It is said that, during the thawing of frozen puffer fish, muscle becomes toxic due to the migration of toxin (tetrodotoxin) from toxic viscera and skin. In this study, the extent of toxification of muscle after thawing at 4°C was examined using slowly or quickly frozen puffer fish Fugu niphobles.
No significant difference in the extent of toxification of muscle was observed between slowly frozen specimens and quickly frozen ones. The muscle of most specimens was found to be toxic (up to 58 MU/g) even immediately after the first thawing. The migration of toxin into muscle from toxic tissues progressed gradually with the time elapsed after thawing; several specimens showed a strong toxicity of more than 100 MU/g after 24 and 48 h from the end point of thawing. The toxicity of muscle at the second thawing was failry low as compared with that at the first thawing probably because of the difference of muscle parts used for toxicity test. In the case of half-thawed specimens, the toxin did not migrate into muscle from toxic tissues.