Visible (VIS)/near-infrared (NIR) spectroscopy was used to investigate spectroscopic changes occurring during storage of Atlantic salmon fillets with and without bacterial growth. A storage experiment was conducted for 11 d postmortem. Bacterial growth was inhibited by soaking a group of salmon fillets in 3 mM NaN3 prior to storage, while a control group retained its normal bacterial growth. Spectra were obtained by directly applying the spectroscopic probe onto the loin part of each fillet stored under conditions accelerating degradation. Principal component analysis (PCA) was used to monitor and compare spectroscopic development of the 2 groups and the results showed that VIS/NIR spectral changes occurred in the control as well as the treated group of samples within a single day after filleting. After 2 d of storage, stored samples were distinguishable from those fresh in both groups and it was only after the microbial spoilage became pronounced (8 to 9 log colony forming unit [CFU]/g) that the spectra of the spoiled control samples could be differentiated from spectra of the treated samples with no bacterial growth. Microbial growth is therefore not the only explanation for the spectral variations prior to microbial spoilage. Nonmicrobial, autolytic changes including possible changes in the physical properties are also contributing. Our results show that VIS/NIR spectroscopy can detect autolytic changes occurring in salmon muscle during the early stage of storage, independent of microbial growth.
Practical application: Important spectroscopic changes occur even when microbial growth is not apparent. This indicates that VIS/NIR spectroscopy may be used to determine the degree of freshness before microbial spoilage becomes relevant.