Untimely rains in certain wine regions of Australia have seen Botrytis appear in some vineyards. A number of queries have been taken by our winemaking team on how to process fruit containing Botrytis and the potential impacts this might have.
Botrytis cinerea is a weather driven fungus which causes the grapevine diseases Botrytis bunch rot and grey mould. High humidity or prolonged rain in conjunction with cool or mild temperatures results in persistent moisture on berry surfaces and promotes infection and disease development. Previously infected sites and sheltered vineyard areas such as hollows are at greatest risk of developing the disease.
Managing Botrytis in the vineyard can be a challenge because many factors contribute to infection and disease development.
Botrytis can cause two problems during winemaking. First, it produces the oxidative enzyme ‘laccase’, which in the presence of oxygen can cause oxidative spoilage. Second, the presence of Botrytis can lead to the presence of a mouldy character in the resultant wine. Preventing this enzyme from causing damage requires techniques that minimise or eliminate exposure to oxygen. Eliminating or minimising the quantity of Botrytis-affected fruit processed will reduce the potential effects of oxidative damage, however, a holistic approach is required which involves many aspects of the winemaking process.
Botrytis infection in red grapes will require different treatments to infection in white grapes. The following processing strategies will help reduce the oxidative damage that can be caused by Botrytis infected fruit.
For both white and red fruit
Minimise the quantity of mould-affected fruit at harvesting. For crops picked by hand, fruit can be selectively harvested and infected fruit avoided. Pickers must be able to recognise Botrytis bunch rot and know which bunches should be avoided.
If the vineyard is to be machine harvested, send hand pickers through the vineyard to remove the worst affected fruit prior to machine harvesting.
Add more sulfur dioxide (SO2) than usual (in the range of 60-80 mg/L) due to the increased risk of oxidation from laccase (although SO2 does not inactivate laccase) and because there is likely to be a higher than usual population of other unwanted microorganisms if Botrytis is present.
Whole bunch press with CO2 cover. Assess press fractions for mouldy taint carefully.
Add pectic enzyme at the higher end of the recommended range and cold settle at low temperature to achieve rapid settling. Extra additions of pectic enzyme might also be required if settling.
Rack and discard the heavy lees.
Trial and add bentonite to remove mouldy characters and settle for 24 hours. Recommended starting rates is 0.5-1 g/L bentonite.
Rack off bentonite lees.
Test for laccase activity: if laccase activity is detected, heat treatment (pasteurising) should be considered to deactivate the laccase enzyme before conducting the fermentation (e.g. 50°C/90 seconds, or 55°C/30 seconds, 60°C/5 seconds, 70°C/1 second).
If heat treatment is not available, initiate fermentation. Addition of 0.1-0.2 g/L of bentonite during fermentation might be beneficial.
As SO2 can inactivate thiamine, the addition of thiamine to the must should be considered.
The lees will contain much of the laccase, so it is important to rack off fermentation lees as soon as possible after the fermentation is complete, and keep wine in stainless steel with inert gas cover.
Test for laccase activity: if positive, further racking can be beneficial in order to remove all fermentation lees. However, if laccase activity is still detected in the wine after subsequent racking, heat treatment of the wine might be necessary.
Minimise the time between crushing and inoculating. Cold soaking should be avoided.
Addition of greater yeast inoculums as a sacrificial culture could assist with binding of free SO2.
Addition of 200-500 mg/L of an oenological tannin at crushing will bind the laccase enzyme (note that tannin addition can change wine style).
Where practical, separate heavy fermentation lees at pressing, utilising the carbon dioxide (CO2) produced during fermentation for coverage, and press to stainless steel not oak.
Rack off gross lees after 24 hours and test for laccase activity. If laccase activity is still detected in the wine after subsequent racking, heat treatment of the wine might be necessary.
These wines are often difficult to clarify and filter due to the presence of long chain polysaccharides which are produced by Botrytis. A suitable enzyme (see “Pectic and lysozyme enzyme preparations available for winemaking – 2009/2010” in Technical Review 182) to assist with clarification and filtration might be required at a later stage.
Further reading on other vineyard-derived moulds can be found here.
Should you require further assistance, please call the AWRI’s Winemaking and Extension Services team on 08 8303 6600 or email: firstname.lastname@example.org