Utilisation of a rapid technique based on front-face fluorescence spectroscopy for differentiating between fresh and frozen–thawed fish fillets

The potential of front-face fluorescence spectroscopy combined with chemometric tools was investigated to differentiate frozen–thawed from fresh fish. A total of 24 fish fillets, i.e., 12 fresh samples and 12 frozen–thawed samples, were investigated. Regarding the frozen–thawed samples, two speeds of freezing and thawing were tested (fast and slow) and for each condition, three whiting fillets were analysed. The fluorescence emission spectra of tryptophan (excitation: 290 nm, emission: 305–400 nm) and nicotinamide adenine dinucleotide (NADH) (excitation: 340 nm, emission: 360–570 nm) were recorded directly on samples. In a first step, the principal component analysis (PCA) was applied separately to the tryptophan or NADH fluorescence spectra. From the NADH spectra, PCA results showed a good discrimination between fresh and frozen–thawed fish samples. But it was not the case with the tryptophan fluorescence spectra. In a second step, factorial discriminant analysis (FDA) was applied to the first five principal components (PCs) of the PCA performed on the two data sets. Considering tryptophan fluorescence spectra, correct classification was observed for 62.5% and 70.8% of the calibration and validation spectra, respectively. A better classification was obtained from NADH fluorescence spectra since 100% correct classifications were obtained for the calibration and validation spectra. It was concluded that NADH fluorescence spectra may be considered as a promising probe for the reliable differentiation between frozen–thawed and fresh fish.