Nucleotide Degradation in Frozen Swordfish Muscle

In frozen swordfish steaks, inosine monophosphate (IMP) dephosphorylation was active at − 8 C and less so at − 18 C. A loss of IMP at − 26 C became apparent during a 2-year storage period. Rates of loss were about 0.24 and 0.029 μmoles per g per week at − 8 and − 18 C, the Arrhenius plot being linear over the temperature range 0 to − 26 C. Inosine ribohydrolase was less active; nevertheless, about 2.5 μmoles per g of hypoxanthine accumulated in 2 years at − 8 C. Taste panel tests indicated a storage life of 3 months at − 8 C, 9 months at − 18 C, and several years at − 26 C. The loss of IMP was slightly slower than the associated taste panel score decrease and thus may have potential as an indicator of frozen storage quality in some fish species, though it did not prove useful for swordfish. Hypoxanthine formation at low storage temperatures is sufficiently slow to allow use of hypoxanthine as an index of prefreezing quality; however, red muscle must not be included in the sample because of its high initial hypoxanthine content, about 1.9 μmoles per g.