“Post-mortem changes”
Factor influencing the rigor mortis progress:
It is possible to distinguish between pre-rigor and post-rigor fish since pre-rigor fish are fully flexible and do not show pressure marks after gentle squeezing. The time involved in each stage of the development, duration and sub sequent resolution of rigor mortis depends on many factors such as species, sizes, catching methods, handling of fish, temperature and the physical condition of the fish.
Exhausted fish and fish kept at high temperatures will enter and pass through rigor quality. Small and active, struggling fish does the same while large and flat fish takes a longer time. If a rigor develops at high temperatures, the rigor tension may become very strong and can cause “gapping”, i.e. weakening of connective tissues and rupture of fillets. The following factors are responsible for rigor mortis progress in fish.
1. Species: Some species take longer time than other to go into rigor, because of their differences in chemical composition. Fish with high lipid content goes to rigor earlier than low fat species. Generally, dark muscle goes quiet earlier than white muscle. Northern whiting goes into rigor very quickly and becomes completely stiff one hour death, whereas red fish stored under the same condition may take as long as 24 hours to develop full rigor.
2. State of activeness: Fish that are very active in their normal habitat may become excited and die in a frenzied state when caught shows a shorter period of rigor mortis. Exhausted fish have little or no delay period.
3. Condition and seasonality: Rigor mortis starts immediately or shortly after death if the fish is starved and the glycogen reserves are depleted, or if the fish is stressed. The poorer the physical condition of the fish (less nourished) before capture, shorter is the time it takes to go into the rigor, because of very little reserve of energy in the muscle to keep it pliable. Spent fish is an example.
4. Degree of exhaustion: Fish struggle in the net for long time before hauled and gutted on board vessel will have much less reserve of energy than those entered the net just before hauling and thus will go into rigor more quickly.
5. Size: Small fish usually go into rigor faster than large fish of the same species.
6. Handling: Manipulation of pre-rigor fish does not appear affect the time of rigor, but manipulation or flexing of the fish while in rigor can shorten the time they remain stiff.
7. Temperature: this is the most important factor governing the time a fish takes to go into and pass through rigor because the temperature at which the fish is kept can be controlled. The warmer the fish, the sooner it will go into rigor and pass through rigor. Although it is generally accepted the onset and duration of rigor mortis is more at high temperature, it has been observed in certain tropical fish that the biochemical change and thus rigor mortis may actually be stimulated at 0 ̊C compared with 22̊C.
Impact of rigor-mortis on keeping quality of fish:
Rigor can affect the quality of frozen whole fish is three main ways: by causing i. gapping in fillets taken from frozen whole fish; ii. toughness and drip loss in frozen fish or fillets and iii. Shrinkage of frozen fillets. This undesirable effects can be reduced or prevented by i. keeping the fish cool, particularly before going into rigor; ii. handling it carefully when in rigor and iii. Freezing fillets from pre-rigor fish as soon as they are cut. Only careful treatment of the fish before and during rigor will result in a higher quality frozen product. For appropriate handling, careful leveling scheme for frozen fish is necessary that enables processors to identify the fish frozen pre-rigor, in-rigor or post-rigor. Reliable and the safest way to avoid negative impacts of rigor is to keep fish chilled at every stage before freezing. If the fish through rigor at chilling temperature, the effects of rigor on the quality will be not so serious.
Gapping:
A fillet is said to be “gapped” when the individual flakes of muscles come apart, giving the fillet a broken and ragged appearance. This happens when the material that binds the flakes together, known as connective tissue, breaks down. There are several causes of gapping, important one is rigor process. As muscle goes into rigor, it attempts to contract but skeleton and connective tissue prevent contraction and tension increase within the muscle. As long as the connective tissues can withstand this increase in tension, the flesh will not gap. But when the tension is greater than the inherent strength of the connective tissue, some gaping will occur.
The temperature of the whole fish as it goes into rigor has marked effect on gaping. The higher the temperature when it goes into rigor, the greater is the rigor tension, the weaker the connective tissue and higher is the gap between the flesh. If the temperature of the whole fish is lowered so much that the fish start to freeze while it goes into rigor, the connective tissues is again weakened due to ice formation and gapping occurs.
Rough handling of fish in rigor can cause gapping. Any attempt to bend or straighten a rigid/bent fish will break the muscle or connective tissues. Pressure from freezer-plates can damage the fish.
Full stomach of fish has impact on gapping. Gapping by freezing in rigor is more in well-nourished fish than spent fish. Gapping is also more in well-fish kept at high temperature and then frozen after they have started to go into rigor.
Prolonged frozen storage may also lead to gapping.
Toughness and drip-loss:
The higher the temperature of fish at rigor, greater will be the drip-loss on thawing and tougher and more stringy will be the muscle when it is cooked and eaten. This will be aggravated when fish are well fed and no exhausted. Whole fish frozen pre-rigor tends to have higher drip-loss than the similar fish frozen in rigor on post-rigor.
Impact of rigor in frozen fillet:
When rigor sets in, fish flesh becomes hard and stiff and the whole body becomes flexible. If the fish is filleted pre-rigor, the muscle can contract freely and fillets will shorten and have a wrinkled surface. Dark muscle may shrink up to 52%, while the white muscle up to 15% of the original length. After rigor is passed, the muscle tissue returns to a relaxed state.
If the fish are filleted pre-rigor, the fillets may shrink; if these fillets are frozen, they will often have a poor texture and enhanced drip-loss. The shape of the fillet becomes distorted and the surface takes on a corrugated appearance. This distortion will remain throughout subsequent freezing and frozen storage. The extent of shrinkage depends on the condition of fish and the temperature at which it is kept.
It is very important that fillets be frozen immediately after they have been cut from pre-rigor fish. If delay between filleting and freezing is unavoidable, the fillets must be kept chilled to reduce the shrinkage, but even at 0̊C some of the fillets will shrink after certain time. Immediate freezing is the only safe way to avoid shrinkage. Pre-rigor fillets should not be chilled by freshwater ice, because shrinkage enhanced by contact with freshwater. The cut surface of a pre-rigor fillet is different from that taken from a post-rigor fish. It is dull, rough and corrugated with a texture that feels like crepe rubber caused by exposure of the cut ends of individual muscle fibers.
When filleting is delayed until after the whole fish has gone into rigor at a low temperature, most of the problems of shrinkage are avoided and the quality of fillet becomes good. But there are some disadvantages too. Mechanical filleting is difficult when the fish are in rigor and hand filleting may give slightly lower yield from fish.
Rough handling of fish in rigor will also cause gaping. In principal, it is safer to filler the fish post-rigor and freeze these fillets but often this is not possible, as it requires a large chilling chamber for the whole fish.
Frozen fillets taken from post-rigor whole fish are normally of uniformly good quality, provided that the whole fish is properly handled and kept adequately chilled.
Thaw rigor:
When fish muscle is frozen pre-rigor and kept for a short time in cold storage, it is till be able contract and go into rigor after thawing. This is called thaw rigor. If this thawing done at high temperature, the muscle then can suffer from the defects associated with high temperature rigor. Thaw rigor is not a problem in case of whole frozen fish because contraction is negligible due to skeleton restrains the stresses. Thaw is really a problem in case of fillets those are frozen and frozen stored pre-rigor. The effects are most serious when the pre-rigor muscles is cooked from the frozen state.
For example, when consumer packs of fillets or fish fingers are prepared from pre-rigor fish. When a fish finger is given a preliminary cook, the flesh can contract and cause it to distort, resulting in difficulty in packing. The texture will be tough and stringy and drip-loss will be high. On final cooking the free water will boil off and causes the better to spatter. Thaw rigor is, however, not the only source of free water in frozen fillets and fish fingers. The effects of thaw rigor are more noticeable when single fillets or small portions are thawed, rather than blocks of fillets.
Thaw rigor, although uncommon in commercial practice, but if met with, the bad impact can be can be avoided very simply. The simplest way is to extend the frozen storage time of the stock of pre-rigor fish. If the fish fleshed is kept at least 8 weeks at -20F or -28F, the flesh has time to pass through rigor in the frozen state. This has no bad impact on the quality of either whole fish or fillets, since both are held rigidly enough while frozen to prevent the muscle from contracting. If the fish has to be taken out of frozen storage in less than 8 weeks, they should be thawed slowly at room temperature. In this way rigor is completed while the fish are in semi frozen state, thus preventing severe contraction of muscle.