Handling and Processing of tuna

Handling and Processing of Tuna

The following are important factors, which need to be considered for handling and processing of

Tuna:

They are

a) Maintenance of Freshness.

b) Fishing Methods.

c) Killing Methods.

d) Bleeding.

e) Dressing.

f) Chilling on board.

g) Freezing method.

a) Maintenance of freshness

Various measures have been suggested to reduce variations in the quality of tuna meat including; choice of fishing methods, limitation of fishing season, standardized methods of handling. Killing and chilling as well as thoroughly sorting and grading techniques. It is important to avoid bruising the tuna in the initial handling process, because of the effect of the blood on the colour of the flesh. Like any other fish species, tuna will undergo rapid spoilage immediately after catching unless they are properly handled. Exposure to high temperatures will further accelerate and shorten the rigor mortis phase, which will lead to rapid autolysis and bacterial decomposition.

Unlike most other fishes, tuna is considered to be a warm-blooded animal, which make them susceptible to small temperature changes.

b) Fishing methods

Fishing methods play an important role in determining the quality of fresh tuna. Ideally the tuna should be caught with least stress to avoid accumulation of lactic acid in the flesh which tends to trigger the onset of rigor mortis and subsequent quality deterioration. Under a serve stress condition where tuna fight intensely for more than 2 minutes just before landing, a considerable amount of lactic acid may build up in the muscle and lead to a rise in acidic condition of the fish meat. Such conditions coupled, with a high handling temperature, would result in a serious defect known as “yake niku” or “burnt flesh”.

Traps are considered to be the best method of catching tuna. Long lining is the second alternative, which allows tuna to calm down and restore their energy before being landed. Purse seining on the other hand, tends to catch tuna in large numbers, leading to physical damage and maximum physical stress.

c) Killing methods

Tuna has to be killed immediately and properly after landing, to prevent further stress. Not with standing, the slight variations in the method of tuna killing, its main purpose is to ensure rapid destruction of the central nervous system which controls body temperature, so that the temperature of the fish can be brought down very quickly during chilling / frozen storage.

d) Bleeding

Although bleeding is considered to be a good practice in tuna handling, it is not really compulsory and the application is very much dependent on the final use of the product. Like wise it does not make any difference whether the fish are bled before killing or vice versa. The most important thing is to process tuna quickly immediately after being caught. The purpose of bleeding is usually to prevent the tuna meat from turning sour quickly due to the accumulation of lactic acid.

e) Dressing

After bleeding, the gills have to be removed, followed by eviceration. Gills are severed at their points of attachment, and the membrane behind the gills as well as the viscera pulled out. The gills are removed by inserting a sharp knife behind the gill cover and sliding forward at the top about 2 inches to the pre operculum. The gill covers are then opened out and the isthmus (Joint of the gills to the front of the head) severed. The gill membrane is separated on the other side and pushed forward end of the gills to become clear. The membrane around the gill area is then removed by cutting in to the skin to a depth of one eighth of an inch.

Evisceration without cutting the belly open minimizes bacterial contamination and maintains the external appearance of the fish. In most cases, evisceration is done by inserting the blade to a depth of one or two inches just behind the base of the pectoral fins in the pectoral groove on both sides of the fish. A shallow cut is then made from the front fin (ventral area to the anus) by stranding the fish and holding one pectoral fin. This cut must be a shallow one inch to the stomach cavity, as too deep a cut could puncture the bile and stain the flesh, or trigger the release of stomach acid which burns and disfigures the cavity walks. The fish are then washed with concomitant removal of remaining kidney, loose membranes or tendons. The fins are removed immediately after evisceration.

f) Chilling on – board

Various methods of chilling can be carried out on board fishing vessels, including simple icing, chilling using chilled seawater as well as refrigerated sea water. Icing is hitherto considered to be the simplest, most efficient and cheapest method of chilling. Using either crushed or flake ice. A sufficient amount of ice (1:1) has also to be assured throughout the handling process to maximize the chilling efficiency of the ice. For yellow fin tuna and big eye tuna caught in tropical waters, it is desirable to chill the fish in a chilled sea water ice slurry (2 parts ice, 1 parts sea water) for 6 up to 12 hours prior to ice storage. To ensure an even low temperature throughout the slurry should be agitated frequently.

g) Freezing method

There are two types of frozen tuna in Japanese trade. The first category are tuna frozen at -20oC to -30oC destined for canning, and the second category tuna frozen at ultra – low temperatures (ULT) of -50oC to -70oC destined for raw consumption. Frozen tuna using ULT method can be stored up to two years without significant quality changes. However, thawed ULT frozen tuna usually has a shorter shelf life compared to chilled tuna. While chilled tuna has a shelf life up to twelve days at 0oC, thawed tuna has a commercial shelf life of only three days before the taste and colour changes become noticeable. Thawed tuna also tends to exudate drip accompanied by rapid textural changes.

Last modified: Sunday, 25 December 2011, 5:38 AM