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Aquaculture 182 2000 261–269 www.elsevier.nlrlocateraqua-online
Muscle activity at slaughter: I. Changes in flesh colour and gaping in rainbow trout
D.H.F. Robb , S.C. Kestin, P.D. Warriss
Department of Clinical Veterinary Science, UniÕersity of Bristol, Langford, Bristol, BS40 5DU, UK Accepted 23 July 1999
Abstract
High activity at slaughter is known to cause changes to the muscle biochemistry in terms of Ž
. muscle pH and adenosine triphosphate ATP content. This experiment looked at how the muscle
activity may affect the flesh quality of fish. Electro-stimulation of a carcass immediately after death can be used as a model for high levels of muscle activity prior to death. This experiment
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showed that electro-stimulation of 1.5 kg rainbow trout Oncorhynchus mykiss immediately after slaughter not only resulted in short times to rigor, but that the flesh colour was affected. The flesh
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Ž .
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became significantly lighter P - 0.001 , less red P - 0.001 and had higher chroma P - 0.001 w
as measured by the CIE CIE, 1976. 18th Session, London, U.K., Sept. 1975. CIE Publication 36, x
U U U
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Paris, France. L a b
system, and the Roche colour card score was lowered P - 0.01
compared to flesh from fish that had shown very low levels of activity before slaughter and were not electro-stimulated after death. The fillets were also more susceptible to gaping following the
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post-mortem stimulation P - 0.001 . This indicated that electro-stimulation after death reduces the quality of the flesh. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Salmonids; Slaughter; Flesh quality; Colour; Rigor
1. Introduction
There is increasing evidence that activity at slaughter affects the flesh quality of fish. Greater activity immediately prior to death results in a more rapid drop in muscle pH as
Ž the white muscle metabolism is predominantly anaerobic, producing lactic acid Lowe et
. al., 1993 . Greater activity also results in shorter times to the onset of rigor. The strength
Corresponding author. Tel.: q44-117-9289312; fax: q44-117-9289324; E-mail: david.robbbristol.ac.uk 0044-8486r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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PII: S 0 0 4 4 - 8 4 8 6 9 9 0 0 2 7 3 - 2
of the muscle is affected, the muscle breaking more easily under strain after increased Ž
. activity Jerret et al., 1996 .
Anecdotal evidence showed that there were differences in the number of fish rejected Ž
. for poor colour from populations of Atlantic salmon Salmo salar that were harvested
over several days. The flesh of all fish should have had approximately the same levels of Ž
the pigment astaxanthin, but the Roche colour scores of some were lower A. Brown, .
pers commun., Norsk Hydro . The fish with the poor colour were harvested under the most severe conditions and could have been showing higher levels of activity prior to
slaughter. It has also been observed that some fish show a greater propensity to muscle Ž
gaping than others. Gaping occurs when the tissues between the muscle blocks the .
myosepta break and the blocks become separated. Commercially it is a problem as it makes the processing of the flesh much more difficult.
Variation in visual colour scores and colourimetric measurements between samples Ž
. with the same pigment concentration was also reported by Little et al. 1979 . Using
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U U U
CIELab CIE, 1976 , CIELUV and Hunter L a b measurements it was concluded that
differences in colour measurements between samples were caused by differences in light scattering and absorption within the flesh. However, the cause of these differences was
not suggested by the authors. Previous work on the effects of slaughter treatments showed an influence on muscle
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pH Lowe et al., 1993 and the times to rigor Azam et al., 1990 . The development of rigor can be followed by measuring the flexibility of the fish. As the fish enter rigor they
are able to bend less and as rigor resolves so they are more flexible. However, anecdotal evidence indicates that there are also potential effects of muscle activity on gaping and
muscle colouration. Muscle gaping can be scored subjectively according to the amount of gaping and this system is used regularly commercially as it is the most convenient
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method available A. Dingwall, pers. commun., Pinneys of Scotland . Muscle coloura- Ž
tion of salmonids can be scored subjectively using the Roche colour card NS9402, .
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U U U
Ž 1994 and objectively using the CIE 1976 or Hunter L a b
systems Skrede and .
Storebakken, 1986; Christiansen et al., 1995 . The Roche card is commonly used commercially, but for research purposes the use of a tristimulus meter is preferable,
giving a repeatable methodology. The measurements of L
U
, a
U
and b
U
give information Ž
. Ž
. on the lightness, angle of hue colour and chroma of the flesh Warriss, 1996 . These
can be used to determine differences between treatment groups or to follow changes in these parameters during storage.
Electro-stimulation of the flesh post-mortem with low frequency, pulsed direct current results in the rapid and repeated contraction of the muscle and can be used as a
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model for very high muscle activity levels Jerret et al., 1996; Fletcher et al., 1997 . This has advantages over using natural activity in that it is repeatable between fish. It is also
likely that the degree of muscle activity produced is greater than normal activity levels would result in, so causing maximum difference between the treatments.
This experiment aimed to investigate the extent that activity affects flesh quality by using two extremes of muscle activity. AQUI-S
w
, an anaesthetic used commercially on fish farms in Australia and New Zealand, anaesthetises fish slowly and was used to
Ž produce a ‘no activity’ or ‘rested’ group Jerret et al., 1996; Robb and Kestin, unpubl.
. data . The active ingredient is 2-methoxy-4-propenylphenol, which is a good anaesthetic
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for rainbow trout Keene et al., 1998 , without causing swimming reactions. A model muscle high activity group was generated using electro-stimulation post-mortem. The
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fish used were rainbow trout Oncorhynchus mykiss , as they are good models for the more valuable salmon flesh, having the same muscle pigments and similar constraints of
commercial flesh quality. They were used preferentially to salmon for this experiment as they were easier to obtain and hold at the experimental facilities at Langford.
2. Materials and methods
