Non-invasive sensing of freshness indices of frozen fish and fillets using pretreated excitation–emission matrices

Abstract

Fluorescence spectroscopy with fiber optic probe was utilized to estimate different freshness indices of whole and filleted horse mackerel (Trachurus japonicus) directly in the frozen state. The actual freshness values were assessed using the nucleotide-based method depending on the determination of ATP and its breakdown derivatives (ADP, AMP, IMP, HxR and Hx) with high pressure liquid chromatography (HPLC). Different spectral treatments (i.e. normalization, smoothing and first and second derivatives) were applied on the acquired excitation–emission matrixes (EEMs) and calibration models were developed using partial least squares (PLS) regression for the original and pre-treated EEM to predict the freshness indices. A searching algorithm was proposed as a smart wavelength selection method to find the optimized excitation–emission wavelength combinations. Results revealed that prediction performance of the developed models on the original and pretreated EEM spectra was in the following order: normalized EEM<original EEM<smoothed EEM<first derivative EEM<second derivative EEM. The excitation wavelength at λEx=390nm was the most efficient excitation wavelength to stimulate fluorophores responsible for fish freshness detection in whole and fillet samples. Under this excitation wavelength, 8 emission wavelengths (440, 450, 480, 500, 530, 600, 640, 710)nm in the normalized EEM spectra were found to be very sensitive in predicting K values in frozen whole fish with RCV2=0.85 and RMSECV=11.15%, and 7 emission wavelengths (500, 520, 530, 540, 650, 660, 670)nm in the normalized EEM spectra of fillet samples were found to be very important in predicting K values with RCV2=0.94, RMSECV=7.34%. The outcomes of this study emphasized the potential of fluorescence spectroscopy technique as an objective, non-destructive and rapid sensing method of fish freshness directly in the frozen state without the need to thaw the samples before testing.