In shrimp, the black spots initially
appear mainly on the cephalothorax,
appendices, and inter-segment
membranes. This is why melanosis
is usually more severe in head-on
shrimp products. After removing the
heads for headless, shell-on shrimp,
care should be taken to thoroughly
wash the tails to eliminate the proteases
that could promote melanosis.
Two biomechanisms in postmortem
shrimp result in the onset of melanosis.
One does not require enzymes,
and in the other, the enzyme tyrosinase
is the principal factor.
Enzymatic activity shows important
variations during the intermolt
cycle, but the main role of polyphenol
oxidase in the crustacean life cycle
is probably related to hardening the
shell in the intermolt cycle. The highest
level of polyphenol oxidase occurs
during stages C and D of the intermolt
cycle. When considering the texture
of the shell, the most suitable stages
to harvest the shrimp are stages C
and early D.
Stress can also initiate melanosis
in shrimp, whether directly influenced
by the general health of the animals
or wounds inflicted during harvesting.
The mechanism of wound healing
in shrimp produces compounds
as a result of the polymerization of
quinine, which exhibits both antibacterial
and antifungal activities.
Nevertheless, the enzyme that induces
this tanning effect does not interfere
in postmortem processing.