Evaluation of the Salt Content of Canned Sardines in Tomato Ketchup by Diffuse Reflection near Infrared Spectroscopy

Abstract

This research aimed to determine the salt content of canned sardines in tomato sauce using NIR spectroscopy compared with traditional methods employing an auto-titrator, which are more consuming in terms of time and chemicals used. Scanning of NIR radiation was performed using an FT-NIR spectrometer with a wavenumber range of 12,500–4000 cm⁻¹ (800–2500 nm) with the diffuse reflection mode. Model development was achieved using partial least-squares regression from three groups of samples, which were the sample group that included salt-adjusted samples, the sample group that did not include salt-adjusted samples and the sample group that included only salt-adjusted samples plus five non-salt-adjusted samples. The model from the sample group that did not include salt-adjusted samples used the first derivative + straight-line subtraction spectra between 9403.8 cm⁻¹ and 4242.9 cm⁻¹ with nine partial least-squares factors providing the lowest root mean square error of prediction (RMSEP), with a coefficient of determination of calibration (R²) of 0.90, root mean square error of estimation of 0.039%, coefficient of determination for prediction (r²) of 0.85, RMSEP of 0.046%, average error of prediction (bias) of −0.002% and ratio of standard error of validation to the standard deviation of 2.58. The sample group that included salt-adjusted samples had a skewed distribution. The model returned an RMSEP of 0.109% on test samples, but the performance-to-interquartile range index indicated that this was a poor calibration. For the smaller salt-adjusted samples plus five non-salt-adjusted samples, cross-validation indicated the potential to evaluate salt in canned sardines (root mean square error of cross-validation = 0.158%). It was found that the vibration of water had an effect on the prediction accuracy as well as the vibration of starch and protein. In addition, the CONH₂ and CO₂H groups, which are common amino acid functional groups, also showed an effect. These findings represent novel findings concerning the use of NIR spectroscopy to determine salt that has no signal of its own in the NIR region.