José Santos, Enartis Vinquiry
Even though the incidence of Botrytis is typically lower in the US than in other wine regions, high levels of infection were found in the 2010 and 2011 vintages. High incidences of Botrytis are also being found in 2013 despite the ideal growing conditions during the summer, particularly in regions of the Pacific Northwest.
When winemakers think about Botrytis infection and the consequences, they think mainly about the presence of laccase, an oxidative enzyme that can cause browning. However, there are other metabolites produced that can decrease and jeopardize the quality of wine. Among these, the most worrisome are the production of glucans and gluconic acid by Botrytis, as well as higher levels of VA due to activity of indigenous yeast or bacteria on damaged fruit..
So, when facing a potential risk of Botrytis infection, what do you need to know?
1. How much is there?
Because Botrytis levels can be difficult to estimate visually in fruit (“rot” is non-specific and the infection can be hidden within grape clusters), it is recommended to quantify the percent of infection.
A test using a Botrytis monoclonal antibody that recognizes a specific Botrytis antigen in grape juice from affected berries is available. Results are reported in percent incidence which allows wineries and vineyards to make better decisions on harvesting, segregation and processing, correlating it to vineyard monitoring results.
2. Has the fruit quality already been compromised in the vineyard?
The underlying question here is “should you even bother?” If you haven’t already made a decision whether to harvest based on the appearance of the fruit or the known level of Botrytis, the following tests can help to objectify your decision-making process:
Gluconic Acid – D-Gluconic acid (gluconate) is a metabolite of Botrytis and can be a useful marker for Botrytis activity in the vineyard. It is associated with a sour taste and has an overall negative effect on quality. Gluconic acid is used globally as a rapid screening tool for assessing fruit quality on incoming fruit, even so far as influencing the price and/or rejection criteria in vineyard contracts.
Countries like France (in particular Champagne), Spain, Portugal, Argentina and Chile have already adopted this as the key parameter to determine how much to penalize the payment on Botrytis infected grapes, as well as a winemaking decision tool. Some wineries reject grapes if the content of gluconic acid is over 0.5 g/L. Levels of 0.2 – 0.3 g/L are a red flag and require action.
Volatile Acidity – VA is a rapid and inexpensive indicator of spoilage from native yeast and bacteria in the vineyard. The measurement of acetic acid is also a good indicator of microbial spoilage.
3. How much laccase activity can I expect?
Because the level of laccase produced varies widely and is not perfectly correlated with the level of Botrytis infection, it is important to understand how “reactive” the laccase will be in your juice or must. Each juice or must will vary in its anti-oxidant capability and resistance to laccase activity.
The different methods available to measure laccase activity also vary, in particular regarding the range and units of measure with which the results are reported, making it difficult to compare results.
Testing laccase alone is a limited approach, as it looks only at one parameter when in fact Botrytis can cause many issues in wine.
That being said, Enartis Vinquiry’s Laccase Activity Test was designed to evaluate the presence of residual and active laccase in musts and wines. It allows for an objective oxidative risk measurement for the consideration of corrective measures (tannin addition, SO2 additions, etc.). We recommend testing before and after any corrective measures are employed to determine treatment effectiveness. In addition, the laccase activity should also be tested post alcoholic fermentation.
4. Will I have problems with clarification or filtration?
Botrytis cinerea produces glucans – sticky polysaccharides that are released into the juice and must at crushing. Glucans can inhibit the clarification of juice and can go on to cause problems with filtration and fouling of membranes in the finished wine stages. The Glucan Screening test is a semi-quantitative test in which we are able to detect the presence of as low as 3 mg/L of glucan in a juice or wine sample. Levels as low as 3 mg/L can, early on, have a negative impact on the clarification and subsequent filterability of the wine.