There are several high risk situations in which vineyards should be conscientious about potential sour rot infections late in the growing season:
- Vineyard sites with annual populations of migratory birds, which can wound grape berries and create an entry point for disease.
- Vineyard sites with high populations of yellow jackets, which can wound grape berries, creating entry points for disease, and potential transfer yeast/bacteria infections.
- Vintage seasons with August and September hurricanes, which can minimize sun exposure while enhancing humidity and precipitation in extreme settings.
Sour rot occurs in an open wound of grape berries where yeast and bacteria can utilize the grapes' nutrient resources. Specifically, several grape yeasts and bacteria including several Zygosaccharomyces species and acetic acid bacteria cause the sour rot infection (Barata et al, 2008). Like many grape diseases, sour rot is not specific to one genus and species of bacteria, and it's difficult for winemakers to know how to deal with such infections.
Based on observation, there is much debate within the wine community about how to make commercially viable wine from sour rot infested fruit. There is a general question among wine producers about the amount of resources required to make a "commercial" wine with sour rotted fruit, and whether or not the investment is worth the outcome.
Sour rotted grapes will emit an odor that smells like acetic acid (vinegar) in the vineyard, during harvest, and potentially produce a wine has a high volatile acidity (VA) with a dominant odor and flavor of acetic acid. Wines produced from fruit with a high concentration of sour rot typically display a sharp acetic acid taste in the finish, and can potentially have a VA over legal limits or a strong nail polish odor, which is produced by ethyl acetate. These sensory attributes can mask any positive features associated with the wine.
In those years in which sour rot is more prevalent (e.g., 2011 in the Mid-Atlantic), monitoring VA after primary and through aging is a must for winemakers and cellar staff. Remember that there are legal limits to VA:
- 1.1 g/L acetic acid for white wines,
- 1.2 g/L acetic acid for red wines.
Further information regarding legal limit specifications pertaining to VA can be found within Title 27 of the CFR for wine.
However, sensorially, most people can sense the vinegar taste at 0.8 to 0.9 g/L acetic acid, regardless of variety, which is below the legal limits associated with VA. Wines at acetic acid concentrations greater than 0.7 g/L typically require some sort of remediation before being bottled. Furthermore, even if the VA is less than 0.7 g/L acetic acid, the threat of accumulated ethyl acetate is pertinent. Ethyl acetate is typically identified in the wine by an odor reminiscent of nail polish, nail polish remover, or glue. While some reverse osmosis (RO) treatments can help minimize the ethyl acetate concentration, it is typically expensive and completely dependent on the RO unit being used to treat the wine. Winemakers that are looking to treat high ethyl acetate concentrations are encouraged to talk to their RO supplier prior to submitting the wine for remediation.
For some, fermenting sour rotted fruit may be essential. Therefore, there are several helpful production steps that may help minimize the aromas, flavors, and tastes associated with sour rot:
- Always sort out diseased fruit in the vineyard and in the winery. Keep in mind that grapes with more than 60% of an infection may be a lost cause for commercial wines, as their VA may go well above legal limits.
- Know whether or not the winery has the financial resources and opportunity to utilize RO technology if the wine should become flawed during production.
- Choose a strong yeast strain, particularly a Saccharomyces bayanus strain. Talk to your supplier for their recommendations on a yeast that can survive and thrive in higher acetic acid conditions. S. bayanus strains of yeast generally ferment quickly and produce a lot of heat. Winemakers need to monitor fermentations religiously when using such fast fermenters. The heat produced during fermentation may blow off some of that acetic acid while out-competing the native yeast and bacteria on the infected clusters. This may help minimize the aromatic and flavor influences of the sour rot infection.
- Maintain an appropriate nutrient strategy during fermentation. Measure the fruit's YAN prior to fermentation, and refer to the supplier guidelines for how to treat fermenting must/juice with low, medium, or high starting YAN levels. Do not follow generic nutrient strategies. It is essential to maintain the healthiest fermentation as possible to try to keep the problem (i.e., sour rot yeast and bacteria) from dominating the fermentation. For recommendations on nutrient strategies, please refer to the Penn State Wine Made Easy Fact Sheet: Nutrient Management During Fermentation .
- Manage sulfur dioxide (SO2) levels, oxygen exposure, and sanitation through fermentation. There are some good recommendations on the webpage: Fermenting with Botrytis 101 . SO2 is only an antimicrobial agent at 0.5 to 0.8 mg/L (molecular) concentrations. This means that monitoring the pH and understanding its relationship to SO2 integration is important. For a better understanding of free SO2 and its use in wine, please see the Penn State Wine Made Easy Fact Sheet: Sulfur Dioxide Management . Keeping oxygen exposure to a minimum will keep sour rot bacteria and yeast levels low, as many of these species require oxygen to grow and proliferate. Utilizing proper sanitation techniques is key to avoiding cross contamination of infected fruit/wine with clean fruit/wine. Good sanitation practices will also help isolated wines with any potential spoilage associated with sour rot.
- Add some tannins prior to fermentation, particularly ones that will add to mouthfeel. Suppliers should have some recommendations for fermentation tannins. Sour rot tends to thin the structure of wines, as well as enhance the sensation of astringency and bitterness, depending on the variety. These tannins will help build structure and integrate into the wine while it is fermenting.
- Add enological tannins to the must prior to fermentation to build structure and alter the mouthfeel of the wine. While tannins also act as an antioxidant source, they have the secondary benefit of softening the acetic acid bite in the finish of the wine, potentially freshening the aromatics, and contributing to structural mouthfeel. Not all tannins are treated equally, so it is suggested that you contact your supplier for appropriate tannin recommendations. Conduct bench trials prior to additions.
- Consider polysaccharide or inactivated yeast product additions after fermentation to soften the finish and/or mouthfeel of the wine. The finish is a large problem with sour rotted wines, as acetic acid is quite harsh on the palate. Polysaccharides have the potential to round out the finish and create a smoother mouthfeel. There is no magic bullet with polysaccharide/inactivate yeast additions, so it is recommended to talk with your supplier for appropriate recommendations. Conduct bench trials prior to making additions.
- Reverse osmosis (RO) is a technology that may be utilized after fermentation and malolactic fermentation (MLF) is complete to reduce the concentration of acetic acid and, potentially, ethyl acetate. Keep in mind that the use of RO does not eliminate the entire concentration of any one chemical compound, but it can help reduce the concentration. Reducing the concentration of the aromatic compound may bring the compound below sensory threshold.
- Blending with wines that have a low VA may be helpful. Always remember to conduct bench trials for blends first.
- Sterile filter any wine that was produced by sour rot infected fruit prior to bottling to ensure bottle stability. Use a 0.45 μm absolute (not nominal) filter prior to bottling.
- Dr. Bruce Zoecklein, emeritus professor from Virginia Tech University, has previously recommended "light whole-cluster pressing, thermo-vinification, separating press fractions, cryoextraction, and post-fermentation heat treatments" for fruit infected with sour rot.
Always remember to create a plan for when and how the wine will be processed when you know that sour rotted fruit exists in the vineyard. Know your time and financial restraints before going into the fermentation. Processes like RO can be quite expensive, and it may not be economical to ferment a wine that is heavily infected with sour rot. Making these decisions prior to receiving the grapes can save a lot of time, resources, and money during the busy harvest season.
It is also important to remember that things like excessive oak use or adding external flavors is not going to subdue a noticeable acetic acid character. While it may help "hide" the flawed wine initially, these are not recommended actions, especially if the VA is above legal limits.