Development of tools to verify origin and varietal nature of wines

Abstract

This project was successful in developing tools to identify the provenance of Australian wines in an international context, based on statistical analysis of selected isotope ratios naturally found in wine. The models developed were able to identify Australian wines when applied across different vintages and varieties of wines, demonstrating their robustness. The project also made important strides in understanding why some methods that have been previously promoted as tools for identifying provenance (particularly those involving analysis of lead isotope ratios and trace metal concentrations) have not been successful.

Summary

Verifying the origin of wines by analysis relies on the use of parameters that not only reflect the geology and water source of the location where grapes are grown, but which are also not altered during winemaking. The project set out to identify analytical parameters based on isotope ratios to verify the geographic origin – or provenance – of Australian wines within the framework of the international packaged and bulk wine trade. Multi-dimensional statistical tools were employed, in particular Orthogonal Projection of Latent Structure-Discriminant Analysis (OPLS-DA) to analyse isotope ratios of boron, lithium, strontium and oxygen, which were determined using appropriate Inductively Coupled Plasma Mass Spectrometry (ICP-MS) methodology and Isotope Ratio Mass Spectrometry (IRMS). A discriminant model was developed using data from 292 Australian wines (from 16 production zones) and 94 international wines (from Europe, South America, Canada, New Zealand, South Africa and USA). The model had a successful classification rate of 97.3% for Australian wines, with a successful classification rate of 92.5% on average for all wine analysed. The main drivers of the model were isotopic ratios of boron, oxygen and strontium; however, some concentration data from selected trace elements including nickel, lithium and chrome was required to achieve these levels of discrimination. Identification of the region of origin of Australian wines, mostly through the impact of subsoil strata on strontium isotopes, was also achieved with a classification success rate of approximately 60%. The lower levels of successful classification for regions is strongly linked to the underlying similarity of geology that contributes to the subsoil composition of many Australian grapegrowing regions. This information will provide important baseline data for any future studies of analytical methods to determine regional origin of Australian wines. Several potentially confounding factors on these isotopic ratios and their ability to differentiate Australian from overseas wines were investigated: Vintage variation was measured using vertical series of three single-vineyard wines from very different regions across Australia over a period of ten years. This temporal variance was tested against that measured for all study samples and found to be

smaller for all isotopic parameters with the except of two lead isotope ratios. This allowed the majority of isotope ratios to be used. Grape variety variation was tested in ten cultivars, both red and white, grown in several locations of South Australia subjected to controlled micro-vinification. Differences in the isotope ratios of Li, O, and Sr differed by below 10% due to grape cultivar and were therefore useable. Neither vintage nor variety showed a significant negative effect on the use of the models to classify if a wine was of an Australian origin. Bottle source and type were considered to have potential to influence the boron isotopic ratios found in packaged wine. However, analysis demonstrated that the bottle did not make a significant contribution in comparison to the differences introduced by wine origin. Bentonite clay fining is a common winemaking practice which can introduce significant amounts of lead into wine. This fact makes the use of lead isotopic ratios unsuitable for classification of white wines. Analysis demonstrated that the lead isotopes were more useful at indicating the regional source of the bentonite used, an internationally traded commodity, than the wines origin. The success of this project in using combined isotopic ratios and statistical analysis to categorise a wine’s origin will allow the provenance of a wine that claims to be Australian to be analytically determined. While by its nature this technique is not suitable for rapid in-market applications, it will provide an important addition to technologies such as smart labelling and methods that compare direct retention samples of known wines. Significantly, unlike many other technologies used in authentication of wine, the use of isotopic ratios can link a wine’s provenance to the underlying physical attributes of its place of origin. This is important in cases where rather than being a direct forgery of an existing Australian product, a wine is simply making claims of Australian provenance. Such cases can expose the consumer to, at best, a disappointing quality outcome and at worst possible health impacts from the use of unapproved additive or ingredients, with additional obvious negative outcomes for the reputation of all Australian wines. While this work identified the overall potential of using a combined groups of isotopic ratios to provide information on wine origin, further work is needed to clarify nature of the links between the final values of these components in wine and the underlying sources in the geology, soil, water and air. Such an understanding will allow the much larger data sets compiled in environmental and geological studies to be used in the determination of the provenance of wine as well as other agricultural products.