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

Abstract

This project lends weight to the concept of terroir and demonstrates how the interplay of site, viticultural management, climatic and environmental effectors can shape aroma compound profiles in cool-climate Shiraz.

The results establish that development of the sought-after ‘peppery’ aroma from rotundone in Shiraz grapes appears to depend on site characteristics and environmental factors that regulate sesquiterpene biosynthesis, rather than genetic determinants of planted vines. From a practical vineyard management perspective, a late harvest date is key, as extensive formation of the immediate rotundone precursor α-guaiene, and subsequently rotundone itself, typically only commences well after veraison.

Summary

Previous research 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 appeared to be temporally stable within an individual vineyard across different growing seasons, with large differences in concentrations occurring across individual vineyards and across different growing seasons. In general, cooler ripening periods and limited bunch exposure appeared to favour the synthesis/accumulation of rotundone in Shiraz grapes. Focusing on premium cool-climate Shiraz, this project built on research at the Mount Langi Ghiran ‘Old Block’ in the Grampians, and expanded that study to other selected premium Shiraz sites across South Australia and Victoria.

In order to explain the magnitude of the observed site-specific differences in rotundone, this research sought to differentiate between 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.

Despite the elusive character of the unique grape aroma compound rotundone in premium Shiraz, detailed concentration-based maps of grape sesquiterpenes including the aroma compound rotundone could be established. Key findings from this project are:

• The broad intra-vineyard variability and distinct spatial structure in grape rotundone is not unique to Mt Langi Ghiran’s Old Block and its heritage vines, but appears to represent a common feature found in growing seasons with elevated grape rotundone concentrations and cool-climate vineyards planted with other clones.
• The spatial distribution of grape rotundone at an Adelaide Hills site planted with the Shiraz clones 2626 and 1127 is closely associated with its precursor, α-guaiene, and a range of other sesquiterpenes and is distinct, yet in contrast to Mt Langi Ghiran, may vary from year to year.
• Analysis of grape samples collected across multiple trial sites and growing seasons established that formation of elevated α-guaiene and rotundone are synchronised and typically initiated very late during berry ripening. This means that the biosynthesis of the precursor α-guaiene is key to the concomitant formation of rotundone in grapes. That is, availability of α-guaiene and initiation of α-guaiene biosynthesis appear to be critical for controlling grape rotundone at harvest.
• Analysis of site characteristics, including microbial populations, established that the spatial (intra-vineyard) variability in grape rotundone at the Adelaide Hills site was not related to established measures of vine performance and canopy, such as vine vigour (as measured using remotely sensed imagery). This result was similar to previous observations for Mt Langi Ghiran’s Old Block. In respect of other site characteristics, results were equivocal, perhaps due to marked differences in seasonal conditions and, in contrast to Mt Langi, a very limited vineyard area orientated away from north.
• Metagenomics analysis of surface soil samples collected over two seasons from the previously identified high- and low-rotundone zones at Mt Langi Ghiran’s Old Block found marked differences in the genetic diversity and composition of the soil bacterial and fungal microbiomes. A few specific taxa/groups of microorganisms were associated with the rotundone-based variation; and bacterial communities in soil from the high-rotundone zone appeared to form a much more complex and connected network than those in the low-rotundone zone soils. Also, short-term mulching effects did not seem to mask this rotundone zone-based variation in soil microbiology. • Functional profiling of the soil microbiomes in the low- and high-rotundone zones at Mt Langi Ghiran’s Old Block through direct metagenomic sequencing analysis showed significant and distinct dissimilarity between the low- and high-rotundone zone samples at all gene classification levels; that is, for genes, gene ontology (GO) and GO and COG (clusters of orthologous groups of proteins) pathway analysis.
• Studies of the formation of aroma compounds from sesquiterpenes established that sesquiterpene hydrocarbons common to grapes are absent from wine (other than a trace of α-muurolene), because they are not extracted into wine-like hydroalcoholic solutions and/or are removed from juice with solids. This means that sesquiterpene aroma precursors from grapes are most likely not chemically converted or metabolised during fermentation, while at the same time grape marc and wine solids are a potential source of sesquiterpene aroma precursors.
• Model experiments demonstrated that exposure to daylight catalyses the chemical oxidation of sesquiterpene precursors. This means that the formation of potent aroma compounds such as rotundone does not necessarily require the previously described enzymatic reactions by grapevines or microorganisms. In addition, the aroma compound iso-mustakone – formed by oxidation from its precursor α-ylangene – was identified through GC-MS-O analysis for the first time in wine, where it contributes ‘celery-like’, ‘spicy’ characters at very low concentrations.

In summary, for the formation of the ‘peppery’ aroma compound rotundone in Shiraz grapes, site characteristics appear to be more important than genetic determinants of planted Shiraz vines, as similar effects were observed in Shiraz vineyards planted with clonal selections, mass-selected vines, and in vines grafted with low-and high rotundone canes.

Overall, this project provides science-based evidence supporting the concept of terroir, demonstrating how the interplay of site, viticultural management, climatic and environmental/biological effectors (albeit the latter are yet to be identified) shape distinct sensory attributes in Shiraz wine.

Further research and insights into the environmental regulation of sesquiterpene biosynthesis in Shiraz are required to preserve the desirable, but elusive, ‘peppery’ flavours in existing ‘cool-climate’ vineyards through preserving the environmental and/or biological effectors. In addition, such knowledge could potentially enable growing Shiraz grapes with ‘cool-climate-like’ flavour attributes in warmer/hotter regions. Ultimately, the findings of this projects represent an important step towards mitigating risks from climate change, biodiversity loss or viticultural management decisions, which might otherwise lead to unintended flavour consequences in Shiraz.