The Sugar/Flavour Nexus in Grapes Uncovering Its Identity and Relevance for Industry
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(Thesis)
Date
2021
Authors
Previtali, Pietro
Editors
Advisors
Ford, Christopher
Wilkinson, Kerry
Dokoozlian, Nick
Wilkinson, Kerry
Dokoozlian, Nick
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Thesis
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Abstract
During grape ripening genotype, environment and human management are interactively at play to shape grape chemical composition, which comprises sugars and a wide number of ‘secondary’ or ‘specialised’ compounds. Among those, polyphenols and aroma compounds are sought in winemaking for their positive sensorial contribution to wines. Interestingly, differences between sugars and specialised metabolites of grapes, primarily involving their localisation across berry compartments, dramatically different concentration ranges and possibly different biological functions, seem to contradict a connection between these berry traits. On the contrary, it can be speculated that either metabolites derive from the same carbon sources and therefore metabolic pathways for the synthesis of sugars or secondary metabolites need to be regulated at some level. If this was the case, understanding regulatory factors forms the basis for the definition of manipulation strategies to achieve the desired grape composition. Previous studies have highlighted the role of grape development in the formation of grape metabolites and described environmental conditions promoting their formation, but there is limited research to corroborate the existence of the sugar/flavour nexus. This thesis begins with a review of the previous literature concerning the formation of grape secondary metabolites, their relationship with sugars and techniques to manipulate sugar accumulation in grapes (Chapter 2). The following chapters describe original research grouped in three sections reflecting different experimental approaches to study the sugar/flavour nexus: the single berries approach (Chapter 3), the traditional approach (Chapter 4 - 5) and the meta-analysis approach (Chapter 6). Grape ripening is highly asynchronous and groups of berries at different stages of development co-exist at a given time within the total berry population. Previous studies on aroma development in grapes analysed mixed berry samples or subsamples of berries sorted according to their density, which inherently causes the averaging of different berries. To investigate the sugar/flavour nexus, an analytical method for the determination of C6 compounds in individual berries has been developed and the concentrations of six C6 compounds and sugars have been analysed in a wide range of single berries collected at multiple stages of development and sampling points (Chapter 3). Profiles of C6 compounds and sugars were strongly correlated to the stage of development, with little influence of the sampling time, canopy side or berry position. Warmer and drier conditions in many wine regions worldwide have accelerated sugar accumulation in grapes, leading to advanced ripening and impaired dynamics between sugars and organic acids or anthocyanins. However, there was a gap in literature as to the effect of accelerated ripening on the development of aroma compounds in grapes. By manipulating crop load and irrigation regimes in Cabernet Sauvignon and Riesling vines, different rates of sugar accumulation were established and grape aroma and phenolic compounds were analysed at sequential maturity stages (Chapter 4). A slower ripening led to a decrease in green compound and higher concentrations of some grape monoterpenes, norisoprenoids and phenolic compounds, showing that delayed ripening is key to restore a balance between traits desynchronized by climate change. Similarly, wine quality was enhanced when a slower rate of sugar accumulation was achieved (Chapter 5). An increasing number of publications on vineyard strategies to delay ripening in the past two decades have reported inconsistencies in the delaying capabilities of these practices. To strengthen the statistical significance of individual studies, quantitative data were gathered across relevant sources and re-analysed with the meta-analysis approach. Using this data-driven approach, the effectiveness of different commercially-available techniques was evaluated and conditions under which the magnitude of the delaying effect was increased were reported (Chapter 6). In summary, this thesis investigated the relationship between sugars and aroma compounds combining different approaches, from the level of single berries, which attempted to account for grape asynchronous development, to vineyard trials designed to manipulate ripening rates at the commercial scale, and finally using novel statistical tools to refine results of primary literature. This work has progressed our understanding of the formation of quality traits in grapes but has also potential contribution to the wine industry through an overview of tools to delay ripening, which in this work was demonstrated to enhance grape and wine quality.
School/Discipline
School of Agriculture, Food and Wine
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2022
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