Fermenting with Botrytis 101

Suggested actions for successful fermentation depending on the extent of Botrytis infection on the incoming grape clusters.
Fermenting with Botrytis 101 - Articles


Botrytis Infection on Pinot Grigio Grapes. Photo by: Denise M. Gardner

The harvest season marks a monumental time where it all comes together for grape growers and winemakers. The lack of rainfall during the last few weeks of the growing season usually transpire into a relatively ideal harvest. Warm, dry days and cool, dry nights allow grapes to mature optimally.

In the eastern U.S., however, each growing season is unpredictable. Preparation for incoming, diseased fruit can help winemakers make well-informed decisions during processing to minimize the influence of disease on wine quality. Botrytis, which typically flourishes in more humid or higher precipitated years, is one disease that winemakers should be prepared to handle with incoming fruit. While management and prevention in the vineyard is obviously the best strategy for dealing with Botrytis, some years it's presence is unpreventable.

In Loinger, C. et al. (1977), Semillon grapes were fermented to assess the effects of Botrytis rot on chemical composition of grape must, the wine quality's sensory attributes, and the chemical composition of wine. This research showed several changes in the must and wine chemical composition chemically, associated several sensory generalizations (i.e., color, aroma, flavor) in wines produced with rot. Additionally, Bruce Zoecklein has noted in his Enology Notes that Botrytis routinely causes a loss of fruitiness in wine, and may produce off-flavors that are "phenolic" and "iodine-like" in their descriptions. Loinger et al.'s guidelines on cluster rot infestation can be summed up as follows:

  • 5 - 10% rot on clusters: noticeable reduction in wine quality; wine quality is still "good" (as opposed to very good with 0% rot on clusters)
  • 20 - 40% rot on clusters: marked reduction in wine quality; wine quality is "low"
  • >80% rot on clusters: wine is commercially unacceptable

The paper concluded, although not tested, that greater than 40% rot on clusters would create a wine in the range of low quality to commercially unacceptable. It is reasonable for winemakers and cellar assistants to evaluate incoming fruit, regardless of disease or rot infestation, and decide if it is worth the time and finances to ferment the received crop.

What is a vintner to do?

Fermenting with Rot: Emphasis - Botrytis (Gray Mold)

Create a sorting table and sort through diseased fruit

Investing in a sorting table may be an integral part in many winery's standard operating procedures during harvest operations. However, if the winery is not set up with a sorting table, taking the time at harvest to carefully pull out diseased fruit manually can save a lot of time and effort during and after fermentation. Remember that the rot organisms will impart characteristics on the wine as it ferments, causing changes in the aroma and flavor composition and depleting the varietal characters associated with properly ripened grapes. The goal in removing diseased fruit prior to fermentation is to minimize the influence of disease-associated attributes on the wine by reducing the amount of contact the product has with diseased component.

Do not contaminate clean fruit with rot infested fruit

Simple cleaning and sanitizing protocols need to be utilized when dealing with rotted fruit. Washing off crusher/destemmers with hot water is not enough to decontaminate the processing equipment from the rot infestation. There are a few easy steps wineries can integrate into harvesting operations to minimize the potential for disease contamination:

  • If you are processing more than 1 variety in 1 day, process the cleanest fruit first. You should end with the "dirtiest" fruit.
  • Wash and sanitize all equipment prior to use. There's no point in allowing dead bugs, old mildews, and dirt in your product. This practice decreases the potential risk that can be detrimental to wine quality as soon as grapes get to the processing area.
  • Physically wash off equipment in between lots of grapes. This is just good practice. Use water to initially clean leftover fruit and debris from the processing equipment, and manually scrub off any debris before properly sanitizing equipment. In our research winery, we use a citric acid/SO2 rinse, but you can also use commercial cleaners and sanitizers. If the sanitizer requires a neutralizing agent or water rinse, make sure you do this before putting in the next lot of fruit. Always follow supplier safety instructions.
  • Make sure you take the time to properly wash and sanitize processing equipment at the end of the day. This keeps your equipment in good condition, reduces biofilm formation, and minimizes chances for any external contamination.

With whites and reds, limit contact time with the skins

Botrytis resides on the skins with the grapes, and minimizing skin contact with the pulp will inhibit extraction of Botrytis-related components into your wine. Remember that red wines require skin contact in order to extract the red pigments (anthocyanins) from the skins. However, there are several available options for winemakers. Pre-fermentation flash pasteurization techniques that will quickly increase red pigmentation and inhibit Botrytis. It should be noted that this technique does impart its own flavor characteristics on the wine, but it will likely be more beneficial than the disease-related flavors and aromas. If flash pasteurization is not an option, consider removing

Whole cluster press your whites, and separate press runs

Fugelsang and Edwards (2007) recommend separating out the first 10+ gallons that are rich in Botrytis metabolites. Separation of press fractions is key here, as it allows more control over the phenolic content, solids, and potential off-flavors associated with Botrytis.

During cold settling of whites, it may be necessary to do a slight bentonite fining

A light bentonite fining in the juice should reduce your laccase enzyme content. Laccase, unlike polyphenol oxidase, is not inhibited by the alcohol content produced during fermentation. Therefore, laccase can cause premature browning of young wines. Grapes high in Botrytis infection are often rich in laccase concentrations. Refer to your supplier's product recommendations for this step if you think it is essential.

Press lightly - this is not the time where you want to over-extract

With high Botrytis-infected fruit, pressing for increased yield is not recommended. The increased pressure will not only over-extract tannins, but also extract more rot-associated compounds and flavors that are detrimental to wine quality.

Use PVPP prior to fermentation to minimize color oxidation

PVPP will help strip out potential browning compounds or their precursor forms before fermentation begins.

Do not undergo a native/natural fermentation: use a commercial yeast strain

With disease-heavy fruit, the microflora associated with the disease are a part of the potential microorganisms that can contribute off-flavors to your final product. It is recommended that winemakers inoculate with a commercial yeast strain that is recommended for Botrysized fruit. Most suppliers will have a recommendation for red, white, and rosé wines under Botrytis conditions. Generically, you want a yeast strain that is robust and can ferment under disease pressure and its associated fermentation inhibitors.

Limit your oxygen exposure to crushed fruit: use Nitrogen and Argon gas blanketing

Although this will slightly inhibit your commercialized yeast strain, the lack of oxygen will completely knock out invasive yeasts and microorganisms, including Acetobacter, which are usually more prevalent in rotted grapes. Ask your supplier if the commercial yeast can handle this treatment.

If you have more than 10% rot on clusters, consider getting a Laccase Test from a wine lab

Laccase is an oxidative enzyme that is produced in higher concentrations with Botrytis infection. Laccase generally progresses the rate of oxidation (i.e., turning wines brown or making them taste like sherry) even in very young wines. The results of the Laccase Test will tell you whether or not you need to go extreme treatments (e.g., heat treatment, early bentonite fining) to avoid rapid oxidation.

Consider the use of high temperature, short time (HTST) treatments

Some people also call this "Flash Vinification" or "Pasteurization" in which the must is heated very quickly to a high temperature and then quickly chilled back down to a reasonable fermentation temperature. The quick cooling time is essential. Too much heat for too much time will not only kill the bacteria and denature harmful enzymes, but it will also denature proteins and other compounds in the fruit. It can also cause "cooked flavors" in the final wine, which are not preferred by consumers.

Add SO2 to inhibit the natural microflora on the clusters

There are several recommendations on how much SO2 to add, which is dependent on the variety (white, rosé, or red) and the extent of Botyris infection. In general, for lower Botrytis infections, a 30 ppm SO2 addition in the harvest bins followed by an additional 20-30 ppm after crushing and destemming should suffice. For larger Botrytis infections, a 40 ppm SO2 addition in the harvest bins followed by an additional 30-40 ppm addition after crushing and destemming is recommended.

Decrease the pH of your red wines

Decreasing the pH will help inhibit natural microflora from growing and proliferating during primary fermentation. However, it is important for winemakers to choose a commercial yeast strain suitable for low pH conditions. Additionally, lowering the pH can have potential sensory implications.

Treat the must with Lysozyme

A Lysozyme addition will not do anything to the Botrytis infection directly, but it will decrease populations of potential lactic acid bacteria spoilage organisms. This may be necessary if the fermentation has a hard time getting started. (It is recommended that winemakers evaluate the lactic acid bacteria levels in the must prior to treating it with Lysozyme. You can do this by simply evaluating the must under a microscope or sending a sample to a wine lab.)

Add tannins prior to red wine fermentations

The use of pre-fermentation tannins will do several things for your fermentation when dealing with Botrytis-infected fruit:

  • Binds with some of the active enzymes that may be destructive towards the fermentation (e.g., laccase).
  • Will offer binding materials for anthocyanins to enhance for better color stability throughout and after fermentation.
  • Enhance the mouthfeel of the red wine. Botrytis (and disease in general) tends to thin the mouthfeel of many wines, which extenuates the off-flavors associated with Botrytis. Building mouthfeel will off-set this phenomena.

Use a β-gluconase enzyme product, especially in white wines during cold settling and before fermentation

This enzyme will help break down some of the solids associated with Botrytis infections. Additionally, this enzyme should help make clarification and filtration easier on the wine after fermentation. Both processes are often challenging for Botrysized white wines.

Manage your free SO2 after primary fermentation and malolactic fermentation (MLF)

Checking the free SO2 concentration every other week or once every month can help inhibit spoilage microorganisms from proliferating during times when the wine is not actively worked on. Remember that the molecular SO2 concentration is always changing and heavily associated with pH. Every time the wine is moved, the free SO2 should be confirmed to ensure that the proper molecular level is being obtained in the wine.

Manage your numbers

Good quality control programs are essential for winemakers even when making wines without disease pressure. However, having analytical records from the start of processing through bottling of the wine will assist winemakers in making proper processing decisions. Get a juice panel (Brix, pH, TA, malic acid, and YAN) prior to fermentation, with, perhaps, some of the additional analyses for heavily-infested Botrysized fruit (i.e., Laccase Test). At the completion of primary fermentation, check your glucose/fructose levels (reducing sugar to ensure that your fermentation is dry to avoid any microorganisms from utilizing left over sugar), volatile acidity (VA) to ensure that it is not high and to know the starting level post-primary fermentation, and malic acid concentration. Make sure that your MLF is complete by getting the malic acid concentration checked enzymatically after a paper chromatography result shows that it has completed. Regularly monitoring SO2 is also an important step.

Trust your palate - don't just add sugar

Some people will add sugar to sweeten wines when less than optimal grapes were received at harvest. If the wine successfully ferments to dryness (<1 g/L RS) and completes MLF, it is recommended that winemakers stabilize the problem wine and taste it periodically over time before making sugar additions. If it has no residual off-flavors from the Botrytis infection, then do a bench trial and determine adequate sugar additions. However, do not underestimate the possibility of blending out the wine into blended wines. Depending on the starting level of Botrytis, there may be some off-flavors that developed during primary fermentation that are not well complemented with sugar additions. However, they may be minimized, or masked, through blending with clean wines.

These are all suggestions that will depend on the extent of Botrytis infection on the incoming fruit, and what you are capable of accomplishing at your facility. If you can estimate that less than 10% of the fruit is infected, minor treatments are usually needed to ensure a successful fermentation and quality product. Above 15% infected fruit is where processing operations can get a bit tricky to maintain wine quality.

Remember that clarification and filtering is a challenge with wines produced from Botrysized fruit. Taking some of these suggestions above, should help minimize filtration problems at later stages in wine processing.

Literature Cited

  • Fugelsang, K.C. and C.G. Edwards. (2007) Wine Microbiology: Practical Applications and Proceedings. (2nd Ed.) Springer: New York, NY. 393 pg.
  • Loinger, C., S. Cohen, N. Dror, and M.J. Berlinger. (1977) Effect of grape cluster rot on wine quality. AJEV. 28(4):196-199.

August, 2011