The document summarizes key post-mortem biochemical changes that occur in fish after death. It discusses how rigor mortis develops as ATP is broken down and actin and myosin cross-bridges form. It then describes how autolysis and bacterial spoilage lead to degradation of lipids, proteins, and other tissues. Several chemical indices used to measure spoilage are also outlined, including TVN, TMA, alpha-amino nitrogen, and peroxide value.

1. WEST BENGAL UNIVERSITY OF ANIMAL
AND FISHERY SCIENCES
A SEMINAR ON:
POST MORTEM BIO CHEMICAL CHANGES IN FISH&
RIGOR MORTIS
SUBMITTED TO, SUBMITTED
FROM,
DR. S.CHOWDHURY SK.SAMIM AKTAR
DEPT. OF FPT M.FSC1ST SEM1ST YR

2. INTRODUCTION
 Fishes are perishable commodities. After the catch when fish dies the
biochemical changes responsible for anabolism stop and only catabolism
processes keep functioning.
 The digestive enzymes still being active,begin to digest fish tissue components
itself. Due to this, fish tissue components e.g. lipids, carbohydrates and
proteins breaks down (autolysis).
 Lipid oxidation causes rancidity of fish flesh which produces foul smell.
 Autolysis also causes fading of fish pigments, development of off flavour and
browning.
.

3. Changes in fish flesh Bio chemistry
 The demise of a fish begins a series of irreversible changes which lead to
spoilage and loss of quality.
 The natural process is as following:
The duration of each phase can change or phases can overlap.
 This depends on storage conditions, especially the temperature which greatly
influences these processes.
 It also depends on the ever increasing microbial population with time.
Slime
secretion
Rigor
Mortis
Dissoiution
of Rigor
Autolysis
Microbial
spoilage

4. Slime Secretion
 Slime is formed in certain cells of fish skin and the process becomes very active just after
fish death..
 Fish which secrete great quantities of slime have poorly developed scales; very often the
quantity of slime reaches 2-3% of the fish mass and that in turn creates problems during
processing.
 Slime contains large amounts of nitrogenous compounds and these provide good
nourishment for micro-organisms originating from the environment.
 Therefore, the slime spoils quickly: first giving an unpleasant smell to the fish, and second
opening the way for further and deeper bacterial penetration into the fish.
 The secretion process stops with the onset of rigor mortis.

5. PRE-RIGOR CONDITION
 After death as circulatory system stops: respiration in organisms ceases to occur,
depleting the amt. of oxygen used in the making of ATP.
 But the oxygenated glycogen rich muscle can remain metabolically active for hours
in pre-rigor condition.
 However, after death, anaerobic breakdown of glucose takes place. The synthesis of
ATP stops and hydrolysis begins as per the reaction given below:
ATPASE
ATP ADP+Pi…………………..(i)
 Rapid decrease in the ATP level and increase concentration of phosphoric acid is
observed.
 Although certain amount of ATP is synthesized by the hydrolysis of creatine
phosphate by the enzyme creatine kinase by the following reaction:
CREATINE KINEASE
Crp+ADP Cr+ATP……………….(ii)

6. CONTD….
 But the creatine reserves in the tissues are not infinite and soon get exhausted.
 The concentration of lactic acid formed due breakdown of glucose is found to be
dependent on the glycogen reserves of the muscle prior to death.
 The concentration of lactic acid formed due breakdown of glucose is found to be
dependent on the glycogen reserves of the muscle prior to death.
 consequence of the accumulation of lactic acid in the muscle is the lowering of pH from
near neutrality (pH 7.0) to the lactic acid range (pH 6.0).
 The decline of pH affects the quality of the fish tissues, in that flesh become firmer and
tendency to drip enhances.

7. RIGOR MORTIS
 At the final phase of ATP breakdown myosin cross bridges interaction are established
firmly between the thick and thin filaments thereby making it non-extensible and
hence non-contractile.
 Muscle consists of several proteins actively involved in contraction. The two major
proteins, actin and myosin, combine in the presence of calcium ions to form
actomyosin.
 ATP then supplies the energy for contraction, and later also the energy for the
removal of the calcium ions via a calcium pump.
 This breaks the actomyosin complex, leaving the muscle ready for a further
contraction.
 Temperature should be remain low to get proper rigor mortis.

8. PIC:MUSCLE CONTRACTION

9. POST RIGOR CONDITION
RAW FISH
SPOILAGE
BIOLOGICAL
CHANGES
ENZYMATIC
CHANGES
MICROBIAL
CHANGES
CHEMICAL
CHANGES

10. ENZYMATIC CHANGES
ENZYMATICCHANGES
GLYCOLYSIS
FLAVOUR CHANGES IN
FISH
BELLYBURSTING
COLOUR CHANGES IN
FISH
PINK SPOILAGE
YELLOW
DISCOLORATION
CHOCOLATE
DISCOLORATION

11. AUTOLYSIS
 The decomposition of the fish occurs as its constituent compounds break down.
 The proteins, nucleotides and sugars break down, bases are released, the pH falls and the
fats are oxidized.
 These make the fish smelly, rancid and tough. Enzymes in the flesh and gut previously
involved in metabolism now catalyse autolytic reactions, in which various compounds
decompose.
 Enzymes in the flesh break down desirable compounds into tasteless or bitter ones, whilst
gut enzymes attack the internal organs, turning them into a soupy mess and allowing
bacteria to enter the flesh.

12. CARELESS HANDLING OF FISH
FISH CELL BROKEN
RELEASE AUTOLYTIC
ENZYMES
PRODUCTION OF SPOILAGE SUBSTANCES
THESE SUBSTANCES CREATE VERY GOOD ENVIRONMENT FOR
MICROBES
STEPS INVOLVED IN AUTOLYSIS PROCESS

13. Bacterial spoilage
 In a living fish, bacteria are present in the gut and skin, but the flesh, which they are
prevented from entering, remains sterile.
 Once autolysis begins, however, the bacteria are able to enter the flesh, whereupon they
multiply rapidly and decompose the muscle.
 The gut contains huge no of bacteria;after acid breaks down the thin wall of the gut they can
easily penetrate in to muscle.
 Anaerobic bacteria (those which operate in the absence of oxygen) produce a particularly
foul type of spoilage which results in an inedible fish.

14. CHANGES OF DIFFERENT TISSUE
COMPONENTS
1. WATER
 Water in the tissues exists in two different forms, free water and bound
water.
 Water molecules attached to other molecules like protein etc. through
strong or weak chemical bonds constitute the bound water.
 The bound water may be an integral part of the molecule.
 Removal of water by any process will result in alterations of the structure
of the molecule.
 In other words, when the structure of such molecule is altered by any
process, the bound water is released.

15. CONTD….
 2. PROTEIN
 The physical and biological properties of protein are due to the unique
structure of these molecules.
 In a living system this structure is kept intact.
 Changes in pH, temperature etc. will affect the structure of the protein
molecules, especially once the system is dead.
 Under such conditions the natural form of protein is lost (denaturation of
protein) which will result in loss of texture, reduced water holding capacity etc.

16. LIPID OXIDATION AND HYDROLYSIS
 The two major deteriorative changes which occur in fish are:
 (i). The enzymatic hydrolysis of lipids (fats) to produce free fatty acids and
glycerol:
 (ii) the oxidation of fish oils yielding the rancid odours and tastes which
are the major problem encountered in fish storage.

17. NEUCLEOTIDE & QAC
NUCLEOTIDE:
 The major nucleotide present in fish is ATP which is degraded to hypoxanthine in
 dead fish and is responsible for imparting bitter taste.
QUARTERNARY AMMONIUM COMP0UNDS:
 Trimethyl amine oxide (TMAO) is the major compound.
 TMAO is responsible for odour and flavour of marine fish and is present in larger
quantities in elasmobranchs than in
 teleosts. It is concentrated more in the red muscles than the white counterpart. TMAO is
 decomposed to Trimethyl amine (TMA) which is a good index for spoilage of fish.

18. SPOILAGE INDICES
In the trade, freshness is usually judged based entirely on the appearance, order, and
texture of the raw fish. Since this assessment depends on the sense, these factors are
called sensory or organoleptic parameters. Some chemical parameter are also used for
measuring the freshness of fish such as:
1. Total volatile base/ total volatile nitrogen (TVN)
2. Trimethylamine (TMA)
3. Alpha – amino nitrogen
4. Ammonia
5. Volatile Reducing Substance (VRS)
6. Hypoxanthine
7. Peroxide Value (PV)
8. Thiobarbituric Acid (TBA) Value
9. K-Value

19.  Total volatile base/ total volatile nitrogen (TVN)
Spoilage of fish is accompanied by the release of several volatile compounds like
trimethylamine, ammonia etc. The concentration of these compounds in tissues may indicate
the degree of spoilage, particularly in later stage of spoilage. The recommended upper
acceptable limits vary from 30 to 60 mg/ 100g fish flesh.
 Trimethylamine (TMA)
Trimethylamine is one of the chemical indices that have the nearest correlation with
sensory evaluation in many species of fish. Trimethylamine oxide (TMAO), a naturally
present component in marine fish, is broken down by bacteria to TMA, related chemically to
ammonia. Very fresh fish has only very low content of TMA and its concentration increase as
spoilage advances.
 Alpha – amino nitrogen
During spoilage the tissue protein are degraded into simple peptides, amino acids etc.,
thereby increasing the content of free alpha – amino nitrogen groups in the muscle. The
estimation of alpha – amino nitrogen, thus serve as an index of extent of spoilage.

20.  Hypoxanthine
The enzymes present in fish tissues decomposed adenosine triphosphate (ATP)
through a series of steps to hypoxanthine. Formation of hypoxanthine in tissue is somewhat
proportion to the extent of spoilage during the early stages.
 Peroxide Value (PV)
Oxidative rancidity is one of the most important factors that determine the
acceptability of fish during processing and storage. Peroxide value is the measure of the
degree of oxidation of the fat and in term determines the spoilage condition.
 Thiobarbituric Acid (TBA) Value
Oxidised fat reacts with thiobarbituric acid to produce a red pigment known as
malonaldehyde. TBA test is used to determine the quantity of malonaldehyde in fish.
Quantity of malonaldehyde in fish indicate its fresheness.
.

21. K-value:
It is an index of freshness and is the % of inosine and hypoxanthine in total ATP
and
its degradation products after death. Inosine (HxR) and Hx reflects poor quality
the
product. It is related to both autolytic and or microbial action and is generally
completed
within a day.

22. CONCLUSION
 As fish is a perishable commodity it spoils quickly.
 Proper care should be taken at the time of handling and preservation.
 Shrimp contains greater amount of free amino acids than fish and also
highly active proteolytic enzymes. Shrimps are highly susceptible to
melanosis, a characteristic black spot development caused by an enzymatic
reaction. They are highly perishable, and require adequate refrigeration
and expeditious handling to prevent decomposition
 Fish should be consumed fresh or should be properly preserved so that its
palatability and nutritive value are not seriously impaired.