Application of near-infrared transmittance spectroscopy in the determination of fat, protein and dry matter in Atlantic halibut fillet

Near infrared transmittance (NIT) spectroscopy (850–1048 nm) has been applied to the measurement of fat, protein and dry matter in wet homogenized Atlantic halibut (Hippoglossus hippoglossus) fillet. A total of 155 fillet cuts (five sample sites) were selected from the start of the investigation (survey I) and 98 fillet cuts (one sample site) were also selected from eight fish groups six months later (survey II). The fish groups originated from a slaughtering quality experiment, where fish size and fat content in the feed had been varied. The multivariate calibration models were tested separately for each survey and after mixing the survey samples. The models were also tested after inclusion of sample temperature or fish weight, or after reduction to spectra of eight wavelengths. The selected fillet cuts from survey II gave calibration models with the best prediction abilities for fat and dry matter, whereas the combination of samples from both surveys gave the best prediction abilities for protein. The constituent ranges (w.w.) were: 10–123 g/kg fat, 165–274 g/kg protein and 234–335 g/kg dry matter. The partial least squares regression resulted in the following prediction errors, expressed as root-mean-square error of cross validation: 2.7 g/kg fat, 5.2 g/kg protein and 4.2 g/kg dry matter. The temperature adjustment had positive influence on the test set validation of protein and dry matter, whereas the fish-weight adjustment had positive influence on the test set validation of fat. Calibration models based on eight wavelengths were not optimal, but showed promising results, applicable in a simpler instrument for future online quality monitoring of Atlantic halibut fillet.