Determinação dos compostos basicos totais de pescado e o seu potencial para avaliação do frescor

AUTOR(ES)
DATA DE PUBLICAÇÃO

1985

RESUMO

This research reports the analytical conditions and application of a new chemical method for the estimation of the freshness of fish. It has been given the name "Total Bases" (TB) to differentiate it from the conventional test for Volatile Bases (V$. The development of the present method was performed with the following species: sardine (Sardinella aurita), mackerel (Scomber scombrus), croacker ((Micropogon fournieri), weackfish (Macrodon ancylodon), corimbatã (Prochilodus reticulatus) and hybrid-tilapia (honorum-nilótica). The determination consists of a selective extraction of nitrogenous basic compounds from fish, muscle with absolute ethanol and solid Ca(OHl2 in the ratio: 1:3:0.08 (f ish:ethanol :Ca (OH) 2). The mixture is homogenized and filtered, providing an extract with the basic compounds in the free state so that they may be titrated directly with HCl, The choice of absolute ethanol and Ca(OH)2 was based on tests with several solvents and solid alkalis to obtain an extract with a minimum of amphoteric substances that intefere with the end point of the titration. The efficiency of the ethanolic Ca(OH)2 extraction was demonstrated by the quantitative recovery of the N from NH4Cl added to the fish sample and also by the content of trimethylamine aid other volatile bases determined for comparison by independent methods . Identification of individual compounds in the TB extracts was first by determining which compounds react with ninhydrin. It was established that, in the extracts of sardine, 70.9% of the N corresponded to ammonia and primary NH2-containing compounds. This value was made up of 47.6% of N from histidine, 10.8% of N from NH2 and 12.6% of N from other amino acids, mainly lysine, alanine and glycine. In the extracts of croaker and weakfish, only 37.2 and 43.2% of N, respectively, reacted with ninhydrin, being made up of 25.1 and 32.3% of N from NH3. Corimbata extracts had 36.2% of ninhydrin-reacting N, consisting of 12.2% NH3 N, 10.0% histidine N. The remaining 14% was from other amino acids. The TB method was applied to studies of storage of iced fish using also the VB method for comparison. Experiments with sardine, mackerel and tilapia for 13 days showed that curves for TB had a greater slope than the respective curves for VB, indicating more clearly the progress of the spoilage. In corimbata the changes of both tests with time were almost identical, while with croaker and weakfish the curves of TB were less demonstrative of loss of freshness than those for VB. BT values in fresh, fish are in the range of 20 to 50 mg of TBN/l00g. After 2 weeks in ice those values may reach 100 mg of TBN/l00g.

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