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

Background
Rotundone is the potent, grape-derived compound responsible for ‘black pepper’ aroma in wine. Previous research demonstrated that the Grampians and Pyrenees regions in Victoria can produce wines with substantially higher levels of rotundone than other Shiraz-producing regions such as Barossa and McLaren Vale. Patterns of rotundone variation appear to be stable within an individual vineyard across different growing seasons. A new collaborative project with CSIRO commenced in 2017, focusing on premium cool climate Shiraz, that aims to define features at the within-vineyard scale that contribute to rotundone formation. The research seeks to identify genetic and biophysical factors responsible for large differences in rotundone concentrations at harvest, and will provide insights into how the rotundone element of terroir, and grape aroma in general, may be influenced and managed at a range of scales.

Aroma compounds in cool climate Shiraz
Gas chromatography-mass spectrometry analysis confirmed the presence of the sesquiterpene, α-muurolene, in Shiraz grapes and wine. However, α-muurolene did not contribute any notable sensory attributes and model oxidation experiments did not provide evidence for the formation of obvious aroma compounds from α-muurolene. Instead, the oxidation of the grape sesquiterpene α-ylangene, a compound commonly found in grapes from ‘peppery’ vineyards, has been demonstrated to result in the formation of a new potent aroma compound present in wine where it contributes ‘celery-like’/‘spicy’ characters.

The role of site characteristics in defining grape aroma compounds
Despite bushfires and COVID-19 restrictions, vintage 2020 field trials at Mount Langi Ghiran were completed as planned. In addition, sentinel grapes were also sourced and exposed to cold temperatures. For this model experiment, controlled conditions were chosen to replicate temperature profiles of cool climate vineyards observed in previous vintages. Some elevated concentrations of rotundone and its precursor α-guaiene have been observed and additional analyses are underway to corroborate the effects of ambient temperatures on grape metabolites.

Further understanding of sesquiterpene hydrocarbons in grapes
Most sesquiterpene hydrocarbons commonly found in grapes are absent from wine. It has now been shown that this is because these compounds are not extracted into wine-like hydroalcoholic solutions and are instead removed from juice as they bind back to solids from grapes. This means that sesquiterpene hydrocarbons can act as flavour precursors in grapes, but not in yeast fermentations.


Project Contact

Markus Herderich, Mark Krstic

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