What is burnt meat syndrome or yake in Japanese?
The stress of capture makes bluefin tuna (and other animals) secrete lactic acid in the muscle, causing the meat to take on a metallic taste when consumed raw and accelerating the degradation process due to a pH drop.
This phenomenon is known as yake in Japanese, meaning “burnt meat syndrome.” It gives the meat an unpleasant metallic taste and a brownish colour. This quality is highly penalised, especially by Japanese markets.
The ikejime technique is a very hands-on, humane form of slaughter that is done individually by a team of experts and requires great precision. The tunas are chosen for their size, by customer demand. Divers enter the aquafarming facility and give the tuna a clean and accurate hit using a bang stick (a sawed-off underwater shotgun).
This minimises the stress signals that are sent out and prevents nervous spasms. Once slaughtered, the tuna is bled into the water by making two incisions where the blood flow is concentrated, promoting the generation of inosinic acid (flavour enhancer). The key to success is in being able to bleed the fish quickly so none of the stress hormones get into the muscles and make them overheat.
After 12 seconds, the fish is taken out of the water and then a sharp wire is pushed into the spinal cord to cleanly eviscerate the animal — preventing any parasites from passing to the muscles — and then soaked in ice water to lower their body temperature.