,
their utility is poor. Regulatory authorities, like the EU Commission, might
specify limits, but they are floundering. Regulators are hooked on TVB and
find withdrawal symptoms of giving it up too severe to contemplate. When
asked by companies my advice on TVB methods I would tell them: if a seller,
use the Conway method and quote that to the purchaser - it gives the lowest
value, if a buyer and you wish to drive down the price, test by the EC
procedure and report that - it gives the highest value. If you are
interested in quality assurance, forget chemical methods and base decisions
on sensory evaluation.
Peter Howgate
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