Portable battery-operated system for determination of free acidity in virgin olive oil
Olive oil quality is affected by different factors such as the extraction process and fruit ripening stage, as well as the storage conditions (light and temperature). A number of parameters (e.g. free acidity, peroxide value), as well as sensory attributes, are assessed to determine the quality of olive oil. Free acidity is the first and most important parameter for oil quality and the official method for its determination is a chemical titration that must be carried out in a laboratory by trained personnel.
This project is aimed at the development of a novel technique for the determination of olive oil free acidity that can be easily implemented as a portable electronic system for quick and accurate measurements in-the-field by operators without specific training. This allows even the smaller production centers (oil mills and packaging centers) that cannot afford an internal laboratory for analysis to carry out free acidity measurements in-situ without the costs and time delays associated with the sample shipping to an external laboratory.
The measurement working principle is based on Electrical Impedance Spectroscopy (EIS), a technique where a sinusoidal voltage is applied to the sample under test and its electrical characteristics are measured in a range of frequencies. In this case the sample under test is the emulsion between an hydro-alcoholic solution (ethanol 60%, distilled water 40%) and the olive oil sample. Preliminary investigations have been carried out using a commercial impedance analyzer (Agilent E4980A) in the frequency range 20 Hz – 2 MHz. The results have shown that a correlation exists between the oil free acidity and the emulsion electrical conductance measured at low frequencies (< 10 kHz).
A portable embedded system for in-situ measurements of olive oil free acidity has been designed and built. The instrument has size 11 x 15 x 5 cm and can be powered by USB port or batteries (3 AAA alkaline batteries 1.5 V). The electronic board, based on the microcontroller STM32L152RCT6A by ST Microelectronics, integrates a LCD screen to output the measurement results, four buttons for user interaction and all the electronics for the electrical measurements (electrical conductance measurements at 200 Hz). The emulsion (15 mL of hydro-alcoholic solution and 1 mL of olive oil) is hosted in a 50 mL Falcon vial modified to feature a couple of stainless steel electrodes (diameter 6 mm, spaced by 12 mm) for the electrical measurements. The instrument has been calibrated and the calibration curve stored in the microcontroller memory. The instrument can perform free acidity measurements in about 30 seconds.
After calibration, the system has been in-house validated by carrying out measurements on a set of 30 olive oil samples of different origins with free acidity values between 0.18 and 4.5 (Grossi, M.; Palagano, R.; Bendini, A.; Riccò, B.; Servili, M.; García-González, D. L.; Toschi, T. G.; Design and in-house validation of a portable system for the determination of free acidity in virgin olive oil. Food Control, 2019, 104, 208-216). The accuracy of the system has been evaluated by comparing the instrument output with the results from the official method. A good correlation was found with the official method (Radj = 0.97) and satisfactory results were obtained in terms of LOD, LOQ, precision and accuracy.