Assessment of fish (cod) freshness by VIS/NIR spectroscopy
F.Sigernes, K. Heia, M. Esaiassen, J.P. Wold and N.K. Sørensen
Norwegian Institute of Fisheries and Aquaculture Ltd., 9005 Tromsø, Norway.
The use of Near infrared (NIR) spectroscopy has proven promising for assessing aspects of fish quality, e.g. fat, water, protein and salt contents. Previously, transmission measurements in the wavelength range 860 to 920 nm have been conducted on samples of cod muscle. A general correlation between attenuated light and storage time of cod fillets on ice were found.
New instrumentation using diffuse reflectance now allows measurements to be done non-constructive on whole fillets. In this study, the main aim was to provide information on how fish fillet (cod) change properties in spectral response in the wavelength region 400-1100 nm as a function of time, days on ice. Since fish muscle is highly absorbing / opaque / cloudy an approach called Diffuse Reflectance also known as Transflection was used. In this mode, monochromatic light strikes the fillet perpendicular to the fillet surface. The light then penetrates the fillet and is absorbed and diffusely reflected in all directions. The diffuse back-scattered light of the fillet was then detected.
5 fresh cods were killed. One fillet per fish was selected and put on ice. The 5 fillets left was frozen for later use. First day of measurements was 24 hours after death. Each measured cycle per day took approximately 2 hours. 5 spectral samples per side of the fillets were recorded. The fillets were then put back to storage on ice. The above procedure was repeated daily over a period of 14 days.
It was easy to observe the spectral difference between dark- and white- fish muscle. The dark muscle had high absorbance in the visible compared to the white muscle. The white muscle scattered visible light better than the dark muscle. In the near infrared region the two muscle types followed the same trend. The diffusion became smaller with increasing wavelength.
The most characteristic or dominant spectral change over time was obtained in the VIS region from the skin-side tail region of the fillets, where the probes field of view covered mostly the dark muscle. A model for predicting stored time from observed spectra was constructed using Partial Least Square Regression (PLSR). The correlation coefficients between measured and predicted time were 0.95 and 0.94 for samples of white and dark muscle, respectively. The standard deviation were found to be close to 24 hrs.
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