Objective measures of quality and provenance in Australian vineyards
This project builds on knowledge gained in Project AWR1202 which identified important chemical predictors for commercial grape quality grading. Based on this successful proof of principle study, there is interest from wine producers to further explore the potential of this approach, using commercial Shiraz vineyards.
This project will initially assess differences between two premium quality grades (Q2 and Q3) of Shiraz fruit and wine from 20 vineyards primarily within the Barossa Valley (with reference samples from other regions in SA). Fruit and wine will be sourced from industry partner, Pernod Ricard Winemakers (PRW), who have historically produced wines across a variety of quality grades, including ultra-premium. The aim is to establish chemical and spectral indicators which define Q2 and Q3 vineyards. Wines will be produced commercially from each vineyard and analysed compositionally and sensorially to determine differences in styles achievable. The primary output of the study will be the ability to determine the key chemical indicators associated with quality rating and wine style in premium Shiraz, and how they vary by quality grade and region. Secondary objectives are to identify vineyard or winery management options for shifting Q3 to Q2, and for reducing the cost of producing Q2 grapes.
From 2018, This project forms part of a new multi-agency collaboration to research Shiraz terroir across a range of scales, primarily in the Barossa Valley. At the sub-regional scale, 23 sites were monitored with fruit undergoing sampling for ripeness, yield assessment, chemical analysis and small-lot winemaking. These sub-regions were identified by the Barossa Grounds Project, and are classified as Northern Grounds, Central Grounds, Southern Grounds, Western Ridge, Eastern Ridge and the Eden Valley (Figure 1). The AWRI’s involvement in the project has been to perform multiple analyses of volatile and non-volatile compounds in the small-lot wines. In subsequent seasons, vineyard management interventions will be imposed on selected sites to determine the potential to optimise a site’s terroir. These management practices will aim to influence vine growth and development, and to establish a cause and effect for environmental impacts on fruit and
Figure 1. Sub-regional sites identified in the Barossa Valley to be used in the multi-agency Shiraz terroir project
Chemical and sensory factors that define Eden Valley wines
The second season of the project was completed. Chemical and sensory analysis were performed on wines from three locations within each of 23 and 24 vineyard sites in the first and second seasons, respectively. In both seasons of the study, the wines from the Eden Valley were the most clearly distinguished from the other Barossa sub-regions using chemical analysis alone. Partial least squares discriminant analysis was performed, and a number of compounds were consistently associated with the Eden Valley wines for both vintages. In both seasons, the primary grape anthocyanin, malvidin-3-glucoside, was lower in the Eden Valley wines, while other non-malvidin anthocyanins were elevated. Changes in the ratios of anthocyanins may be a response to the interactive effects of sunlight, temperature and, potentially, elevation. For wine volatile compounds, Z-3-hexanol, E-2-hexenal and an ester hexyl acetate (associated with ‘green’ characters) were elevated in Eden Valley wines in both vintages. The key wine odorant β-damascenone (‘red fruit’/‘floral’) was also consistently lower in Eden Valley wines. Interestingly, ‘vegetative’ sensory attributes were elevated in Eden Valley wines in both seasons and since E-2-hexenal has a strong ‘vegetative’/‘green’ aroma, this may point to its significance in defining wine style from this part of the Barossa. However, there are interactive effects between wine volatiles, and the lack of β-damascenone may also be relevant since it can potentially mask the expression of ‘vegetative’ attributes. In subsequent seasons the existing project work will continue, along with efforts to understand the effect of vineyard management interventions on specific sites.
The impact of mulching on wine chemical composition
An experiment was conducted on an Eastern Ridge site to assess whether a viticultural intervention (mulch) could substantially change the characteristics of wines from a single vineyard, relative to the differences observed between vineyards. Mulch was applied to three areas within the vineyard, while another three corresponding areas were untreated and used for comparison. Wines made from mulched sections had lower tannin concentration (including skin tannin, overall tannin molecular mass and degree of polymerisation). Wine colour density was not affected by the mulching treatment, but polymeric pigment and chemical age were lower in mulched areas, and there were higher concentrations of free monomeric anthocyanins in the wine. This indicated that a greater proportion of anthocyanin had reacted with tannin to form polymeric pigments in the areas that did not receive mulch. Other wine components that differed between the treatments were metals, with mulched areas being clearly defined by increased potassium, while concentrations of other metals including iron were lower. Although the effect of the mulch treatment was evident, the wines were nonetheless compositionally similar relative to wines from the other sites in the study. This suggests that strong site-specific ‘terroir’ effects may exist. Further viticultural interventions are planned for subsequent seasons to explore the influence of management practices as a component of ‘terroir’.