Spoilage of Fish

introduction image

Spoilage of fish is a process of deterioration in the quality of fish that changes its appearance, odour and taste. Fish spoilage can be caused by the breakdown of biomolecules like protein, amino acid and fats that are naturally found in the fish. Thus a fish can be spoiled by either chemical or biological degradation. In chemical degradation, protein, fats, amino acids etc. are decomposed, whereas, in biological degradation, microorganisms carry out the degradation.

Other than bacterial and chemical degradation, enzymatic and mechanical damage are also types of spoilage. There are certain factors like high moisture, protein and fat content, improper handling etc. which favours the spoilage of fish. The common cause of fish spoilage is bacterial contamination and chemical oxidation of protein, fats etc.

The microorganisms involved in fish spoilage refers as SSOs (Specific Spoilage Organisms) which results in numerous unwanted metabolites being produced in the food cause undesirable appearance, flavour and odour. The growth of SSOs strongly depends upon the nutritional components of fish, moisture content, high temperature etc.

Content: Spoilage of Fish

  1. Definition of Fish Spoilage
  2. Causes
  3. Types
  4. Assessment of Fish Spoilage

Definition of Fish Spoilage

It can define as the contamination of fish, which results in the undesirable change in the colour, texture, flavour, odour, and appearance etc. Spoilage of fish also refers as “Putrefaction”.

Fish spoilage can occur as a result of enzymatic degradation, bacterial degradation, chemical decomposition and mechanical damage.

We can characterize the spoiled fish by observing the colour change, fishy smell, sliminess in the skin and scales, firmness of the flesh, discolouration of the backbone etc.


Spoilage of fish basically caused by the action of microbes, enzymatic activity and oxidation of nutritive elements present in the fish. In addition to these, there are some other factors which are also responsible like:

  • Improper handling
  • The high moisture content of the fish
  • Weak muscle tissue
  • Ambient temperature


There are commonly three types of fish spoilage, namely autolysis, bacterial and chemical spoilage.

types of fish spoilage


It can define as the process of the enzymatic degradation that results in the cell damage of fish and release of autolytic enzyme which degrade the cell components like proteins, fats etc. and thereby changes the flavour of fish. The change in the flavour of fish can be due to the conversion of ATP to hypoxanthine and decomposition of fish.

Conversion of ATP to hypoxanthine: This conversion adds bitter taste, and we can estimate the degree of freshness by knowing the hypoxanthine content.

Decomposition of fish: It leads to belly bursting of the fish, which is caused by the enzymatic action of digestive enzyme found in the gut of fish.

Autolysis can change the appearance and odour of the fish by two ways:

  1. Cause black spot formation: Autolysis can cause black spot formation in some shrimps by the enzymatic action on the amino acid. The black spot forms due to the formation of melanin pigment, which results in poor appearance.
  2. Cause foul smell: Autolysis can produce foul smell by the enzymatic degradation of muscle protein by a proteinase into amino acids and further forms ammonia, carbon dioxide, amines, fatty acids etc. This production of secondary metabolites produces indole, skatole etc. which results in the production of foul smell from a fish.

Bacterial Spoilage

A fish acquires a load of bacteria in the gills and on the surface. When a fish gets died, the bacteria already present in the fish attacks the flesh and result in the formation of undesirable products. The microbial growth on fish depends on the type of water from where it caught from.

The bacteria cause fish spoilage by:

Reducing TMAO to TMA: Reduction of trimethylamine oxide into trimethylamine produces an offensive odour.

Degradation of amino acid to primary amines: It can cause food poisoning.

  • Histidine – Histamine
  • Glutamic-acid – Arginine

Degradation of urea to ammonia: It also produces an offensive odour.

Chemical Spoilage

Chemical spoilage of fish is favoured by high temperature. Oxidative rancidity is a common cause of chemical degradation.

chemical degradation of fish
Protein degrades into amino acids, amines, ammonia and hydrogen sulphide by the action of proteolytic microorganisms. carbohydrate degrades into acids, alcohols and gases by the action of fermentative microorganisms. fat degrades into fatty acid and glycerol by the action of lipolytic microorganisms.

Assessment of Fish Spoilage

We can access the quality of fish by the three consecutive methods:

quality assessment of fish

Physical method

Torrymeter is a device that is placed vertical and provides a digital reading of the fish quality whether it is aged or fresh. From the digital readings or value of Torrymeter, we can estimate the freshness of fish.

A low value indicates the presence of a high number of the microbial population. 10 is the highest value for the freshly caught fish, and below 3 is the value of spoiled fish. The value of 6 on Torrymeter is acceptable by the consumer.

Subjective method

It also refers as “Sensory method” which is assessed by sensory organs that represents the customer view.

Organoleptic test

This test includes the quality assessment of fish by the sense of sight, smell, touch etc.

We can check the quality of fish by a sense of sight by seeing the eyes, gills and skin surface like:

  • The eyes should be clear and vibrant. If there is any discolouration around the eyes and cloudiness in the eyes, then it indicates that the fish is not fresh.
  • The gills should be red-pink in colour.
  • The skin should be shiny, not slimy.
  • The skin or the surface of fish should be clear, and there must not be any discolouration.

We can also check the quality of the fish by the sense of touch like by touching the flesh and scales of the fish:

  • The flesh should be tight, elastic, but not slimy.
  • The scales should be intact with the skin.

We can also check the quality of the fish by the sense of smell:

  • The smell of fish should be neutral and fresh. There should not be fishy, sour and ammonia-like smell.

Biochemical method

It includes the following methods

Proximate testing: It is a prevalent method, in which the fish components like moisture, protein, lipid etc. are regularly checked from the time of fish harvesting. This method does not give a satisfactory assessment and thus not accepted.

Hypoxanthine value: After the death of fish, break down of an ATP (Adenosine triphosphate) into ADP, AMP, IMP and finally into hypoxanthine occurs. The value of hypoxanthine increases during the storage of fish. Hypoxanthine value gives an estimate for the freshness of fish. A fish is considered to be spoiled when the hypoxanthine value reaches 7-8 micromoles/g.

Trimethylamine (TMA) value: Fish contains a considerable amount of trimethylamine oxide (TMAO), but when it gets spoiled, the TMAO reduces into TMA. The value of TMA with a level of 1.5mg / 100g indicates that the fish is moderately spoilt.

Ammonia production: The production of ammonia indicates that there is an extent of spoilage.

Peroxidase value: It helps in the measurement of oxygen rancidity. Peroxidase value less than 10 indicates the good quality of fish, but value more than 20 indicates rancidity.

Thiobarbituric acid value (TBA): It also helps us to determine the oxygen rancidity. TBA value less than 2, is accepted by the consumer.

Biological method

It includes total plate count method (TPC), which is used in the quality assessment of fish by giving an estimate of bacterial presence. In this method first, grind the fish and then dilute the sample by following serial dilution. Prepare media for the growth of microorganisms present in the fish, where we can use both ordinary and selective media.

For the enumeration of microorganisms in marine fish, agar media is used whereas for the processed fish, tryptone glucose beef extract agar media is used. Other than this, selective media like SS-agar can be used for the detection of coliform bacteria (E.coli, Shigella sp, etc.) in the fish.

After media preparation, perform pouring method and incubate the plates for 24 hours at 35-37 degrees Celsius. Count the number of bacteria per plate, by multiplying with the dilution factor. Total plate count method thus gives a count for the bacterial population present in the fish, which can be pathogenic and non-pathogenic. Hence, total plate count method does not determine the edibility of the fish.

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