1. Change of odor
The fish odors deteriorate changing from a smell of staleness to ammonia and eventually to a repulsive smell of putrefaction manifested by many components of food. Rancidity can be observed in fatty fish due to fat oxidation.
2. Change of gills color
The gills area is considered the most susceptible part to the microbial putrefaction. Therefore, the color and smell change as the fish starts to decay. The dark pink color of the gills can be easily observed as it changes to bright pink and then to brown and eventually to yellowish grey depending on the degree of spoilage.
3. Skin color
The shinny color which distinguishes the fish changes to sandy brown. Other colors may form a visible slimy layer on the surface, a feature commonly found in chilled fish.
4. Meat texture
The meat texture converts from rigid and tight into tender and juicy, and it becomes loosely connected to the bones. This change is caused by proteolysis attributed to the microbial growth and the auto analysis initiated by the meat's own enzyme activities.
Other signs of spoilage are manifested by atrophic sunken eyes, with their brightness fading away. In addition, the tail area changes to the reddish brown color.
Signs of fish freshness
1. shinny popped eyes with glassy irises
2. pinky gills
3. glossy non-slimy skin
4. Stiffness of flesh
5. The smell characterizing fresh fish.
6. For the cuts, the surface should be wet and white but not brown.
Tips to prevent or slow down fish spoilage
1. The fish should be cooled immediately after they are caught on the fishing boat.
2. The guts should be removed with extreme caution, cleared away from the remaining parts of the fish.
3. Fish should be treated with chlorinated water (at concentrations of 2-5 ppm of chlorine).
4. The fishing boat should remain clean and it can always be sanitized with chlorinated water of 10-15 ppm.