Grape harvest bio-protection on red wine making process
We had mentioned in one of our recent articles that grape harvest bio-protection [ read “Harvest bio-protection: state of play in 2023” ] is a key issue in the winegrowing world.
But how can it be applied in practice? What are the processes, and what conclusions can we draw?
At SITEVI 2021, the Institut Rhodanien presented the results of an experiment seeking to define the limits of bio-protection on a red wine process.
A reminder of grape harvest bio-protection
A major issue in sustainable development, bio-protection helps winegrowers adopt processes that have less impact on the planet and people, and produce healthier, sulphite-free wines. Bio-protection encompasses microbiological processes which can be used to substitute SO2 during the pre-fermentation phase.
About the experiment and the procedure
The aim of the experiment, conducted on a harvest of Syrah grapes, was to identify the possible limits of bio-protection in the event bacterial spoilage in the must and high-risk winemaking practices (at the pre-fermentation maceration stage).
Good to know
The control samples were an untreated wine (no SO2, no bio-protection) and a wine with 3g/hl added sulphite at vatting stage.
After harvesting, the grapes were crushed and destemmed. At this stage, 10,000 cells of spoilage yeast/ml were added and left overnight. The spoilage yeasts added were Brettanomyces and Hanseniaspora, the most commonly found on grapes. The next morning, SO2 or bio-protection yeasts were added to the vats: pre-fermentation maceration began, at 12°C, for 3 days, with daily punching of the cap. This was followed by yeasting with saccharomyces cerevisiae. The experiment then consisted of monitoring the 16 vats during alcoholic fermentation.
Monitoring and development of spoilage
On the first day of pre-fermentation maceration, the musts were highly contaminated, with a concentration of acetic bacteria, positive yeasts and spoilage yeasts in excess of 10^5 per ml. Will bio-protection really succeed in doing anything? That's the big question at this stage.
In the middle of alcoholic fermentation, there were still just as many acetic bacteria and there was a marked increase in positive yeasts and spoilage yeasts.
At the end of the process, there was very little difference between all the vats. Whether it had been given added sulphite or not, the wine contained acetic bacteria and spoilage yeasts. In all cases, the wine was unmarketable, as it contained high levels of acetic acid and volatile phenols.
The aim of the experiment was to identify the limits of bio-protection, and these were indeed identified! By going this far in the experiment, we can clearly assert that bio-protection does not work miracles on spoilt grapes.
Conclusion of the experiment on the red wine process
Bio-protection is an interesting alternative to SO2 and offers another way of working, but it is only effective under certain conditions. All the same, it requires a microbiological pre-diagnosis before harvesting to help make the best decision. It should be noted that although it is considered a key lever in SO2-free winemaking, it cannot be used in isolation.
This project enabled us to gain a better understanding of the mechanisms involved and is contributing to the development of new processes to combat wine spoilage in the long term. This is another subject that will be central to the next edition of SITEVI in 2023 !