V-Animal products-D-Fish Processing-1
FISH PROCESSING
Since 1978, New Zealand has had exclusive fishing rights to all its surrounding waters
up to 320 km away from the land. This has resulted in fish becoming an important
resource, and much research has been done to understand the processes involved in fish
ageing, and ways to prevent these processes.
After a fish dies, the flesh quickly becomes rigid, in a process known as rigor mortis.
This rigidity then dissolves, and the fish flesh decomposes.
Rigor mortis
When muscles contract, they absorb calcium ions. After a fish has died, calcium ions
leak into the muscles, causing them to contract. However, ATP, the biochemical
energy carrier that provides energy to relax the muscle again, is no longer present, so
the muscles remain rigid. This locking of the muscles, called rigor mortis, can be
slowed down by chilling the fish immediately after death.
Dissolution of rigor
Over time this rigidity disappears, but by then the fish has significantly deteriorated.
Autolysis
The decomposition of the fish occurs as its constituent compounds break down (called
autolysis). The proteins, nucleotides and sugars break down, bases are released, the pH
falls and the fats are oxidised. These make the fish smelly, rancid and tough.
Tests for freshness
By measuring the pH of the fish and the levels of various compounds present the
amount of autolysis that has occured, and hence the freshness of the fish, can be
measured.
INTRODUCTION
Since the 320 km Exclusive Economic Zone was established in 1978, giving New Zealand
one of the largest fishing zones in the world, the significance of fish as a resource has risen
greatly. Many areas of research, such as the best processing methods, the quality and
composition of the fish landed, and the effects of handling, storage and packaging on fish
quality, have become of great importance.
The Seafood Research Unit, Crop & Food Research was established in 1979 in response to
the industry's need for research. Laboratories are located in Auckland and Nelson. Major
areas of interest include:
Catching and handling. The understanding and control of the physiological changes
occuring during the handling of fish and their effect on quality.
Chemistry and biochemistry. Detection of spoilage indicators and toxic components and
quality evaluation of seafood to establish management guidelines for quality systems.
V-Animal products-D-Fish Processing-2
Microbiology. Improvement of product safety and quality, and assistance with hygiene and
sanitation.
Seafood toxins. Depuration of algae from shellfish bioassays.
Fish proteins. Changes in the structure and biochemistry of proteins and hence the texture of
fish muscle during frozen storage.
Sensory evaluation. Determination of shelf-life changes and consumer evaluation of seafood.
In this article we shall discuss the major biochemical and microbiological changes occurring
post mortem, their effects on the quality of the fish, plus the handling, storage and processing
procedures by which these can be reduced and the quality of the fish maintained.
CHANGES IN FISH FLESH BIOCHEMISTRY POST MORTEM
The demise of a fish begins a series of irreversible changes which lead to spoilage and loss of
quality.
The natural process:
Rigor mortis → Desolution of rigor → Autolysis
can be slowed down if correct handling and storage procedures are followed.
Step 1 - Rigor mortis
Muscle consists of several proteins actively involved in contraction (Figure 1). 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.
Relaxed muscle
Ca2+
ATP Actomyosin
complex
Contraction
ATP
-ATP
Rigor
Figure 1 - Muscle contraction reactions
On death, the circulatory system stops and the ATP levels drop. Calcium ions leak, forming
actomyosin. However, there is insufficient ATP for the calcium pump to operate, and so the
actomyosin complex remains unbroken. The muscle is now in a continual state of rigidness,
known as rigor mortis.
V-Animal products-D-Fish Processing-3
Step 2 - Autolysis
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.
Bacterial attack
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.
Anaerobic bacteria (those which operate in the absence of oxygen) produce a particularly
foul type of spoilage which results in an inedible fish.
A number of chemical changes take place during autolysis, and these are outlined below.
Protein Denaturation
Denaturation of protein involves the destruction of its secondary, tertiary and quaternary
structure, reducing the protein to a simple polypeptide chain. A number of factors, including
slow freezing and variability of storage conditions, cause this denaturation. A denatured
protein has not only lost its ability to function as an enzyme, but also its "water-holding"
ability. This results in denatured fish flesh dripping excessively when thawed (a situation
known as "drip-thaw"), and appearing white, dull and spongy, and upon chewing becoming
fibrous and tasteless.
Decreasing flesh pH
A living fish has a flesh pH of 7.0. However, after death residual glycogen is broken down
via glycolosis to pyruvic acid and then lactic acid. As this happens, the flesh becomes more
acidic. If the pH remains above 6.6, the texture is reasonably soft, but below this level the
flesh becomes firm and eventually unacceptably tough.
TVB-Total Volatile Base
TVB is a measure of the total amount of a variety of nitrogen-containing substances which
are produced during storage. An example of a volatile base present in the flesh is a
trimethylamine (TMA), which is formed from the reduction of trimethylamine oxide. Marine
fish contain a small amount of trimethylamine oxide, the function of which is unknown. This
odourless and tasteless compound is reduced by invading bacteria to TMA, which is
characterised by its "fishy" smell. TMA, though, only becomes useful as a quality index
during the middle and late stages of spoilage after the bacteria have invaded the fish.
Trimethylamine oxide is converted in the muscle tissue into dimethylamine (DMA) and
formaldehyde by enzyme action during frozen storage. This formaldehyde is able to crosslink
with protein, denaturing the muscle structure. This fish loses water when it is thawed,
and when cooked has a tough and fibrous texture.
Nucleotide Breakdown
This involves the enzymatic breakdown of the energy carrier ATP, as outlined below:
V-Animal products-D-Fish Processing-4
O
N
N
N
N
OH OH
H H
H H
OCH2
NH2
P
O
OH
P O
O
OH
P O
O
OH
HO
+
O
N
N
N
N
OH OH
H H
H H
OCH2
NH2
P
O
OH
O
H
H O
N
N
N
N
OH OH
H H
H H
P OCH2
O
OH
O
O
HN
N
N
O HN
+
OH
O
OH OH
H H
H H
OCH2
HN +
N
N
O HN
H
+
ATP-ase
myokinase
AMP-deaminase
5' nucleotidase
nucleoside hydrolase
nucleoside phophorylase
ATP
ADP
AMP
Inosine IMP
Hypoxanthine Ribose
Hypoxanthine Ribose-1-phosphate
O
N
N
N
N
OH OH
H H
H H
OCH2
NH2
P
O
OH
P O
O
OH
H O
P
O
OH
HO OP
O
OH
HO OP
O
OH
HO OO
N
N
N
N
OH OH
H H
H H
OCH2
O
H
OH
O
OH OH
H H
H H
P OCH2
O
OH
HO
V-Animal products-D-Fish Processing-5
Thus as spoilage proceeds the amount of ATP present decreases, causing rigor mortis.
Liquid 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:
CH2 OH
CH OH
CH2 OH
CH2 O
CH O
CH2 O
O
C R
O
C R
O
C R R C
O
OH
+ 3
Fat Glycerol Fatty acid
(ii) the oxidation of fish oils yielding the rancid odours and tastes which are the major
problem encountered in fish storage.
TESTS FOR FRESHNESS
Several of the changes described above can be chemically monitored to determine the
freshness of the fish, and these are used in conjunction with taste testing.
Protein denaturation
During cold storage the proteins undergo molecular cross-linking with formaldehyde (see
above), causing the muscle protein to become less soluble. Deterioration can be assessed by
adding 5% NaCl (common salt) and then separating and measuring the soluble and insoluble
proteins present. Increasing insoluble protein indicates longer storage and greater
deterioration, mainly of texture.
Nucleotide breakdown
The concentrations of ATP, ADP, AMP, IMP, inosine and hypoxanthine (i.e. the different
compounds formed over time as ATP breaks down - see above) can be measured by high
performance liquid chromatography. The ratio of inosine and hypoxanthine to the total
amount of the above substances is represented as a percentage value, K. A K value of less
than 20% represents fresh fish, whilst anything greater is indicative of spoilage.
Decreasing pH
pH is a possible test of textural strength, with anything below 6.6 resulting in noticeably
firmer flesh than that of fresh fish.
TVB - Total Volatile Base
The amount of TVB is measured by distilling a fish extract, and determining the base
concentration by titration against acid. A fresh sample of Jack Mackerel would have a value
of 19-21mg TVB N/100g, whilst an ageing sample would be nearer 30 mg TVB N/100g.
V-Animal products-D-Fish Processing-6
The amount of DMA can be measured by spectrophotometry, giving an indication of the
storage time.
Lipid oxidation and hydrolysis
Oxidation leads to rancidity, the degree of which is commonly evaluated by measuring the
free fatty acid and peroxide concentrations. Hydroperoxides can be measured by mixing the
fish oil with potassium iodide, and measuring the amount of iodine liberated by titration
against thiosulphate. The hydroperoxides oxidise the iodide to iodine, which is liberated
according to the following equation:
2I- → I2 + 2e-
A further measure of oxidation is the TBA test. This involves extracting some fish muscle
into trichloroacetic acid and treating it with with thiobarbituric acid (TBA). The TBA reacts
with malonaldehyde, a substance formed during oxidation, to form a red compound. The
intensity of the red colour, which is proportional to the concentration of the malonaldehyde,
can be measured using a spectrophotometer.
C CH2
C
H
O
H
O
malonaldehyde
Written for volume two by Linda Boyd and John Ryder; revised by Ron Wong (Crop and
Food Research) and edited by Heather Wansbrough.
FISH PROCESSING
Since 1978, New Zealand has had exclusive fishing rights to all its surrounding waters
up to 320 km away from the land. This has resulted in fish becoming an important
resource, and much research has been done to understand the processes involved in fish
ageing, and ways to prevent these processes.
After a fish dies, the flesh quickly becomes rigid, in a process known as rigor mortis.
This rigidity then dissolves, and the fish flesh decomposes.
Rigor mortis
When muscles contract, they absorb calcium ions. After a fish has died, calcium ions
leak into the muscles, causing them to contract. However, ATP, the biochemical
energy carrier that provides energy to relax the muscle again, is no longer present, so
the muscles remain rigid. This locking of the muscles, called rigor mortis, can be
slowed down by chilling the fish immediately after death.
Dissolution of rigor
Over time this rigidity disappears, but by then the fish has significantly deteriorated.
Autolysis
The decomposition of the fish occurs as its constituent compounds break down (called
autolysis). The proteins, nucleotides and sugars break down, bases are released, the pH
falls and the fats are oxidised. These make the fish smelly, rancid and tough.
Tests for freshness
By measuring the pH of the fish and the levels of various compounds present the
amount of autolysis that has occured, and hence the freshness of the fish, can be
measured.
INTRODUCTION
Since the 320 km Exclusive Economic Zone was established in 1978, giving New Zealand
one of the largest fishing zones in the world, the significance of fish as a resource has risen
greatly. Many areas of research, such as the best processing methods, the quality and
composition of the fish landed, and the effects of handling, storage and packaging on fish
quality, have become of great importance.
The Seafood Research Unit, Crop & Food Research was established in 1979 in response to
the industry's need for research. Laboratories are located in Auckland and Nelson. Major
areas of interest include:
Catching and handling. The understanding and control of the physiological changes
occuring during the handling of fish and their effect on quality.
Chemistry and biochemistry. Detection of spoilage indicators and toxic components and
quality evaluation of seafood to establish management guidelines for quality systems.
V-Animal products-D-Fish Processing-2
Microbiology. Improvement of product safety and quality, and assistance with hygiene and
sanitation.
Seafood toxins. Depuration of algae from shellfish bioassays.
Fish proteins. Changes in the structure and biochemistry of proteins and hence the texture of
fish muscle during frozen storage.
Sensory evaluation. Determination of shelf-life changes and consumer evaluation of seafood.
In this article we shall discuss the major biochemical and microbiological changes occurring
post mortem, their effects on the quality of the fish, plus the handling, storage and processing
procedures by which these can be reduced and the quality of the fish maintained.
CHANGES IN FISH FLESH BIOCHEMISTRY POST MORTEM
The demise of a fish begins a series of irreversible changes which lead to spoilage and loss of
quality.
The natural process:
Rigor mortis → Desolution of rigor → Autolysis
can be slowed down if correct handling and storage procedures are followed.
Step 1 - Rigor mortis
Muscle consists of several proteins actively involved in contraction (Figure 1). 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.
Relaxed muscle
Ca2+
ATP Actomyosin
complex
Contraction
ATP
-ATP
Rigor
Figure 1 - Muscle contraction reactions
On death, the circulatory system stops and the ATP levels drop. Calcium ions leak, forming
actomyosin. However, there is insufficient ATP for the calcium pump to operate, and so the
actomyosin complex remains unbroken. The muscle is now in a continual state of rigidness,
known as rigor mortis.
V-Animal products-D-Fish Processing-3
Step 2 - Autolysis
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.
Bacterial attack
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.
Anaerobic bacteria (those which operate in the absence of oxygen) produce a particularly
foul type of spoilage which results in an inedible fish.
A number of chemical changes take place during autolysis, and these are outlined below.
Protein Denaturation
Denaturation of protein involves the destruction of its secondary, tertiary and quaternary
structure, reducing the protein to a simple polypeptide chain. A number of factors, including
slow freezing and variability of storage conditions, cause this denaturation. A denatured
protein has not only lost its ability to function as an enzyme, but also its "water-holding"
ability. This results in denatured fish flesh dripping excessively when thawed (a situation
known as "drip-thaw"), and appearing white, dull and spongy, and upon chewing becoming
fibrous and tasteless.
Decreasing flesh pH
A living fish has a flesh pH of 7.0. However, after death residual glycogen is broken down
via glycolosis to pyruvic acid and then lactic acid. As this happens, the flesh becomes more
acidic. If the pH remains above 6.6, the texture is reasonably soft, but below this level the
flesh becomes firm and eventually unacceptably tough.
TVB-Total Volatile Base
TVB is a measure of the total amount of a variety of nitrogen-containing substances which
are produced during storage. An example of a volatile base present in the flesh is a
trimethylamine (TMA), which is formed from the reduction of trimethylamine oxide. Marine
fish contain a small amount of trimethylamine oxide, the function of which is unknown. This
odourless and tasteless compound is reduced by invading bacteria to TMA, which is
characterised by its "fishy" smell. TMA, though, only becomes useful as a quality index
during the middle and late stages of spoilage after the bacteria have invaded the fish.
Trimethylamine oxide is converted in the muscle tissue into dimethylamine (DMA) and
formaldehyde by enzyme action during frozen storage. This formaldehyde is able to crosslink
with protein, denaturing the muscle structure. This fish loses water when it is thawed,
and when cooked has a tough and fibrous texture.
Nucleotide Breakdown
This involves the enzymatic breakdown of the energy carrier ATP, as outlined below:
V-Animal products-D-Fish Processing-4
O
N
N
N
N
OH OH
H H
H H
OCH2
NH2
P
O
OH
P O
O
OH
P O
O
OH
HO
+
O
N
N
N
N
OH OH
H H
H H
OCH2
NH2
P
O
OH
O
H
H O
N
N
N
N
OH OH
H H
H H
P OCH2
O
OH
O
O
HN
N
N
O HN
+
OH
O
OH OH
H H
H H
OCH2
HN +
N
N
O HN
H
+
ATP-ase
myokinase
AMP-deaminase
5' nucleotidase
nucleoside hydrolase
nucleoside phophorylase
ATP
ADP
AMP
Inosine IMP
Hypoxanthine Ribose
Hypoxanthine Ribose-1-phosphate
O
N
N
N
N
OH OH
H H
H H
OCH2
NH2
P
O
OH
P O
O
OH
H O
P
O
OH
HO OP
O
OH
HO OP
O
OH
HO OO
N
N
N
N
OH OH
H H
H H
OCH2
O
H
OH
O
OH OH
H H
H H
P OCH2
O
OH
HO
V-Animal products-D-Fish Processing-5
Thus as spoilage proceeds the amount of ATP present decreases, causing rigor mortis.
Liquid 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:
CH2 OH
CH OH
CH2 OH
CH2 O
CH O
CH2 O
O
C R
O
C R
O
C R R C
O
OH
+ 3
Fat Glycerol Fatty acid
(ii) the oxidation of fish oils yielding the rancid odours and tastes which are the major
problem encountered in fish storage.
TESTS FOR FRESHNESS
Several of the changes described above can be chemically monitored to determine the
freshness of the fish, and these are used in conjunction with taste testing.
Protein denaturation
During cold storage the proteins undergo molecular cross-linking with formaldehyde (see
above), causing the muscle protein to become less soluble. Deterioration can be assessed by
adding 5% NaCl (common salt) and then separating and measuring the soluble and insoluble
proteins present. Increasing insoluble protein indicates longer storage and greater
deterioration, mainly of texture.
Nucleotide breakdown
The concentrations of ATP, ADP, AMP, IMP, inosine and hypoxanthine (i.e. the different
compounds formed over time as ATP breaks down - see above) can be measured by high
performance liquid chromatography. The ratio of inosine and hypoxanthine to the total
amount of the above substances is represented as a percentage value, K. A K value of less
than 20% represents fresh fish, whilst anything greater is indicative of spoilage.
Decreasing pH
pH is a possible test of textural strength, with anything below 6.6 resulting in noticeably
firmer flesh than that of fresh fish.
TVB - Total Volatile Base
The amount of TVB is measured by distilling a fish extract, and determining the base
concentration by titration against acid. A fresh sample of Jack Mackerel would have a value
of 19-21mg TVB N/100g, whilst an ageing sample would be nearer 30 mg TVB N/100g.
V-Animal products-D-Fish Processing-6
The amount of DMA can be measured by spectrophotometry, giving an indication of the
storage time.
Lipid oxidation and hydrolysis
Oxidation leads to rancidity, the degree of which is commonly evaluated by measuring the
free fatty acid and peroxide concentrations. Hydroperoxides can be measured by mixing the
fish oil with potassium iodide, and measuring the amount of iodine liberated by titration
against thiosulphate. The hydroperoxides oxidise the iodide to iodine, which is liberated
according to the following equation:
2I- → I2 + 2e-
A further measure of oxidation is the TBA test. This involves extracting some fish muscle
into trichloroacetic acid and treating it with with thiobarbituric acid (TBA). The TBA reacts
with malonaldehyde, a substance formed during oxidation, to form a red compound. The
intensity of the red colour, which is proportional to the concentration of the malonaldehyde,
can be measured using a spectrophotometer.
C CH2
C
H
O
H
O
malonaldehyde
Written for volume two by Linda Boyd and John Ryder; revised by Ron Wong (Crop and
Food Research) and edited by Heather Wansbrough.
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