Olive fruit ripening characterisation based on electrical capacitance measurements

• Development of a field meter for processing olive samples on-site. • Olive electrical capacitance correlates with fat content, regardless of moisture. • The study reveals high intra-sample natural variability. Olive fruit ripening involves the accumulation of fat content through lipogenesis. The completion of this complex process is considered a reliable indicator of the optimal time for harvesting. While chemical and magnetic resonance analyses, among other, can accurately determine fat content, these methods are costly and require specialized personnel, making them impractical for large-scale testing. Alternatively, visual grading methods are widely used, although recent studies have shown that the external appearance of fruits may not always reliably indicate ripeness. This paper investigates the influence of olive fruit maturation on their electrical behaviour, specifically on their ability to store electrical charge. To this end, a low-cost and portable field meter capable of measuring the electrical capacitance of olive fruits was designed and developed. Subsequently, 110 olive samples were measured weekly from September to harvest time in November. These samples were also subjected to chemical analysis for reference. The analysis of this data revealed high intra-sample variability, consistent with recent studies. Notably, strong correlations of up to 0,9741 emerged between capacitance measurements and gold-standard fat content on dry matter values after accounting for intra-sample variability. In this case, a Root-Mean-Square-Error value of 2,66% was calculated when using the regression line to estimate fat content on dry matter from capacitance values. Furthermore, no significant differences were observed between the distributions of the reference values and the estimated values. These findings pave the way for the development of an affordable tool to accurately assess the ripening stage directly in the field, and to expand our understanding about the ripening process.