Nucleotide Degradation and Organoleptic Quality in Fresh and Thawed Mackerel Muscle Held at and above Ice Temperature

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Abstract
In mackerel, by the time of initial sampling, adenine nucleotides had been deaminated to inosine monophosphate (IMP) in the ordinary muscle; in the red muscle the degradative sequence was even more advanced, as indicated by high initial levels of inosine. Postmortem rates of degradation of IMP to hypoxanthine through inosine were similar in both types of muscle; at ice temperature the rates were slower than in cod but faster than in swordfish. A delay in icing of 6–8 hr after catching accelerated the gradual decline in eating quality with replacement of the characteristic fresh mackerel flavor by tastelessness. IMP dephosphorylation paralleled development of tastelessness, and spoilage (organoleptic) had developed prior to accumulation of appreciable amounts of hypoxanthine. At higher temperatures, 5–20 C, rates of IMP dephosphorylation, hypoxanthine accumulation, and quality loss were markedly increased. Thawing did not influence subsequent deterioration rates, but ascorbic acid dips delayed darkening of the flesh in the thawed samples.Excellent correlation of taste with both IMP and hypoxanthine content, and with various simple measures of IMP dephosphorylation, was obtained under the various handling conditions investigated, including delayed icing, holding at elevated temperatures, and after thawing. The simple tests — ultraviolet absorption at 248 mμ of a Dowex treated perchloric acid extract, and ratio of ultraviolet absorption of extracts at 251 mμ after Dowex treatment to that before treatment — proved as good indices of progressive quality loss to the unacceptability level as the more complex estimation of IMP or hypoxanthine.