Rot Protocols

“Like when you bring your umbrella, it won’t rain, or put snow tires on your truck it won’t snow, you give me the protocols and I won’t have any rot!” (Tim Jordan, Barren Ridge Vineyards)

It is in this spirit that I include a section on handling compromised fruit. It is good planning to have a protocol in place in case fruit gets infected with Botrytis or sour rot. Not all fruit is infected for the same reasons, and some of these interventions are more useful for one kind of infection than another, but there are some things to keep in mind in terms of winemaking with compromised fruit.  I have included a summary as well as references and links to additional resources, should you need them.

 

The culprits

The primary forms of infection you will see are Botrytis and sour rot. Though also associate with noble rot, in Virginia, Botrytis is really bunch rot. This fungus infects the skins of grapes and compromises their integrity, leaving grapes susceptible to co-infection with acetic acid bacteria and other fungi such as Candida. Botrytis produces the enzyme laccase, an oxidizing enzyme that is not inhibited by alcohol and therefore persists after fermentation is complete. It also produces enzymes that degrade esters and terpenes, lowering the varietal character and overall fruitiness of wines. Botrytis uses much of the grape’s natural nitrogen and thiamine, and produces botrycine, a polysaccharide that triggers yeast to produce acetic acid later in fermentation. Not noble at all.

Sour rot can also be a problem in Virginia. Sour rot is caused by a constellation of microorganisms that opportunistically infect grapes whose skins have been compromised. Birds and yellowjackets are common culprits. These infections contain a number of microbes including Acetobacter and Zygosaccharomyces, with the overall effect of producing acetic acid.  Telltale signs are the smell of vinegar in the vineyard, though ethyl acetate production is also a problem in the wine.

Whichever rot you are dealing with, there are several things you can do as a winemaker to limit their effects. You will not be able to return the fruit to its former glory, but you may be able to save the wine enough to have a salable product. Here are some things to consider:

 

General approaches

Sorting: In the vineyard and in the winery, take the opportunity to remove any diseased fruit. This will constitute a loss of volume, however, it is better to have less volume of a good wine that more volume of a bad wine.  Loinger et al (1977) studied the effect of Botrytis on Semillon grapes and found that 5-10% infection rate gave a noticeable reduction in wine quality, but still produced a wine considered to be “good” by trained tasters. However, wine made from fruit that was 20-40% infected was considered to have “low” quality.  Some authors also suggest adding 30 ppm SO2 to the grapes in the field to reduce laccase and microbial activity. 

Limit contamination: Whether dealing with a white or a red, limit contamination with proper hygeine. This is not the time to skip cleaning steps or leave the press to clean until the next day. Physically remove any debris by rinsing and scrubbing, then chemically sanitize.  Microbial contamination doesn’t need much to spread to other batches. Also, always process infected fruit last.  

Limit oxygen: Laccase and Acetobacter both need oxygen to do their damage. At all steps, limit oxygen availability to limit the effects of these two mechanisms of spoilage.

 

White Wine Considerations

• Limit skin contact, whole cluster press. Botrytis lives on the skins of grapes.
• If there is a lot of juicing when you load the press, consider disposing of the first 10 gallons or so, as this juice is rich in Botrytis metabolites.
• Press lightly and separate out press fractions. Botrytis increases the production of glucans, polysaccharides that can lead to poor filterability. These can be treated with enzymes, but are best to keep separate to maintain quality in the larger lot.
• Treat with SO₂ (20-30 ppm) at the hopper and/or in the press pan. Laccase will cause browning quickly.
• Settle quickly to remove skin components
  • Use settling enzymes at the high rate (or specialized enzymes, check with your manufacturer
    • Use a B-gluconase enzyme in white to break down solids associated with Botrytis and make it easier to clarify and filter later.
  • Treat with lysozyme if you don’t intend to go through malolactic fermentation. This will inhibit Lactobacillus, an opportunistic co-infection of diseased grapes.
  • Alternatively, treat with a chitosan-based antimicrobial to limit spoilage organisms.
  • Consider fining with bentonite to remove laccase.
  • Consider fining with PVPP to remove substrates of laccase that lead to browning.
  • Consider fining with tannin for antioxidant protection
  • Cold temperature limits laccase and microbial activity
• Rack clean. Target <100 NTU if possible.
• Get the fermentation going quickly
  • Inoculate promptly with a higher rate of yeast (30 g/hL)
  • Use rehydration nutrients. Infections will eat yeast nitrogen and thiamine, leaving a poor nutritional environment.
  • Choose a yeast strain that has low nutrient needs, is a low SO2 producer, and tolerates higher VA. Bayanus strains are popular choices. Ask your manufacturer for recommendations.
  • Ferment 5 degrees F warmer than usual. This will speed things up, potentially allow volatilization of some VA and limit production of esters that might amplify moldy flavors.
  • Test YAN and feed the fermentations with complex yeast nutrients (not just DAP).
• Adjust the acid to as low a pH as you dare. This will limit Acetobacter and other spoilage organisms.
• Limit lees contact
  • Consider racking once during fermentation.
  • As soon as the fermentation is complete, add SO₂, settle, and rack off lees.
• Maintain proper SO₂ levels during aging to limit laccase and contaminating microbes.
• Check filterability before bottling, as glucans can clog filters.

 

Red Wine Considerations

Many of the same principles apply, but reds are difference as you cannot fully limit skin contact.

• SO₂ is tricky here, as oxygen is needed for tannin polymerization and color fixation. However, SO₂ is useful for inhibiting laccase, spoilage microbes, and binding oxygen.
• Plan for short maceration time. No cold soak, no extended maceration. This will lead to loss of tannin and color, though.
• Treat with a chitosan-based antimicrobial to limit spoilage organisms. This will require inoculation for malolactic fermentation.
• Ferment on neutral oak chips to contribute tannin for color fixation
• Use fermentation tannins to bind laccase and provide antioxidant protection. These also help build mouthfeel and structure that have been limited by infection. Add half at the crusher and half at the first or second punchdown or pumpover.
• Natural yeast derivatives help build colloidal structure, which is affected by infection.
• Consider using enzymes to allow fast extraction of polyphenols with less working of the skins.
• Inoculate with low VA low SO₂ producing yeast with low nutrient needs at a higher rate (30 g/hL) with proper rehydration.
• Adjust acid to lower pH as soon and aggressively as you can.
• Measure YAN and add complex nutrients.
• Ferment warmer (5 degrees F) to help blow off VA and speed up fermentation.
• Rack off lees during fermentation using delestage.
• Press lightly and separate press fractions.
• Allow for good settling and rack off fermentation lees clean.
• Inoculate malolactic fermentation with SO₂ tolerant strain. Use malolactic nutrient.
• Add SO₂ and rack off lees after malolactic fermentation.
• Maintain SO₂ rate during aging to prevent laccase and microbial activity.

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References:

1. Penn State Extension “Fermenting with Botrytis 101”
2. Penn State Extension “Managing Sour Rot in the Cellar”
3. Production considerations with Rot Compromised Fruit, Bruce Zoecklein, Vintner’s Corner