Fish processing technology

1. Fish silage
 The product of the process of preserving and storing wet biological
material in a silo (a pit or airtight container) is called silage.
 Silage production is considered one of the best ways of preserving
agro and animal wastes.
 The word “silo” has traditionally been used in conjunction with
green forage, preserved either by added acid or by the anaerobic
production of lactic acid bacteria.
 The term “fish silage” has been adopted for analogous products of
whole fish or parts of fish.
 Fish silage can be defined as a liquid product made from whole fish
or parts of fish to which no other material has been added other
than an acid and in which liquefaction of the fish mass is brought
about by enzymes already present in the fish.

2.  Fish silage is produced by adding inorganic or organic acids to the
comminuted fish or by the acids produced by the anaerobic
microorganisms introduced into the system.
 Fish silage produced under ideal conditions can be kept for more
than 1 year under tropical conditions.
 The first commercial production of fish silage was started in Denmark
in 1948.
Types of Fish Silage:
 The principle of silage production is the preservation of materials from
microbial degradation by addition of chemicals or other agents of
chemical or microbial origin or microorganisms. Silages are classified
according to the nature of the agents employed for the production.

3. Acid Silage:
 Among mineral acids, sulphuric acid (50-75%) or a mixture of sulphuric
and hydrochloric acids is used to produce silage. The produce retains
the fresh acidic smell even after months of storage at tropical
temperatures. This silage has to be neutralised prior to feeding animals.
 The most commonly used organic acids are propionic, acetic and formic
acids. A 3% by weight of formic acid (85-90%) is added to the well-
ground fish mince. Silage preserved with formic acid has a shelf-life up
to 1 year at ambient temperatures of the tropical countries.
 As organic acids are more expensive than mineral acids, use of a
mixture of inorganic and organic acids for silage production is also
recommended.
 Cheap mineral acids like sulphuric acid or hydrochloric acid are used
to lower the pH and organic acids like propionic or formic are added to
it for antimicrobial activity.

4. Biological Silage:
 Biological silage production employs the principle of fermentation by
microorganisms that produce lactic acid.
 Streptococcus faecalis, Lactobacillus plantarum, L. brevis, L. cerevicae,
and L. mesenteroids are some of the bacteria commonly used in silage
production.
 Among them, L. plantarum is the best suited for fish silage production.
It can tolerate a very low pH and high salt content up to 8%.
 They ferment the sugars present in the medium to organic acid,
predominantly lactic acid, thus lowering the pH.
 At low pH, growth of putrefying organisms is inhibited by competitive
inhibition and also by the action of certain antibiotics produced in the
system.

5.  As fish contain only a small quantity of starch or carbohydrate, they are
added in silage production to facilitate microbial fermentation.
 Different workers have used cereal meal, molasses, topioca meal, ragi
and other materials, in silage.
 The process of conversion of carbohydrate to lactic acid by
fermentation is anaerobic.
 It can be divided into three stages. The first stage is the hydrolysis of
starch to maltose by amylase.
 In the second stage, maltose is converted to glucose by maltase. Both
these stages require starch hydrolyzing enzymes and hence require
addition of substances like molasses or fermented cereals like ragi to
facilitate microbial fermentation.
 The last stage is conversion of glucose to lactic acid by the bacteria.

6.  Small amounts of acetic acid, propionic acid and ethyl alcohol are also
produced in this process.
 Lactic acid bacteria destroy spoilage organisms by competitive
inhibition and also by producing antibiotics and other antimicrobial
chemicals.
 The lactic acid-producing bacteria are divided into two groups,
homofermentative and heterofermentative. The homofermentative
bacteria convert every glucose molecule into two molecules of lactic
acid, whereas heterofermentative ones produce lactic acid, acetic acid,
ethyl alcohol, mannitol, dextrans and carbon dioxide.
 The primary catabolism of hexoses by lactic acid bacteriaI in both types
of fermentation is reported to be different. Hence, in silage production,
it is desirable to use a starter culture, containing predominantly
homofermentative lactobacillus. Usually, fish contains only a very small
quantity of lactic acid-producing bacteria. This is one reason why a
starter culture containing lactobacillus is invariably used in microbial
fermentation of fish.

7. Raw Materials:
 Fish of all species and fish wastes can be converted into silage.
 Silage is produced by of using by-catches from shrimp trawlers, fresh
fish and fish offal.
 Fatty fish may pose technological problems as the silage produced is
susceptible to rapid lipid oxidation.
 When silage is produced from fatty fish, often the formation of an
emulsion phase is seen. This emulsion occludes sizeable quantifies of
proteins that are thus lost as they are not easily recoverable even on
centrifugation. Fatty silage gives a fishy taint to milk of cattle, eggs of
poultry, and meat of pigs and chicken.
 Fatty fishes are not generally preferred in silage production.

8.  When fatty fishes are used, lipids are seen floating on the top of the
silage, rendering the silage anaerobic.
 In the industrial production of silage, de-oiling equipments are used
to remove excess fat.
 Lipid oxidation results in the formation of several volatile carbonyl
compounds. Some of them interact with proteins and make the silage
nutritionally poor.
 A lean fish with lipid content less than 2% (w/w) is a desired raw
material for silage production.
 Silage prepared from spoiled fish when fed to chicks caused mortality
and poor growth.
 But silage produced from a 1:1 mix of spoiled fish and good fish gave
good growth and no mortality.

9. Production of Acid Silage:
 The cost is the decisive factor in selecting an acid for silage
production.
 All equipments, tanks and vessels must be acid resistant.
 For small-scale production, not many equipments are needed.
 The pulped material can be mixed manually with an adequate quantity
of acid and stored in a container.
 For large-scale production, the following equipments are needed:
1) Meat mincer for mincing the whole fish into suitable sizes.
2) Mixer or homogeniser for homogenising the fish mince and acid
together.
3) A mechanical stirrer for agitating the slurry in the tank.

10. 4) Mechanical pump for measuring and adding acid to the slurry.
5) Tanks for silage production and subsequent storage: Silage can be
stored in drums made of steel or polyethylene.
6) De-oiling equipment: After the liquefaction of the silage, the fish oil
floats on the top. The excess oil must be removed, since high oil
content in silage is likely to create nutritional problems
 The whole fish is comminuted in a mechanical mincer.
 Add the required quantity of acid or acid mixture (normally 2—3%
w/w) and mix the slurry well.
 After this process the whole material becomes a good paste.
 Store the paste in a drum or large tank for 15—20 days with daily
stirring.
 It has a shelf-life of over 1 year under normal conditions of storage.
 The whole process can be automated.

11. Commonly used acid or acid mixtures are:
1) Formic acid (90%) 3% (w/w) of fish mince
2) 2) Mixture of 2.5% (w/w) of sulphuric, formic and propionic acids
(1:1:0.5),
3) A mixture of 3% (w/w) of 90% formic acid and 95% propionic acid, (1:1
w/w).
4) Three per cent w/w of 90% formic acid, 95% propionic acid and
concentrated sulphuric acid (1:0.5:2 v/v).
5) 15% (v/w) of sulphuric acid (25 or 30% strength).
6) A mixture of formic acid (1%) and hydrochloric acid having pH 2 to 3
 Between formic acid and sulphuric acid formic acid is the choice,
because it gives a higher pH compared to one obtained with sulphuric
acid

12. For the successful production of add silage, the following precautions are
recommended
1) The material should be reduced in size, preferably to pieces no longer
than 3—4 mm in diameter.
2) Acid should be thoroughly dispersed throughout the minced fish to
avoid pockets of untreated material where bacterial spoilage can
continue.
3) Periodic agitation is necessary to bring about rapid liquefaction.
4) Temperatures of at least 20°C are desirable, since below this
temperature liquefaction is rather slow. The enzymes responsible for
liquefaction can be inactivated as temperature rises but samples
heated to 40°C have been found to liquefy rapidly.

13.  During autolysis, proteins are broken down to peptides and amino
acids.
 When the pH is around 3.0 both exo- and endopeptidases present in
the digestive tract of fish as well as in tissues are active, bringing
about effective autolysis needed for good silage.
 The preservative principle in silage is due to the reservoir of un-ionised
molecules of acids that can cross the bacterial cell membrane and
once inside dissociate in the cytoplasm, bringing down the pH and
thereby the death of the cell.
 For this reason, organic acids like formic and propionic acids, which
exist mostly in the un-ionised state even at fairly low concentrations,
are better than inorganic acids, which remain un-ionised at high
concentrations.
 While bacterial growth is readily inhibited by either kind of acid, fungal
growth is inhibited by only organic acids at the concentrations
encountered in fish silage.

14. Biological Silage:
 During the production of biological silage, the whole fish or fish waste is
comminuted to get a uniform mix.
 Add molasses (10% w/w) and 30% water (w/w) to this mix and stir well. The
resultant thick slurry is cooked for 20 minutes.
 Cooking kills all undesirable microorganisms present in the fish paste. Transfer
the slurry to large bitumen-coated vessels or cement tanks.
 Innoculate with a starter culture of the lactic acid bacteria (e.g., Lactobacill!us
plantarum) 18—20 hours old and stir well.
 Keep it for 15—20 days, stirring daily, after which the silage is ready. It has a
shelf-life of up to 1 year under normal conditions of storage.
 The precooked silage shows rapid fermentation and reaches a pH of 4.4 within
72 hours, while uncooked silage attains a pH of only 5.
 Also, uncooked silage shows more degradation of proteins by autolysis as
indicated by the alpha amino nitrogen values.

15.  Fish is a poor source of carbohydrates needed for lactic acid bacteria to
ferment and produce lactic acid in the silage.
 Usually, molasses or other carbohydrate materials are added for this purpose,
Fish: molasses ratio of 100:5 is usually followed.
 The use of molasses or tapioca is ideally suited to tropical countries as they
are abundantly available.
 A satisfactory pH around 4 can be obtained only when molasses concentration
is in the range of 10% and above.
 Lactic acid bacteria ferment sugars and produce lactic acid, which lowers the
pH as low as 4.5. At this low pH, growth of many putrefying organisms; moulds
and pathogens like Salmonella and Clostridium botulinum are prevented or
inhibited.
 If sufficient carbohydrate is not present in the medium, required levels of acid
will not be produced. This is the reason why silage with 5% added molasses
got putrefied within 7 days. Nutritionally, biological silages are superior to acid
silages.

16. Biochemical Changes Associated with Silage Production and Storage
Autolysis:
 In a dead fish, autolysis is a natural process aiding putrefaction.
Enzymes present in the digestive tract as well as in tissues are
responsible for autolysis.
 After post-mortem changes, the muscle pH falls to as low as 4 because
of the anaerobic breakdown of muscle glycogen producing lactic acid.
 At this pH, most proteases are active and cause the breakdown of
muscle proteins.
 In fact, proteases are mainly responsible for autolysis.
 Apart from these changes, lipid hydrolysis is also taking place
concomitantly by action of lipases releasing free fatty acids.

17.  Proteolysis is a complex phenomenon mediated by several proteases
present in the fish.
 Each protease has a different pH optimum and temperature optimum.
Most digestive proteases present in the fish have optimum activity at 45—
50°C.
 But autolysis takes place at lower temperatures. The critical temperature
for the commencement of autolysis is around 20°C for cold-water fish and
around 30°C for tropical fish.
 Enzymatic breakdown of proteins make the product bitter because of the
production of bitter peptides.
 It was also reported that liberated fatty acids from lipid hydrolysis cause
solubilisation of proteins.
 Consequently, fatty fish can give fewer yields of liquefied proteins.
 The yield of solubilised proteins may vary depending on the raw material,
the flesh giving the lowest and viscera the highest.
 Proteølysis gives a large residue containing unhydrolysed proteins.

18.  Although the fish viscera contains a number of proteinases, it is
usually the acid proteinases like pepsin and cathepsins B and D,
that actively contribute to the autolytic process because of the acid
pH of the silage.
 The pH of the silage also determines the extent of the breakdown
and nature of the proteolytic products formed.
 A low pH (3.0 or lower) limits the hydrolysis to mostly endo-
proteinases resulting in a lesser degree of hydrolysis (65—70%), and
a greater amount of longer peptide fragments.
 A higher pH (3—4), extends the proteolytic range to exopeptidases
as well, increasing the degree of hydrolysis (up to 80%) and
producing mostly amino acids and small peptide fragments.
 Upon liquefaction of the silage, which can take from 3 to 7 days,
depending on the temperature and nature of raw material, the silage
separates into 3 or 4 layers. An oily layer floats at the top,
sometimes with an underlying emulsified layer. A middle aqueous
layer forms most of the silage and sediment or sludge containing
undigested protein, scales and bones is found at the bottom.

19. Nature of Acids:
 It is seen that different acids, organic and mineral, have different
rates of liquefaction giving varying amounts of silage.
 In pH 2 (in ensilation with sulphuric acid) only acid endo-peptidases
and a weak exo-peptidases activity are present, slowing the
formation of amino nitrogen during antolysis. Correspondingly, the
amount of short peptides and amino acids are higher in silage with
formic acid than in the one with sulphuric acid
 At pH 3.0 the rate of autolysis and yield of silage were markedly low.
 Herring silage was better autolysed when formic acid was used
alone rather than either sulphuric acid or phosphoric acid, because
the former gave pH 4.5 and the latter 3.1.
Lipid Oxidation:
 Unsaturated long chain fatty acids released from fish by lipid
hydrolysis by lipases absorb oxygen and undergo rapid auto-
oxidation, releasing a large number of volatile carbonyls and
making the silage rancid.
 The rate of lipid oxidation is directly related to exposure to sunlight,
presence of pro-oxidants and concentration of heavy metals,
temperature, and other factors.

20.  Oxidized lipids are responsible for the poor nutritional quality of the
silage.
 Hence, silage produced from fatty fish has shorter shelf-life than one
produced from lean fish.
 Addition of antioxidants like BHA, BHT and ethoxyquin can
substantially retard the development of rancidity but are seldom added
in silage.
 Oxidized lipids also interact with proteins rendering them
unacceptable to proteases.
 Presence of Microorganisms:
 Preprocess handling of fish plays in important role in the microbial
quality of silage produced.
 The raw material may harbour pathogens. However, cooking or
pasteurisation can destroy most bacteria.
 Many organisms usually seen in fish are sensitive to acidity and
pathogens like Vibrio cholerae and Salmonella are totally destroyed at
pH 4—5.
 But spores of many organisms are likely to occur. Hence, it is
recommended that silage should be properly cooked prior to feeding
animals.

21. Degree of Autolysis in Silage:
 During ensilation it is the proteolytic enzymes of fish or its offal which
carry out extensive hydrolysis of fish proteins. As a result amount of
protein fraction gradually decreases while that of non-protein fraction
increases.
 The degree of autolysis and protein solubilisation in silage varied with
the nature of raw materials, ranging from 80% in temperate fishes to
40—45% in tropical fishes like silver bellies.
 The undigested proteins appear to be peptide aggregates held together
by non-covalent forces.
 The undigested protein in the sludge at the bottom of the silage is seen
to be as high as 50% of the total protein in case of tropical fish.
 The exact reason for this incomplete proteolysis is not fully understood
so far; but pH, temperature, duration of ensilation and nature of raw
materials appear to play an important role.
 A high degree of hydrolysis, results in a greater liquefaction and
digestion of fish and consequently higher yields of the aqueous phase
of silage.
 But this is not desirable due to several reasons.

22.  The less soluble amino acids can separate from the fish silage on
standing, leaching losses are greater upon incorporation of such
silages into feeds and their assimilation is poorer.
 Due to these reasons, several attempts have been made to limit the
hydrolysis in fish silages.
 Heating to inactivate enzymes is an often used approach, while
addition of formalin is another.
 However, use of formalin is not to be recommended as it impairs the
performance of animals fed such silages.
Composition of fish silage:
The range of composition is: moisture – 70 to 81%. crude protein – 15 to
17%, ash – 2 to 4% and oil – 0.5 to 13%.

23. Nutritional value of silage:
 The silage concentrate is a highly digested protein hydrolysate which is
convenient as a protein supply for weaning calves and pigs as well ass
poultry.
 When high amounts of fish silage protein are fed to mature ruminants or
fish, the animal production and growth are reduced.
 This is probably due to adverse effects of highly hydrolysed protein in
the metabolism of these animals. However, 5 – 10 % of the feed protein
may be substituted by silage protein without negative effects.
 Actually, there are indications that health, fertility and general
appearance are improved when some fish silage protein is included.
 At low levels of inclusion in diet, silage does not produce any ill effects
on growth of chicken and pigs and serves as ideal substitute for fish

24. Fish meal versus fish silage
Fish meal
Fish silage
Capital cost High Low
Manpower
requirement
Require engg. and tech.
staff
Require unskilled
workers
Storage Require more space than
silage
Require more space
than meal
Smell at production
centre
More Less
Transport Cheap Costly
Marketing Established, well known Not well known

25.  Fish silage has an inherent defect, its liquid consistency, which makes
it difficult to transport to distant places and to store.
 Feeding experiments in India showed that it was extremely difficult to
convince the farmers who rear poultry, pigs and cows about the
efficiency of fish silage as a protein supplement because of this
disadvantage.
 To overcome this problem, a solid feed mix was compounded out of
boiled fish silage and rice bran powder in the ratio 1:3 and sun-dried.
 The resultant dry powder has about 9% moisture and 21% protein.
 It is easily transported and has extended shell-life at ambient
temperatures in the tropics.
 The rice bran contains all vitamins, particularly the B group, and many
other micronutrients required for animals. This is an added advantage

26.  Feeding trials conducted at the Livestock Research Station at
Kattupakam (under the Government of Tamil Nadu, India) on pigs and
poultry for a period up to 6 months showed that the feed mix gave
excellent weight gain and feed conversion.
 Another problems encountered in the use of silage is its low solids (dry
matter) content, which can increase the cost of transportation.
 Two approaches have been tried to overcome this problem;
Concentrating the separated aqueous phase of silage to a higher solids
content is practiced in Norway and Denmark, while the silage is mixed
with other feed ingredients and dried to yield a stable product in many
southeast Asian countries including India.
 A notable advantage of acid preserved silages is the absence of fly
infestation while drying, which can be a formidable problem in when
drying fish in the open for fish meal production.