Article by Enology-Grape Chemistry at Virginia Tech

Summer rains are the norm for the mid-Atlantic region. As such, fruit rot issues are a common concern. Even with careful fruit sorting in the vineyard and winery rot can be a big issue.

The following are a few production considerations to keep in mind in a season where the threat of fruit rot is high.

  1. Crop level: Avoid over-cropping, which could delay maturity. Delayed maturity can increase the incidence of fruit rot and fruit pH.
  2. Fruit culling: Cull as much fruit rot out as possible in the field.
  3. Fruit sorting: Sort fruit at the winery. A very small concentration of rot can have a large impact. It is not the incidence of rot, but the level of various rot metabolites that determine how much rot is acceptable. The best rule of thumb: no rot is acceptable. Many wineries now conduct double or triple sorting to help assure only the best fruit goes into the fermenter.
  4. Rinse fruit: You may consider rinsing the fruit with water if the fruit delivered to the winery is high in rot and you are unable to adequately sort. That will help to lower some of the sour rot metabolites and late season spray residues. This may not be practical for large volumes but has been shown to have a positive impact. This practice can slightly lower the Brix level as a result of dilution.
  5. Use optimum sanitation: Sanitation is a key to successful wine production, particularly in wet seasons. Sanitation of lugs and processing equipment is essential.
  6. Muté production/cryoextraction: A small quantity of muté produced from non-degraded fruit can help recover and replace lost aroma and aroma intensity resulting from sour bunch rot.
  7. Dehydration: Only extremely “clean” fruit should be used for this style of wine production. This is an excellent option particularly if fruit maturity at harvest is not optimal
  8. Pressing: Whole cluster press whites by discarding the initial juice. Press very lightly and take press fractions.
  9. Pressing: Whole cluster press whites by discarding the initial juice. Press very lightly and take press fractions.
  10. pH adjustment: Adjust the juice pH – the lower, the better. Expect about 2.0 g/L TA will drop out during fermentation or shortly following completion. In some wet seasons, high pH values caused many to have both biological and oxidative problems. If you have compromised fruit get the pH down pre-fermentation! Not lowering the pH far enough when fruit quality is compromised is a frequent problem.
  11. Sulfur dioxide: Keep the initial sulfur dioxide level low during pressing. You want the low molecular weight tannins to polymerize or bind together. Then raise the sulfur dioxide, depending on the fruit condition and pH. Note that sulfur dioxide can bind thiamine-an important yeast nutrient. In rot-degraded fruit, nutrient management is essential-see below.
  12. Cold settle: Adequate white juice cold settling with the use of pectinolytic enzymes will help lower the level of rot metabolites.
  13. Tannin addition: You could add enological tannins to help clarify juice and bind with some of the rot-produced enzymes. Tannins can act as oxygen buffers and may bind with enough protein to lower the bentonite requirement needed for wine protein stabilization. This is an important consideration for rather delicate varieties such as Pinot gris and Sauvignon blanc.
  14. Pectinolytic enzymes: The addition of pectic enzymes aids in clarification, which is particularly important if juice is produced from compromised fruit.
  15. PVPP: Add PVPP inline to the juice if there is a high level of grape tissue degradation. This nylon-based fining agent has the ability to bind with small tannins which can be both harsh and bitter.
  16. Ascorbic acid: For varieties where the oxidation potential is large, add ascorbic acid to the juice. See Enology Notes for details on the use of this anti-oxidant.
  17. Nutrient management: Test the YAN (yeast assimilable nitrogen) content and make adjustments accordingly. Rots deplete YAN. Note that rots also lower the micronutrient levels. As such, the addition of a complex nutrient formulation, not simply DAP, is wise. Some complex nutrients also contain glutathione, a strong anti-oxidant (See Enology Notes). Oxygen is a yeast nutrient. Make sure that you yeast culture is properly oxygenated. Oxygenate the fermenting wine in the early stages of fermentation.
  18. Measure the NTUs: You want to ferment fairly-clean juice. Measure the NTUs (nephelos turbidity units) if you can. If you measure, you will want about 100 to 150. If the juice is not clarifying, you may want to add enzymes or more tannin. Don’t add them together.
  19. Inoculation: Inoculate with a high volume of a vigorous, not too N-dependent yeast strain. Use more than the standard 24 g/hL or 2 lb/1000 gallons yeast addition rate. Make sure the starter is properly prepared, and understand that oxygen is a yeast nutrient
  20. Co-fermentation: If you are planning on an MLF co-ferment. Check with your suppliers regarding yeast and MLF strain compatibility. If you do not desire an MLF, consider the use of lysozyme.
  21. Fermentation temperature: Begin the fermentation at a slightly warmer temperature to help lower the concentration of undesirable aroma characters, and to assure a rapid yeast fermentation
  22. Mid-fermentation racking: Rack mid fermentation. This helps to remove wine from the primary lees. In the case of extreme rot incidence, more frequent racking may be desired to both remove lees and to oxygenate.
  23. Rack immediately post-fermentation.
  24. Consider short vatting reds, avoid cold soak and extended post-fermentation maceration. Use short vatting, and possibly délestage, to help remove fermenting wine from lees. Ferment in the presence of non-toasted wood and carefully review the steps listed above. If cold soak is done it should only occur with clean fruit. A cultured yeast should be added after adjustment to the cold soak temperature to avoid temperature shock