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, the project’s scope will be expanded as part of a multi-agency collaboration to research Shiraz terroir across a range of scales, primarily in the Barossa Valley. At a regional scale this project will collect simple canopy and environmental metrics and fruit for basic maturity and quality assessment from at least 100 sites in the Barossa, including sites in the Barossa Grounds program. At the sub-regional scale ~20-25 sites will be monitored in greater detail, and fruit will be sampled for maturity, yield assessment and small lot winemaking. Sensory analysis will be performed on the small-lot wines and commercial wines. Fruit and wine will also be analysed for compounds relating to wine quality and style which in turn will be related to the vineyard, environmental and sensory parameters. 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 wine style.
Predicting wine quality grade from grape-based measurements
The initial goal of this study was to apply a limited set of analyses to grape samples sourced across two premium Shiraz quality grades to attempt to identify objective chemical measures which could delineate them. Determining the relevance of within-vineyard variability or relative homogeneity as a predictor of quality was an additional research goal. The initial dataset incorporated pre-vintage grade assignments, and using these it was found that the quality grades could not be distinguished by chemical measures. What was clearly evident from the pre-vintage analysis was that some vineyards were highly variable, with certain analyses showing as wide a range as previously observed across growing regions. Post-vintage, the final quality grade was assigned retrospectively, based on sensory assessment of wines by the producer. The final quality grades spanned four categories rather than the initial two assigned pre-vintage and were: 1 (highest quality), 1.5, 2, and 3 (lowest quality). Using multivariate statistics, only grade 3 could be successfully separated from the better quality streams. Grape-based measures which were important predictors of grade 3 were high total (homogenate) tannin, low total anthocyanin, high °Brix and high nitrogen measures including ammonia, alpha-amino nitrogen and certain amino acids. Interestingly, when small-scale ferment information was included in the model, it was found that grade 3 grapes produced wines higher in volatile acidity and lower in certain esters. Non-targeted spectra from homogenates and juice were also used to develop multivariate models. It was found that mid infrared spectra could be successfully used to discriminate grade 3 grapes. The preliminary work showed that objective measures were of use only to identify parcels of fruit which met quality targets based on vineyard assessment, but which lacked the necessary chemical composition to reach the expected wine quality grade. Ongoing work is looking more deeply at the question of vineyard variability, as well as seeking to better understand why particular vineyards fail to meet quality targets.
Defining the sensory attributes of premium Shiraz wines
Fruit from the premium Shiraz vineyards used for the grape objective measures study was kept as separate parcels in the winery, and commercial wines were prepared from each using older oak barrels to minimise oak effects on wine flavour and aroma. These commercial wines were then graded by a winemaker panel, and put through descriptive sensory analysis. Twenty-six attributes were assigned which could clearly describe the wines, and then multivariate statistical analysis of the results was performed. As for the objective chemical data in grapes, it was found that wines of grade 3 could be successfully distinguished from higher grades (1, 1.5 and 2) by their sensory attributes. Generally, grade 3 wines were lower in the attributes ‘opacity’, ‘purple colour’, ‘dark fruit’ aroma, ‘vanilla’/‘chocolate’ aroma, ‘woody’ aroma, ‘dark fruit’ flavour, ‘vanilla’/‘chocolate’ flavour, ‘woody’ flavour and ‘viscosity’ than higher quality grades. Grade 3 wines also had higher brown colour, ‘tinned vegetable’ aroma, ‘drain’ (reductive) aroma, ‘stalky’ aroma and ‘red fruit’ flavour. Using the grape data to predict wine sensory attributes, it was found that grape nitrogen measures, including certain key amino acids, were more important than other grape measures in defining the sensory attributes relevant to the study, and were better predictors of final quality grade. This points to nitrogen management in the vineyard as being critical to ensure wine quality outcomes are maintained, particularly in premium categories.