Influencing wine style and efficiency through management of oxygen during wine production
Abstract
Aeration applied during active fermentation is known to have effects on fermentation performance and wine style in white and red wines respectively. An understanding of the practical approaches to achieve these effects was lacking at the project outset. A series of pilot-scale trials and collaborations with industry partners was undertaken to refine our understanding of optimal schemes for white and red ferment aeration and to elucidate practical strategies for implementing those schemes. These investigations have informed the development of educational resources that are available to winemakers interested in using aeration during the production of their wines.
Summary
A major aim of the project team was to focus on aeration solutions that did not require information about dissolved oxygen concentration or redox status during fermentation. Experimentation with various types of equipment for the measurement of dissolved oxygen status indicated that their implementation was likely to be impractical, economically unviable or technically challenging in many instances. Therefore, laboratory-scale and pilot-scale trials were undertaken to define airflow rate and duration metrics that could be applied with minimal investment.
Fermentations of red and white grapes respond very differently to aeration and therefore require different approaches, both in regard to the intent of the application and the practicalities of implementation. For these reasons they are discussed separately in this report.
A compilation and analysis of data across four years of red wine vintage trials showed the limited effect of aeration on the efficiency of red wine alcoholic or malolactic fermentation. The lack of an observable effect on fermentation duration led to a shift in focus to examine its effect on red wine style. It was shown that increasing airflow rate during fermentation resulted in a step-wise increase in the intensity of red fruit attributes and a decrease in attributes related to sulfide compounds compared to the non-aerated wines. The effects on ‘viscosity’ or ‘dark fruit’ aroma and flavour were limited.
Aeration applied over an extended period at low intensity, or repeated short-duration high-intensity applications had equivalent compositional and sensorial effects. The red fruit attributes were consistently associated with the enrichment of ethyl-2- and ethyl-3-methylbutanoate in wines that had been aerated during fermentation.
Red wines made without aeration or with limited oxygen exposure were typically more opaque and astringent. Wine colour density and either hue, non-bleachable pigment or total anthocyanin were strongly associated with ‘opacity’. Total phenolics and other non-volatile measures such as mean degree of polymerisation of tannin (mDP) and total tannin were positively related to wine ‘astringency’. In addition, descriptors such as ‘drain’ aroma, ‘earthy’ aroma, ‘bitterness’ and ‘vegetal’ were more commonly associated with wines made using little or no aeration. Hydrogen sulfide was strongly implicated in reductive aroma attributes such as ‘drain’, ‘vegetal’ and ‘earthy’ in some cases combined with MeSH or its acetate ester.
Optimal strategies for the aeration of red wine fermentations are those that can be coupled with cap management operations. Effective aeration coincident with cap management requires that these operations are undertaken frequently, at least daily but ideally more often, due to the generally short duration of cap management.
Investigations of the effect of oxygen exposure during the processing of white grapes were completed and published during the project period, showing the possible stylistic potential and limitations of oxygen management during processing. Subtle increases in floral and confection characteristics were demonstrated in white wines with limited oxygen exposure during pressing and handling operations. Strict protection of wine from oxygen exposure also minimised yellow colour development. However, these wines were more sensitive to post-fermentation oxidative colour development.
In contrast to the effect of aeration on red fermentations, the effect on white wine fermentations was almost entirely related to fermentation efficiency. Aeration intensity beyond that required to stimulate fermentation performance was invariably detrimental to wine quality. Low-intensity aeration sustained over a longer duration was determined to be the optimal approach for white ferments. Attempts to apply shorter aeration treatments at defined times during fermentation were less effective at reducing fermentation times than longer aerations.
The concept of aerating white ferments to reduce fermentation time was extended to non-inoculated ferments, where timing and aeration intensity were explored, mindful of the potential repercussions on microbial community structure. Optimal fermentation performance benefits from aeration of non-inoculated ferments were achieved when aeration coincided with the onset of S. cerevisiae ascendency within the microbial community.
Built-in sparger systems lend themselves better to white fermentations due to the absence of a cap and their more uniform nature. Such systems are also less prone to damage during the post-fermentation operations associated with white fermentation.
Knowledge building about the implementation of aeration at scale was undertaken through collaboration with industry partners. These trials included comparisons of aeration using rack and return with inline sparger systems, and initial evaluations of the contribution of Pulse-Air devices to ferment aeration. Whether Pulse-Air-like devices can be configured to be used efficiently for aeration operations remains an open question.
A substantial set of educational resources was generated on the effects of aeration during fermentation in red and white wines. These resources include the different ways of undertaking aeration and the methods available to measure those effects. Extension materials related to aeration were continuously developed as a direct consequence of experimental work undertaken during the project. In the final year of the project, team members provided content for an adoption program across various formats, including webinars, podcasts, demonstration videos and web-accessible fact sheets.