Introduction
What is rigor?
What causes rigor?
How long docs a fish stay in rigor?
How does rigor affect handling and processing?
Controlling the effects of rigor
Thaw rigor
Can thaw rigor be prevented?
Does rigor affect the quality of smoked fish?
Which is best, freezing before rigor, in rigor or after rigor?
Summary
This advisory note explains briefly what rigor is, how it occurs and how it can affect the quality of fish. The effects of rigor on the handling and processing of fish, particularly frozen fish, are described in detail. The note recommends a number of ways in which adverse effects on quality can be reduced or prevented by correct handling before, during and after the onset of rigor.
Although the information given refers mainly to cod, all white fish behave in a similar way, and the advice should prove of value to all fishermen and processors who are concerned with the processing of newly caught white fish, either at sea or on shore.
Rigor is only one factor among many that can affect the quality of fish frozen very soon after capture; other factors, for example blood discoloration, are not discussed here. General advice on the freezing of fish at sea is given in Advisory Note 34, and on the handling of blocks of sea-frozen fish in Advisory Note 2.
Rigor or, to give it its full name, rigor mortis means the stiffening of the muscles of an animal shortly after death. The word rigor is used throughout this note because it is shorter and easier to use than either death stiffening or rigor mortis.
Immediately after death the muscles of an animal are soft and limp, and can easily be flexed; at this time the flesh is said to be in the pre-rigor condition, and it is possible to make the muscles contract by stimulation, for example by means of an electric shock.
Eventually the muscles begin to stiffen and harden, and the animal is then said to be in rigor. The muscles will no longer contract when stimulated, and they never regain this property.
After some hours or days the muscles gradually begin to soften and become limp again. The animal has now passed through rigor, and the muscle is in the post-rigor condition. Sometimes rigor is said to be resolved; this is simply another way of saying that the muscle has passed through rigor to the post-rigor stage,
Rigor in fish usually starts at the tail, and the muscles harden gradually along the body towards the head until the whole fish is quite stiff. The fish remains rigid for a period which can vary from an hour or so to three days, depending on a number of factors described later, and then the muscles soften again.
Rigor results from a series of complicated chemical changes in the muscle of a fish after death; the process is not yet fully understood, and research is still going on, but it is known that factors like the physical condition of the fish at death, and the temperature at which it is kept after death, can markedly affect the time a fish takes to go into, and pass through, rigor.
While the fish is alive, cycles of chemical changes take place continuously in the muscle; these provide energy for the muscle while the fish is swimming, and also produce substances necessary for growth and replacement of worn-out tissue. The compounds that bring about, and control, these changes are known as enzymes.
The enzymes in the flesh go on working even after the fish is dead, and some of them act on those substances that normally keep the muscle pliable and lifelike. During life the muscle would contract and become rigid if its two main protein components were allowed to interact and bond together, but the bonding is prevented by the presence of substances that keep the muscle pliable, rather like the way in which oil lubricates the moving parts of a machine and prevents it from seizing up.
For so long as the muscles contain a reserve of energy, these substances can be replaced by one set of enzymes as fast as they are destroyed by another; thus the muscles stay pliable for a time after death. But once the energy reserves are used up, the replacement stops and depletion results. The protein components are then able to interact, the muscle attempts to contract, and it eventually becomes hard and rigid.
The interaction of the protein components is also influenced by the accumulation of lactic acid, which is produced in the muscle when the energy reserves break down. The relative importance of the two factors, depletion of one set of substances and accumulation of another, is not fully understood, but they are known to vary with the type of animal and with how well nourished and rested it was at the time of death.
The time a fish takes to go into, and pass through, rigor depends on the following factors: the species, its physical condition, the degree of exhaustion before death, its size, the amount of handling during rigor and the temperature at which it is kept.
Species: Some species take longer than others to go into rigor, because of differences in their chemical composition. Whiting, for example, go into rigor very quickly and may be completely stiff one hour after death, whereas redfish stored under the same conditions may take as long as 22 hours to develop full rigor. Trawled codling, 18-22 inches long, gutted and stored in ice, usually take 2-8 hours to go into rigor.
Condition: The poorer the physical condition of a fish, that is the less well nourished it is before capture, the shorter will be the time it takes to go into rigor; this is because there is very little reserve of energy in the muscle to keep it pliable. Fish that are spent after spawning are an example.
Degree of exhaustion: In the same way, fish that have struggled in the net for a long time before they are hauled aboard and gutted will have much less reserve of energy than those that entered the net just before hauling, and thus will go into rigor more quickly.
Size: Small fish usually go into rigor faster than large fish of the same species.
Handling: Manipulation of pre-rigor fish does not appear to affect the time of onset of rigor, but manipulation, or flexing, of the fish while in rigor can shorten the time they remain stiff.
Temperature: This is perhaps 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. For example, gutted cod kept at 32-35°F may take about 60 hours to pass through rigor, whereas the same fish kept at 87°F may take less than 2 hours.
To sum up, small fish with low reserves of energy, that is exhausted and in poor condition, and kept at a high temperature will enter and pass through rigor very quickly. On the other hand, large, rested, well-fed fish kept at a low temperature will take a very long time to enter and pass through rigor.
The following table gives some indication of rigor time for different species. All times are from direct observation, but the limits are not fixed and it is more than likely that times outside these limits could be met with in practice. For instance, some fish may already be in rigor when they are landed on the deck of the fishing vessel either because they have completely exhausted themselves struggling in the net, or because they have died through asphyxiation perhaps as much as three hours before the trawl was hauled.
|
Species |
Temperature |
Time from landing on deck to entering rigor |
Time from landing on deck to end of rigor |
|
trawled cod
|
in ice |
2-8 |
20-65 |
|
37 |
4½-8½ |
54-64 |
|
|
42-44 |
5 |
45 |
|
|
62 |
2-5½ |
16-20 |
|
|
87 |
|
1½-2 |
|
|
rested cod from aquarium |
37 |
14-15 |
72-96 |
|
trawled redfish |
in ice |
22 |
120 |
|
trawled whiting |
in ice |
1 |
20 |
|
trawled plaice |
in ice |
7-11 |
54-55 |
|
trawled coalfish |
in ice |
18 |
110 |
|
trawled haddock |
in ice |
2-4 |
37 |
Although the problems of rigor also affect processing in the meat and poultry industries, the problems in the fish industry are more acute because we have no control over the nutritional condition or the degree of exhaustion of the fish before they come on board the vessel. Rigor creates problems mainly for those sections of the industry concerned with freezing fish at sea, either as whole fish or as fillets, and for those who handle very fresh inshore fish at the port for freezing very soon after landing. Rigor problems are not normally encountered when handling the bulk of the chilled wet fish landed at the ports, because this fish will already have passed through rigor while in ice on board ship. The only time when problems could occur with wet fish is when they have been left lying on deck at a high temperature until they have gone into rigor. The explanation of this is given later.
Rigor affects frozen whole fish and frozen fillets in different ways:
FROZEN WHOLE FISH
Rigor can affect the quality of whole fish in three main ways, by causing gaping in wet and frozen fish, and toughness and excessive drip loss on thawing in frozen fish.
Gaping
A fillet is said to gape when the individual flakes of muscle 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 appear to be several causes of gaping, one of which is the rigor process. As muscle goes into rigor, it attempts to contract but, because the skeleton and the connective tissue prevent contraction, tension increases within the muscle. As long as the connective tissue can withstand this increase in tension, the flesh will not gape, but when the tension becomes greater than the inherent strength of the connective tissue, some gaping will occur.
The temperature of the whole fish as it goes into rigor can have a marked effect on the amount of gaping; the higher the temperature when it goes into rigor, the greater is the rigor tension and the weaker the connective tissue becomes. Thus the higher the temperature the more the flesh will gape. Furthermore, with cod, there is a critical temperature of about 63° F, above which the contractions become so strong and the connective tissue so weak that the tissue breaks down completely, resulting in a fillet so ragged that it is completely unacceptable. Below 63°F, the lower the temperature the less damage is done by the contractions. Other species also have critical temperatures, not necessarily the same as for cod. This gaping is apparent on filleting, whether or not the fish has been frozen and thawed, but is worse in frozen fish, whether frozen in rigor or after rigor.
Strangely enough, if the temperature is lowered so much that the fish starts to freeze while it goes into rigor, the connective tissue is again weakened, this time by the formation of ice, and gaping occurs. Gaping caused by freezing fish that are going into rigor is more likely to occur in well-nourished fish, where the contractions are stronger than in spent fish.
Rough handling of fish in rigor can also cause gaping, because any attempt to bend a rigid fish will break the muscle or the connective tissue. Damage of this kind is most likely to happen when the fish are being loaded into freezers at sea, and attempts are made to straighten bent fish while they are stiff. Pressure from the freezer plates can also damage rigid fish lying in distorted positions in the freezer.
Rigor, however, is only one of several causes of gaping, since gaping is often seen in fillets taken from whole fish frozen post-rigor, and also in wet fish that have never been frozen. Here gaping is due to prolonged storage, and the longer the fish has been kept the worse it becomes. Some fish are inherently softer than others, and simply handling the fish during freezing, thawing and filleting can cause considerable gaping. Softness of the flesh can be influenced by the type of feeding, the fishing ground and the stage in the spawning cycle. All these factors may be superimposed on the gaping caused by rigor, thereby making it worse.
Gaping due to rigor, then, is most likely to occur in well-fed fish kept at a high temperature and then frozen after they have started to go into rigor, or in fish that are roughly handled while they are in rigor.
Toughness and drip loss
The higher the temperature at which a fish goes into rigor, the greater will be the drip loss on thawing and, when the fish is cooked and eaten, it will be tough and stringy; this effect will probably be aggravated when the fish are well fed and not exhausted. However, it is not rigor alone that causes toughness and high drip loss in thawed frozen fish; the flesh may be inherently tough or it may have been toughened by incorrect freezing, cold storage or thawing.
Whole fish frozen pre-rigor tend to have a higher drip loss than similar fish frozen in rigor or post-rigor, but this may be due to what is known as thaw rigor, which is explained later.
FROZEN FILLETS
Unless precautions are taken, fillets cut from a fish before it goes into rigor will shrink; the shape of the fillet then becomes distorted and the surface of the flesh takes on a corrugated appearance. These distortions will remain throughout subsequent freezing and thawing.
When a whole fish goes into rigor, the muscle tries to contract but is prevented from doing so by being anchored to the rigid skeleton, thus setting up the stresses that lead to gaping but, as soon as the fillet is cut off, the restraint of the skeleton is removed and the fillet shrinks.
The extent of the shrinkage depends on the condition of the fish and on the temperature at which it is kept. When a fillet is cut from a well-fed, pre-rigor fish and then kept at a high temperature before freezing, it may shrink by as much as 30-40 per cent of its original length; on the other hand, a fillet taken from a pre-rigor fish in poor condition and then frozen at once will hardly shrink at all.
Since we have little control over the condition of the fish, except by avoiding fishing grounds where fish in poor condition are likely to be found at certain seasons, 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 then the fillets must be kept chilled to reduce shrinkage, but even at 32°F some of the fillets will shrink after a time. Immediate freezing is the only safe way to avoid shrinkage. Pre-rigor fillets should not be chilled by means of fresh water or freshwater ice; shrinkage is increased by contact with fresh water.
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 fibres. Pre-rigor fillets are unsuitable for smoking because the rough, dull surface does not take on a good gloss during the process.
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, but nevertheless there are some disadvantages. Mechanical filleting is often difficult when fish are in rigor, and even hand filleting may give a slightly lower yield from fish in rigor compared with fish that are soft and flexible. In addition, gaping may be caused by forcibly straightening bent fish before cutting them, and chilled buffer storage space has to be provided to keep the whole fish until they go into rigor.
Frozen fillets taken from post-rigor whole fish are normally of uniformly good quality, provided the whole fish has been properly handled and kept chilled; the main disadvantage is the long time in buffer storage, up to three days, which makes extra demands on space and labour.
Rigor affects the toughness of, and drip loss from, frozen fillets in the same way as with whole fish; the warmer the fish when it goes into rigor, the greater will be the drip loss and the tougher will be the cooked fillet. Just as with frozen whole fish, pre-rigor frozen fillets will lose more drip than comparable fillets frozen in rigor or after rigor.
The safest and most reliable way of avoiding the undesirable effects of rigor is to keep the fish chilled at every stage before freezing. Provided the fish pass through rigor at a low temperature, the effect of rigor on quality will not be serious.
Having said that, it is necessary to mention the possibility of accelerating the rigor process by raising the temperature of the fish under carefully controlled conditions. It is possible by warming a fish to shorten the time it takes to go into, and pass through, rigor; the space required for buffer storage can thus be reduced, but only at the expense of some loss of quality due to the higher temperature, and the possibility of increased gaping. As explained earlier, the maximum temperature for accelerating rigor in cod is 63°F if irreparable damage is to be avoided. On the whole it is probably safer in commercial practice at sea not to attempt acceleration of rigor, but to keep the fish chilled until they enter the freezer.
When muscle is frozen pre-rigor and kept for a short time in cold storage, it is still able to contract and go into rigor after thawing. This is known as thaw rigor and, when the thawing is done rapidly at a high temperature, the muscle can then suffer from the defects associated with high temperature rigor.
Thaw rigor is rarely a problem in thawed whole fish because freezing and cold storage have usually sapped the energy reserves sufficiently to weaken the contractions in the muscle; the skeleton restrains the muscle but the stresses are insufficient to break the connective tissue. If damage is seen at all in thawed whole fish, it usually occurs near the tail, where thawing is most rapid.
However, when pre-rigor fillets are thawed, the muscle is free to shrink as soon as the ice within the flesh has melted, and the fillets become shrunken and corrugated and lose a large amount of drip. The effects are most severe when the pre-rigor muscle is cooked from the frozen state, as, 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 the fish finger 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 cause the batter to spatter. Thaw rigor is not of course 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 is uncommon in commercial practice but, when it is met with, the ill effects can be avoided fairly simply. The simplest way is to extend the cold storage time of the stock of pre-rigor fish. Provided they are kept for at least eight weeks at minus 20° F, the flesh has time to pass through rigor in the frozen state; this has no bad effect on the quality of either whole fish or fillets, since they are both held rigidly enough while frozen to prevent the muscle from contracting.
If the fish have to be taken out of store in less than eight weeks, they should be thawed slowly at room temperature; in this way rigor is completed while the fish are in a semi-frozen state, thus preventing severe contraction of the muscle.
Fillets from fish frozen whole before rigor should yield a smoked product of excellent quality provided thaw rigor effects are avoided. Fillets from whole fish frozen during or after rigor should also yield a good smoked product, provided gaping has been avoided.
Frozen fillets are not normally smoked, since they do not make such a good smoked product as fillets from thawed whole fish; pre-rigor fillets are particularly difficult to smoke satisfactorily, because they fail to take a good gloss due to the rough texture of the cut surface. Pre-rigor fillets from very fresh inshore fish should not be smoked, since treatment in the kiln can cause high temperature rigor effects, resulting in very shrunken smoked fillets.
There is no one simple answer to this question, since there are arguments for and against freezing at any of these stages. None of the three stages of rigor is clearcut; the process is a gradual one, beginning from the moment the fish dies, and the effects of rigor are therefore very much a matter of degree.
On the freezer trawler the answer really is to have a processing system that is sufficiently flexible to handle properly fish in any stage of the rigor process, and a reliable labelling scheme that enables shore factories to identify fish frozen pre-rigor, in rigor or after rigor, so that the raw material can be handled appropriately.
The following table lists the advantages and disadvantages of freezing whole fish or fillets in all three conditions of rigor:
Frozen whole fish
|
|
advantages |
disadvantages |
|
frozen pre-rigor |
buffer store not required |
thaw rigor gaping possible |
|
frozen in rigor |
uniformly good quality obtainable generally |
buffer store required |
|
frozen post-rigor |
uniformly good quality obtainable generally |
buffer store required |
Frozen fillets
|
|
advantages |
disadvantages |
|
frozen pre-rigor |
buffer store not required |
large processing capacity required to deal with high catching
rates |
|
frozen in rigor |
excellent quality possible |
buffer store required |
|
frozen post-rigor |
uniformly high quality |
large buffer store required for up to 3 days |
Rigor changes occurring in fish before it is frozen may affect the quality in three main ways:
1. toughness and high drip loss in frozen whole fish or fillets;These undesirable effects can be reduced or prevented by:
2. gaping in fillets taken from frozen whole fish; and
3. shrinkage of frozen fillets.
1. keeping the fish cool, particularly before it goes into rigor;Careful treatment of the fish before and during rigor will result in a higher quality frozen product with a correspondingly better market value.
2. handling it carefully when in rigor; and
3. freezing fillets taken from pre-rigor fish as soon as they are cut.
