Use of visible and near infrared spectroscopy with a view to on-line evaluation of oil content during olive processing

The aim of this preliminary feasibility study was to verify whether visible/near infrared(vis/NIR) spectroscopy could be used to predict the oil content of intact olives entering the mill, and of olive paste, pomace and paté during the milling process. Three different extraction methods (3-phase decanters, 2-phase decanters and 2.5-phase decanters) were considered, and two optical devices were tested: (i) a process device for non-contact analysis and (ii) a system equipped with an immersion probe for contact measurements, both working in the spectral range 400–1650 nm. 35 samples of olives were collected during the experimental tests, 50 samples of olive paste, 50 samples of pomace and 50 samples of paté. The collected samples (olives, olive paste, pomace and paté) were used to calculate partial least squares (PLS) regression models. Results regarding the non-contact analyses were encouraging, except for the measures on olives. On pomace, satisfactory models were calculated for the vis/NIR range [Ratio Performance Deviation (RPD) > 2], and a good model with R2 = 0.81 and RPD = 2.68 in validation was calibrated in the NIR range. The device equipped with an immersion probe achieved good predictive models for the oil content prediction on paté (R2 and RPD values ranged 0.77–0.82 and 3.00–3.43). The predictive models could be easily applied in an on-line system to monitoring the entire extraction plant and to perform a feed-forward control, allowing a reduction of oil leakage to minimise the oil losses and to maximise the extraction yield.