Hence it is not suitable for "in situ" measurements in small oil mills. One of the most promising methods is based on optical near-infrared spectroscopy (NIR) where the optical absorbance, i.e. the fraction of intensity of the incident light that is absorbed by the oil sample, is used to estimate the oil acidity. An oil sample is placed in a cuvette and analyzed by a spectrophotometer on a wide range of wavelengths. The results (i.e. the absorbance data for every tested wavelength) are thus processed by a statistical algorithm, such as Principal Component Analysis (PCA) or Partial Least Squares regression (PLS), to estimate the oil acidity. The feasibility to measure olive oil free acidity and peroxide value by NIR spectroscopy in the wavenumber range 4,541 to 11,726 cm−1 was reported. Many commercial spectrophotometers exist  that can be used for analysis of different quality parameters in olive oil. The main advantage of NIR spectroscopy is the possibility to carry out the analysis on raw olive oil samples, without any chemical pretreatment. The main drawbacks are the high cost of commercial spectrophotometer and the need of calibration for different types of oil (produced by olives of different varieties, different geographical origin, etc.). Another approach is based on electrochemical impedance spectroscopy (EIS). EIS is a powerful technique that has been widely used to characterize different food products such as the analysis of milk composition, the characterization and the determination of the freezing end-point of ice-cream mixes, the measure of meat ageing, and the investigation of ripeness and quality in fruits.