Many of us experienced the Derecho storm on August 10, which increased chances for damaged fruit to be making its way to the winery this harvest. This storm didn’t bring large amounts of rainfall, rather extreme winds caused much of the damage. With damaged fruit comes concern for unwanted/untimely native yeast fermentations and development of volatile acidity (VA) in the grapes. VA is accumulation of acetic acid (vinegar) and in the vineyard this is deemed as sour rot. Typically, sour rot occurs after large amounts of rain when berries have reached ≥ 15 °Brix and temperatures are in the 60s (°F). However, splitting caused by drought or other damage leaves berries prone to rot.
Splitting of the berries exposes the native/wild yeast on the berry skin to the sugars in the fruit, which they ferment to ethanol. Acetic acid bacteria are generally in low levels on the fruit surface but the bacteria have the ability to produce acetic acid from glucose or from the ethanol produced by yeast in broken berries. Fruit flies are also generally part of the equation but their specific action is unknown. Wild yeast on grapes can also produce high amounts of acetic acid and ethyl acetate (smells like nail polish/solvent).
It is imperative to inspect and monitor the fruit for suspected damage, while still in the vineyard if possible, certainly upon arrival to the winery. Common harvest parameters of pH, titratable acidity, and °Brix are measured from vineyard berry samples to determine ripeness. In addition, visual inspection and smelling and/or testing for VA, should be considered for any fruit that is compromised. A small amount of VA production by yeast is expected in the range of 0.2-0.4 g/L following fermentation. Keep in mind that the sensory threshold for VA is 0.7 g/L, so any VA that develops in the fruit prior to fermentation is added to what will be produced by yeast during fermentation.
Other measures to consider if damaged fruit arrives to the winery are use of sulfur dioxide (SO2), adjustments to cold soaking and cold settling, use of dry ice or gases, and nutrient management. Upon entry to the winery, inspect and sort fruit to remove visibly damaged clusters. To help reduce microbial concentrations on fruit, use higher levels of SO2; the recommendation for clean fruit is 30-50 parts per million (ppm), damaged fruit may benefit from 50-75 ppm. Limit or eliminate cold soaking periods for reds and cold settle whites as quickly as possible at lower temperatures. Using dry ice or inert gas to blanket the juice/must helps to inhibit growth and prevent oxidation of juice. Take measurements of yeast assimilable nitrogen to find out nutrient needs of the juice/must and make any necessary additions. The use of higher levels of SO2 can lead to a deficiency in the amino acid thiamine, which can slow yeast growth and fermentation rate.
Bordelon, B. 2016. Grapes: The Sour Rot Situtation. Facts for Fancy Fruit. 16-11. https://fff.hort.purdue.edu/article/grapes-the-sour-rot-situation/
Moyer, M. 2020. Grape (Vitis spp.)-Sour Rot. https://pnwhandbooks.org/node/2787
Osborne, J. 2014. Dealing with Compromised Fruit in the Winery. Vines and Wines. https://winesvinesanalytics.com/features/article/136671/Dealing-with-compromised-fruit-in-the-winery
Waterhouse, A. 2004. What’s in Wine? Volatile Acidity. https://waterhouse.ucdavis.edu/whats-in-wine/volatile-acidity