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.

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.

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.

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.

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.

‘edible’ = skin and bone-free fillet.

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:

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.

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.