Trials show the impact of oxygen

Small but important steps are being taken in the quest to better understand how early oxygen exposure during winemaking might influence wine style, improve fermentation efficiency and prevent reductive odours.

Researchers from the Australian Wine Research Institute (AWRI) have released the results of their first pilot-scale winery trials looking at how different amounts of oxygen enter passively into must depending on how winemaking is undertaken.

AWRI Research Scientist Dr Martin Day and colleagues used two pressing modes (inert and aerobic) and two forms of post-pressing handling (reductive or oxidative) to create four distinct wines from the same Chardonnay grapes, allowing the effects of oxygen timing to be closely examined.

Both variables were shown to impact on a wine’s final chemical composition and sensory characteristics, in this 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 post-pressing handling method', the researchers say in an article prepared for the February 2015 edition of Grapegrower & Winemaker magazine.

'This seems to suggest that, at least for Chardonnay, there is little need to invest so much time and money protecting juice and fermenting wine from oxygen, if the wine is made with aerobic pressing. Other white varieties may behave differently so caution should be used before throwing out the dry ice altogether.

'On the other hand, if a juice is produced by inert pressing then 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.'

In the next phase of the work, during the 2015 vintage, the researchers will focus on making deliberate but controlled oxygen additions during fermentation.

The trials are part of an AGWA-funded project, led by Dr Day, to investigate the effects of early oxygen exposure, develop tools for monitoring oxygen exposure, and provide practical advice on methods to introduce oxygen during winemaking. Fact sheets covering these last two topics are now available from the AWRI website.

Complementary small-scale laboratory experiments are also being carried out by AWRI Senior Research Scientist Dr Simon Schmidt. This work is seeking to understand how oxygen addition during fermentation affects yeast, and what the impact of this is on wine flavour.

'We already know that oxygen helps yeast perform better as fermenters. Only small additions of oxygen early in fermentation are required for this: adding more doesn’t give a bigger boost. In contrast, the production of flavour compounds by yeast depends on both how much oxygen is added and when it is added', Dr Schmidt said.

'The eagerly-awaited forthcoming vintage trials are a good way of seeing how the outcomes of these lab experiments work in practice at a working winery.'