An analytical approach to Sr isotope ratio determination in Lambrusco wines for geographical traceability purposes

Highlights

• A low temperature digestion procedure was tested for wines mineralisation.

• Modena Lambrusco PDO wines collected from four vintages, 2009–2012, were analysed.

• Results were compared with isotopic values of soils and wines coming from different district.

• Tracing and discriminatory power of ⁸⁷Sr/⁸⁶Sr was investigated.

Abstract

Geographical origin and authenticity of food are topics of interest for both consumers and producers. Among the different indicators used for traceability studies, ⁸⁷Sr/⁸⁶Sr isotopic ratio has provided excellent results. In this study, two analytical approaches for wine sample pre-treatment, microwave and low temperature mineralisation, were investigated to develop accurate and precise analytical method for ⁸⁷Sr/⁸⁶Sr determination. The two procedures led to comparable results (paired t-test, with t < t_crit). Furthermore, the precision of the whole analytical procedure was evaluated by using a control sample (wine sample), processed during each sample batch (calculated Relative Standard Deviation, RSD%, equal to 0.002%. Lambrusco PDO (Protected Designation of Origin) wines coming from four different vintages (2009, 2010, 2011 and 2012) were pre-treated according to the best procedure and their isotopic values were compared with isotopic data coming from (i) soils of their territory of origin and (ii) wines obtained by same grape varieties cultivated in different districts. The obtained results have shown no significant variability among the different vintages of wines and a perfect agreement between the isotopic range of the soils and wines has been observed. Nevertheless, the investigated indicator was not enough powerful to discriminate between similar products. To this regard, it is worth to note that more soil samples as well as wines coming from different districts will be considered to obtain more trustworthy results.

Introduction

In oenology, the concepts of geographical origin, traceability and authenticity of a product are often strictly tied with its quality. Generally, the quality is often linked to the history of the product, namely its geographical origin, the used raw materials, etc., and the producer wants to convey this information since this represents a peculiar added value, useful for the enhancement of the product itself. Actually, there are different European rules (Regulation (EC), 1898, Regulation (EC), 2002, Regulation (EC), 2006, Regulation (EC), 2008) with the aim to regulate and guarantee the authenticity and geographical traceability of food, but unfortunately, these are often not enough to avoid frauds or false labelling declarations, and to assure an honest marketing competiveness. Hence, several researches have been addressed to the development of analytical methodologies, able to link the product to its territory of origin by means of different parameters such as inorganic/organic chemical patterns and isotopic signature (Adami et al., 2010, Almeida and Vasconcelos, 2003, Di Paola-Naranjo et al., 2011, Durante et al., 2013, Laaks et al., 2012, Riovanto et al., 2011). Among the different indicators, ⁸⁷Sr/⁸⁶Sr isotopic ratio, Sr-I.R., has provided excellent results for different types of food matrices (Asfaha et al., 2011, Baroni et al., 2011, Swoboda et al., 2008, Voerkelius et al., 2010), and in particular for wine (Almeida and Vasconcelos, 2004, Barbaste et al., 2002, Di Paola-Naranjo et al., 2011, Durante et al., 2013, Marchionni et al., 2013). Nevertheless, the development of a geographical traceability model is often troublesome since different aspects have to be considered: the representativeness of the sampling procedure, the local geochemistry of the soil, the climate conditions, the winemaking processes etc., and the model is always strictly related to the reality in which the food lies.

This work is part of a long-term project dealing with the development of authenticity and geographical traceability models of two typical Italian wines, PDO (Protected Denomination of Origin) Lambrusco from Modena district and TRENTODOC (http://www.progettoager.it (Ager Project)). In particular, this study reports the results coming from the optimisation of the analytical procedure as far as the pre-treatment of wine matrices is concerned and summarises the preliminary data obtained on a representative set of PDO Lambrusco wine samples.

In order to carry out the determination of the studied indicator, a preliminary sample pre-treatment is often required to prevent or avoid any matrix influence on the experimental measurements. Therefore, a comparison of the performance of two different analytical approaches was tested.

In this context, the most commonly used pre-treatment for the ⁸⁷Sr/⁸⁶Sr determination in wine consists in the mineralisation of the sample. This digestion procedure can be performed by means of different analytical techniques, such as dry ashing (Di Paola-Naranjo et al., 2011), evaporation to dryness and dissolution with HNO₃ and HCl (Barbaste et al., 2002), UV-irradiation (Almeida & Vasconcelos, 2004) and by microwave apparatus (Microwaves: Theorical Aspects, 2011).

During the last decade, microwave assisted digestion techniques have become widely used for the pre-treatment of different kind of samples, since they allow the simultaneous processing of a large number of samples, the reduction of contamination and losses of volatile analytes by using an easy setting and control of the instrumental conditions. On the other hand, the presence of a high amount of ethyl alcohol, as in wine, can give explosive characteristics to the sample when it is exposed to microwave energy irradiation inside the closed reaction vessels. In fact, during the heating, the presence of concentrated nitric acid contributes to the formation of a highly reactive vapour mixture of ethanol and nitric acid in the upper part of the vessels. In particular, HNO₃ can rapidly oxidise ethanol producing carbon dioxide, nitrous oxides and water, increasing the amount of gas, which produces an instantaneous increment of the pressure (Tasev, Karadjova, & Stafilov, 2005). Owing to the increasing velocity of the process, the reaction can have explosive effects. Since these reactions take place at relatively low temperature, less than 100 °C, it is very difficult to find safely operative conditions. To avoid the problem of explosion of the closed reaction vessels, a two steps procedure must be arranged: in the former, the wine must be gently treated to eliminate the alcohol fraction (de-alcoholised) while in the latter, a digestion procedure must be worked out. Obviously, this experimental approach could be tedious and time consuming with a low productivity in terms of mineralised samples per hour.

An alternative digestion procedure with HNO₃, based on a low temperature mineralisation, was investigated in order to work in safe conditions and to analyse more samples. The obtained results were compared with those coming from the microwave-assisted digestion in order to evaluate the influence of the residual organic compounds on the separation procedure and on the instrumental determination of ⁸⁷Sr/⁸⁶Sr.

To the authors’ knowledge, this is the first report developing an easy analytical methodology for the pre-treatment of the studied matrix, obtaining high precision measurements, in terms of repeatability, reproducibility and time variability of the monitored indicator. The ⁸⁷Sr/⁸⁶Sr isotopic ratios were determined by a double focusing magnetic sector multicollector, MC-ICP/MS.

As concern Lambrusco wines, they represents one of the main typical products of Modena district and obtained the Controlled Designation of Origin (DOC) in 1970, replaced by the Protected Designation of Origin in 2009. Previous results (Durante et al., 2013) on these matrices allowed to optimise the procedure for the separation of Sr from its isobaric interference, namely Rb, and to study Sr-I.R. variability along the soil-branch and grape-juice coming from the production chain of Lambrusco wines. In this work, it has been performed a strontium isotopic characterisation of Lambrusco wines collected from four vintages, namely 2009, 2010, 2011 and 2012, in order to investigate the temporal variability of the investigated indicators and to obtain an objective ‘geographical isotopic fingerprint’ for the investigated wines. The tracing and discriminatory power of this indicator were also investigated by comparing the data with those coming from soils of Modena province and Lambrusco wines obtained by the same grape varieties cultivated in different districts.