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From: Handbook of Salmon Farming, (editors S. M. Stead & L. Laird). Springer-Praxis, Chichester. Pp 187-

202.

CHAPTER 5

Post-harvest handling and processing

5.1 INTRODUCTION

Farming of Atlantic salmon can now be considered as a developed system of food production.Considerable research and development by the industry itself, and by associated industrieslike feed manufacturers and the pharmaceutical industry, has resulted in an efficient industrythat can produce fish at costs less than many species of fish caught from the wild.Aquaculturists, and the industries that support aquaculture quite naturally, andunderstandingly, put emphasis on the economic production of salmon, but it must beremembered that production of fish is not an end in itself. The product is intended as food andthe aquaculture industry will be successful as long as consumers find the product acceptablein terms of satisfaction given as a food, and on price. Consumers in economically advancedcountries are presented with a wide variety of foods, and have the economic means to beselective in what they buy, and salmon competes with other foods and not just other fishspecies. Customers for farmed salmon appear to like what they are buying and over the lastdecade or so Atlantic salmon production, on a world basis, has increased by around 14%,compound, a year. This is a remarkably high rate of sustained growth for a food commodity.

Good quality begins at the farm and salmon farmers are well aware of the importance of goodquality in marketing their products. A variety of quality assurance schemes have beendeveloped by the industries in countries with significant productions of farmed salmon bothfor the raw material and for processed products like smoked salmon. This chapter discussesthe harvesting and subsequent handling, processing, distribution and marketing of the salmon,and particularly of the interaction between husbandry practices and these processes.

5.2 SLAUGHTERING AND IMMEDIATE POST MORTEM HANDLING

5.2.1 Slaughtering procedures

The methods most commonly used for slaughtering fish in commercial salmon farming are

2percussive stunning and carbon dioxide (CO ) anaesthesia, though other procedures such asanaesthesia by drugs, electrostunning, chilling in ice water, spiking the brain, and taking thefish out of water to suffocate are available. These procedures might not render the fishimmediately physiologically dead, but render them unconscious, and thus easy to handle.Considerations of humaneness in the slaughtering process require that the procedure becarried out swiftly with minimum stress to the animal, and that the fish be rendered deeplyunconscious to the extent that the fish cannot recover consciousness (Farm Animal Welfare

2Council, 1996). The first two procedures listed above, percussive stunning and COanaesthesia, at least meet these requirements when carried out under good practices.

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In percussive stunning, the culled fish, which have previously not been fed for up to 10 daysto ensure that the gut is empty, are hit on the head with a small club, referred to as a 'priest' inScotland, just behind the eyes. This operation requires some degree of skill and dexterity tocarry out cleanly on a struggling fish; a misdirected blow can fail to immobilise the fish, candamage the head of the fish detracting from its appearance, or bruise the fillet behind thehead. The stunning is usually carried out at the cages and the fish are brought ashore in tanks

2of water. CO anaesthesia can be carried out at the cages, but it is more usual to bring the live

2fish ashore The fish are transferred to tanks containing water saturated with CO gas andusually the gas is also vented into the tanks. The salmon become quiescent and unconsciouswithin a few minutes when they can be removed from the gassing tank.

It is common practice after stunning to cut the gill arches on one or both sides and to drop thefish into a tank of water to bleed. Preferably the water in the tank should be cooled by

2refrigeration or with ice. Though the salmon after correct stunning or CO anaesthesia areirrecoverably unconscious, the heart continues beating for a time and blood drains efficientlyfrom the flesh. Exsanguination avoids blood spotting of the fillet and is important for thequality of products sold as fillets or smoked fillets. A disadvantage of electrostunning is thatit causes blood to lodge in the vessels, and sometimes causes bruising at the backbone.

5.2.2 Handling after slaughtering

Salmon intended to be distributed as gutted fish should be eviscerated as soon a possible afterdeath, and before they have entered rigor mortis (see discussion below). The belly cavityshould be opened by a cut from between the pectoral fins to the vent and the viscera removedcleanly. The swim bladder and underlying kidney tissues should be scraped and brushed out,and the belly cavity thoroughly washed.

The fish, gutted or not, should be iced as soon as possible. Before packing, the salmon shouldbe graded for size and for quality according to the requirements of the intended markets.Sometimes the producer will grade to standards drawn up by a grower's organisation orsimilar trade organisation, sometimes to standards set by the buyer. Appearance will figureprominently in the specifications when salmon are to be displayed whole or gutted, which isoften the case for retail markets. The fish must have a good conformation, a regularly-shapedhead, clean, undamaged skin, and not show signs of maturity. Fish with misshapen heads, orwith slight skin defects might be suitable for fillets products.

5.3 IMMEDIATE POST MORTEM CHANGES, AND EFFECTS OFSLAUGHTERING PROCEDURES ON THEM

The most obvious immediate post mortem phenomenon observed in the salmon is the onsetand resolution of rigor mortis. The stiffening starts a few hours after death and increases inintensity to a maximum rigidity about 12-24 hours after death, followed by a resolution of therigor taking a further one or two days. The biochemical events leading to rigor mortis and itsresolution are complex and not fully elucidated. An important factor in the timing of onset ofrigor, and its intensity, is the glycogen content of the muscle at death. Glycogen is the energyreserve in the muscle and is depleted during any struggling before death. Fish which arerested and unstressed before death will have high glycogen contents in the muscle and the

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time into rigor mortis, and its intensity, will be long, and strong, compared with fishexhausted and stressed before death.

It is known from experience with catching fish in the wild that the capture method - trawling,seine netting, trolling, fish traps - and nutritional condition of the fish at the time of catchinginfluence the glycogen content of muscle and hence the progress and intensity of rigor. It hasbeen reported that stress associated with slaughter also affects the rigor process in farmed fishand, apart from any considerations of humane slaughter, fish should be slaughtered with theminimum of stress to give the best fillet quality. Most of the work on the effects ofslaughtering method on rigor mortis, and associated biochemical changes, and particularly onthe spiking procedure (iki jima), has been carried out in Japan and many of the papers are inJapanese. None of the reported experimental work on the iki jima procedure has involvedsalmon, (though various laboratories have experimented with iki jima for salmon, the resultshave not yet appeared in the open press), but there is no reason to suppose that the generalfindings will not apply to this species. There are two variations of the spiking procedure. Thesimplest is just to stab through the skull between the eyes into the brain using a small knife orspike, or mechanically using a builders nailing gun (Boyd et al, 1984). The other is to stab thebrain as described then pass a thin wire into the neural canal of the spinal column to destroythe spinal cord (Nakayama et al, 1996, 1997). Nakayama et al (1996) using red seabreamshowed that destruction of the spinal cord delayed full rigor tension until about 24 hours afterdeath compared with 10 hours in the fish spiked without spinal cord destruction.

Stress in the context of depletion of muscle glycogen can be related to the amount of activityjust prior to death. Spiking and neural cord destruction results in complete relaxation of themuscle and maintenance of high glycogen contents; the opposite extreme of considerabledepletion of glycogen is given by removal of fish from the water and allowing them tostruggle and suffocate to death without any prior stunning or anaethetisation. Highly stressedfish go into rigor very quickly, and the duration of rigor is short compared with procedures inwhich fish die in a relatively unstressed condition (Berg et al, 1997). Of course, any methodof slaughtering will involve some stress and it is a matter of degree. A fish which suffocatesto death out of water will struggle considerably; one which is stunned will struggle less so.Reflex muscular activity might not cease entirely with stunning; the muscle can twitch andthe fish might show swimming movements in the bleeding tank. Destruction of the spinalcord after spiking inhibits even this reflex muscle activity (Nakayama et al, 1996, 1997) andresults in the least exercise stress of any of the slaughtering procedures referred to here.

There is little or no advantage in having long times into and through rigor for most systems ofhandling, processing and distribution of salmon after harvesting. Salmon should not behandled while in rigor, and the 2-3 days it usually takes for complete resolution of rigor cansometimes be a nuisance. Times of delivery of salmon from farms to processing plants areoften short in many countries and usually less than that required for resolution of rigor.Consequently the salmon might have to be held back in a chill store for a day or two before itcan be processed. However, if the salmon is to be sold entire to the customer, for example, acatering outlet, the fact that the fish is still in rigor would be an indication that the fish wasvery fresh. For some specialist outlets, for example, Japanese style sashimi restaurants, theproduct should be prepared from pre-rigor fish, and spiking followed by spinal corddestruction would be the preferred slaughtering procedure. Other than the extremes of the iki

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jima process and death by suffocation, the length of time into, and through rigor, does notdiffer much among slaughtering procedures, and probably does not need to be taken intoaccount when selecting the most convenient procedure.

Any gutting any packing of the salmon should be carried out before the fish goes into rigor ashandling salmon in rigor can result in damage to the fillet. Salmon should not be filleted untilthe rigor process is fully resolved. A fillet removed before rigor is fully resolved will have arough, sandpaper-like, appearance, and will not take on a good gloss if it is smoked. Thecooked product will have a tough, rubbery texture. The times for onset and resolution of rigorreferred to above are for salmon stored in ice and times are shorter at ambient temperaturesaround 20 C, but warming fish up to hasten the rigor process is inadvisable as the rigoro

process at high temperatures can cause vigorous contraction in the muscle that can result insevere gaping, (breaking of the tissue at the junction of the blocks of muscle), in the fillet.

5.4 EFFECTS OF HUSBANDRY PRACTICES, INCLUDING SLAUGHTERING,ON COMPOSITION AND QUALITY OF SALMON

5.4.1 Comparison of farmed and wild fish

Food writers will often claim that the culinary properties of farmed salmon are inferior tothose of wild. There does not seem to be any basis for this proposition based on consumerstudies, and it must remain an opinion of the particular writer. According to FAO fisherystatistics, only about 1% of Atlantic salmon is produced from the wild and the great majorityof consumers of this species would not have had the opportunity to make any comparisonbetween farmed and wid salmon. My own experience of evaluating both wild and farmedsalmon is that there is no systematic difference in eating quality of the two varieties. Schallichand Gormley (1996) reported that a panel of 28 persons showed no significant preference foreither wild or farmed steamed Atlantic salmon caught/cultured in Ireland in a preference test.Sylvia et al (1995) compared wild and farmed chinook salmon in a consumer study, (189consumers), and found only a small preference for the wild variety.

5.4.2 Fat content

There is one aspect of the composition of farmed salmon for which the farmed product can bemarkedly different from wild salmon and that is in the fat content. Surveys of fat content offarmed salmon grown in Scotland, Ireland and Norway gave average values of 10.1%(n=495), 11.9% (n=587) and 15.0% (n=145) respectively (Bell et al, 1998; Refsgaard et al,1998; Schallich and Gormley, 1996). (Numbers in brackets are the numbers of fish sampled).The fat content of wild Atlantic salmon varies with biological factors like season andmaturation, but wild salmon caught in Scotland have fat contents around 8%. Schallich andGormley (1996) reported an average fat content of 12.3% in 295 wild salmon marketed inIreland. Fat content affects the quality of both unprocessed and processed products. Salmonwith very high fat contents will exude oil from cut surfaces or when minced. High fatcontents give rise to a soft texture to the feel of the raw fillets, and to soft texture on thepalate and an oily mouthfeel in the cooked product. The fat content affects the uptake of saltin preparation of smoked products - the higher the fat content, the slower the uptake - and theeating quality of the finished product. Smoked salmon prepared from high fat content fish,

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more than about 10% fat in the raw fillet, will have a soft texture, an oily mouthfeel, and willweep oil into the package in the case of packaged products. A typical process specification forprocessors will ask for an fat content of 8-12% in the flesh.

Several factors influence the fat content of farmed salmon. One is the oil content of the diet.On average, in Scottish farms, a feed of 15% oil content gives fish with about 7% fat contentin the flesh, one of 30% gives about 11% fat content. There is an annual cyclic effect; fatcontent is highest in winter, lowest in late summer. The range, on average over the year, isabout 4 percentage units. Salmon tend to get more fatty as they get larger; the range, onaverage, between salmon of 2.5 and 4 kg, is about 4 percentage units (Bell et al, 1998). Amore detailed account of the effects of ration on fat content of salmon is given elsewhere inthis book.

Apart from the high values of average fat content, the variation in fat content among salmonis large. In the three studies on fat contents referred to earlier the standard deviation of fatcontents over all of the salmon tested approached 3 percentage units. This means that over allthe fish analysed within a survey, 95% of the values lay between ± 6 percentage units of theaverage value, that is, for example, between 4% and 16% for 95% of individuals in the surveyof Scottish salmon referred to in Bell et al (1998). This variation includes various factors thatcan affect fat content including variations between farms; the variation within a particularbatch from one farm, within a restricted size range, and harvested together will of course besmaller, around ±3 percentage units of the batch mean for 95% of the fish. A processor canexpect the fat contents of individual fish within a batch of farmed salmon from a singlesource with an average fat content of 10% to range from 7% to 13%.

A processor for whom fat content in the processed product is important should bear thesefactors in mind when sourcing supplies. For example, a salmon smoker should avoid suppliesfrom farms using high oil content feed, particularly large fish harvested in the winter. Asatisfactory raw material is more likely to be obtained from a farm using low oil contentfeeds, but then smaller fish from such a farm harvested in late summer could have fatcontents too low, approaching 5%, for optimum quality in the smoked product. There areinstruments for non-destructively measuring the fat content of salmon with sufficientaccuracy and speed for processors to scan individual fish within a batch and select foroptimum fat content (Kent, 1990; Kent et al, 1996).

5.4.3 Composition of lipids

The nutritional benefits of fish oils are well established, and nutritionalists recommendconsumption of fish or fish oils as a preventative for some degenerative diseases andinflammatory conditions. The protection comes from the polyunsaturated fatty acids,(PUFAs), which are present in higher concentrations in fish oils than in vegetable oils. Manystudies have shown that oils from farmed fish are also high in PUFAs, and to a small extenttheir amounts are affected by diet. The effect of diet on the composition of oils in farmed fishis complicated, but it has long been known that the fatty acid composition of muscle lipidsreflects that of the diet, and a formulated diet containing fish oils will produce oils with highPUFA contents in the farmed fish . Vegetable oils are deficient in the long chain length,polyunsaturated fatty acids, but fish are capable of elongating and desaturating fatty acids,

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and studies have shown that the lipids of fish fed on vegetable diets or on diets containingvegetable oils have high levels of PUFAs, though perhaps not quite as high in those fed dietsin which the oil is derived solely from marine sources (Henderson and Tocher, 1987).

The high content of unsaturated fatty acids in fish oils does not have a direct influence on theprocessing of salmon, but does have a bearing on the stability of the lipid during storage,particularly frozen storage. The unsaturated nature of the lipids makes them prone tooxidation and development of rancid flavours during frozen storage. Vacuum packaging inoxygen-proof film, as is often done for smoked salmon, is effective in protecting againstoxidation, but is not all that feasible a process for other products. A natural chemical presentin fish flesh, vitamin E, is an effective antioxidant. Studies on incorporation of vitamin E inthe form of "-tocopherol into diets of farmed fish have shown that it is taken up in the fleshand provides some protection against oxidation of the lipids in the flesh even when thePUFAs have been enhanced by the diet (Bell et al, 1998).

5.4.4 Other components

The protein content of salmon flesh, once past the juvenile stage, is little influenced by theamount or composition of the diet, or by husbandry factors short of extended starvation(Shearer, 1994). It is common practice to cease feeding salmon a few days prior to culling,but this has no appreciable impact on the protein content of the flesh. The reason forwithdrawing food seems to be to allow the gut to empty to give a cleaner evisceration, and topurge any off flavours induced in the flesh by the feed. The water content of the flesh isinversely related to the oil content such that the sum of the water and oil contents in salmonflesh is approximately constant at 79% (Fig. 1).

Fig. 1. Water/fat relationship of Scottish farmed Atlantic salmon. Author’s unpublished data.

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Hence feeding practices will affect water content of the fillet because they affect oil content.The variation in water content will have some effects on properties of the fillet duringprocessing. The fillet will take up less salt in total in low water content fillets during thebrining process, but this will be a minor factor compared with the effects of oil content on therate of penetration of salt and on the rate of drying in the kiln. The variation in water/oilcontents will also influence physical properties like thermal conductivity, but these effectswill not be of any practical importance.

The colour of farmed salmon is imparted by the carotenoids, astaxanthin and canthaxanthin,in the feeds. The amounts of these pigments fed to the salmon is carefully controlled by thefeed manufactures and by the farmers both as to the amount in the feed and the timing of theiruse in order to achieve the required colour in the fillets. Pigmentation of the flesh is checkedby reference to standard colour cards. A full account of the use of pigments in feeds isprovided elsewhere in this book.

5.4.5 Effects of slaughtering practices on post mortem quality

The effects of slaughtering methods and procedures on the rigor-mortis process has beendiscussed in the earlier section on Immediate post-mortem Changes, and Effects ofSlaughtering Procedures. Once rigor has resolved, neither the quality of post-mortem salmonflesh, nor the behaviour of salmon flesh during subsequent handling, storage and processingare affected by the slaughtering procedures used.

5.5 HANDLING, STORAGE, DISTRIBUTION AND PROCESSING OF CHILLEDSALMON

5.5.1 Loss of freshness and spoilage

Salmon, in common with all fish, lose freshness, and spoil, during storage after harvesting.Temperature has a large effect on spoilage rate - the rate approximately doubling for each 5 Co

rise in temperature of the fish - and it is very important to maintain the fish as near to 0 C aso

possible. The effects of spoilage can be observed in various characteristics of the salmon. Thefirst signs are the onset and resolution of rigor mortis already described. During storage in icethe skin progressively loses its bright appearance, becomes dull, and the sharp distinctionbetween colours is lost. After about 10 days in ice a yellow slime appears that gets thicker asthe fish is further stored. The eyes, originally convex, become flat, then sunken. The fleshsoftens and loses its elasticity so that finger indentations in the flesh are retained rather thanspringing back. The gills at first have a sharp marine smell which fades over the first week orso in ice. After that, unpleasant odours formed by bacterial degradation of the gill tissuesappear, and strengthen on further storage. By about 15-18 days in ice the salmon will lookand smell offensive enough for it to be considered unfit for human consumption, but beforethat point, at about 7-8 days in ice, the appearance of the fish will have lost enough of itsfresh characteristics for it not to be considered a good quality product. Table 1 shows ascoring system for evaluating the freshness of raw iced whole salmon.

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Table 5.1. Sensory scoring sheet for freshness of iced salmon - raw fish

score odour of gills appearance of eyes appearance of gills

10 marine, shellfish, seaweed, sharp

9 freshly cut grass, fresh fruit, floral convex, clear, bright dark red

8 oily, freshly cut grass

7 musty, muddy, mousyflat, clear

red/brown, slightbleaching6 leathery, beery, yeasty

5 sour milk flat/slightly sunken,cloudy

brown, bleached4 sour, stale fruit, stale vegetable

3 sour sink, rancid sunken, cloudy,discoloured

brown, bleached,yellow slime2 ammonia

These are the characteristics of the raw salmon; the eating properties change as well, Table 2.Fresh, post-mortem salmon, after cooking has a slightly sweet, metallic, meaty flavour. Thetexture is firm and succulent, though there can be an oily mouthfeel in high oil contentsalmon, above 10% fat content. The intrinsic fresh flavours are maintained over the first weekor so of storage, though they decrease in intensity to some extent. Indeed, it is difficult evenfor experienced sensory assessors to detect consistent changes in flavour of stored salmon inthis period. Unpleasant flavours are due to the action of bacteria and appear at later stages ofstorage. Fish flesh, unless the fish is diseased, is sterile, and spoilage flavours will appear inthe flesh by diffusion of compounds from the growth of bacteria on the skin, and by the actionof bacteria penetrating from the skin and the belly cavity. In salmon, this will occur afterabout 8-10 days in ice when slight musty and stale oil flavours can be detected. Chemicalchanges in the oil component can contribute to the unpleasant flavours, The spoilage flavoursbecome more unpleasant, and stronger, on further storage and by 15-18 days the cooked fishtastes definitely sour. Salmon fillets and steaks will spoil faster than whole fish, gutted or not,and it is good practice to store salmon in the whole form and leave preparation of productsuntil as late in the supply chain as possible.

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Table 5.2. Sensory scoring sheet for freshness of iced salmon - fish cooked by steaming or inmicrowave. Assessors should interpolate between scores as required by the perceived odourand flavour.

score odour flavour

10 baked, meaty, fresh oil strong meaty, sweet, fresh oil, metallic

8 earthy, musty loss of sweetness and meatiness, slightly musty

6 musty, sour musty, sour

4 sour, stale fruit sour, bitter

2 rancid, sweaty putrid, nauseating

5.5.2 Chilled storage and distribution

Following sorting, and gutting if specified, the salmon should be packed in ice made frompotable water or clean seawater. The fish should be handled carefully to avoid damage to theskin and flesh and should be laid out straight. Allowing the salmon to go though rigor bent ortwisted can give rise to gaping - breaking of the connective tissue between muscle segmentsso that there are gaps between the segments. The salmon should be allowed to go throughrigor mortis without any further disturbance as handling salmon in rigor can also causegaping of the fillet. It is common to stow the salmon in boxes and the boxes should be longenough to take the salmon without having to bend the fish. The salmon can be stowed in bulkin layers of fish and ice, but the layers should not be more than about 50 cm deep withoutsupport to prevent pressure damage to the flesh. The salmon should be mixed with anadequate amount of ice to cool the fish down and to keep the fish chilled during storage anddistribution. About 2.5 kg of ice are required to cool 10 kg of salmon at about 15 C to iceo

temperature, but it is not always easy to estimate the extra amount required to keep the fishchilled during subsequent distribution and storage; the degree of insulation of the transportboxes, the insulation or refrigeration of the carrying vehicle, ambient temperatures, and thelength of the distribution chain are important factors. Experience is a better guide than tryingto estimate these factors, but generally salmon would be iced in a ratio of 3:1 to 2:1 fish: icefor a journey of a few days in insulated or refrigerated vehicles.

Ice can be manufactured in various forms, but as far as cooling capacity is concerned there isno difference between them. Criteria for selection are ability to make good contact with thefish, avoidance of physical damage to the fish, and ease of application. Flake ice meets allthese requirements; its small, flat, plates make good contact with the surface of the fish andeffect rapid cooling, and do not damage the skin of the salmon. Flake ice is often readilyavailable in bulk and can be conveniently made on the premises by small ice-makingmachines. Another form of ice than has some advantages is slush ice. This is prepared fromwater to which some salt has been added and is prepared as a slurry of ice in dilute brine. Thismaterial can be pumped, makes excellent contact with the fish, and does not cause anyphysical damage, apart perhaps for some dulling of the appearance of the skin. When firstprepared, slush ice has a temperature a little below 0 C because the salt lowers the freezingo

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temperature of the mix. This lower temperature slows the rate of spoilage slightly comparedwith storage in normal ice, but this advantage wears off during storage as the ice melts andthe brine drains away.

Typically, salmon are distributed in non-returnable expanded polystyrene boxes custom-madeto hold salmon. The salmon should be iced in the boxes by distributing about a third of theanticipated amount of ice required on the bottom of the box, laying in the salmon, then addingthe rest of the ice. Legislation already in force, or intended, in some countries requirescompanies to recover and recycle waste, though at present most fish processing companiesand merchants are probably below the threshold of turnover at which the regulations apply.Expanded polystyrene is difficult to recycle or recover economically and it is possible therecould be a move towards the use of returnable plastics boxes. Unfortunately the fish boxescurrently used for distributing fish are too short to take whole salmon, other than perhaps forthe smaller size ranges, without bending them, and it might be necessary to use custom-madereturnable boxes for the purpose of transporting salmon. The normal fish box is not insulated,and it would be necessary to use at least insulated, if not refrigerated, vehicles fordistribution.

5.6 FREEZING AND FROZEN STORAGE

Fish spoil rapidly even when chilled and in situations where fish are caught at long distancesfrom ports, or where supplies can fluctuate, fish must be frozen for transport or long termstorage. A major advantage of aquaculture over catching fish from the wild is that the farmedfish can be produced near the markets and be supplied as the market demands. Consequentlythe need for freezing and frozen storage of farmed fish is much less than it is for fish fromwild stocks. Nevertheless, some salmon is frozen, particularly by processors, to even outseasonal variations in supplies from farms and to provide buffer stocks for times of highdemand. In addition, some consumer products are sold as frozen products. Freezing andfrozen storage of fishery products have been practised for decades - indeed, wild salmon wasamong the earliest fishery products to be frozen and stored - and the technologies involvedare well understood.

Good manufacturing practices require that fish be frozen rapidly in equipment designed to doso, and be stored at a low temperature to avoid deterioration in quality during storage. Wholesalmon are typically frozen in air blast freezers in which cold air at around -40 C is blowno

over the fish in a recirculating tunnel system. The salmon are laid out singly on trays or racksin the tunnel, or perhaps hung from rails. The core temperature of the fish, that is at the centreat the thickest part, should be reduced to below -20 C before it is taken out of the freezer foro

transfer to the cold store. A salmon 12.5 cm thick will take about 5 hours to freeze in anefficient air blast freezer (Aitken et al, 1982). Fillets, including closely wrapped fillets likevacuum packs to be described later, can also be frozen in air blast freezers. Products inregularly-shaped packs such as convenience products are typically frozen in plate freezers. Inthese, the packs are frozen between hollow plates through which refrigerant, again at about-40 C, is circulated. The plates are brought together under slight pressure so that the producto

is frozen from two sides. A pack of fillets 4 cm thick in a carton will take about 1½ hours tofreeze in a plate freezer.

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Frozen salmon, in common with other fishery products, are not completely stable duringfrozen storage, and appearance, flavour and texture can change during storage. The rate ofdeterioration depends on several factors, but after a few weeks or months under poor storageconditions, frozen fillets after thawing, and fillets cut from frozen whole fish after thawing,will exude liquid, have a coarse appearance, and will be paler than the original. Oil and fattytissue will take on a bronze appearance and have a rancid odour. The cooked product willhave a tough and stringy texture, and a bitter, rancid flavour. The flavour changes, which areusually the limiting defects determining the storage lives of frozen salmon products, are dueto oxidation of the unsaturated oils. This oxidation can be reduced considerably by exclusionof air from the frozen product by heavy glazing, that is by spraying the frozen salmon withwater or dipping briefly in water after freezing and before storage, or, in the case of smallerproducts by tightly wrapping in plastics film. Salmon frozen for buffer stocks and laterprocessing should be frozen as whole fish rather than as fillets. Storage temperatureconsiderably affects the rate of deterioration of frozen fish, and salmon, with its propensityfor rancidity development, should be stored below -25 C and preferably below -30 C. Undero o

these conditions, assuming protection against oxidation, salmon products should have astorage life for good quality of between 6 and 12 months, and perhaps longer, depending onthe product.

5.7 PROCESSING, AND PACKAGING FOR RETAIL SALE

Typically, salmon is distributed from the farms to processors or wholesale markets as intactor as gutted fish; processing at production sites is uncommon. Merchants at wholesalemarkets pass on the salmon to retailers or catering outlets, and might carry out simpleprocessing operations like gutting if necessary, filleting and steaking. It is important to keepthe salmon chilled during all stages of distribution and marketing in order to maintain quality.Customers at the retail level will expect otherwise whole salmon to be gutted, which might becarried out at the retail premises. Outlets like traditional fishmongers or fish counters atsupermarkets will offer salmon as steaks, a common form for retailing and cooking salmon,or as fillets, often prepared on the premises, but possibly at the wholesale merchant's factory.Fillets might be further portioned as required by the customer into pan-sized pieces of one ortwo servings. This is all traditional fishmongering practice.

Prepackaged salmon and salmon products are now common in supermarkets and multiplestores. There are various ways of packaging for retail sale. The simplest is the overwrap. Forthis, the salmon, ranging from whole fish to steaks and fillet portions, are laid in a shallowtray, typically made of expanded polystyrene, and wrapped in cling-film. The pack is catchweighed and labelled. This packaging can be done in the store or fishmonger's shop and is aconvenient way for the product be picked up and carried by the customer. Another form usesa deep, rigid tray made of clear plastic. The product, usually steak or fillet portion, one ormore pieces, is laid in the tray, usually on an absorbent pad to take up exuded liquor, and atransparent lid is heat sealed on. A label is fixed onto the lid or the whole tray can be insertedinto an outer sleeve which is printed with brand information, and nutritional and culinaryinformation. These types of packs will be prepared in factories and delivered to the retailoutlets.

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An extension of packaging in the rigid tray is Modified Atmosphere Packaging (MAP). Inthis, the air in the pack is replaced by a mixture of gases intended to extend the storage life ofthe product (Davis, 1993). For example, a common mix used for fish products is 30%nitrogen, 40% carbon dioxide and 30% oxygen. Under appropriate conditions carbon dioxidesuppresses growth of spoilage bacteria and extends the storage life of the products. Higherconcentrations of carbon dioxide are more effective in extending storage life, but lead toproblems with the sensory properties of the fish - acidic tastes and a tingling mouthfeel - andwith collapse of the pack as the carbon dioxide dissolves in the fish product. 40% carbondioxide in the mix is a suitable compromise between storage life extension and consumeracceptability. Sometimes a mix of 40% carbon dioxide/60% nitrogen, that is without anyoxygen, is advocated for packaging fatty fish like salmon to reduce potential for rancidity, butthat should not be a problem for salmon over the times the products are likely to be ondisplay. Though MAP in 40% carbon dioxide under appropriate conditions can extendacceptable storage life of fish by 50% or so compared with air packs, this potential is notrealised in practice. The studies that have demonstrated this extension have been carried outunder laboratory conditions with the packs stored at 0 C. The effectiveness of carbon dioxideo

for extending storage life drops off rapidly with increase in storage temperature and isinsignificant above about 5 C. It is difficult to operate chill display cabinets in stores belowo

4 C and consequently MAP under these conditions provides little benefit over packaging ino

air.

There is perhaps little justification for trying to extend the storage life of chilled salmonproducts under the conditions for processing and distributing such products in Britain, andindeed many other countries. Salmon can usually be delivered to processing plants andmerchants within a day or two of slaughter, hence they are very fresh at this point in thesupply chain. Indeed, perhaps too fresh as the salmon could still be in rigor and the processorwould have to wait until rigor is resolved before the fish can be filleted. Simple filleting,steaking and packaging can be accomplished easily within a working shift, and given goodtransport systems, products can be on the supermarket shelves the following day, that iswithin 3 or 4 days after harvesting. The policy of supermarkets, at least in Britain, is not tohold chilled fish products for more than two days so the total time from farm to sale is lessthan 6 days. Given good chilling practices throughout the processing and distribution chainthis is well within the good quality storage life of chilled salmon without resource to anyprocess to extend storage life. Though packaging in MAP might not be of any advantage inretailing chilled salmon products, packaging in rigid containers is an attractive way ofmarketing the product, and a convenient way for the customer to buy and carry the product.

Another common form of packaging for retail sales is vacuum packaging. The product is laidon a coated paper board, inserted into a plastics sleeve, the air evacuated from the pack, andthe sleeve sealed. The pack looks attractive and vacuum packaging is particularly useful forproducts which might have extended storage life with regard to microbiological spoilage, butfor which storage life is determined by lipid oxidation and development of rancid flavours.Smoked salmon is an example, and vacuum packaging of this product is widely used for bothchilled and for frozen storage.

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5.8 PROCESSED PRODUCTS

The best known of processed salmon products is smoked salmon. The salmon is salted beforesmoking and the most commonly used procedure is dry salting. Skin-on fillets are preparedand the skin is slashed, or few small areas of skin are removed to facilitate salt penetrationfrom the skin side. The fillets are laid, skin side downwards, on a layer of fine grained salt 2cm or so deep. More salt is spread over the cut surface. The amount to use is determined fromexperience, more for a thick fillet than a thin one. The salt is graded for amount from head totail of the fillet to even out uptake of salt throughout the fillet, about 1 cm thick at the headend, a sprinkling at the tail end. The fillets are allowed to take up the salt in a cool or chilledroom for 12-18 hours depending on size of fillet (Aitken et al, 1982). High fat content in thefillet slows down uptake of salt and needs to be allowed for. Salty liquor will drain off thefillets which will lose 8-10% of their weight in the process. The salted fillets are washed toremove adhering salt and allowed to drain on racks. The fillets are then smoked for up to12 hours depending on size of the fillet at a temperature of 27 C. This is cold smoking;o

salmon is rarely hot smoked, that is smoked under conditions where the temperature rises toaround 60ºC so the product is cooked. Traditionally, fillets were hung up to smoke in anaturally-ventilated smoke house, but nowadays fillets are smoked on racks in mechanicalsmoking kilns. The fillets will dry during smoking and this is important for imparting thedesired texture to the finished product. Typically there is a further weight loss of about 9% atthis stage. The smoked product is allowed to cool, then inspected and packaged as sides, orsliced and vacuum-packed.

There are variations on this basic procedure. The fillets can be brined in strong salt solutionsrather than being dry salted, and flavourings like molasses, whisky and rum, can be added tothe dry salt or to the brine bath. The smoking time, the smoke density, and weight loss duringsmoking can be varied to produce various degrees of cure. Traditionally, salmon, as was thecase with other fish, was smoked as a means of preservation without refrigeration. Thetendency now is for consumers to want less strongly flavoured and less salty products whichleads to lighter cures being currently made than was the case two or three decades ago. Lesshard cures of course results in shorter storage lives, and the typical current cures require thatsmoked salmon should not be considered as a long-life product that can be held withoutrefrigeration and it should be held under refrigeration below 5 C.o

Smoked salmon is intended to be consumed without any cooking and the product must beprepared under hygienic conditions to ensure it is safe to eat. One hazard is the possibility ofthe growth of Clostridium botulinum organisms and the production of botulinum toxin. It istherefore recommended that smoked salmon be prepared to contain at least 3.5 % salt contentin the water phase and kept below 5 C to inhibit toxin production. If smoked salmon is to beo

held or distributed above this temperature for any appreciable time, for example, delivery bymail order, the cure should aim for 6% salt in the water phase.

Gravlax is a lightly cured salmon product originating from Sweden and popular inScandinavia, though obtainable much more widely than that. It is prepared by sprinkling thesurfaces of a pair of fillets with some salt, sugar and white pepper. The fillets are laidtogether, cut side to cut side, head to tail with a layer of dill between. The pair of fillets is putunder light pressure and allowed to marinade in chill conditions for 1-2 days. The fillets are

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then thinly sliced and served as a delicatessen product. Gravlax does not have a long storagelife, and should be chill-stored and consumed soon after preparation.

A feature of the retail marketing scene in Britain in the last two or three decades, and also inother countries, is the increasing sales of ready prepared meals which are already cooked andjust need reheating, or require no more than cooking in an oven or in a microwave. They canbe presented as just a single component of a meal or as a part of a complete meal. Farmedsalmon is well represented among the convenience fish products.

5.9 OPPORTUNITIES FOR MARKETING FARMED SALMON PRODUCTS

Farmed salmon has advantages over species from capture fisheries in that it is availablethroughout the year in amounts that can be tailored to production schedules, and in specifiedsizes. It can be assured of being very fresh, and meeting trade specifications for quality.Quality assurance, consumer satisfaction, and regularity of supply, which are so important formultiple retailers, are much simpler to arrange in the case of farmed salmon than it is for fishcaught in the wild. Salmon has an attractive appearance and a luxury image. The increasedinterest in cooking, fostered by television programmes and magazines, is stimulating aninterest in the use of less familiar species of fish other than the standard cod and haddock, andsalmon often features in the recipes. Convenience fish dishes incorporating traditional whitefish are often prepared from portions sawn from frozen blocks, (usually incorrectly referred toas 'steaks'), which gives the product a factory-made appearance, whereas the conveniencesalmon products are prepared from natural fillet portions or from natural steaks.Supermarkets report an increasing support by the consumer for more natural, and lessprocessed, foods and farmed salmon products meet this demand. Some salmon farms canoffer organically produced salmon.

Forecasts of fish supplies into the next decade or so show no, or at best, a negligible, increasein supplies from wild stocks, but an increasing demand for fish (FAO, 1997, 1999). The level,or decreasing, supplies of fish from the wild accompanied by an increasing demand mustinevitably result in increasing prices for the products. Indeed over the last decade or so, retailprices of traditional fish products in economically advanced countries at least, have increasedfaster than prices of food in general. At present, early 2000, farmed salmon is generally,though not invariably, slightly more expensive than traditional white fish species like cod,haddock and some species of hake, though the gap is closing, but is distinctively lessexpensive than many other species of prime fish. A higher yield of edible flesh is obtainedfrom salmon compared to fish like cod - 60% of skinless fillets compared with 45% for cod.

It appears then that there are various factors related to both supplies and markets which arefavourable to the marketing of salmon products. At present, farmed salmon is processed andmarketed as wild fish are. As the problems of producing salmon are solved, the industryperhaps should give some attention to matters related to post-harvest handling and themarketing of the product; there is scope for development of new products and new marketoutlets. The studies on the incorporation of "-tocopherol in the diet of salmon to stabilisecolour and lipids during frozen storage is an example of the interaction between husbandrypractices and post-harvest properties that can enhance the acceptability of the product. Abilityto control fat content of the harvested fish within specified limits would be of benefit to

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processors. Selective breeding to produce fish more attractive to the consumer than wildcounterparts could be considered. An example here, is breeding to reduce the size andnumbers of pin bones, which is known from consumer surveys to be a strong negativeinfluence on purchases of fish.

REFERENCES

Aitken, A., Mackie, I.M., Merritt, J.H. and Windsor, M.L. (1982). Fish Handling &Processing, 2nd edn., HMSO, Edinburgh, Scotland.

Bell, J., McEvoy, J., Webster, J.L., McGhee, F., Millar, R.M. and Sargent, J.R. (1998). Fleshlipid and carotenoid composition of Scottish farmed Atlantic salmon (Salmo salar). Journalof Agricultural and Food Chemistry, 46, 119-127

Berg, T., Erikson, U. and Nordtvedt, T.S. (1997). Rigor mortis assessment of Atlantic salmon(Salmo salar) and effects of stress Journal of Food Science, 62, 439-446

Boyd, N.S., Wilson, N.D., Jerrett, A.R. and Hall, B.I (1984). Effects of brain destruction onpost harvest muscle metabolism in the fish kahawai (Arripis trutta). Journal of Food Science,49, 177-179.

Davis, H.K. (1993). Modified atmosphere packaging of fish. In: Principles and Applicationsof Modified Atmosphere Packaging of Foods, R. T. Parry, ed, Blackie Academic andProfessional, London, pp 189-228.

Farm Animal Welfare Council (1996). Report on the welfare of farmed fish.FAWC, Government Buildings, Tolworth, Surbiton, Surrey KT6 7NF

FAO. (1997). The State of World Fisheries and Aquaculture. 1996. Food and AgricultureOrganization, Rome, Italy.

FAO. (1999). The State of World Fisheries and Aquaculture. 1998. Food and AgricultureOrganization, Rome, Italy.

Henderson, R.J. and Tocher, D.R. (1987). The lipid composition and biochemistry offreshwater fish. Progress in Lipid Research, 26, 281-347.

Kent, M. (1990).Hand-held instrument for fat/water determination in whole fish. FoodControl, 1, 47-53.

Kent, M., Christie, R.H. and Lees, A. (1996). A portable fat meter suitable for live salmonidfish. In: Microwave Aquametry (ed. Kraszewski, A.), pp. 387-394. IEEE Press, New Jersey,USA.

Nakayama, T., Matsuhisa, M., Yamaura, M., Sumiyoshiyama, T. and Ooi, A. (1997). Delayedexample in rigor mortis of spinal cord destroyed plaice detected by measurements of isotoniccontraction and isometric tension. Fisheries Science, 63, 830-834.

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Nakayama, T., Toyoda, T. and Ooi, A. (1996). Delay in rigor mortis of Red sea-bream byspinal cord destruction. Fisheries Science, 62, 478-482

Refsgaard, H.H.F., Brockhoff, P.B. and Jensen, B. (1998 ). Biological variation of lipidconstituents and distribution of tocopherols and astaxanthin in farmed Atlantic salmon(Salmo salar). Journal of Agricultural and Food Chemistry, 46, 808-812.

Schallich, E. and Gormley, T.R. (1996 ). Condition factor, fat content and flavour of farmedand wild salmon. Farm & Food, 6(3), 28-31.

Shearer, K.D. (1994). Factors affecting the proximate composition of cultured fish withemphasis on salmonids. Aquaculture, 119, 63-88.

Sylvia, G., Morrisey, M.T., Graham, T. and Garcia, S. (1995). Organoleptic qualities offarmed and wild salmon. Journal of Aquatic Food Product Technology, 4, 51-64.