Fish Handling and Processing

 

Fish Handling and Processing

What is fresh? Correct use of ‘fresh’ can be
(a) Newly produced , not stored or preserved and
(b) Having its original qualities unimpaired, i.e. not deteriorated in any way.  Fish which have been carefully frozen and thawed and are otherwise of high quality might well be considered fresh by the second definition, although not by the first. Both aspects of the definition must be kept in mind for consideration of freshness.

Some guides for descriptions of fresh fish characteristics include the following:

Flesh: 

Firm, elastic flesh not separating from bones, indicates that fish and flesh   has been handled carefully.

Odour: 

Fresh and mild. A fish just taken from the water has practically no fishy (ammoniacal) odour. The fish odour becomes more pronounced with passage of time, but it should not be disagreeably strong when the fish are bought.

Eyes:

Bright, clear and full. The eyes of fresh fish are bright and transparent; becomes cloudy and often turn pink in stale fish. The eyes often protrude in fresh fishy but become sunken with increasing staleness.

Gills:

Bright red. The colour gradually fades with age to a light pink, then grey and finally brownish or greenish.

Skin:

Shiny, with colour gradually not faded. Most fish are iridescent in appearance when taken from water. Each species has its characteristic markings and colours which fade with increasing staleness.

1. Flesh: fresh-cut in appearance, the colour should resemble that of freshly dressed fish. It should be firm and moist in texture, without traces of browning about the edge and without dried-out look.
2. Odour: Fresh and mild, not fishy, pungent or ammoniacal
3. Wrapping: If the fillets or steak are wrapped, the wrapping should be of moisture-vapour-proof material. There should be little or no air space between the fish and the wrapping.

Why Fish Spoil

Certain irreversible changes begin to take place immediately after fish dies. Within some hours, the muscles gradually harden along the fish until it is quite stiff. The fish can remain rigid for a number of hours or a few days depending on various factors. Thereafter, the muscles then ‘soften’ or become pliable again. The stiffening is called rigor mortis and is brought about by enzymes in the muscle. It is important in relation to filleting operations. Enzymes also cause complicated series of breakdowns of other tissue components, called autolysis (or self digestion). Bacteria in addition, and un-gutted fish digestive juices, invade flesh to start the process of putrefaction. Lastly, fats is broken down by oxygen and can give rise to rancidity.

          

Autolytic spoilage

Food supply ceases and energy source depletes at death. The enzymes do not ‘die’; they can continue to operate but, since energy is required to build larger units, the function which the enzymes perform (post mortem – after death) is to break (down) compounds into smaller units. This breakdown is called autolysis. Autolysis can affect flavour, texture, and sometimes, the aesthetics/appearance of flesh.

Flavour: 

The characteristic sweet, meaty flavour of fresh fish is (at least in) partly due to a compound called inosinic acid; its breakdown through autolysis results in loss of this flavour. Another compound, hypoxanthine, which is produced from breakdown of inosinic acid, contributes to the bitter flavours of spoiled fish. Autolysis also contributes to bitter flavours by providing a supply of compounds which the bacteria convert to unpleasant flavours (and odours).

Texture: 

The stiffening of fish (rigor mortis) and the subsequent softening  (of fish) are caused by autolysis. Rigor is of great significant in fish processing particularly in freezing operations for very fresh fish i.e. freezing at sea. In rigor the fish can stiffen into distorted shapes and they can be difficult to load between freezer plates. Forcibly straightening the fish can lead to serious textural damage in the flesh when filleted. Fillets, cut before rigor and then frozen can contract during storage giving a tough rubbery texture.

Appearance: 

Yellowish-brown discolorations which are sometimes present in frozen flesh could be due to autolysis.

Bacterial spoilage

Bacteria present on the surface and in the guts multiplies rapidly and invade the flesh, when fish dies.  In dead fish, bacteria can breakdown the muscle itself and also will ‘feed’ on the smaller units produced by autolytic action. The increase in numbers of bacteria result in heavy slime on the skin and gills; an unpleasant ammoniacal, sour odour and eventual softening of flesh. Frequently gut wall will burst.

The bacterial load present on the fish when caught will continue to multiply (even if thoroughly chilled in ice) until the fish is consumed. However, during handling they are likely to pick up more bacteria, from washing in polluted water, careless gutting, dirty containers (boxes), etc. However careful you are in handling the fish, there will always be bacteria present but, with care, the members can be controlled.

Flesh from living fish is aseptic i.e. it is sterile. An aseptically removed flesh maintained at O° C for up to 6 weeks has no obvious organoleptic changes. Autolytic changes will, of course be occurring during this period.

           

Table1.  Yield from several species of fish (from considering a wide variety of data source)

Species	Dressed-fish (%)	Liver (%)	Viscera Less Liver (%)	Other trimmings(%)
Average species Flounder Ling cod (the big head) Sockeye salmon	65 67 54 73	2 1 1 2	8 7 8 6	25 25 37 19

Source: Stansby M. E. 1976. Industrial Fishery Technology. Robert E Krieger Publishing Co. Huntington, New York. 415pp.

            Dressed fish average 78% flesh, 21% bone and 6% skin.

Table 2.  Proximate composition for edible portion fish in general

‘edible’ = skin and bone-free fillet.

Statistics calculat(%)	Moisture (%)	Protein (%)	Oil (%)	Ash (%)
Average Range Ratio high to low	74.8 28-90 3.2	19 6.28 4.7	5 0.2 – 64 320	1.2 0.4-1.5 3.8

Oil content vary (even within the same species) with Season of year, geographical area, age, sex and size of fish. The primary causes of variation are degree of energy expenditure and food intake.

It is more meaningful to classify fish into categories because of the variation V12:

Fig. 3.  Varying oil content of some species of fish

Category	Type	Oil Content (%)	Protein Content (%)	Prototype
A B C D E	Low oil – high protein Medium oil – high protein High oil – low protein over Low oil – very high protein Low oil – low protein	Under 5 5-15 over 15 5 <5	15-20 15-20 <15 >20 <15	Cod Sockeye Siscowet lake Skipjack Halibut Clams, oysters

Fig. 4.  Types of composition for some important species

Species	Primary category	Secondary Category
Anchovies Bullhead and catfish Carp Clam Cod Crab  Flounder Mackerel Menhaden Mullet Salmon (Atlantic, chum pink, silver) Salmon (king) Scallop Shrimp Tuna (albacore, bluefin) Tuna (skipjack, yellowfin) Whiting Yellow pike	B A A E A A A B B A B B A A D D A A	C - B - - - - C C - A C - D B - - -

Oxidation of fish

In fatty fish, chemical changes involving oxygen from the air and fat of the fish may produce rancid odour and flavours. This problem is of importance when storing frozen fish for fairly long periods. Glazing before cold storage helps to alleviate the problem.

Fish Handling

Effect of fishing methods

Fishing method may affect freshness. A normally live fish e.g. tuna, mackerel, may become excited and die in frenzied state when seined. Similarly, certain types of gears e.g. gill nets may kill the fish after and exhausting struggle. Such exhausting activity before death results in rapid development of rigor mortis followed by earlier signs of deterioration during icing. On the other hand, many salmon are caught by surface hook and line, brought (to the boat) brought quickly up and dispatched quickly with a blow on the head. These don’t deteriorate fast. Halibut caught on a bottom hook and line usually come to the surface easily and are quickly killed. Such ‘clean kills’ are significant in extending freshness and quality. Refrigeration brine immersion and electric shocker to stun or kill the fish immediately after harvest is used to control quality in modern aquaculture operations.

Physical damage: 

Fishing gear and handling of the fish when the gear are brought aboard often contribute to bruising or tearing of the flesh during transfer of fish in and out of the boat with spear and spearhead, gaff hooks, fish-pughs or forks are responsible for lots of unsightly and unsanitary holes in some fish before processing. Quick bacterial spoilage follows in these (pugh) marks. Rough weather on the trip back to port after fishing and excessive ice pressure in the bins accelerate the deterioration and increases the shrinkage of fish.

Condition: 

fish is usually in a better condition if caught from tidal than still water (such fish are said to be doing more ‘exercise’).

Dressing: 

Actively feeding fish when caught requires prompt dressing and icing or processing by or other methods to reduce greater incidence of autolytic spoilage by digestive enzymes. Dressing involves removal of gills, viscera and scales (where present) immediately after catching and removal of bones in some cases. The gut cavity should be washed with clean water (or clean sea water if at sea) before icing, 50ppm chlorine in sea water (in Atlantic trawlers) is more effective than plain sea water in rinsing blood and slime from the fish. Dressing is deemed impractical in some fisheries where fish value and size are small gutting and washing of fish here however, been (shown demonstrated to be very important.

Methods for preserving fish to reduce spoilage

Biological systems which operate bacterial and autolytic spoilage are only possible under certain optimum conditions.  Altering the conditions can therefore provide ways of preventing or reducing spoilage. Since bacteria require water and are sensitive to heat, salt concentration and pH, (there are) a number of approaches can be used to prevent bacterial spoilage. Control of autolytic action (in fishing industry) is by lowering temperature. The enzymes could also be inactivated by other means e.g. irradiation with rays or by poisoning with chemicals.

Temperature control:

In cold water fish enzymes and bacterial action are optimum at 5 - 10° C; and warm water fish between 25-30° C. Lowering temperature prolongs storage life by reducing bacterial and enzyme activities.

Lowering temp.

Chilling: Holding fish at above or just below freezing point i.e. reducing temperature of fish from 25° C to 1-4° C in the tropics. Ice is ideal for chilling. Fish should be chilled as soon as possible. Ice is used for “short term” storage though in some species it may be as long as one month.

It appears that generally

(1) Freshwater fish have a longer shelf life on ice than marine species e.g.

Tilapia (Freshwater fish)         -           22-28 days

Mrigal carp (Freshwater fish) -           35 days

Nile perch (Freshwater fish)   -           20 days

Snapper (Brazil) (tropical marine) -     11-16 days

Spanish mackerel (tropical marine) -   18days

Bonga (tropical marine)                  -     20 days

(2) Tropical species keep longer than temperature or coldwater species on ice e.g.

Cod (temperate marine)                      -           12-15 days

            Haddock (temperate marine)              -           12-15days

            Whitings (temperate marine)               -           9-12 days

            Trout (temperate freshwater)              -           10 days

            Channel catfish (temperate freshwater)          12 days

(3) Non-fatty fish keep longer than fatty fish species

Freezing – Long term storage i.e. for months, years because

autolytic and bacterial action are almost arrested.

Read for more information

Eyo, J. E. and B. O Mgbenka. 1997. Methods of fish preservation in rural communities and

                beyond.  Pages 16 – 62.  In H. M. G. Ezenwaji, N. M. Inyang and B. O. Mgbenka (eds.)

                Proceedings of the Anambra State Ministry of Agriculture, Awka and UNDP-sponsored

                workshop on Women in Fish Handling, Processing, Preservation, Storage and Marketing,

                13 - 17 January, 1997.