GB2384969A

GB2384969A - Fish Exsanguination

Info

Publication number: GB2384969A
Application number: GB0203215A
Authority: GB
Inventors: Steven Charles Kestin; David Henry Francis Robb
Original assignee: University of Bristol
Current assignee: University of Bristol
Priority date: 2002-02-12
Filing date: 2002-02-12
Publication date: 2003-08-13
Also published as: GB0203215D0

Abstract

A method of exsanguinating a fish comprises the steps of orienting a fish having a cut therein such that the cut is in the lower half of the fish and allowing bleeding of the fish in the oriented position for a time sufficient to achieve effective blood drainage. Preferably, the cut is at the lowest point of the orientated fish and in a major vessel in the anterior end region of the fish e.g. the gill arches. The fish may be orientated by hanging and the bleeding may be carried out for at least two hours. Preferably the fish is stunned before orientating and cooled in the oriented position. This method is particularly suited to a salmon to reduce the number of blood spots in slices of smoked salmon.

Description

FISH EXSANGUINATION This invention relates to a method of exsanguinating a fish, particularly, although not exclusively, a salmon or trout, with particular emphasis on reducing the number of blood spots present in the fish.

A major cause of downgrading in the production of smoked salmon is the presence of small haemorrhages or spots of residual blood in the flesh. These often cause rejection of the product by consumers as they appear as unsightly dark brown blemishes in the translucent flesh. These blemishes are known in the industry as"blood spots"and are one of the most important single causes of economic loss in smoked salmon production.

Despite the volume of production, most smoked salmon is still produced traditionally. Briefly, salmon are killed and exsanguinated, chilled to less than 40C and gutted before the onset of rigor ("Operating manual for the product certification schemes for Scottish quality farmed salmon and smoked Scottish quality salmon", Scottish Quality Salmon Ltd. , Inverness, Scotland). The gutted fish are stored on ice until rigor has resolved, and then filleted, leaving the skin on the fillet. Fillets are covered in a layer of dry salt for 12 to 16 hours, rinsed, dried and then smoked for 12 to 24 hours in the fumes from smouldering hardwood. After smoking and a few days maturation, the outer surface (pellicle) of the smoked fillets is removed, the pin bones extracted and the flesh sliced very thinly and vacuum packed. The industry claim that blood spots are present on the surface of fillets before slicing but that the more costly blood spots are those which are found within the muscle mass during slicing. They further indicate that it is not possible to detect fish with a high prevalence of blood spots until after smoking. This makes blood spotting particularly costly as not only is the value of fish lost, but also the value of all the processing.

Several types of blood spot or haemorrhage can be recognised, but the most prevalent are those associated with the blood vessels along the mid-line and belly flaps (Figure 1). It is widely believed that these blood spots are

caused by residual blood left in blood vessels after exsanguination and that effective exsanguination reduces their prevalence.

There is therefore a need to reduce the number of blood spots which are found in fish.

Thus, according to the present invention, there is provided a method of exsanguinating a fish comprising the steps of: (a) orienting a fish having a cut therein such that the cut is in the lower half of the fish when it is oriented; and (b) allowing exsanguination of the fish in the oriented position for a time sufficient to achieve effective blood drainage.

The fish can be cut before, after, or even during the step of orienting. It is preferable that the steps of cutting and orienting are carried out reasonably closely together.

Preferably, the cut is at substantially the lowest point of the fish when in the oriented position, although in one embodiment the head of the fish would prevent it from being the lowest point. In any case, the cut should preferably be in the region of the fish between its head and tail. The cut is preferably made in any major vessel, and is preferably made at the anterior end of the fish. For example, the cut could be made in the heart, isthmus, throat or gill or as a result of beheading. In the embodiment in which the cut is made in a gill of the fish, any number of gills may be cut, although 8 is preferred. In general, the more vessels that are cut, the more effective is the exsanguination, and the better is the reduction in the number of blood spots found.

Preferably, the fish is oriented by hanging, for example from its tail such that the gills or other cut vessel are at the lower end region of the fish. Any orienting method could be used and other examples are supporting the fish in a rack or the like, or restraining a fish for example in a tube or the like.

The fish may be oriented at any angle which encourages exsanguination, although angles of greater than 300 to the horizontal, or more preferably greater than 450, are preferred. In a particularly preferred embodiment, the fish is held substantially vertically with its anterior end lowest.

It has been found that the heart of the fish may or may not continue to function during the early stages of exsanguination, and the invention has been found to be effective in either case.

In order to achieve effective exsanguination, it is generally required that the fish is exsanguinated for at least two hours and preferably for about 24 hours.

In a preferred embodiment, the fish is stunned prior to step (a). This is for animal welfare reasons. The fish may then be treated in other ways known in the art, for example gutting, filleting, storing on ice (or otherwise cooled), washing, salting and smoking etc. Exsanguination is generally needed before filleting, salting and/or smoking. Gutting and washing can be carried out before or after the orienting step. In one embodiment, a commercial operation may include the steps of stunning, gill cutting, transporting, gutting and orienting the fish in that order. Preferably the fish is oriented soon after killing or stunning. For example, the fish could be oriented shortly after killing, then transported, washed and gutted and filleted, washed again if required and then oriented again. Storing on ice, or other cooling, can be carried out in any convenient manner and at any suitable time during the process. For example the fish could be cooled, preferably stored on ice, in the oriented position, advantageously using the ice to support the fish on its nose or other part of the anterior end of the fish.

Preferably, the method of the present invention is applied to salmon, although it could be applied to any other fish, one example being trout.

The invention will now be described, by way of example, with reference to the following drawings, and in which: Figure 1 shows the presence of blood spots in a slice of smoked salmon ; Figure 2 is a graph showing the presence of blood spots when killed fish are laid on their side and turned at various times; and Figure 3 is a chart showing the presence of blood spots when killed fish are oriented in a specific manner.

Figure 1 is discussed above and shows the appearance of blood spots 1 which may be observed in a slice of smoked salmon 2, and which are typically

associated with the blood vessels along the mid-line 3. Similar blood spots or flecks could be observed in slices of salmon taken from the belly flap (not shown).

Example 1 (Comparative) From a pen of slaughter weight Atlantic salmon (Salmo salar) 80 fish were selected at random, netted out by hand and killed by the current industry recommended method (a percussive blow to the head followed by exsanguination by cutting the four gill arches on the left side of the head). All fish were then tagged and carefully laid on ice in boxes on their left side and covered with further ice. The boxes of fish were transferred to a cool room and after 6 hours of storage on ice, all fish were eviscerated and then replaced on ice.

At intervals shown in Table 1 ten fish, chosen at random, were turned from their left side down and placed with their right side down (see Figure 2).

Some groups of fish were turned prior to gutting and others were turned after gutting. However in each case, after gutting, care was taken to ensure that fish were placed with the correct side down. All fish were filleted 72 hours after slaughter and the fillets individually tagged and laid skin side down in trays. Blood spots in each fillet were then visualised and counted. In this respect, fillets were salted for 16 hours (having been laid skin down on a rack and covered in an even layer of dried salt 3mm thick and stored at 4OC), after which the fillets were washed, dried and sliced once 5mm above the skin.

The number of blood spots in the three regions: midline, belly flap and other were counted on the surface of each fillet and on the slice.

From the raw data, the total number of spots on each fillet and each fish were calculated. The total number of spots per fish in each group was then compared using analysis of variance (NOVA). The number of spots on the left fillet in each group was then compared using ANOVA and similarly the number of spots on the right fillet. Finally, within each group, the number of spots on the left and right fillets was compared using a paired Student's t-test.

To attempt to identify the dynamics of blood in the fish post mortem in more detail, the blood spot counts in the midline and the belly flaps of the left fillet slice, the left fillet surface, the right fillet surface and the right fillet slice was compared by ANOVA for each treatment.

There was no difference in the total number of spots counted in any group (Table 1). Fish laid on the left side for the duration of the study (group 6) had substantially and significantly more blood spots on the left side than on the right side (that was uppermost for the whole study) (mean = 53 and 7, respectively, p < 0.001). Similarly, fish which were turned immediately at slaughter (group 1) had substantially and significantly more blood spots on the right side than on the left side (that was uppermost for the whole study) (mean = 67 and 10 respectively, p < 0.001). Figure 2 illustrates the distribution of blood spots in all groups. Summarised, it illustrates that the pattern of blood spotting changed progressively with time of turning. In fish turned relatively soon after slaughter, most blood appeared to transfer from the left side (originally down but uppermost after turning) to the right side. Thus groups 2 and 3, that were turned after 6 and 12 hours, had most blood spots in the right side (47 and 38 respectively). However, during the course of the study, the capacity for blood to redistribute after turning was progressively limited so that in groups 4 and 5, that were turned after 24 and 48 hours, most spots were found in the left side (41 and 58 respectively).

Midline spots changed with time turned. However, the greatest number of spots was always close to the skin. This implied that the blood drained within the flesh during salting, when the fillets were stored skin side down. This was different to the pattern observed in the belly flaps, where the number of spots counted on the surface and after skinning was not different.

After all active and passive blood loss had ceased; a volume of blood remained in vessels within the musculature. This remaining blood tended to pool in the lowest vessels within the fish. If the dead fish were reoriented, residual blood drained to pool in the lowest vessels again. The ability of blood to redistribute decreased progressively from slaughter until 72 hours post mortem. These studies clearly indicate that gravity plays a major role in

determining the location of residual blood and thus where blood spots are likely to be located when the fish are eventually processed.

That the fish turned later during the storage period showed less redistribution in their pattern of blood spotting indicates that either: a) blood remains fluid for up to 72 hours but drainage within fillets is a slow and progressive process, or b) the blood clots progressively, thus progressively limiting the volume available for redistribution.

Table 1: Fillet blood spot counts in fish laid first on their left side and turned onto their right side.

Blood Spot Count

Group Time fish turned Left Fillet Right Fillet Total n on to right side Mean s. e Mean s. e Mean s. e 1 0 hours 10 3.6 57 9.6 67 9.8 10 2 6 hours 19 3.3 47 7.2 65 9.3 8 3 12 hours 22 6.3 38 8.8 60 13.7 10 4 24 hours 41 10.9 29 6.1 70 16.6 10 5 48 hours 58 4.8 15 4.9 73 9.1 8 6 72 hours 53 3.4 7 1.1 60 3.8 10 Example 2 Figure 3 shows the total number of blood spots following a variety of treatments. In each case, the salmon was killed by the standard industry method described in Example 1 above and was then treated by a variety of exsanguination methods as follows : (a) the viscera were left in, and the salmon oriented head downward at an angle of 450 ; (b) the viscera were removed, and the salmon was oriented head downward at an angle of 450 ;

(c) the viscera were removed, and the salmon was placed on its back; (d) the viscera were left in, and the salmon was oriented substantially vertically by hanging with its head downward by a tie around its tail ; and (e) the viscera were left in, and the salmon was placed with its left side down.

The total number of blood spots was counted by a salting method similar to that described in Example 1. Salting is carried out for 24 hours and is followed by rinsing and drying. Blood spots can be counted immediately or any time, for example during the next 2 or 3 days, providing the fillets are kept cool so they do not go off.

It can be seen that orienting the fish in the manner of treatments (a) and (b) reduces the number of blood spots observed relative to treatments (c) and (e), but the number of blood spots is significantly reduced by hanging vertically, as in method (d) which is the preferred method. The reduction in the number of blood spots according to the invention relates to the physiology and/or anatomy of the blood vessels in relation to the angle of drainage, the effect of gravity on residual blood, and also the phenomenon of rigormortis which can squeeze some of the blood out.

Claims

CLAIMS 1. A method of exsanguinating a fish comprising the steps of: (a) orienting a fish having a cut therein such that the cut is in the lower half of the fish when it is oriented; and (b) allowing exsanguination of the fish in the oriented position for a time sufficient to achieve effective blood drainage.
2. A method according to claim 1, wherein the cut is at substantially the lowest point of the fish in the oriented position.
3. A method according to claim 1 or 2, wherein the cut is in the anterior end region of the fish.
4. A method according to any preceding claim, wherein the cut is in a major vessel of the fish.
5. A method according to any preceding claim, wherein the cut is in a gill of the fish.
6. A method according to claim 5, wherein eight gill arches are cut.
7. A method according to any preceding claim, wherein the fish is oriented by hanging.
8. A method according to any preceding claim, wherein the fish is oriented at an angle greater than 300 to the horizontal.
9. A method according to any preceding claim, wherein the fish is oriented substantially vertically.

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10. A method according to any preceding claim wherein exsanguination is carried out for at least two hours.
11. A method according to any preceding claim, wherein the fish is stunned prior to step (a).
12. A method according to any preceding claim, wherein the fish is cooled in the oriented position.
13. A method according to any preceding claim, wherein the fish is a salmon.