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close this bookFish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)
View the document(introduction...)
View the documentSummaries
View the documentAcknowledgements
View the documentIntroduction
View the documentSalting of fish: salt
View the documentSalting of fish: methods
View the documentDrying of fish: basic principles
View the documentDrying of fish: methods
View the documentSmoking of fish
View the documentMarinades
View the documentFermented fish products: a review
View the documentBoiled fish products
View the documentFish canning: theory and practice
View the documentFreeze drying
View the documentIrradiation
View the documentMiscellaneous products: crustaceans
View the documentMiscellaneous aquatic products used as food
View the documentFood by-products
View the documentNon-food by-products
View the documentNew and delicatessen products
View the documentFish meal
View the documentFish silage
View the documentChemical and physical methods of quality assessment
View the documentOrganoleptic (sensory) measurement of spoilage
View the documentMicrobiology of spoilage
View the documentMicrobiology of fish spoilage
View the documentPublic health microbiology
View the documentInternational standards for fisheries products
View the documentLarge-scale fish landing facilities
View the documentSmall-scale landing facilities: design and operation
View the documentRetail sale facilities
View the documentFisheries extension services: their role in rural development
View the documentTraining in the field
View the documentAppendix

Fermented fish products: a review

The fermentation processes are those in which organic catalysts (enzymes or ferments) break down complex organic molecules to simpler ones. The enzymes responsible for digestion in the higher animals were once referred to as digestive ferments. Alcoholic fermentation is a process in which sugars are converted to alcohol with the evolution of carbon dioxide; yeast enzymes are commonly used and this process is important in the production of beers, wines and spirits.

If wet fish protein, that is fish flesh, is protected from contamination by microorganisms, and if the enzymes present in the flesh are rendered inactive, the flesh does not break down. It would, in fact, remain stable for a considerable period. Many of the processes used in fish preservation aim at keeping the fish flesh as near as possible to its original condition. With fermentation, however, we are considering methods by which the wet protein is broken down to simpler substances which are themselves stable at normal temperatures. In some of the processes we shall be considering, breakdown is only partial and is controlled by the addition of salt; thus the process is designed to produce a particular flavour as well as to preserve the product. Sometimes the breakdown is effected by enzymes present in the fish (autolysis); sometimes micro-organisms are involved. In many cases, the breakdown is hydrolysis or 'splitting-with-water'.

Three quite different types of product can be recognised:

1. Products in which the fish retain, substantially, their original form or in which large chunks of fish are preserved.

2. Products in which the original fish are reduced to the form of a paste.

3. The so-called fish sauces in which the flesh is reduced to a liquid.

Very few of these processes are employed outside Asia.

Fermented fish

Any fish which are subjected solely to a salting process are likely to be subject to a degree of fermentation. The degree of fermentation depends on factors such as whether the fish are completely or partially gutted; the proportion of salt used; the fat content of the fish; which additives, if any, are added during the salting process; and, finally, the temperature at which the salted fish are kept. The temperature is particularly important: using precisely the same process but holding the fish at a higher temperature than is normal may produce a quite different result.

Herring

Herring (Clupea harengus) were formerly used for a variety of different types of product. The commonest of these was pickled herring. The fish were typically dry salted in barrels, the proportion of salt varying from 15 to 36 per cent in different cures. A brine formed and covered the fish; in most cures, the barrels were topped up after a period with fish and brine from the same day's curing. There were special cures known as, e.g., Dutch, Scotch and Icelandic. Fish pickled in this way could be kept for more than one year at European ambient temperatures. The fish flesh was then only very moderately softened; the ripening process took several months. Such products typically contained 10-12 per cent salt; the only bacteria which would survive in these conditions would be salt-tolerant or halophilic. When fermentation was to be encouraged a proportion of sugar was added to the salt, and spices such as peppers, mace, coriander, hops, cinnamon, ginger, cardamom and even sandalwood were also added. Benzoic acid was sometimes used as a preservative; boric acid was also used until recently by some packers but is now generally prohibited.

Anchovies

Cured anchovies are a delicacy popular in the Mediterranean area. The only genuine anchovies are made from Engrautis encrasicolus by salting and fermentation. Although similar processes are sometimes applied to sardines and to sprats, the product is not of comparable quality. The best anchovies for curing weigh about 35 - 40 fish to the kg and they should have as high a fat content as possible. They are headed and gutted by pulling off the head without any particular care. The fish are salted down in layers, using from 5 to 6 kg of salt per 10 kg of anchovies. A final layer of salt is put on top and a weighted wooden disc is used to keep the fish well pressed. A brine forms, the fish sink down under the brine and additional fish and salt may be added a few days later. The fish are kept in the brine under pressure for a period of at least 6 - 7 months. During this time, water and fat are pressed out of the fish and form a layer of brine covered with fat. The liquid overflows and is collected and later used to spray the anchovies during the cure. The process can be carried out in sterile containers using sterile salt and it would thus appear that no micro-organisms are involved in the process. Traditionally, the cured anchovies are sold from the containers in which they have been manufactured; these vary from wooden barrels holding 50 to 200 kg to hot-dipped tin plate cans (plain on the inside and laquered on the outside) holding 20 kg; however, the fish are now sometimes filleted and packed in small retail cans.

Mackerel

In the tropics fermented fish are often made from the various mackerel species, especially Scomberomorus commerson. In India, a specialised cure known as the Colombo cure is used in South Kanara. Absolutely fresh fish are gutted, the gills are removed and the fish washed in seawater. They are then rubbed with salt (ratio 1:3) and put in cement tanks. About 8 kg of the fruit of Garcinia camboges, which is similar to tamarind, is added per tonne of fish. The fruit is extremely acid. Fish remain in the brine which forms for 2 - 4 months and are then exported packed in wooden barrels. The fish are reported to keep well for a year or more. The same fish used to be salted in Aden in cement tanks using about 1 part of salt to 3 parts of fish and the brine was allowed to flow away. No acid fruit pulp was used. These fish were exported in a very soft condition, sewn up in palm leaf bags, to the East Coast of Africa. A number of other products, including Makassar fish, are made in Asia; many of these include a proportion of cooked rice.

Shrimp and fish pastes

These should not be confused with the fish pastes made in Western countries, for little, if any, fermentation takes place in these; they are typically made merely by comminuting fish flesh, which might be smoked as a preliminary.

The processes used in making fermented fish pastes are all essentially similar to one another. Typically, the fish or shrimps are pounded with a proportion of salt so that a paste results; this is subjected to varying periods of sun drying before being packed to mature in a container from which air is excluded. Sometimes a period of sun drying follows salting before any comminution is attempted. The moisture content of a typical paste varies from 35 to 50 per cent so that almost half the water present in the fresh material would have been lost during processing.

Typical pastes include the ngapi of Burma, the pra hoc and various mams of Cambodia, belachan of Malaysia and the trassi of Indonesia. Trassi may be made from shrimp or fish. Details of the process vary almost from village to village but, typically, where sun drying is possible, the raw material is sun dried for a day before it is salted. After sun drying, the material is traditionally pounded in a wooden mortar; the following day the dough is exposed for a second time. Later the product is pounded and mixed with salt. In some cases, the raw material is salted in the catching vessels and is only later subjected to sun drying. The Malaysian process for making belachan is similar: the raw or partially dried material used to be crushed in wooden trays by a treading process similar to that in which wine is made; nowadays, the process has been mechanised and butchers' choppers are used for the mixing and pounding.

In a typical process, Acetes shrimp (udang gragok, udang team ng) and smaller proportions of mysid shrimp are used. The process is as follows:

1. The fish and larger prawns are sorted from the catch so that only the smaller shrimp remain.

2. The small shrimp are mixed with salt in bamboo baskets or wooden tubs using a ratio of 4 - 5 kg salt: 100 kg wet shrimp.

3. The pickled shrimps are spread out in thin layers on mats to dry in the sun.

4. Drying continues for 4 to 8 hours; about 50 per cent of the moisture is evaporated. The material is again sorted and rubbish, fish and crabs are removed.

5. The salted shrimps are minced and then pressed into a paste in wooden tubs or boxes. It is important that all air bubbles are excluded at this time.

6. The minced paste ferments for up to 7 days and is then dug out of the tub and spread to dry in the sun for 3 - 5 hours. The paste is then minced again and returned to the wooden tubs where it ferments for about a month. It is then minced for a third time and packed in blocks wrapped in cellophane or brown paper.

The yield of paste is 40-50 per cent of the raw shrimp weight. A typical analysis of good quality belachan is as follows:

pH

7.6 - 7.8

Moisture

27 - 40 per cent

Ash (including salt)

20 - 24 per cent

Salt

13 - 18 per cent

Protein

30-40 per cent

Artificial colouring matter such as Rhodamine B has sometimes been used to produce the desired deep pink colour. Some curers have been observed to be most liberal in their application of this poisonous dye. There are no records of anyone being poisoned by these materials but it would be advisable to replace them with one of the safer food colours.

Fish sauces
In these processes, the fermentation of the fish is carried out for a longer period than in the manufacture of fermented fish and fermented fish paste; the sauces are liquids containing a mixture of amino acids and other protein degradation products. They are thus similar to soya sauce; like soya sauce, they have high salt contents and this is a limitation to their use as a food; they are used principally as condiments for flavouring rice dishes, indeed they may be eaten with plain boiled rice. They are also freely used in vegetable cookery.

In the manufacture of the classic sauces such as the nuoc-mam of Cambodia and the nam-pla of Thailand, fish are mixed with salt in tubs or vats and left to stand for periods varying from 5 to 18 months. Figures 1 and 2 illustrate the type of vat used for the manufacture of nuoc-mam. The best sauces are made from anchovies (Stolephorus spp.); a typical process is as follows:


Figure 1 - Vietnam: Section of the vat used for making nuoc-mam (Courtesy Institut Oceanographique de Nha-Trang)

1. The fish are washed in sea water and then mixed with salt in tubs or vats make of wood or cement, the ratio of fish to salt being from 1:5 to 1:1. A weight is also placed on top to keep the fish below the brine.

2. The vat stands for 5 to 18 months then the clear liquid is skimmed off the top or drained through a spigot near the bottom of the container.

3. The liquid is then filtered and bottled or barrelled and exposed in the sun until it is ripe.

4. The liquid sauce is finally packed in bottles or earthenware containers for distribution.

After the first liquid has been drawn off at stage 2 the residue may be extracted several times with salt water to make a lower grade product. Sometimes this is followed by extraction with boiling brine. The yield is typically about 90 per cent of fish sauce by weight. The chemical composition of low grade and high grade nam-pla (Thailand) is given in the table below.


Low grade

High grade

Salt (g/100cm³)

28.4

28.15

Total nitrogen (g/100cm³)

0.92

1.92

Ammonia nitrogen (g/100cm³)

0.28

0.28

TMA nitrogen (g/100cm³)

0.02

-

Organic nitrogen (g/100cm³)

0.62

0.64

Formol nitrogen (g/100cm³)

0.83

1.13

Amino nitrogen (g/100cm³)

0.55

0.85

pH

5.75

5.58


Figure 2 - Vietnam: General view of a nuoc-mam vat at Nha-Trang (Courtesy Institut Oceanographique de Nha-Trang)

A first-grade product should contain 20 - 23 g/litre of nitrogen and, of this, 50 per cent should be formol titratable and not more than 15 - 20 per cent titratable as ammonia. The salt content should be 20 - 25 9/100 9 and the pH should be below 6.0.

A good deal of research has been undertaken in an effort to speed up the process of manufacture of fish sauces. The process proceeds faster as the temperature increases up to about 45°C; unfortunately, the liquids that are produced at such high temperatures do not have the characteristic flavour of the classic sauces. This fact is well known amongst the producers who endeavour to keep the areas in which the vats are situated relatively cool. Although some work has been carried out on the flavouring substances of fish sauces, no accelerated process has so far been brought into commercial use.