Introduction
The amount of vineyards affected throughout South East Australia by Botrytis cinerea in vintage 2011 was very high. In this article we look at the effects this fungus can have on wine quality, how to test the impacts and what can be done to overcome these effects.
What is Botrytis Cinerea?
Botrytis cinerea is a parasitic fungus that can affect many plant species but is particularly well known in viticulture where it can form mould on grapes. Strawberries and tomatoes are other fruits that can also be badly affected (if you have ever had grey mouldy strawberries in the fridge: that’s Botrytis cinerea).
The name Botrytis cinerea is derived from Latin and means “grapes like ashes”; the ashes referring to the common form of the fungal infection which gives a grey fuzzy mould on fruits. The organism is found in all wine growing regions worldwide but is not a problem in hot and dry conditions.
What does it do to grapes?
The common form of Botrytis cinerea on grapes is called grey rot. This mould forms in cold wet weather (15 oC to 20oC and relative humidity over 90%) and can severely reduce fruit quality. Varieties with thinner skins and tight clusters are more prone to Botrytis cinerea infection; white varieties that are more likely to be infected include Chardonnay, Riesling and Sauvignon Blanc. Red varieties can also be affected.
In some particular weather circumstances the Botrytis cinerea infection forms what is known as noble rot. High humidity is needed for infection but then drying conditions need to follow for the dehydrating effect that can increase the sugar content by up to 30% above normal levels due to the permeability of the berry skin. This effect is desirable in making dessert wines with interesting flavours from some white varieties and has been long known in old world winemaking; classic dessert wine examples are Sauternes, Tokay and Trockenbeerenauslases.
What does it do to juice?
The main effects on juice are on the acids and sugars. Both tartaric and malic acid can be metabolized by the mould, however the dehydration of the berry can mask this concentration lowering effect.
Because the berry skin is affected secondary infections of wild yeasts and bacteria can also take place. This can have the effect of sugars being consumed and other metabolites produced. Compounds such as ethyl acetate, acetaldehyde, glycerol and acetic, lactic and gluconic acids can be present in the juice and subsequent wine because of these multiple infections.
Botrytis cinerea (as well as the wild yeast and bacteria) consumes amino acids, ammonia and some vitamins and so can have a further negative effect on yeast fermentation of sugars in the juice by leaving fewer nutrients available for the yeast. Careful supplementation, preferably with a complex nutrient, should be performed during the yeast fermentation.
The oxidative enzyme laccase is produced by Botrytis cinerea (as do other fungi) and is a particularly problematic side effect of the infection. Laccase causes oxidation of phenolic compounds and so can have a major impact on resultant colour in red wine with browning being the main issue. The non flavonoid compounds present in juice and wine, p-coumaric and caffeic acids, are oxidised to quinones by laccase and the polymerisation products produced give a brown colour. Browning is also an issue in white wines.
Laccase treatment
Laccase is a particularly troublesome enzyme as it cannot be totally removed by the normal practises of sulphur dioxide or bentonite additions. The efficacy of sulphur dioxide is exacerbated by the increased presence of other metabolites that cause binding: for example acetaldehyde. Laccase continues to be active, particularly if oxygen is present, even with ethanol at the levels encountered in finished wine.
The actions that a winemaker can take to reduce the impact of laccase are limited but are well known and have been practised for many years. These include:
- addition of sulphur dioxide
- bentonite addition
- low temperatures
- inert gas
- pasteurisation
As laccase can still be present in the wine made from Botrytis cinerea affected grapes, this can cause ongoing problems in both red and white wines.
Glucans
Another effect of Botrytis cinerea in wine is the production of glucans – large chain polysaccharides. Glucans are formed by the action of Botrytis cinerea enzymes on pectins. The ethanol in the wine causes the resulting glucans to conglomerate which causes clarification and filtration problems. Commercial glucanase enzymes, such as Rapidase Glucalees, can be used to reduce this problem.
What tests can be done?
The level of Botrytis cinerea infection can be gauged to some degree in the vineyard by visual inspection. However sometimes the infection can be on the inside of bunches and not readily observable. Segregation of infected fruit is the first and best method of eliminating Botrytis cinerea problems; however this is not always practicable.
The presence of Botrytis cinerea in juice can be tested for by Enzyme-Linked Immunosorbent Assay (ELISA) technology. This vintage at Vintessential we trialled the Envirologix ELISA test strip that gives a rapid and qualititiatve result for a particular antigen produced by Botrytis cinerea. The strip can also be read in an optical bench top instrument to give a semi-quantitative result in % incidence of Botrytis cinerea. This technology looks very promising as it is relatively cheap, robust and can be used in the field with minimal equipment. It could be well deployed by growers, grower organisations and fruit buyers at the fruit receival stage to give an objective measure of Botrytis cinerea incidence and so minimise the angst caused by fruit rejection based upon visual estimation and to help make processing decisions.
Another test employed by many winemakers for red wine is to place a small amount of wine in an open dish at ambient temperatures for several hours and to see if the wine turns brown; thus giving them a rough guide as to how much laccase is present.
Laccase can also be tested for its oxidative ability in a more accurate test but requires a spectrophotometer (a common piece of laboratory equipment in wineries). The basis of this test is the oxidative impact of laccase on a chemical that gives a purple colour when oxidised, with the resultant colour change giving a guide as to the level of laccase present. This vintage our labs have been very busy performing this test.
Glucans can also be determined in a semi-quantitative way. This measurement can be a great help in deciding which treatment is required to reduce filtration problems.
Conclusion
Botrytis cinerea has been very problematic this vintage. The effects of the infection include fruit spoilage by mould, introduction of the oxidative enzyme laccase and the formation of the long chain polysaccharides glucans that give problems with clarification and filtration. Several ways of testing for Botrytis cinerea are available to assist the winemaker in making decisions as to how much impact has occurred and on how to overcome the issues raised.
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