Lees are a central part of winemaking – for good and bad – but until now we have lacked a detailed understanding of their physical properties.
A Wine Australia project just completed at the Australian Wine Research Institute has filled that knowledge gap, but was unable to find a way to avoid the process of moving juice or wine and cleaning a tank each time settled lees need to be removed.
‘Settled lees often occupy less than 10 per cent of the volume of a tank so in this project we were interested to see if it was possible to rack this out from underneath the clear juice or wine, which could then stay in the same tank,’ said project leader Dr Simon Nordestgaard. ‘However, the trials weren’t successful enough to justify developing a prototype device or tank modification.’
The research team was able to remove lees from underneath clear liquid using brewery-style cylindroconical tanks, and to a lesser extent agitating the surface of the lees with a sweeping arm. However, both methods would be impractical to implement as it would require significant new capital purchases.
The detailed data collection and analysis that preceded the trials, along with discussions with wineries about usual practices, did offer interesting insights and the final report makes a number of recommendations. These include:
- more work be carried out on the optimisation of draining and pressing processes to minimise grape-derived lees,
- alternatives continue to be sought and implemented for bentonite to heat stabilise white wines, as it is the second major source of lees after grape solids, and
- flotation be considered as an option for juice clarification by medium and large wineries currently using settling.
‘Flotation appears to be a means of achieving low juice lees volumes compared with settling, with less capital investment than other separation technologies and it is being increasingly used,’ the final report says.
The report also notes that oak chips in red ferments that end up in settling tanks can cause processing difficulties and suggests that wineries experiencing this problem consider either larger oak chips, tighter screens or alternatives to oak chips.
To Dr Nordestgaard, the most striking discovery was how different red ferment lees were compared with all the other lees. ‘They were much thicker and had a much higher yield stress, which is the stress you have to put in to make something flow. And this occurred even when the red lees had the same percentage solids content as other lees.’
That may not surprise winemakers and cellar hands who have the job of removing red ferment lees from tanks every year, but it does have implications for equipment manufacturers.
‘A big change happening in the sector is that people who have traditionally used rotary drum vacuum filtration to recover juice and wine from lees are now moving towards cross-flow lees filters to avoid potential quality losses,’ Dr Nordestgaard said.
‘With more interest in lees cross-flow filtration it’s important to know how this will handle the different types of lees.’
As part of the project, Dr Nordestgaard ran a workshop on lees recovery techniques at last year’s Australian Wine Industry Technical Conference.
The final report, which can be accessed on the Wine Australia website, includes detailed information on the various trials, including links to short YouTube videos.