Process monitoring of reduced-sodium table olive fermentation using vibrational spectroscopy

Abstract

Table olives are a key product of the Mediterranean diet, yet their production by fermentation remains a traditional process characterized by high variability. Reliable monitoring tools are therefore needed to ensure product safety, quality, and regulatory compliance. This study evaluated mid-infrared (MIR) and Raman spectroscopy as process analytical technologies (PAT) for monitoring cv. Kalamata natural black table olive fermentation over 145 days. Also, as the industry aims to reduce sodium content in line with public health recommendations, the proposed monitoring approach was tested in different NaCl/KCl substitution ratios (0–75 %). Classical microbiological analyses confirmed that lactic acid bacteria (LAB) dominated all fermentations, with KCl substitution supporting faster acidification and higher titratable acidity, consistent with enhanced LAB growth. Both spectroscopic techniques captured temporal biochemical dynamics: MIR was more sensitive to salt substitution effects, while Raman provided complementary information on gradual temporal changes. Substantial intra- and inter-olive variability underscored the biological heterogeneity of the matrix as the main source of spectral noise. ANOVA simultaneous component analysis (ASCA) confirmed fermentation time as the dominant factor of variance, with substitution exerting smaller but significant effects. Partial least squares (PLS) regression enabled indirect prediction of brine parameters (pH, titratable acidity, LAB counts) from olive spectra, while the evolving window dissimilarity index (EWDI) improved detection of deviations in fermentation trajectories compared to traditional multivariate statistical process control (MSPC). Overall, the integration of MIR and Raman spectroscopy within a PAT–MSPC framework provides a robust, non-invasive strategy for real-time monitoring of olive fermentations, supporting the development of reduced-sodium products without compromising safety or quality.