Under anaerobic conditions, ATP within the muscles ceases to recycle. When about two-thirds remain, rigor mortis sets in. But this depends on the species, temperature and physical handling, this may take between 10 and 20 hours.
By lowering the temperature of brain-dead iki-jime fish, it is possible to slow down enzymic activity and maintain the ATP. To prolong the pre-rigor state, the spiked iki fish are cooled by lowering their temperature in a slurry tank only five to six degrees below the normal living water temperature for that particular fish.
For the shime fish, however, it is desirable that rigor mortis sets in fast and strong and lasts as long as possible. These fish are therefore treated by ice slurry immediately after spiking. Bleeding and ice slurrying are the most effective and fastest methods for chilling fresh fish for rigor mortis shime fish.
Depending on what fish are to be treated, and where, the ice slurry is made using either ice and sea water or fresh water with salts. In fact, using just fresh water for cooling and chilling is not ideal for presentation, regardless of the quality of the fish, as it causes leaching of the skin colour, clouding of the eyes and browning of the gills.
It is important to control the salinity of the slurry if fish are to be stored in it for a long time. Ideally, for most fish, storage time should be less than three hours. Japanese fishermen are generally prepared to achieve the right salinity and water temperature according to species by using a thermometer and specific gravity gauge to judge if extra salt and ice are necessary. For example, about half the salinity of sea water (1.8 %) is used for snapper, while a salinity similar to sea water (3.5 %) is used for squid.
Salts in an ice slurry bring down the freezing point, and sea water with a salinity of around 3.5 % will freeze at about minus 2 degrees Celsius. To avoid freezing the fish, the salinity of the slurry is carefully monitored.