Myosin denaturation in “Burnt” Bluefin tuna meat

Abstract

A quantitative conversion of tuna meat into muscle homogenate made it possible to study myosin denaturation in Bluefin tuna meat. Myosin denaturation was accessed by measuring Ca²⁺-ATPase activity, salt-solubility with and without Mg-ATP, monomeric myosin content, and amount of subfragment-1 (S-1) and rod produced by chymotryptic digestion. Commercially available tuna used in this study showed a pH around 5.4–5.7. Myosin in the meat lost the salt-solubility measured in the absence of Mg-ATP; however, such myosin showed full salt-solubility when released from actin in the presence of Mg-ATP. Incubation of tuna meat at 30 °C for up to 90 min caused obvious myosin denaturation. However, the tuna meat dialyzed against neutral pH buffer showed practically no myosin denaturation by the same heating. It was suggested that exposure to lowered pH to around 5.5 and increased temperature of 30 °C led myosin denaturation. Myosin denaturation in the “Burnt” Bluefin tuna sample was analyzed. A significant myosin denaturation was observed with the part showing the “Burning” symptom, the inner part of the tuna meat near the spine. Myosin in that part showed almost no Ca²⁺-ATPase activity, no salt-solubility even with Mg-ATP, no recovery of monomeric myosin, and almost no production of S-1 by chymotryptic digestion. However, myosin denaturation was not detectable for the meat taken from outer parts of the same tuna near the skin with normal appearance. It was demonstrated that “Burning” of tuna meat occurring in the deep part of the body is accompanied by myosin denaturation. The above results suggested that insufficient cooling of the deep part of body would be the reason for “Burning” of tuna meat.