Aquaculture Research, 1998, 29, 367–372 Muscle colour development in Arctic charr, Salvelinus alpinus (L.), monitored by fibre-optics and electrical impedance H J Swatland, F Darkin, S J Naylor, L Caston & R D Moccia Department of Animal & Poultry Science, University of Guelph, Guelph, Ontario, Canada Correspondence: H J Swatland, Department of Animal & Poultry Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1 Abstract Farmed Arctic charr, Salvelinus alpinus (L.), (n = 270) with a wide range of carotenoid muscle pigmentation were produced by feeding astaxanthin at different levels (0–120 mg kg –1 feed). Steaks were scored subjectively for pigment concentration (dark = high score). Internal reflectance spectra were measured with a relatively non-destructive 1- mm-diameter fibre-optic probe. Colour scores were only moderately correlated with reflectance (R = 0.66 and P  0.01, using data at 500, 610 and 520 nm) because the small-diameter probe had a short light-path through the tissue and was highly responsive to scattering. However, in fish without astaxanthin in their diet, this sensitivity to microstructural causes of scattering revealed that fibre-optic reflectance increased (P  0.01) with age from 400 nm (r = 0.68) to 440 nm (r = 0.40), and from 530 nm (r = 0.30) to 700 nm (r = 0.56). In agreement with these results, colour scores decreased with age (r = –0.52; P  0.001; n = 85), as did electrical resistance 24 h post-mortem (r = – 0.42 at 120 Hz, r = –0.39 at 1 kHz and r = –0.54 at 10 kHz; P  0.001). Resistance was correlated with colour score (r = 0.40 and P  0.001 for resistance at 10 kHz) and with fibre-optic reflectance (R = 0.42 and P  0.01 for resistance at 120 Hz and 1 kHz versus reflectance from 420 to 680 nm). Thus, without astaxanthin in the diet, muscle colour and tissue integrity at 24 h deteriorated with the age of the fish. Introduction The Arctic charr, Salvelinus alpinus (L.), is a large salmon-like fish that can be commercially farmed © 1998 Blackwell Science Ltd. 367 with rapid growth in cold conditions (DFO 1991). The commercial importance of carotenoid colouration in salmonid muscle is widely appreciated in the aquaculture industry and by food retailers (Simpson 1982): intense colouration makes the product more attractive for many customers, but is relatively expensive to induce in captive stock by dietary supplementation. Carotenoid pigmentation may be measured objectively by colourimetry (Foss, Storebakken, Schiedt, Liaaen-Jensen, Austreng & Streiff 1984; Skrede & Storebakken 1986; King 1996), but this involves exposing several square centimetres of muscle; less destructive methods are required for routine monitoring of pigment accumulation during growth and for grading the finished product. The present research was undertaken to determine if a relatively small-diameter fibre-optic probe capable of penetrating the skin of an intact fish with minimal damage could be used to measure internal carotenoid colouration of muscles. Another objective was to investigate light scattering from tissue microstructure as a source of error in measuring dissolved carotenoid pigment: when light scattering from microstructural sources such as precipitated sarcoplasmic proteins or shrunken myofibrils is high, the muscle appears pale and normal pigmentation from dissolved chromophores is obscured (Swatland 1995). Electrical impedance was used as a rapid method to assess the state of the muscle: in fish muscle post-mortem, cell membrane capacitance is lost and electrolytic resistance decreases, so that high capacitance and resistance indicate a high degree of cellular integrity (Jason & Lees 1971; Kent & Jason 1975).