Non-Invasive Rapid Method to Evaluate Tunisian Virgin Olive Oil Based on color Measurement by Digital Images and Chlorophyll-Carotenoids Content

Abstract

Color and pigment analyses are not required by the majority of olive oil marketing standards. However, it is a basic attribute that is highly associated, by most consumers, with the idea of quality. In this study, we aim at providing a fast non-invasive method for routine analysis that can be used on a large scale in the laboratories of olive oil analysis starting from pigment quantification and color range measurements. A selection of 172 virgin olive oil samples obtained in Tunisia between 2018 and 2019 were used for this purpose. Chlorophyll and carotenoid contents were analyzed using a UV spectrophotometer standard method while color range was measured using digital images taken under controlled conditions. All samples showed high significant differences in chlorophyll and carotenoid contents (p <0,01) confirming that the visual selection of the set of samples was satisfactory for this study. Chlorophyll content varied from 3,0 to 28,3 ppm for samples SM137 and SM96 respectively, while carotenoids ​​oscillated between 0,7 and 6,2 ppm for SM138 and SM100 respectively. Principal component analysis using chlorophyll and carotenoids contents along with RGB-CYMY color measurements showed a higher significant correlation P<0,05 between pigment contents and Red, Green, Blue and Yellow colors. Bivariate tests suggest that although color and pigments are correlated, color range assessment using digital imaging may represent a more sensitive method to discriminate olive oil according to cultivar, geographical origin, maturation index and year of harvest.