(1) Temperature of the flesh. A rise in the temperature of the air and water
hastens the onset of rigor mortis. As the temperature increases, the faster the fish will
pass through this stage toward deterioration. Rapid cooling of fish immediately after
catching slows the onset of rigor mortis and therefore prolongs freshness.
(2) Size. All other things being equal, the smaller the fish, the more rapid the
onset of rigor mortis.
(3) Method of catch. Any method causing fish to struggle hastens the onset
of rigor mortis. In the struggle, muscle cells convert the stored glycogen to energy
causing a sudden release of lactic acid in the cell. During life, this lactic acid is normally
carried away by the blood. If large amounts of lactic acid are present in the cell at
death, there is an almost immediate onset of rigor mortis.
(4) Method of handling. Stunning the fish prevents a violent death struggle.
Evisceration and packing the body cavity with ice immediately after catching will hasten
chilling and delay rigor mortis.
c. Autolysis. Autolysis is the softening of the fish flesh caused by enzymes
which are normally present in the body. It starts immediately following death and
continues until final decomposition. Enzymes are chemical substances produced by
living cells. They produce changes in other substances without being changed
themselves. After death, the body systems that regulate enzyme action no longer
function; however, enzyme action continues.
(1) Digestive enzymes. Fish, as with all living animals, derive essential
nourishment from eating plants or other animals. To accomplish this, the animal must
break down these complex foods into simple compounds that are capable of passing
through the wall of the intestine into the blood stream where they can be assimilated by
the animal's own body. This process, called digestion, is a chemical process performed
by digestive juices containing enzymes. Each enzyme has a specific function; some
break down protein contained in the food, others act on fats, and still others on
carbohydrates. For example, a fish was eating just before it was caught. It has
secreted digestive juices and enzymes to digest its food. However, since there are no
body processes to control the functioning of the enzymes when the fish dies, the juices
attack the intestinal walls and escape into the muscles of the belly walls where they
continue to act.
(2) Body tissue enzymes. The enzymes present in the body tissue enable
the body to draw upon stored body reserves for maintenance or energy supplying
materials if food is not available to the body. Tissue enzymes are also essential in the
healing process by dissolving the damaged cells so that they can be carried away in the
blood. The old cells are then replaced by new cells to heal the injured part. Worn-out
cells are similarly broken down by tissue enzymes and removed by the blood. Organs
such as the pancreas and the stomach undergo self-digestion very rapidly due to the
action of the enzymes they normally produce.