c. Chemical Changes Associated with Storage.
(1) Oxidative rancidity. Oxidative rancidity is the result of oxygen combining
with polyunsaturated fats of fish, producing a change in color, flavor, and odor. The
change starts as a light yellow color, progressing through a golden yellow, to a light
reddish brown. The fish has a bitter flavor and emits an odor similar to paint. This
poses a problem with fish containing over 6% fat. The median superficial muscle will
normally be affected more than the great lateral muscle. Proper glazing and air-tight
packaging will prevent oxidative rancidity.
(2) Freezer burn. Freezer burn is a loss of moisture that leaves white dried
fibrous areas on the surface of the product that resembles styrofoam. As the condition
progresses, these areas become larger and deeper. The surface takes on a spongy
texture. Proper glazing and air-tight packaging will prevent this condition.
(3) Storage breakdown. Storage breakdown, discussed in textbooks as
protein denaturation (as there is loss of nutritional value), is the result of extended
storage or less than adequate storage conditions. The protein breaks down resulting in
a light brownish discoloration associated with a bland flavor and storage odor.
1-10. NATURAL CHEMICAL CHANGES
a. General. In order to understand the importance of handling fish properly, it is
necessary to understand the changes through which they go from the time of catch and
death until they are finally consumed. The flesh of all animals goes through three
changes after death: rigor mortis, autolysis, and final spoilage or putrefaction. Quality in
fish is more closely related to freshness than any other factor. Once fish are caught,
deterioration immediately sets in and the spoilage process continues, even though the
fish are iced down, until the product is no longer acceptable for food. The fact that fish
are cold-blooded animals complicates retarding of deterioration. Enzymes and bacteria
normally found in fish are accustomed to functioning at lower temperatures than those
found in warm-blooded animals. For this reason, control of enzymes and bacteria in
fish through chilling is not as effective as it is with red meats. The enzymes of these
cold-blooded animals are as active at a temperature of 32o F as those of warm-blooded
animals at 70o F.
b. Rigor Mortis. Rigor mortis is the first change that takes place in flesh after
death. When present, it is an important sign of freshness and wholesomeness. Rigor
mortis is caused by a breaking down of the glycogen of the muscle cell into lactic acid,
resulting in a firming of the flesh. The body comes rigid. This condition lasts from a few
hours to as much as 4 days. When pressure is applied, an indentation is left on the
surface. However, once the pressure is removed, the area returns to its original shape.
Since rigor mortis is a sign of freshness, it is important that steps be taken to prolong
this condition. The slower the onset, the longer rigor mortis will continue. The quicker
the onset, the sooner its disappearance. The following factors are directly related to