Best Practices for Working with Rot
Last updated: 9/2023
Applies to: Producers dealing with disease-compromised fruit including those with mildew and rot (particularly rot from Botrytis cinerea). This article serves as an introduction to the complete guides which contain process and product recommendations. The full best practices guides are available as pdf downloads at the beginning and end of the article.
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For the full product and process recommendations, please download the pdfs above. Read on for an introduction to working with grapes affected by rot. While this guide was specifically created to address winemaking issues caused by rot from Botrytiscinerea, these are also best practices for handling grapes affected by mildews.
WHAT CAUSES ROT?
The most common source of rot in grapes is Botrytis cinerea. Botrytis is a mold that grows under humid, warm conditions, it is often associated with dense, closed canopies and can manifest throughout growing season. Botrytis bunch rot infects the berry through grape surface wounds and produces an array of compounds that can impact wine quality and processing, such as laccase, glucans, and botryticines.
Botrytis bunch rot should not be confused with sour rot. Sour rot although displaying similar symptoms is associated with an acetic acid aroma from within the rot infected clusters.
HOW DOES ROT AFFECT WINEMAKING?
- Laccase is a fungal enzyme which is resistant to SO2 and difficult to remove with traditional bentonite fining. Laccase causes premature browning due its ability to oxidize a large group of phenolic compounds resulting in the formation quinones. In this process, laccase will also destroy aromatic precursors.
- The fungal infection can deplete YAN.
- Glucans can cause significant issues with clarification, filtration, and building stable color in rosés.
- “Slip-skin” is associated with extreme cases of Botrytis and can make the fruit very difficult to process.
- Fruit compromised by Botrytis bunch rot is susceptible to further secondary bacterial infections which can cause stability and spoilage problems in the winery.
ASSESSING ROT SEVERITY
The first step toward successfully dealing with rot is to complete an assessment of the severity. This can be done using the steps below:
(PRE-HARVEST) Visual test: count # of infected clusters/vine and determine the % of the fruit infected.
- <1% proceed as normal
- 1 - 4% further sorting
- 5-20% treat with care as fruit needs special consideration
- >20% extreme measures to save fruit
(PRE-FERMENTATION) Qualitative test for laccase activity: place three samples (~50ml) of juice in clean glasses and cover. After 24 hours access for changes in color and quality, as well as the impact of SO2 and temperature.
- Glass one – Control
- Glass two- Add 60ppm SO2 and leave at cellar temperature
- Glass three- Add 60 ppm SO2 and place in the refrigerator
After 24 hours assess for changes in color and quality. If browning occurs in any of the glasses then that indicates the presence of laccase activity. You may also have an oily film on the surface. If the glass with SO2 or glass chilled in the refrigerator are less brown than the control, then that can give you insight for how to proceed
(PRE-FERMENTATION) Quantitative (laccase unit) test: this analysis can be done in-house if appropriate laboratory equipment is present or can be outsourced to an external laboratory like ETS. The results are given in laccase units per mL, and based on those results certain winemaking practices should be adapted:
- 1 unit/mL: Exercise caution and increase SO2 dose
- 2-5 units/mL: Proactive use of enzymes and tannins at medium dosage level
- >5 units/mL: Aggressive intervention via increased SO2, using enological enzymes and tannins at higher end of dosage recommendations
(PRE-FERMENTATION) Sensory evaluation: it is always a good idea to make notes on the taste and smell of the fruit so that you can determine the impact on wine quality.
IMPORTANT – CROSS CONTAMINATION
Throughout all steps of the winemaking process, appropriate steps to avoid cross-contamination should be taken as all juice is incredible susceptible to damage from laccase. These lots should be isolated as quickly as possible. Picking bins and crush equipment should be thoroughly cleaned after use. In the winery, all tanks, equipment, and sampling apparatuses (if applicable) should be cleaned after working with these lots.