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Fish
spoilage results from three basic mechanisms: Enzymatic autolysis, oxidation,
microbial growth.
Autolytic
enzymatic spoilage: Shortly
after capture, chemical and biological changes take place in dead fish due to
enzymatic breakdown of major fish molecules autolytic enzymes reduced textural
quality during early stages of deterioration but did not produce the
characteristic spoilage off-odors and off-flavors. This indicates that autolytic
degradation can limit shelf-life and product quality even with relatively low
levels of spoilage organisms. Most of the impact is on textural quality along with
the production of hypoxanthine and formaldehyde.
The
digestive enzymes cause extensive autolysis which results in meat softening, rupture
of the belly wall and drain out of the blood water which contains both protein
and oil. Peptides and free amino acids can be produced as a result of autolysis
of fish muscle proteins, essential. Which lead towards the spoilage of fish
meat as an outcome of microbial growth and production of biogenic amines.
Oxidative spoilage: Lipid
oxidation is a major cause of deterioration and spoilage for the pelagic fish
species such as mackerel and herring with high oil/fat content stored fat in
their flesh. Lipid oxidation involves a three stage free radical mechanism:
initiation, propagation and termination. Initiation involves the formation of
lipid free radicals through catalysts such as heat, metal ions and irradiation.
These free radicals which react with oxygen to form peroxyl radicals. During
propagation, the peroxyl radicals reacting with other lipid molecules to form hydroperoxides
and a new free radical. Termination occurs when a buildup of these free
radicals interact to form nonradical products. Oxidation typically involves the
reaction of oxygen with the double bonds of fatty acids. Therefore, fish lipids
which consist of polyunsaturated fatty acids are highly susceptible to
oxidation. Molecular oxygen needs to be activated in order to allow oxidation
to occur. Transition metals are primary activators of molecular oxygen.
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In
fish, lipid oxidation can occur enzymatically or non-enzymatically. The
enzymatic hydrolysis of fats by lipases is termed lipolysis (fat
deterioration). During this process, lipases split the glycerides forming free fatty
acids which are responsible for: (a) common off-flavour, frequently referred to
as rancidity and (b) reducing the oil quality. The lipolytic enzymes could
either be endogenous of the food product (such as milk) or derived from
psychrotrophic microorganisms. The enzymes involved are the lipases present in
the skin, blood and tissue. The main enzymes in fish lipid hydrolysis are
triacyl lipase, phospholipase A2 and phospholipase B. Non-enzymatic oxidation is
caused by hematin compounds (hemoglobin, myoglobin and cytochrome) catalysis
producing hydroperoxides. The fatty acids formed during hydrolysis of fish lipids
interact with sarcoplasmic and myofibrillar proteins causing denaturation
Microbial
spoilage: Composition
of the microflora on newly caught fish depends on the microbial contents of the
water in which the fish live. Fish microflora includes bacterial species such
as Pseudomonas, Alcaligenes, Vibrio, Serratia and Micrococcus.
Microbial growth and metabolism is a major cause of fish spoilage which
produce amines, biogenic amines such as putrescine, histamine and cadaverine,
organic acids, sulphides, alcohols, aldehydes and ketones with unpleasant and unacceptable
off-flavors. Trimethylamine (TMA) levels are used universally to determine
microbial deterioration leading to fish spoilage.
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