Quantification and characterisation of residual blood in fish muscle : impact of slaughtering methods

Abstract

Omorganisering i oppdrettsnæringen har ført til større og mer mekaniserte anlegg med høy slaktekapasitet. I dag kontrolleres normalt restblod i fiskefilet ved visuell inspeksjon. Dette er arbeidskrevende og innebærer subjektive evalueringer som muliggjør høy grad av unøyaktighet. Fargen på fiskemuskelen er også et viktig kriterium når kunder vurderer kvalitet, og disse fargeforandringene skyldes ofte restblod. Oppgaven belyser, i første omgang, de ulike enhetsoperasjonene ved slakting av fisk og hvilke effekter de har på selve utblødningen. I tillegg belyser den muligheten for å bruke en objektiv og hurtig målemetode for restblod i fiskemuskel, som er basert på visuell- og nærinfrarød spektroskopi. Restblod fører til store økonomiske tap for fiskeindustrien som følge av nedklassifisering. Området er lite belyst, og denne oppgaven søker å bidra med ny kunnskap rundt hvilke faktorer som påvirker mengden restblod i fiskemuskel. En kjemisk målemetode ble brukt som referanse og sammenlignet opp mot dagens visuelle inspeksjon samt en visuell- og nærinfrarød (VIS/NIR) spektroskopi metode. Sistnevnte innebærer måling av absorbsjon/refleksjon av lys fra bl.a. fargepigmentet hemoglobin, og metoden benyttes allerede i annen næringsmiddelindustri. Oppgaven viser at mengden blod i fiskemuskel påvirkes av stress før slakting, avlivingsmetoder, kjøling og lagring. Spesielt ble det funnet at bedøving ved bruk av slag før bløgging er optimalt med hensyn til restblod. Bruk av karbondioksid anbefales derimot ikke. VIS/NIR metoden anbefales ikke brukt industrielt, men metoden kan forbedres og automatiseres. Det foreslås å anvende avbildende VIS/NIR spektroskopi for å vurdere restblod i fiskefilet, med spektra fra fiske hemoglobin som referanse.

The quality of farmed fish is assessed according to multiple criteria including freshness, fat content, blood spots, flesh colour and gaping. Blood spots and residuals have become more frequent in fish fillets, after the reorganisation of the industry with a shift from small manually operated slaughterhouses to larger and more mechanized production lines. This has led to increased degree of rejection by consumers and financial losses. Today, blood residuals are mainly detected by manual visual inspection. However, this method is labour-intensive and involves subjective evaluation and thus a high degree of inaccuracy. The main objectives of this work was to find and establish an appropriate method to measure residual blood and blood spots in fish muscle and to study factors contributing to the problem in order to help the industry improve slaughter procedures. A chemical method used to quantify haem pigment in meat was adapted and established for quantifying blood in fish muscle. However, use of chemical methods involve toxic chemicals and are destructive to the product, they are therefore not suited in industrial food production. Nevertheless, the chemical method demonstrated that the amount of residual blood and blood spots were influenced both by pre-slaughter activity, killing procedures, chilling and storage conditions. In particular, percussive stunning prior to bleeding was proved better in terms of residual blood compared to carbon dioxide (CO2) stunning. In terms of fish welfare, CO2 anaesthesia is not recommended, as it creates vigorous activity among the fish before they were proper stunned. Pre-mortem activity also resulted in more blood in the fish muscle. The primary chromophore that gives the blood its characteristic colour is haemoglobin (Hb). Visual and near infrared (VIS/NIR) spectroscopy is a rapid technique and may be a valuable tool for the assessment of the blood residuals in fish fillets. Thus, with the chemical method as a reference, it was anticipated that VIS/NIR spectroscopy could be used to quantify haem pigment in whole fish fillets. A correlation between the VIS/NIR and chemical measurements of Hb in fish muscle was found. Most of the information regarding Hb in the fillet was found in the visible range (400–700 nm), but the 400–1100 nm ranges resulted in the lowest error of prediction. Even so, the error of prediction was still too high to recommend this method for industrial application. Detection limits and correlation of VIS/NIR measurements are influenced by the substance used for calibration. Mammalian Hb has been used as reference when measuring blood Hb. However, when comparing the optical spectra of mammalian and fish Hb, significant differences with respect to absorbance in the visible range (450-700 nm) was found. The fish Hb was also influenced by pH, resulting in deoxygenating and increased auto-oxidation and thus pH related differences in absorption spectres were evident. The knowledge gained through this work, regarding blood residuals in fish muscle, suggests measures in the slaughterhouse to improve bleeding. Added knowledge to improve online or automated VIS/NIR inspection of blood residuals was also gained. With regard to fish welfare and muscle quality, the stunning methods used today are disputed. Further studies are needed to improve these methods and their goal should be to satisfy both the requests for fish welfare and to improve muscle quality. Further studies should also focus on possible industrial application of using non-contact imaging spectroscopy to assess residual blood in fish fillets, using fish Hb spectra as a reference. In addition, studies investigating possible differences in the Hb spectra between fish spices are both academically interesting and may also have relevance if VIS/NIR spectroscopy is to be applied industrially.