Analytical MethodsMethodological development for 87Sr/86Sr measurement in olive oil and preliminary discussion of its use for geographical traceability of PDO Nîmes (France)
Introduction
Olive oil is known for its beneficial effects on health. Containing mono-unsaturated fats and antioxidants, such as flavonoids and polyphenols, olive oil has earned a reputation for protecting the cardiovascular system. Its use is also well known in the field of beauty care and cosmetics. Owing to these qualities, olive oils are increasingly consumed (3.2 million tonnes in 2010) and the area under culture is expanding (8 million ha) (IOC) (International Olive Council). In France, around the city of Nîmes (Gard department), many olive orchards are moving towards controlled cultivation to produce an olive oil recognised by the European Union as having a Protected Designation of Origin (PDO). This designation provides consumers with a guarantee of the geographical origin, the quality, the know-how and strict conditions used in controlling the production and manufacturing of a given product. The PDO label gives the oil a market value, but it has been subject to numerous attempts at counterfeiting and fraud. The PDO for olive oils from Nîmes mainly concerns the Picholine olive variety, which is cultivated only on plots referenced by the decree of 17th November 2004 by the National Institute of Designations of Origin (INAO in French). These plots are subject to very precise specifications that help to distinguish Nîmes olive oil from all other produce originating from neighbouring or more distant countries. Ten mills are recognised in the Nîmes country to produce PDO olive oils from these referred plots. However, to our knowledge, it is impossible to perform geographical identification on olive oils produced from the same olive variety. Indeed, current methods of identifying an olive oil are based on their fatty acid and triglyceride composition (Tsimidou & Karakostas, 1993; Aranda et al., 2004, llivier et al., 2003, Ollivier et al., 2006), on volatile compounds (Cosio et al., 2006, Haddi et al., 2011) or trace elements (Araghipour et al., 2008, Benincasa et al., 2007, Jimenez et al., 2004). Although discrimination is possible, none of these methods is effective in identifying a specific geographical origin (Janin, Medini, & Techer, 2014) with the aim of eliminating the risk of fraud. This lack of a geographical authentication protocol not only gives rise to a significant risk of fraud, but also to health risks associated with pesticides and uncontrolled cultivation methods which do not comply with food health standards. For instance, the Spanish toxic oil syndrome affected more than 20,000 people, with 300 deaths recorded in 1981 (Capote, Jiménez, & de Castro, 2007).
A new way to assign an olive oil to its geographical growing area involves using a tracer of bedrock geology, such as strontium isotopes. Indeed, strontium (Sr) is natural element present in rocks and soils at various concentrations (mainly in trace amounts), with an isotopic composition linked to the nature, age and origin of the material. This element is also encountered in plants at lower contents depending on the species considered (Cougthrey & Thorne, 1983). The Sr in plants is assumed to be absorbed from the soil mainly during germination. Different sources of strontium in soil can be used by plants: Sr in organic or inorganic complexes (Capo, Stewart, & Chadwick, 1998), Sr in free solution (Semhi, Clauer, & Chaudhuri, 2012) or Sr from the underlying groundwater. Based on the assumption that the transfer of Sr from soil to plants takes place without isotopic fractionation (Graustein, 1988), various approaches have been proposed to identify the geographical origin of plant products by determining their 87Sr/86Sr ratio, which might reflect the values observed in the soil. However, a more recent study conducted by De Souza, Reynolds, Kiczka, and Bourdon (2010) points out a slight isotope fractionation during Sr absorption by plants (such as Rhododendron and Vaccinium), with a preferential assimilation of light isotopes. This fractionation allows us to distinguish the Sr isotope signature of plants and their growing soil when expressed in terms of δ88/87Sr values, even when the 87Sr/86Sr ratios are closely similar (De Souza et al., 2010). The role of fractionation to explain these Sr isotope compositions is debateable, since the measurements were performed in a natural weathering system whose mechanisms and parameters are poorly constrained. Despite these drawbacks, the identification method based on the 87Sr/86Sr ratio has been successfully applied to food products such as wine, orange juice, coffee, ginseng and rice (Choi et al., 2008, Kawasaki et al., 2002, Kelly et al., 2002, Lancelot et al., 1999, Lurton et al., 1999, Rummel et al., 2010, Techer et al., 2011, Weckerley et al., 2002). These studies mainly treat the plant samples by digestion in a microwave apparatus to dissolve the organic components. The extraction of elemental Sr is then performed using Sr-specific resin. These protocols were performed on hydrated samples with Sr contents higher than ca. 25 μg g−1 (Cougthrey & Thorne, 1983), but none of them were applied to a fatty matrix very poor in Sr such as olive oil (between 1.5 ng g−1 and 50 ng g−1) (this study; Benincasa et al., 2007). Indeed, because of its hydrophilic nature, strontium is only present in very low amounts in lipid matrices such as olive oil.
The objective of this study is to assess the feasibility of Sr extraction from a complex matrix such as olive oil, and to carry out isotopic measurements to implement further a geographical identification approach using Sr isotopes. Existing protocols for purification of Sr are unsuitable due to the viscosity, high organic load and low strontium content of the lipid matrix, coupled with the hydrophilic nature of Sr. Therefore, the first approach proposed here involved setting up a methodology for olive oil treatment for Sr extraction. The extraction tests were performed on Nîmes PDO olive oil and the analyses were checked against a NBS987 standard solution. The feasibility of the measurements was then corroborated by considering two Moroccan olive oils. The Moroccan samples selected in our study are mainly derived from Picholine olives, the same variety used in the case of the Nîmes olive oil. The data obtained contribute to preliminary discuss the use of Sr isotopes in conjunction with the geology of growing soils to reveal a possible link between the 87Sr/86Sr signature of olive oils and their production areas.
Section snippets
Materials and reagents
Analytical reagent-grade acids, HNO3 (Panreac, 69%, Hiperpur TMA) and HCl (Panreac, 35%, Hiperpur TMA) were used after double sub-boiling distillation in FEP Teflon bottles. H2O2 (Fluka Analytical, ⩾30% RT, for trace analysis) and H3PO4 (Merck, 85%, Suprapur) diluted with de-ionised water (resistance ⩾ 18.2 MΩ) were also used.
All the chemical treatments and strontium extraction procedures on olive oils were performed in an ISO Class-6 clean room to reduce risk of external contamination.
Olive oil treatment for Sr content measurements
4 g of olive
Nîmes olive oil (PDO)
The treatment protocols and measurement of Sr isotopes by TIMS were tested using four Nîmes PDO olive oils produced by three mills recognised amongst the 10 recognised by the Nîmes PDO specifications (mill of ‘Pierredon’, of ‘the Costières’ and of ‘Beaucaire’). The oils were produced in 2011, 2012 and 2013 (Table 2 and Fig. 2b). These four olive oils are derived mainly from the Picholine olive variety (70%). ICP/MS measurements performed on three selected samples yield Sr contents of 13.9, 2
Conclusion
Olive oil is a viscous matrix rich in organic matter and with low Sr content. Despite this low abundance of elemental Sr, isotopes of Sr could be considered to discriminate and identify the geographical origin of a given product. This involves establishing the feasibility of Sr extraction for isotopic measurements by mass spectrometer. In the present paper, we develop and describe a protocol of strontium extraction from olive oils.
The procedure involves a chemical treatment of the olive oil in
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