Quality Assessment of Olive Oil by 1H-NMR Fingerprinting

ALONSO SALCES, ROSA MARIA; MARGARET V. HOLLAND; CLAUDE GUILLOU; HÉBERGER, K.

Olive Oil

InTech

Año: 2011;

Olive oil is a highly appreciated edible oil, which is an important component of the Mediterranean diet, and is recognized for its potential health benefits. Much analytical work has been done on the authentication and quality assessment of this high added value agricultural product, as well as on the detection of its adulteration for both economic and health considerations. However, these issues continue to be major analytical challenges. In this context, the European Commission launched the TRACE project (http://www.trace.eu.org/) through the Sixth Framework Program under the Food Quality and Safety Priority with the aim of providing reliable analytical strategies to address this kind of problems. At present, 77% of the global production of olive oil takes place in the Mediterranean basin, mainly in Spain, Italy and Greece. Virgin olive oil (VOO) is permitted to be marketed under a Protected Designation of Origin (PDO), Protected Geographical Indication (PGI), or Traditional Specialty Guaranteed (TSG) label, on the basis of their area and methods of production [Council Regulations (EEC) No 2081/92 and No 2082/92]. In this context, the characterization of the geographical origin of VOO is becoming increasingly important. The European Commission has already registered in the “Register of protected designations of origin and protected geographical indications” 95 PDO and PGI olive oils. As can be expected, given the financial benefits associated with these prestigious labels, it is very likely that economic fraud occurs (e.g., labeling a non-PDO product as a PDO one or adulteration with olive oils that do not fulfill the PDO requirements). Other fraudulent practices are the adulteration of olive oils with low-grade oils and the mislabeling of the geographical origin of olive oils. The EU is about to establish new labeling rules that will make origin labeling compulsory for virgin and extra virgin labeled olive oil. So, oil produced from olives from just one EU country will have to be labeled with the name of the country of origin. Therefore, analytical methods are urgently needed to guarantee the authenticity and traceability of PDO and PGI olive oils, as well as the country of provenance of the olive oil, to help prevent illicit practices in this sector, and to support the antifraud authorities dealing with these issues. Another matter of major concern regarding the quality of edible oils is their oxidation, not only from the technological and economic point of view but also for safety reasons, due to the undesirable properties of some compounds produced during this process. Most of the studies on the oxidative stability of olive oil employed questionably high-temperatures which, unfortunately, cannot be considered reliable to predict the stability of olive oils under normal storage conditions at room temperature. Olive oil stability at room temperature is of great interest, for instance, in order to know its storage shelf-life. Because VOO is relatively stable to oxidation due to its particular chemical composition, there has been apparently little or no control of this in the past. This chapter reports our research work on the use of 1H-NMR fingerprinting, combined with pattern recognition techniques, for the quality assessment of VOOs. Two major issues have been studied, i.e. the geographical origin of VOO and the stability of VOO at room temperature. For the geographical characterisation of VOO, authentic VOOs from seven countries (Italy, Spain, Greece, France, Turkey, Cyprus and Syria) and from three different harvests (2004/05, 2005/06 and 2006/2007) were analyzed by 1H-NMR. The 1H-NMR fingerprints of the oils were examined by multivariate data analysis techniques, such as principal component analysis (PCA), linear discriminant analysis (LDA) and partial least square discriminant analysis (PLS-DA), to determine their geographical origin at the national and PDO level. In particular, the PDO Riviera Ligure and the main VOO producing countries (i.e. Spain, Italy and Greece) were used as examples to prove the potential of this analytical methodology to detect the mislabelling of non-PDO oils as PDO VOOs or the mislabelling of the origin of VOOs at national level. Thus, LDA and PLS-DA achieved consistent results for the characterization of PDO Riviera Ligure VOOs. PLS-DA afforded the best model: for the Liguria class, 92% of the oils were correctly classified in the modelling step and 88% of the oils were properly predicted in the external validation; for the non-Liguria class, 90% and 86% of hits were obtained respectively. Greece and non-Greece VOOs were properly classified by PLS DA: more than 90% of the samples were correctly predicted in the cross- and external validations. 1H-NMR fingerprinting was also studied to evaluate the stability of olive oil at room temperature while protected from light. The 1H-NMR spectra of the VOO aliquots kept under these conditions, over a certain period of time, were analyzed by principal component analysis (PCA) in order to study their evolution. None of the 1H-NMR signals present at time zero disappeared or experienced significant decreases or increases within the 3 years and seven months of the study. Only small changes in the signals and the appearance of some low intensity signals indicate that some oxidative and hydrolytic degradation of the VOO started after one year. The presence of 1H-NMR signals of hydroperoxides (primary oxidation products) with relative small intensities indicates that the oxidative degradation was taking place at a very low rate and yield. Moreover, the characteristic resonances of aldehydes (main secondary oxidation products) were not detected in the VOO over the studied period, indicating that the secondary oxidation process had not yet occurred. These results confirm the high oxidative stability of VOO at room temperature. On the other hand, slight changes in the 1H-NMR signal intensity of triglycerides and sn-1,3-diglycerides indicate that some hydrolytic degradation of the VOO had begun. This study is a proof-of-concept that 1H-NMR is a useful tool to study and evaluate the oxidative stability of edible oils in a quality control context at any temperature, since any toxic substances which may be generated during the degradation process can be detected and even quantified.The results achieved confirm that 1H-NMR fingerprinting of olive oil is a valuable tool for the traceability of VOOs from a food quality point of view.