In trying to assess fish quality, great care must be taken to carefully investigate the many variables that have an impact on this protein source. One must be prepared to examine each species individually, its environment, composition, harvesting and handling. It must be understood that no single index of quality has been standardised and that there are microbiological, chemical, biochemical and physical interactive changes.
A number of different tests can be used for estimating the degree of spoilage in fish. These include total bacterial numbers, total volatile bases, TMA, total volatile reducing substances, indoor sensory analyses, refractive index of the eye fluid, electrical parameters of the fish flesh, volatile acids, volatile ammonia and total volatile nitrogen. All of these however, require considerable judgement, if they are to be interpreted correctly.
Bacteria growing on the surface of the fish tissue produce volatile amines. One such volatile base is trimethylamine (TMA), a reduction product of the component trimethylamine oxide. TMA has been used as an indicator of general fish spoilage. Other volatile amines produced include ammonia and small amounts of monomethylamine and dimethylamine. While the fish is still in rigor dimethylamine begins to form and after rigor trimethylamine is formed.
Although not universal in acceptance or acceptability the TMA determination has become one of the established procedures for determining fish quality. It has been proposed that TMA levels between 5 and 10 mg/100 g tissue should be considered the maximum allowable levels in international trading. It should be observed that TMA, as would be expected of the bacterial product, is not useful in determining quality deterioration, which occurs during frozen storage. It should also be observed that the TMA value are dependent upon the storage temperature of the fish and will vary accordingly.
Another school of thoughts centres around dimethylamine (DMA) as a basis for quality assessment and in many cases DMA has been successfully used to measure quality of frozen fish. DMA has been shown to be produced autolytically at sub-zero temperatures. While DMA formation has been shown for cod, haddock, husk, hake and Alaskan pollock, no DMA has been found in similar studies of frozen lobsters, scallops or shrimps after extended storage at -5°C.
Chemical tests for dimethylamine are most valuable in the early stage of spoilage and trimethylamine is most sensitive as an indicator in the later stages of spoilage. Determination of bacteria counts, while they are of value to research, requires too much time before results are known for routine testing.
Thiobarbituric acid (TBA) has been used to assess the development of oxidative rancidity. Oxidation of fat-containing foods leads to the formation of malondialdehyde or derivates of this compound. The reaction of malondialdehyde with TBA is an effective means of measuring the extent of auto-oxidation, but unfortunately it seems too unreliable as an index of freshness. Such a wide variation exists between species and within species and so many other additional factors effect the development of rancidity that this determination seems not to be the answer of how to judge rancidity.
All the above-mentioned quality assessment methods must be carried out by specially trained experts, which means that the storeperson can use only sensory evaluation of appearance, odour and texture. Very few specifications exist, as there are obviously differences from species to species and therefore a general specification cannot be very detailed. Torry Research Station, Aberdeen, UK, has developed specifications for cod, herring, flat fish and red fish and the specification on cod is given as an example in Appendix 2. It must, however, be observed that this sensory judgement also requires a specialist taste panel.