A New Chemical Criterion for the Quality Control of Fish: Trimethylamine/Total Volatile Basic Nitrogen (%)

Abstract

Trimethylamine (TMA) and the total volatile basic nitrogen (TVBN) were determined in 169 samples of sea-fish (herring, cod, whiting, and mackerel) at all stages of decomposition. The comparison of these two parameters with the ratio P=TVBN/TMA (%) showed that P provides a useful index of freshness. It is relatively constant between species, its dispersion is less than that of TMA and it increases more rapidly than TVBN at the start of decomposition. Statistical analysis of the experimental results showed that there is a non-linear correlation between P and the decomposition index (i). The comparison of the line-equations defining log P as a function of i for different species of marine Teleostei led us to show that there is little or no intraspace variation of this correlation under determined conservation conditions. It is noted that for a given species, this correlation is temperature-dependant. Statistical exploitation of these results permit us to determine the maximal admissible values for P which can be used as a support for the development of new standards. Although a detailed organoleptic inspection is usually sufficient to determine the freshness of intact refrigerated fish, for sliced, frozen, and processed fish, or in the case of a dispute, laboratory examinations are indispensable and must rapidly provide objective data for the assessment of quality. Fish decomposition is mainly due to bacterial growth which results in the production of various substances, some of which are not normally found in live muscle tissue, while others, which are already present in the muscle, increase logarithmically in parallel with microbial growth (20). The assay of some of these substances may provide useful data for the evaluation of fish freshness. The determination of total volatile basic nitrogen (TVBN) first proposed by Boury (4,5) in 1935, is widely used today to estimate the degree of decomposition of fish. The efficiency of TVBN assay has been confirmed by several authors (8,9,11,23,27), although it does not fully satisfy sanitarians. In a previous paper (18) we showed that there may be a wide dispersion of TVBN levels for a given decomposition index, and that changes in 'Lahoratoire des Services Veterinaires. ^Lahoratoire Central d'Hygiene Alimentaire. TVBN were slow at the onset of decomposition. TVBN levels, although they are a useful parameter, are therefore subject to certain valid criticisms. Other chemical determinations have also been tested as decomposition criteria, though they all have disadvantages. Some of these tests, such as the hypoxanthine test, do not give consistent results (6,14,31). Others are only significant when the product is spoiled and inedible, such as H2S, indol, volatile acids (24), and histamine (22). Another example is the so-called K ratio (inosine + hypoxanthine/total ATP breakdown products) which increases in the first stage of decomposition and then rapidly tends towards a plateau (10,21,25). Many other initially promising tests were later found to give ambiguous results; trimethylamine (TMA) assay provides a good example. The TMA content provides an accurate indication of bacterial spoilage in some species (1,2,17) though it is not always reliable as certain other factors modify the TMA production. During this study we investigated the changes in the levels of TVBN and TMA during fish decomposition. Although neither TVBN or TMA are sufficient when taken alone and are often difficult to interpret, we examined the possibility of using them simultaneously to evaluate the degree of freshness by studying the correlation between the ration P= TMA/TVBN (%) and the level of decomposition. MATERIALS AND METHODS Species studied The degree of freshness and the TMA and TVBN levels were evaluated in four common fish species: Clupea harengus (herring), Gadus morrhua (cod), Merlangus merlangus (whiting), and Scomber scombrus (mackerel). The fishes were stored either at 0°C on an ice bath or in the refrigerator at a temperature of 7°C ± 1°C.