The colour of farmed salmon hasattractedsomuchattentionwithinthe aquaculture industry as it canbe appreciated that the typical
pink-reddishcolourofsalmonissymbolicofquality and value, with retailers demandingstringent criteria for farmed fish such asAtlanticsalmonandtrouttomeetconsumerexpectations.
Indeed this has often proved controver-sial in the media when the farming of fishhas been criticised on many grounds withnegativestatementsalludingtotheuseofsyn-theticagentsandevendyessuggestedasthesourceof artificial colours being prominentlyexpressedbythemisinformed.
Carotenoidscompriseacomplexgroupofxanthophyllsandcarotenes,whicharediverseintheirchemicalstructuretoprovidethehostof yellow to orange red, and pink coloursfoundextensivelyinnatureasalsoseen in theplumageofbirdssuchasflamingos.
The main carotenoids ofimportance to salmon andtrout are astaxanthin and can-thaxanthin, which are specificin their mode of activity andin their manner of metabolism.Astaxanthin is recognised to bethepredominantredcarotenoidfound in salmonids in naturewith canthaxanthin also usedcommerciallytoamorelimitedextent.
A dietary requirementSalmon and trout do not possess the
inherentmetabolicability tosynthesisethesepigments but instead require them withinthe diet as preformed molecules that areabsorbed and subsequently deposited in tis-sues(Bjerkeng,2000).
Itshouldalsobenotedthatpigmentationleadingtothereddish-pinkandorangecolourofintegumentisalsofavouredforcertainfishthat can deposit carotenoids or metabolic
derivatives intheskinsuchasredseabreamand red tilapia (Gouveiaet al, 2002).This isalsoacharacteristicofhighvalueandaccept-abilitytoconsumerswhoperceivethistobesuperiorcomparedtoun-pigmentedfish.
The efficacy of flesh colouration by dif-ferent carotenoids is a function of complexphysiologicalprocessesfollowedbyaseriesofbiochemical events that involve metabolismprimarily in the liver aswell as the intestinaltract(Pageetal2005,PageandDavies,2006).
Evidence from the research investigationsofWhiteetal(2003a)suggeststhatamajorfraction of absorbed astaxanthin is trans-formed intovitaminA in the intestinal tissueof rainbow trout and most likely salmon aswell.
In addition there is increasing evidencethatcarotenoidsare involved ingeneregula-tionandsignal transduction therebyaffectingthe entire metabolism of fish (Azzi, 2007;
Lordan et al 2008). They are also potentanti-oxidants that can interact with vitaminE (α-tocopherol) and can prevent fatty acidoxidationduetofreeradicalgenerationwithintissues.
There are also positive benefits on thepostmortemqualityoffishunderfrozenstor-ageconditionswhen fedhigh levelsof caro-tenoids thusprolongingshelf lifeofproducts(Jacobsenetal(2011).Inthiswayastaxanthinandcanthaxanthinhavepro-vitaminandmet-abolic functions well beyond their standardroleforpigmentationandcouldthereforebe
Davies (2005) previously reviewed sev-eral aspectsof biochemical andphysiologicalparameters affecting salmonid pigmentationwith particular emphasis on post-prandialabsorption kinetics and retention efficiencyand explained that considerable amounts ofastaxanthinandcanthaxanthin isexcretedbyfish and therefore efficiency of utilisation isquite low(~20percent)compared toothernutrients such as proteins, amino acids, vita-minsandminerals.
Therearemanyproductionrelatedfactorsthatcan influencethedegreeofpigmentationof farmed salmonids. These include species,race or stock type, intra-population variation,age of fish, type and quantity of carotenoidsingestedover specificperiod, seasonaleffects,maturation, health and state of physiologicalstress,dietaryformulation,regulatedfeeddep-
rivationperiodsmaintenancefeeding,slaughterconditions, visualisation of colour, processingconditions:storage,smoking,cookingetc.
Thehighgrowthratesachievedinmodernsalmonproductioncanresultinareductionofpigmentation and uneven distribution withinthe flesh. It is well known that considerablevariations can occur in the ability of fish toretaincarotenoidswithinthemuscleandthiscansometimesmanifestasverylowpigmen-tation or indeed excessive levels in differentregionsofmuscle.
Indeedthe factthatseasonalchangescan
The quest to keep Salmon in the pink
naturally by Simon J Davies, School of Biosciences, Plymouth University England, UK
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modulate various selected muscle qualityparameters,includingthedegreeofpigmenta-tion in salmon was noted by Nordgardenet al. (2003). These investigators concludedthat rapid period of growth achieved undercontinuous lighting compared to naturalphotoperiod resulted in elevated growth ofsalmon and increased oxidative stress withmarked reductions in fillet vitamin E levels(a-tocopherol)andastaxanthin.
Environmental factorsOther environmental factors that may
affect the efficiency of pigmentation wouldlikelyincludewaterqualityandflowratesthatwouldinteracttomodulategrowthandnutri-ent retention. Stocking density and exerciseare known to affect fish performance andoptimum fish stocking densities will producesuperiorgrowthand feedconversion leadingtoimprovedfishquality.
Similarly, adequate flow rates, waterexchangerateswillpromote firmer fleshedfish and may enhance pigmentation infish under intensive production systems asdescribed previously in relation to musclequality. Colouration and its relationship toflesh quality in farmed trout and salmonwasextensivelyreviewedbyDavies(2008)whichaddressedcarotenoidfunctioninfishas well as the physiological, biochemicalaspects and aesthetic attributes leadingtowards optimal pigmentation of salmonidfish.
Our knowledge of fish nutrition hasexpanded considerably in recent times andtherehavebeennumerousinvestigationsthathave addressed the effects of dietary levelsofbothastaxanthin,canthaxanthinseparatelyorincombinationonthefleshcolourationoftroutandsalmon.
The type of feed, level of feeding andmatrixeffectsoncarotenoiduptakewillaffectthedegreeofpigmentationinandcansignifi-cantlyalterthecolourcharacteristicsresultinginpossibletaintingwithbackgroundcolour.
Anoptimumdietarylevelofabout65-mg/kg astaxanthin is preferable in general to
achieve acceptable results. A minimum sizethreshold exists to initiate pigmentation insalmon and trout and maintain depositionduring the initial freshwaterstagesofgrowthin fish of above 80-100 grams mean bodyweight.Thisextendswithsubsequenttransferas smolts to seawaterwith salmon typicallyfed65mg/kgtotalcarotenoidseitherasastax-anthin or in combination with canthaxanthinwiththelatternotexceeding25mg/kginthemixture(EUdirectives).Maximumpermittedlevels for astaxanthin in theUSA is80mg/kgoffeed(FDA,2010).
However, revision of such levels is thebasis of much scientific activity to optimisetheiruseandminimisewastageandcost.
Over the last decade, feed manufactur-ershavebeenable to significantly lower theinclusionofpigmentinfeed.Instead,itisrec-ommendedbysome,thatathree-phasepig-mentationstrategywithpost-smoltsbeingfedpigmentatarelativelyhighlevelof60-75mg/kg with a transition to an intermediate levelof40-50mg/kg fromabodyweightof2-3kgbeforeafinalregimemaintainedonafinishingdietpriortoharvestof25-35mg/kgofdietarycarotenoidsconcentration(Sinnot,2006).
There are some scientific rationales forsupporting higher diet pigment levels in thelaterstagesofgrowthforthefeedingoflargeadultfish.Thisisbasedontheviewthatlargersalmon can pigment more efficiently thansmallerfish.Sincepigmentationdevelopsquiterapidlyfrompost-seawatertransfer,pigmen-tationrategenerallyslowsdownwhenfishgetbiggeralthoughtheabsorptiveefficiencymayactuallyincrease.
Itisstillthereforewisetopromoteastrat-egybasedonusinghigherlevelsinearlyphasefeeding with a reduction to a maintenancelevelinfishapproachingharvest.
The impact of plant oils on pigmentation
Choubertetal(2006)havereportedthatthepigmenting efficacyof astaxanthin fed torainbow troutwas affectedby the composi-tionofdietaryoilpresent.
Someevidencesuggestsreducedpigmen-tation efficiency may result in fish fed dietswith elevated plant oils over extended peri-odsduringgrowth.
However,consumertestsseemtoindicatethattherearenosignificantdifferencesinpub-lic perception regarding the overall appear-anceofsalmoncolourwhenfeddietregimesthat include appreciable levels of vegetableoil sources to replace fishoils (Rosenlundetal.2003).
This is of importance given the trendof using higher amounts of plant oil blendsfor a major part of the production and thestrategyofusing‘fishoil’enhanced‘washout’diets in the final stage to harvest to achievehigh omega 3 fatty acid concentrations inthefleshofsalmonpriortomarket.Quintonetal (2005)conducted trialswith salmon toascertain the influence thatgenetics canplayin affecting the absorptionofdietary carote-noids,metabolism and the efficiencyof fleshdeposition.
These workers in Canada evaluated thegeneticparametersindifferentyearclassesofAtlanticsalmonreachingharvest.
Theydetermined sexualmaturationchar-acteristics and associations to colour score,astaxanthin, canthaxanthin, oil, and moisturecontentsofflesh.Positivegeneticcorrelationswere found between body weight and withpigmentretentionlevels.
Obviously, there is the scope to exploitmore efficient stocks for their pigmentationcapacity using selective breeding programs.Such findings could result inmore even andconsistent flesh colour for salmon and troutandtailoredtodifferentproductionsystems.
Less intensive conditions for farmed fish
Traditional use of commercially syntheticsourcesofastaxanthin incompounded feedsaddsgreatlytotheircostsandthevalueoftheresultingproducts.
The annual sales of synthetic astaxanthinforaquaculturealonewereestimatedatmorethanUS$200millionat2010.
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Extruder OEE for the Production of Fish FeedExtruder OEE for the Production of Fish Feed
Recently,however,therehasbeenagrow-ingdemand in theproductionof farmed fishunder less intensive conditions, and withmoreemphasisonnaturaladditivesandsup-plements in the diet. As such, a number ofinvestigationshave reported the feasibilityofvarious single cell products such as the redyeastPhaffiarhodozymaandHaematococcuspluvialisalgaewithrespecttotheirpigmenta-tion ability compared to the synthetic formcurrently available to the industry (Lagocki,2001).
InvestigationshaveconfirmedthatPhaffiacaneffectivelypigmentsalmonidfishbutonlyfewproducts arecurrently available for thispurpose.Choubertetal(2006)founddiffer-enceswithrespecttothecolourofrainbowtroutfedHaematococcuspluvialiscomparedtosyntheticastaxanthinwithhigherreportedcolourandfleshretention.
The prevailing consensus has been thatthesyntheticcommercialastaxanthinisread-ily available for assimilationby fish, and is amorestableandconsistentproduct.
It should also be noted that the astax-anthin present in H pluvialis is found ascomplexesterswhichconferdifferentassimi-lationpropertiescomparedtoothersourcesadding to variable results under practicalconditions (Bowenet al,2002;Whiteet al,2003b),thereiscurrentinterestinseaweedsand extracts from macro-algae which cancontain appreciable levels of carotenoids aswellaswasteproductsfromkrillandshrimpprocessing.
However, these materials are inconsist-ent in availability and carotenoid levelsmayvary with seasonality. However, specialityseaweeds may have a promising future asfeed additives combining functionality asprebiotics and contributing to natural caro-tenoidintake.
Algal and yeast sourcesDespite the potential of both algal and
yeast sources of carotenoids being able toeffectively pigment salmon and trout, thesehave been prohibitive in terms of theirconsiderable costs and variable qualitiescomparedtosyntheticproductswithconsist-entcharacteristics.Consequently,anexcitingdevelopment is the product Panaferd-AX®produced by a leading Japanese companyNippon Oil Corporation. Panaferd-AX®contains the dessicated cells of Paracoccuscarotinifaciens, a soil-inhabiting bacteriumwhichnaturallycontainscarotenoids.
Thebacteriaareculturedbyfermentationand have been selected to yield high caro-tenoid concentrationswithout theneed forgeneticmodification.Althoughastaxanthinisthemajor pigmentwithin Panaferd-AX®, itcanalsoexpressappreciablelevelsofnaturaladonirubinandcanthaxanthin.Bothofthesecarotenoidscanalso supportadditionalpig-
mentationto fish thatdepositthese inmuscle andskin.
The redcarotenoid-rich bacte-rium Paracoccuscarotinifaciens ispermitted as a sensitive addi-tive for use in salmon and trout with amaximum content of 100mg, expressedas the sum of astaxanthin, adonirubin andcanthaxanthin per kg complete feed. Thecurrent carotenoid composition of theproduct is specified as 3–5g canthaxanthin,10–15g adonirubin and 20–23g astaxan-thin/kg. The applicant proposes to modifythe ranges of canthaxanthin to 1–5g, andthat of adonirubin to 7–15g/kg product,while maintaining the astaxanthin range.TheEuropeanFoodSafetyAuthority(EFSA)oftheEuropeanCommissionhasexpertlyreviewed the terms of authorisation of theproduct for fish andverified its efficacy andsafety.
Indeed a number of trials in Scotlandby leading feed manufacturer’s and salmonproducers have proved most encouragingwith favourable results obtained in produc-tioncages.
Flesh pigment deposition and stabilitycomparabletothatofsyntheticpigmentationregimesprovedacceptableandquitecompa-rablewithorthodoxproducts.Thefeedbackreceived so far on fish pigmented withPanaferd -AX®hasbeen consistently goodfromtheretailerandconsumerstandpoint.
"To have all the colour in our salmonderivedfromnaturallyoccurringorganismsistheculminationofyearsofhardwork,"saidmanaging director Nick Joy of Loch DuartsalmoninScotlandwhohavebeenpioneer-ingtheproductintheirbespokenfeeds.
"As a farmer, I am proud that we rearwell-nourished salmon that look and tastegreat,"hehasstated.
Consumers and retailers are driving the agenda
It seems that Panaferd® will benefitsalmonproducersgloballybyofferingoppor-tunitiesfordiversificationinthemarketplace.Since the product has full FDA and EUapprovalforuseandisavailableforapplica-tion in the industry it’s likely to be usedincreasinglyinmain-streamproduction.
It is the requirements of the consumerandretailersthataredrivingtheagendaandgeneratingtheneedformoreinformationinthisareawithmediaattentiontocolourationoffarmedfishandcrustaceanspecies.
The question of producing a ‘pink/ red’
fleshedsalmon inthesamemannerasawildsalmonaccumu-latespigment is awidely acceptedprincipleadvocated by a number of market surveysin which the pink flesh colour of salmonscoresconsistentlyhigherasadesirablefac-torsecondonlytofishfreshness(Baker,andGünther,2004)
In terms of feed costs it is well knownthattheaddedcostsofincludingastaxanthininfeedsamountstoanextra10-15percent,which manifests as an additional 4-6 penceper kilogram produced, that is a cost ofUK£40,000-UK£60,000 per 1000 tonnes ofproduction.
InEurope, it is commonpractice topig-mentrainbowtrouttoadefinedlevelatpor-tionsizealthoughthis isnotsousual in theUnited States where un-pigmented (white)fleshed rainbow trout is more favoured bytheconsumer.Several feedcompanieshaveevaluated the costs of pigmenting rainbowtrout towards harvest with strategies formore economic approaches advocated byseveral feed companies and fish farmingoperations.
Colouration is an emotive issueThequestionoffishcolourationinfarmed
aquatic species will always be an emotiveissue since it is easily visualised and can bemeasured by suitable techniques rangingform direct chemical analysis in feeds andflesh or by a selection of optical methodsbased on colour scale assessments andadvancedimageanalysis.
Theaquafeedindustryandfishnutritionistmustberesponsiveandembracetheoppor-tunities for innovative products includingnatural pigmenting agents. With the needtoconsider ‘organic’certification for farmedfishandthequestforsustainableingredients,research is needed to evaluate the efficacyof carotenoids from a wide spectrum ofpotentialsources.
This will be a continuous challenge asfeed formulations become refined and asnewcandidatespeciesbecomeavailable foraquaculture. ■
Referencesavailableonrequest
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