Project 4.4.4

Rotundone and its role in defining terroir in iconic Australian cool climate ‘peppery’ Shiraz

Project summary

Focusing on premium cool-climate Shiraz, and targeting vineyards with old planting material which may be unique to Australia, this project extends the collaborative research between AWRI, CSIRO and Mount Langi Ghiran. It seeks to define genetic features in grapevine planting material (which may be transferable through propagation and between vineyards) and/or environmental features (which are site-specific and might be influenced by management practices), that are contributing to distinctive aroma attributes in wine.

The project builds on research at the Mt Langi ‘Old Block’ in the Grampians, but expands that study to other selected premium Shiraz sites across South Australia and Victoria. Research to date has demonstrated that the Grampians and Pyrenees regions can produce wines with substantially higher concentrations of the ‘spicy’ flavour compound rotundone than other notable Shiraz-producing regions (e.g. Barossa and McLaren Vale). Patterns of rotundone variation appear to be temporally stable within an individual vineyard across different growing seasons, with large differences in concentrations occurring across individual vineyards (~15-fold differences observed) and across different growing seasons (~30 to 40-fold differences). In general, cooler ripening periods and limited bunch exposure appear to favour the synthesis/accumulation of rotundone in Shiraz grapes. However, it appears unlikely that seasonal conditions, temperature and bunch exposure alone, can explain the magnitude of the observed site-specific differences in rotundone. Hence this project aims to identify genetic and/or environmental factors which alone, or in combination, may shape grape composition and wine flavour in Shiraz.

Building on previous research between the AWRI, CSIRO and Mount Langi Ghiran, this project will:
• generate rotundone and α-guaiene maps for Shiraz vineyards in the Grampians, Pyrenees, and/or Adelaide Hills, in addition to the Mount Langi Ghiran ‘Old Block’, and characterise their variation in vine performance and the underlying land via EM38, elevation and vigour mapping
• investigate potential genetic determinants for rotundone concentration using reciprocal bud grafts and/or potted vines in greenhouses
• investigate unique key aroma compounds in Shiraz wine from cool-climate terroirs, the role of photochemical oxidation in the formation of aroma compounds from sesquiterpenes, and the kinetics of guaiene and rotundone accumulation/degradation
• investigate the potential relationship between soil microbes and inherent spatial variation in grape composition and rotundone concentration.

Latest information

Grape aroma compounds in cool climate Shiraz
The first high resolution grape aroma map for a Shiraz vineyard in the Adelaide Hills showed significant berry rotundone concentrations ranging from 10 to 530 ng/kg in vintage 2017. It also demonstrated substantial within-vineyard variability, a clear spatial structure of rotundone (as observed for other sites in different regions) and for the first time, a clear spatial structure of rotundone’s precursor α-guaiene. No clone effect was observed for rotundone in grapes; that is, the absolute grape concentration and spatial patterns were independent of the two Shiraz clones present in this block.

Analysis of sesquiterpenes in grapes from vineyards in Victoria and the Adelaide Hills also established that it is the biosynthesis of the precursor α-guaiene which is key to the subsequent formation of the rotundone in grapes. That is, availability of α-guaiene appears to be the driver of grape rotundone at harvest, not the oxidation reaction from α-guaiene to rotundone. The results confirm that both α-guaiene and rotundone concentrations in grapes are initially very low and stable from veraison onwards and only significantly increase close to harvest. In addition, the timing of the onset of α-guaiene and rotundone formation in grapes was very well synchronised in vintage 2017 and occurred at the same time for both compounds irrespective of final rotundone concentrations. This means that it is plausible that a singular environmental event triggers biosynthesis of α-guaiene across an entire vineyard.

Project Contact

Markus Herderich

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