2. Learning objectives
• Examine the post harvest changes in fish
– Postmortem changes
• Intrinsic
– Rigor mortis
– Autolysis
• Extrinsic
3. Post-harvest changes
• Quality of harvested fish depends on these
factors
– Intrinsic
• Species, size, sex, composition, spawning, presence of
parasites, toxins, contaminants with pollutants and cultivation
• Biochemical characteristics that influence autolysis, rapid
microbial proliferation and spoilage are
– Low glycogen
– High unsaturated lipids
– Soluble nitrogen compds
– Extrinsic
• Location of catch, season, methods of actch (gill net,
handline, longline, or trap, etc), on-board handling, hygienic
conditions of f/vessel, processing and storage conditions
5. Postmortem changes
• Live fish muscle is relaxed and elastic
• Immediately after death, rigor mortis
sets in
– rigor in fish usually starts at the tail, and the
muscles harden gradually along the body
towards the head until the whole fish is quite
stiff. The fish remains rigid for a period which
can vary from an hour or so to three days,
depending on a number of factors described
later, and then the muscles soften again.
– occurs due to changes in the physiology of
muscles when aerobic respiration ceases
6. • Fish muscle has a low glycogen reserve and trimethylamine oxide is
efficient at buffering against acidity so that the ultimate pH at the end
of rigor is pH 6.4-6.8
– Trimethyl amine oxide (40-120mg/kg fresh), reduced after death by
bacteria to “fishy” smelling trimethylamine (high in sea fish)
– Fish muscle autolysis is low due to
• Low proteinases; cathepsin have low pH optimum at 4.3
– Not active at fish muscle pH 6-7
• Seasonal changes such as spawning, feeding cycle, degree of
struggle during capture, etc. affect the post-mortem condition of the
fish.
• The pH of the flesh influences fish quality, for example, frozen fish
defects include “gaping”, brown discoloration, and shrinkage which
results in “drip” formation and toughening
– Gaping is separation of muscle bundles in fish fillets giving them a ragged
appearance
7. PRE-RIGOR MORTIS
• Live fish
– Cycles of chemical changes take place continuously in the muscle
• these provide energy for the muscle while the fish is swimming, and also
produce substances necessary for growth and replacement of worn-out
tissue.
• enzymes are compounds that bring about, and control, these changes
• Low glycogen in muscles contributes to small pH drop
– pH 6.2 is maintained (fresh: pH 6.0-6.5)
– The enzymes in the flesh go on working even after the fish is dead
• some of them act on those substances that normally keep the muscle
pliable and lifelike.
– During life, the muscle contract and become rigid if its two main
protein components were allowed to interact and bond together
• but the bonding is prevented by the presence of substances that keep the
muscle pliable
• Fish exhausted by lengthy struggle (stress prior slaughter) give meat poor texture
and a low keeping quality
– Rigor mortis duration is shortened
• pH remains high
• Muscle type makes maturation time shorter
• Factors highly favorable for rapid spoilage are;
– Structure of muscle
– Tendency to generate alkaline pH rxn in muscle
– High probability of microbial infection during fishing and dressing
8. RIGOR-MORTIS
• Rigor mortis is shorter in cold-blooded (hours-a day) animals
• Duration of rigor mortis depends on species, tempt, and condition of
fish when caught
– Stiffness is delayed when caught and put on ice immediately and stay
chilled
• However freshness is extended, as bacterial spoilage only occur after rigor
mortis has passed
• Flesh that goes thru rigor mortis (stiff to relaxed muscle) has better
texture and flavor
• Water-holding capacity of proteins is increased
– Makes flesh juicier
• Freezing immediately after catch without chilling to allow rigor mortis
results in tough texture
• Cooking fish prior to rigor mortis also result in tough texture
9. Factor for on set of rigor
mortis
• Species:
– Some species take longer than others to go into rigor, due to differences in their chemical composition.
• E.g. Whiting go into rigor very quickly and may be completely stiff one hour after death, whereas redfish stored under the same
conditions may take as long as 22 hours to develop full rigor.
• Trawled codling, 18-22 inches long, gutted and stored in ice, usually take 2-8 hours to go into rigor.
• Condition:
– The poorer the physical condition of a fish, that is the less well nourished it is before capture, the shorter will be the
time it takes to go into rigor; this is because there is very little reserve of energy in the muscle to keep it pliable. Fish
that are spent after spawning are an example.
• Degree of exhaustion:
– In the same way, fish that have struggled in the net for a long time before they are hauled aboard and gutted will have
much less reserve of energy than those that entered the net just before hauling, and thus will go into rigor more
quickly.
• Size:
– Small fish usually go into rigor faster than large fish of the same species.
• Handling:
– Manipulation of pre-rigor fish does not appear to affect the time of onset of rigor, but manipulation, or flexing, of the fish
while in rigor can shorten the time they remain stiff.
• Temperature:
– The most important factor governing the time a fish takes to go into, and pass through, rigor because the temperature
at which the fish is kept can be controlled.
• The warmer the fish, the sooner it will go into rigor and pass through rigor.
– E.g gutted cod kept at 32-35°F may take about 60 hours to pass through rigor, whereas the same fish kept at 87°F may take less than 2
hours.
– Tempt difference between water and storage of fish
• > difference – the shorter time of death to rigor mortis
10. Onset and duration of rigor mortis in various fish species
Species Condition
Temperature
°C
Time from death to onset of rigor
(hours)
Time from death to end of rigor
(hours)
Cod (Gadus morhua) Stressed 0 2-8 20-65
Stressed 10-12 1 20-30
Stressed 30 0.5 1-2
Unstressed 0 14-15 72-96
Grouper (Epinephelus malabaricus) Unstressed 2 2 18
Blue Tilapia (Areochromis aureus) Stressed 0 1
Unstressed 0 6
Tilapia (Tilapia mossanibica) small 60g Unstressed 0-2 2-9 26.5
Grenadier (Macrourus whitson) Stressed 0 <1 35-55
Anchovy (Engraulis anchoita) Stressed 0 20-30 18
Plaice (Pleuronectes platessa) Stressed 0 7-11 54-55
Coalfish (Pollachius virens) Stressed 0 18 110
Redfish (Sebastes spp.) Stressed 0 22 120
Japanese flounder (Paralichthys
olivaceus)
0 3 >72
5 12 >72
10 6 72
15 6 48
20 6 24
Carp (Cyprinus carpio) 0 8
10 60
20 16
Stressed 0 1
Unstressed 0 6
11. 11
General influence of rigor on fish is that it makes
the fish stiffen
Rigor mortis does not affect whole fish that is
iced on board and during transportation to the
factory.
This is because rigor mortis has passed during
holding in ice on board and transportation to the
factory.
Factors affecting Rigor Mortis
Method used for stunning and killing
Temperature
Influences of rigor mortis on
fish
12. 12
The shape of the fillets becomes
distorted and the surface of the flesh
takes on a corrupted appearance
a-The fillet is cut off before rigor
mortis => the length is
reduced 24 %
b-The fillet is cut off after rigor mortis
=> the length is
reduced a little bit
b
a
Shrinkage of the fillets
13. • Rigor mortis
– is a process not completely understood but thought to
be related to the activation of one or more of the
naturally-occurring muscle enzymes
• digesting away certain components of the rigor mortis complex
• The softening of the muscle during resolution of
rigor mortis (and eventually spoilage processes) is
coincidental with the autolytic changes
– rigor mortis occur simultaneously with autolysis
– Autolysis is "self-digestion"
• contributes to varying degrees to the overall quality loss in
addition to microbially-mediated processes.
14. AUTOLYSIS
• Major processes
occur during
autolysis include
– degradation of ATP-
related compounds in
a more-or-less
predictable manner.
15. • Glycogen or fat is
oxidized or "burned"
by the tissue
enzymes in a series
of reactions to
produce CO2, H2O
and adenosine
triphosphate (ATP).
• Under anaerobic
conditions, ATP may
be synthesized by
two other important
pathways from
creatine phosphate
or from arginine
phosphate.
•
•Normal pathway for the production of muscle energy
(ATP) in most living teleost fish (bony finfish).
16. • Respiration takes place in two stages:
– an anaerobic and an aerobic stage.
• latter depends on the continued presence of
oxygen (O2) which is only available from the
circulatory system.
• Most crustaceans are capable of respiring
outside the aquatic environment by
absorption of atmospheric oxygen for
limited periods.
• Note
– Aerobic energy is restricted to vertebrate muscle
(teleost fish) while the anaerobic is characteristic
of some invertebrates such as the cephalopods
(squid and octopus).
17. • ATP production ceases when the
creatine or arginine phosphates are
depleted.
• Octopine is the end-product from the
anaerobic metabolism of
cephalopods and is not acidic (unlike
lactate),
– thus any changes in post mortem pH not
related to the lactic acid prodn from
glycogen.
• Note
– For most teleost fish, glycolysis is the
only possible pathway for the production
of energy once the heart stops beating.
18. • ATP is produced in glycolysis,
– but only 2 moles for each mole of
glucose oxidized as compared to 36
moles ATP produced for each mole of
glucose if the glycolytic end products
are oxidized aerobically in the
mitochondrion in the living animal.
• After death, the anaerobic muscle
cannot maintain its normal level of
ATP, and when the intracellular level
declines from 7-10 µmoles/g to 1.0
µmoles/g tissue,
– the muscle enters rigor mortis.
19. • Rigor mortis sets in when the muscle ATP
level drops to 1.0 µmoles/g.
• ATP is not only a source of high energy
which is required for muscle contraction in
the living animal
– but also acts as a muscle plasticizer.
• Muscle contraction is controlled by
– calcium and an enzyme; ATP-ase (found in
muscle cells).
20. • When intracellular Ca+2 levels are 1 µM,
Ca+2 - activated ATP-ase reduces the
amount of free muscle ATP which results
in the interaction between the major
contractile proteins, actin and myosin
– (autolysis)
– results in the shortening of the muscle,
making it stiff and inextensible.
• A fish in rigor mortis cannot normally be
filleted or processed due to stiffness of
carcass
– Thus cannot be manipulated and is often
contorted, making machine-handling
impossible.
21. • Degradation of ATP to form
– adenosine diphosphate (ADP),
– adenosine monophosphate (AMP),
– inosine monophosphate (IMP),
– inosine (Ino)-
• is a nucleoside when hypoxanthine
is attached to a ribose ring (also
known as a ribofuranose) via a β-
N9-glycosidic
– and hypoxanthine (Hx).
• The degradation of ATP
catabolites proceeds in the
same manner with most fish
– but the speed of each individual
reaction (from one catabolite to
another) greatly varies from one
species to another
• and often progresses coincidentally
with the perceived level of spoilage.
22. •Saito et al. (1959) were the first to observe
rigor mortis process and to develop a formula
for fish freshness based on these autolytic
changes:
•where [ATP], [ADP], [AMP], [IMP], [Ino] and [Hx] represent the relative
concentrations of these compounds in fish muscle measured at various
times during chilled storage.
23. • The K or "freshness" index gives a relative
freshness rating based primarily on the
autolytic changes which take place during
post mortem storage of the muscle.
– Thus, the higher the K value, the lower the
freshness level.
24. • Note : It is now widely accepted
– IMP (inosine monophosphate)
• is responsible for the desirable fresh fish flavour
which is only present in top quality seafood.
– Hx (hypoxanthine)
• is considered to have a direct effect on the
perceived bitter off-flavour of spoiled fish
• ATP loss is associated with rigor mortis.
25. • Figures indicate the
catabolic pathway for the
degradation of ATP
through to inosine which
is entirely due to autolytic
enzymes.
• Enzymes include:
– ATP-ase;
– myokinase;
– AMP deaminase;
– IMP phosphohydrolase;
– nucleoside
phosphorylase;
– inosine nucleosidase;
– xanthine oxidase.
26. Hx determination in certain species
• Hx
determination
would likely
not be useful
for such
species as
swordfish and
redfish.
27. POST-RIGOR MORTIS:
• Spoilage of fish and seafood is caused by microbial,
oxidative and enzymic spoilage which elicits deleterious
changes in odour, flavour and texture.
• Type and rate of spoilage is multifactorial. Some influences
on early spoilage are:
– A pH greater than 7 is optimum for bacterial and enzymic activity,
for example, fish which have struggled for prolonged periods during
netting operations will have a low glycogen content.
– Evisceration, e.g. small pelagics are not gutted, hence proteolytic
enzymes along with microorganisms rapidly invade the fish’s
tissues.
– Oily fish are susceptible to oxidative rancidity.
– Cold water species of fish, which have a microflora of psychrophilic
bacteria, are predisposed to earlier spoilage under chill conditions.
28. Summary of Autolytic Changes in
Chilled Fish
Enzyme(s)
Substrate Changes Encountered Prevention/Inhibition
glycolytic enzymes glycogen
production of lactic acid, pH of tissue
drops, loss of water-holding capacity in
muscle
high temperature rigor may result in gaping
fish should be allowed to pass through rigor at
temperatures as close to 0°C as practically possible
pre-rigor stress must be avoided
autolytic enzymes, involved
in nucleotide breakdown
ATP
ADP
AMP
IMP
loss of fresh fish flavour, gradual
production of bitternes with Hx (later
stages)
same as above
rough handling or crushing accelerates breakdown
cathepsins
proteins,
peptides
softening of tissue making processing
difficult or impossible
rough handling during storage and discharge
chymotrypsin, trypsin,
carboxy-peptidases
proteins,
peptides
autolysis of visceral cavity in pelagics
(belly- bursting)
problem increased with freezing/thawing or long- term
chill storage
calpain
myofibrillar
proteins
softening, molt-induced softening in
crustaceans
removal of calcium thus preventing activation?
collagenases
connective
tissue
gaping" of fillets
softening
connective tissue degradation related to time and
temperature of chilled storage
TMAO demethylase TMAO
formaldehyde-induced toughening of frozen
gadoid fish
store fish at temperature <= -30°C
physical abuse and freezing/thawing accelerate
formaldehyde-induced toughening
29. CONCLUSION
• Issue
– Post – harvest physical handling (is important)
• accelerates the autolytic changes in chilled fish.
– Systems for conveying fish and for discharge from the
vessels must be designed
• so as to avoid physical damage to the delicate tissues.
– Iced fish should never be stored in boxes deeper than 30
cm and that fish boxes are not permitted to "nest" one on
top of the other
• in order to minimize autolysis and extend rigor
