Preceding actin denaturation accelerates myosin denaturation in tilapia myofibrils in frozen storage

Abstract

Myosin and actin denaturation (0.1 M NaCl, 20 mM Tris–HCl, pH 7.5) of tilapia myofibrils (Mf) during frozen storage at −10, −20, and −40 °C was studied. Ca²⁺-ATPase inactivation was rapid at −10 °C but very slow at −20 and −40 °C. Myosin retained its solubility at 0.5 M NaCl even after Ca²⁺-ATPase inactivation. The amount of subfragment-1 generated from the Mf by chymotryptic digestion decreased with decreasing Ca²⁺-ATPase inactivation. The amount of rod produced in the frozen Mf remained at a high level, explaining the high salt solubility of myosin. Actin denaturation occurred in the Mf during freezer storage, as revealed by chymotrypic digestion patterns that showed a decreased actin content in the digest, and occurred much faster than Ca²⁺-ATPase inactivation. Analysis of tilapia meat during freezer storage revealed that Ca²⁺-ATPase inactivation was very slow in the frozen tissue and was ninefold slower at −10 °C than at −20, and −40 °C. Practically no actin denaturation occurred in the frozen meat. Based on these results, we conclude that native actin in frozen meat protects myosin from denaturation during the frozen storage period but that such protection by actin does not extend to Mf due to rapid actin denaturation. Consequently, the Ca²⁺-ATPase inactivation rate in Mf represents that of myosin itself—with no protection provided by actin. Therefore, the myosin denaturation rate obtained with Mf should not be used for frozen meat. Mf suspended in 0.1 M NaCl is not a suitable model by which to obtain the myosin denaturation rate in frozen fish meat.