Fish Microbiology and Quality Assurance

Unit 8 - Spoilage of fresh and processed fish and fishery products

8.1.2. Characters of spoiling fish
Change in external characteristics as the fish spoilage progresses can be used to indicate spoilage. The sequences of changes taking place as the spoilage proceeds are,

• Bright characteristic colour of fish fades, and fish becomes discoloured (appear dirty yellow or brown).
• Increase in slime on skin especially on gills.
• Eyes gradually sink and shrink, pupil becomes cloudy and cornea turns opaque.
• Gills turn light pink and finally to pale yellow colour.
• Softening of the flesh and exude juice when squeezed and easily indented by pressing with fingers.
• Flesh can be easily stripped from along the back bone / vertebral column.
• Release of odourous substances- the normal, fresh, seaweedy odour will change to sickly sweet, stale fishy odour due to TMA and other malodorous substances. Fatty fishes also show rancid odour.

Spoilage of crustaceans (Shrimps, Crabs and Lobsters)
Spoilage of crustaceans is essentially same as fishes. Spoilage differs depending on the handling and chemical composition. Crustaceans differ from fish in having carbohydrate (about 0.5%), higher content of free aminoacids than fish, and enzymes that rapidly break down proteins. Bacterial flora of crustaceans includes bacteria from the water from which harvested and also contaminants acquired during fishing, handling, transportation, processing etc).
Spoilage generally begins at the body surface. Presence of higher concentration of free aminoacids and nitrogenous extractives make them susceptible to rapid attack by spoilage bacteria. Initial spoilage is by production of large amounts of volatile base nitrogen (VBN). Some amount of VBN is also produced from the reduction of TMA. Subsequent spoilage results in production of off-odour substances, making it unfit for consumption.

Spoilage of Molluscs
Spoilage of molluscs is different from fish/shrimp because of difference in chemical composition. These have high carbohydrate content and low total nitrogen when compared to fish/shrimps. Carbohydrate content, mostly in the form of glycogen, is noticed in clams (3.40%) and oysters (5.60%). Fermentative type of microbial spoilage is noticed because of presence of glycogen. These also contain high levels of nitrogen bases (free arginine, aspartic acid, glutamic acid) than fish. Higher carbohydrate content is responsible for different spoilage pattern of molluscs over other seafood. The filter feeding molluscs have high bacterial load and are involved in spoilage.
The fermentative type of spoilage causes reduction in pH as spoilage progresses. This decrease in pH is used as inductor of extent of spoilage. Thus, pH is used as best objective criteria for examining microbial quality of oysters. Besides pH, organoleptic quality and microbial load are also desired as microbial quality indices. Using pH scale as microbial quality indicator, oysters can be grouped as good (pH 6.5 – 5.9), off (pH 5.8), musty (pH 5.7 – 5.5), and sour/putrid (pH 5.2 or below).