Influencing wine style through management of oxygen during winemaking

Abstract

Effective management of oxygen during winemaking can help create diverse wine styles. Oxygen exposure can be readily modulated throughout the winemaking process and a range of approaches are available to manage it. However, many of these are not based on scientific knowledge of their effects on fermentation and wine style, or are not underpinned by a clear and holistic understanding of the benefits and financial impacts across the entire wine production chain.

The aim of this project was to establish the impact of early use of oxygen at crushing or during fermentation on wine style, and on the efficiency of malolactic fermentation, using both model systems and pilot-scale fermentations. In addressing these questions this research also improved understanding of how oxygen management during processing and fermentation impacts on fermentation efficiency and fast track ageing of wine. Adoption of the outcomes from this research represent a significant opportunity for the Australian wine sector to manage oxygen exposure effectively, enhance stylistic diversity, improve fermentation efficiency and reduce costs derived from excessively reductive handling of wines.

Five pilot-scale vintage trials and numerous controlled laboratory experiments were carried out during this investment period. In parallel, several industry partners trialled the use of air additions at small, medium and large-scale wineries across the country.

The benefits of adding sizeable amounts of oxygen to red ferments include a reduction in the need for adding nitrogen supplements (a significant cost saving in itself) and prevention of low levels of sulfidic off-odours, thus bringing bright fruit characters to the forefront of the wine bouquet. In addition, softening of tannins during fermentation may reduce maturation time before bottling and make the wine available for market several months earlier.

In white winemaking, the research showed that oxygen additions can increase fermentation efficiency without having negative effects on sensory outcomes. This kinetic rather than stylistic effect could have a major impact on the efficiency of fermentation by allowing a wine to finish fermentation several days earlier than normal while maintaining style through unaltered fermentation temperatures. This is a particularly valuable outcome considering the growing need to manage fermentations in compressed vintages.

Summary

Effective management of oxygen during winemaking can help create diverse styles that are attractive to a range of different consumers. Many approaches to oxygen management are currently practised and oxygen management has predominantly been focused on: post-fermentation treatments; management during bottling; and the effects of closure selection on post-bottling development. However, the effects of oxygen management during the process of winemaking (from crushing through fermentation) are not well understood. The limited information at hand is mostly about the management of fermentation efficiency and reliability. Oxygen exposure is a valuable and readily available management option throughout the winemaking process and many practical approaches are available to manage it. However, many of these are not based on scientific knowledge of their effects on wine style, or are not underpinned by a clear and holistic understanding of the benefits and financial impacts across the entire wine production chain. For example, managing oxygen exposure at crushing and juice stage may be very capital-intensive and expensive (e.g. inert crushers), whereas management during fermentation can be achieved with modest capital modifications.

The aim of this project was to establish the impact of early use of oxygen at crushing or during fermentation on wine style, and on the efficiency of malolactic fermentation using both model systems and pilot-scale fermentations. These are critical to delivering the best quality product possible. Questions at the outset of this project included: how much oxygen (O2) gets into juice through production? What does juice exposure to oxygen do to final wine style/composition? What does oxygen exposure during fermentation do to wine style? Can oxygen measurement be improved during winemaking or can markers for exposure be found?

This project focused on influencing fermentation efficiency and/or wine style through management of oxygen during winemaking. Five pilot-scale vintage trials and numerous controlled laboratory experiments were carried out during this investment period. In parallel, several industry partners have trialled the use of air additions at small, medium and large-scale wineries across the country.

The benefits of adding appreciable amounts of oxygen to red ferments have been demonstrated to remove the need for adding nitrogen supplements (a significant cost saving in itself) and prevent low levels of sulfidic off-odours, bringing ‘bright fruit’ characters to 

the forefront of the wine bouquet. In addition, softening of tannins may reduce maturation time before bottling and make the wine available for market several months earlier.

In wines made in 2012, greater oxygen exposure during fermentation produced wines with more ‘aged’ characteristics with respect to greater hue, fewer anthocyanins, lower tannin concentrations and smaller tannins with more modified structure. These changes were similar to those induced by 12 months of bottle-ageing in wines deprived of oxygen during fermentation. Treatments with 40% O2 and air scored lowest for ‘bitter’ and for ‘astringency’, while the protective N2 treatment scored highest for ‘astringency’. This suggests that increased oxygen exposure during winemaking may reduce the need for extended wine ageing, saving winemakers costs associated with tannin fining and extended storage, and possibly increasing consumer preferences. Recent research on white wine phenolics adds to the body of evidence that oxygen is likely to impact wine texture: these results established that two of the major phenolics in wine (grape reaction product [GRP] and caftaric acid) that are influenced by oxygen exposure also modulate the perception of astringency and oiliness.

Additional beneficial effects of oxygen additions to ferments included decreased metal concentrations in wine post-ferment which may benefit the wine’s shelf life and evolution, and significantly faster rates of malolactic fermentation which might provide a practical tool to assist in the reliable completion of malolactic fermentation.

In white winemaking, the research showed that oxygen additions can increase fermentation efficiency without having negative effects on sensory outcomes. Modulating the extent of oxygen exposure at the very earliest stages of juice preparation has been an important tool in understanding the effect of oxygen in white winemaking. Although the project did not set out to assess the merits of inert pressing, experiments highlighted some subtle effects that can be achieved from pressing under low oxygen conditions, if not totally inert environments. This is an area that should receive some further investigation, particularly looking at must from a range of grape varieties.

In a vintage 2014 experiment different oxygen levels that could be achieved simply through pressing and handling operations were investigated in Chardonnay, without further oxygen additions being conducted. The choice of pressing mode and the extent to which juice or wine was protected from oxygen during handling were both shown to affect a wine’s final chemical composition and sensory characteristics, in this particular case potentially affecting ‘floral’ and ‘citrus’ characters. For juices prepared through normal (i.e. aerobic) pressing, no significant differences were introduced through the choice of handling method. This seems to suggest that, at least for Chardonnay, there is little need to invest too much time and money protecting juice and fermenting wine from oxygen, if it has been produced through aerobic pressing. However, other white varieties 

may behave differently so caution should be used before dispensing with inert gas blanketing altogether! On the other hand, if a juice is produced by inert pressing it was shown that sufficient phenolics remain to be affected by further oxygen exposure during normal handling. Inertly pressed juices therefore need continued protection through reductive handling, if oxidation is to be avoided.

Having observed and quantified the chemical and sensory differences that occur through passive oxygen exposure in this study, trials during the 2015 vintage focused on making deliberate but controlled oxygen additions during fermentation which have potential for greater impact on wine style. These experiments demonstrated that addition of oxygen during white wine fermentation has positive benefits, with the main impact on the kinetics of fermentation rather than style of wine. This could lead to significant improvements in the efficiency of fermentation by allowing a wine to finish fermentation several days earlier than normal, while maintaining style through unaltered fermentation temperatures. This is a particularly valuable outcome considering the growing need to generate fermentation efficiencies during compressed vintages.

The sensory effects of adding oxygen in the 2015 experiments were minimal. The preferred timing of oxygen addition appeared to be in the first half of fermentation when sugars had dropped by 20% of the starting concentration. It was still beneficial, however, to make a late addition, even once the sugar concentration had dropped by 80 %. Although this did not give a considerable boost to the fermentation, it ensured that the ferment achieved dryness safely. The sensory analysis confirmed that there were no negatives issues associated with using a reasonable amount of oxygen.

The positive impacts of adding oxygen during red wine fermentation was initially explored in 2012 and are detailed above. During the 2016 vintage trials, the type of fermenter used and the way the aeration was carried out was modified to demonstrate how this could be achieved in wineries not equipped with rotary fermenters and with minimal capital outlay. The timings used in the 2015 trial were replicated with additional treatments of a daily dose and a post-press addition. In order to achieve the positive benefits of enhancing the ‘bright red fruit’ attributes through suppression of low-level reductive aromas, it was shown to be important to use an early aeration during the first few days of active fermentation. In addition, to achieve a decrease in astringency by softening the tannin, a repeated exposure may be necessary.

In summary, by adopting the outcomes from this research significant opportunities can be realised to manage oxygen exposure effectively, enhance stylistic diversity, improve fermentation efficiency and reduce the costs derived from excessively reductive handling of wines.

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This content is restricted to wine exporters and levy-payers. Some reports are available for purchase to non-levy payers/exporters.