Uso da colorimetria por imagem digital para a determinação de índices de qualidade em azeites de oliva

Abstract

Olive oils are subject to oxidation and hydrolysis reactions. These reactions produce hydroperoxides and free fatty acids, compounds that directly affect the quality, classification, and pricing of products. The extent of oxidation and hydrolysis can be measured by the chemical quality parameters called peroxide value (PV) and free acidity (FA), respectively. The standard methods for determining these indicators are based on titrations, with low analytical complexity, but that requires a lot of chemicals and are susceptible to errors in detecting the endpoint. To overcome such disadvantages, this research sought to develop two innovative methods based on Digital Image Colorimetry (DIC) using smartphones to determine PV and FA in olive oils. In the first method of PV determination, analytical curves were built using a standard hydrogen peroxide solution. Potassium iodide was used for the colorimetric reaction, and the solutions were imaged. The parameters of the RGB, CMYK, HSV, L*a*b*, and XYZ color systems were calibrated against the milliequivalents of O2 in the standards, and the analytical curves were used to determine the PV of samples. In the second method, to determine FA, an oleic acid solution was used to construct standard addition curves, using cupric acetate to generate a bluish-colored derivative. The solutions were imaged, and the percentage of oleic acid in the standards was calibrated against the respective parameters of the RGB color system. Finally, the FA of each sample was determined by curve extrapolation. The results showed more reliable and simpler methods than those commonly used, with R2 of 0.97 and 0.99 and RMSE of 3.29 meq O2/Kg and 0.08 %, respectively. The methods also contribute to reducing costs and analytical time in laboratories. The results of this research were added to data from other vegetable oils and published in the journals Journal of Food and Composition Analysis (DOI 10.1016/j.jfca.2022.104724) and Food Chemistry (DOI 10.1016/j.foodchem.2024.138555).