Determination of Oil and Moisture Content in Intact Olive Fruits by Portable near Infrared Spectroscopy

Abstract

Introduction W orld olive oil production and consumption has increased steadily in recent years mainly due to scientific demonstration of the beneficial properties of olive oil for the protection of human health as well as increased appreciation of its sensory and cooking properties. Currently, Spain is the main olive oil producing country in the world, accounting for almost half of the total world olive oil production (around 3 million tonnes). In olive growing for oil production, the evaluation of oil content and moisture in olive fruits are key parameters. Oil content, as a percentage of fresh weight, determines the final oil yield of the orchard and is generally used for payment to growers. Fruit moisture influences the extraction process, particularly in some cultivars which exhibit a tendency to produce emulsions and have low oil extractability. Oil and moisture contents change during the ripening period and are affected by both the cultivar and environmental conditions. In the northern hemisphere, accumulation of oil in the fruit starts in August and ends in December and is normally used to determine the time for harvest. However, conventional laboratory techniques for the determination of these parameters are based on destructive measurements, are time-consuming and require considerable sample manipulation, which can be challenging for large sample sets. As an alternative, near infrared (NIR) spectroscopy offers many advantages, such as the rapid and simultaneous nondestructive analysis of many traits with low analytical cost per sample and without the use of chemical reagents. Recently, several works have demonstrated the analytical possibilities offered by NIR spectroscopy at a number of different steps during the olive oil extraction process. Application of NIR analysis to fruits, pastes and oils have been reported for quantitative analysis of some components or discrimination of samples according to cultivar, origin, classification and authentication of olive oils. Vibrational spectroscopic techniques have been suggested to monitor oil accumulation in olive fruits by providing a rapid tool to classify olives depending on their maturation stage on the basis of spectroscopic criteria which are more objective than the visual inspection of colour changes traditionally employed. In recent years, the development of low cost miniaturised spectro photometers has provided a real possibility of portable measurement devices, offering the possibility of direct analysis of intact fruits in the field which would allow new applications to be developed. In this article, the feasibility of using portable NIR technology for the determination of olive oil and moisture contents has been evaluated. To do that, a wide range of samples was used to test the robustness of the calibration models—this included fruit samples from different plant materials collected on-tree in the field and under laboratory conditions, in different years and harvesting periods.