Crush Pad Operations
Dealing with Diseased Grapes in the Winery
Rain close to harvest can produce conditions perfect for the growth of Botrytis, while damage from heavy rain or hail and harvesting provides opportunities for organisms both large (fruit flies) and small (bacteria and yeast) to proliferate and cause spoilage. Although Botrytis damage itself can be detrimental to grape quality, secondary issues created by damage to the grape skin can also be significant. The large array of microorganisms present on the grape berry surface typically have little impact on grape and wine quality. However, if the integrity of the grape berry surface is compromised through rain/hail damage or Botrytis infection, then rapid growth of oxidative yeast and acetic acid bacteria can occur. For example, the acetic acid bacteria Acetobacter and Gluconobacter are typically present in low populations on healthy berries, but their populations can be significantly higher on damaged or Botrytis-infected grapes. The main spoilage issue associated with these bacteria is excessive production of acetic acid, although Gluconobacter may also produce high quantities of gluconic acid. Typically, wine grapes with tighter cluster are more susceptible to disease infection such as Riesling, Chardonnay, Chenin Blanc, Sémillon, Pinot Noir, Zinfandel, and Barbara.
Botrytis
Botrytis infected grapes can result in (a) browning due to the laccase enzyme, (b) problematic fermentations, and (c) post-fermentation clarity and stability problems (Figure 2.12) (Section 15.3). In the case of red grape varieties, Botrytis also brings about significant deterioration/ reduction in total anthocyanins. Alcoholic fermentations are affected by Botrytis by several mechanisms.
Acetobacter
The main spoilage issue associated with Acetobacter is excessive production of acetic acid, which is more commonly known as vinegar (Section 15.2). Acetobacter will not grow during the alcoholic fermentation, but it may still be present in the wine; under conditions of low sulfur dioxide content and exposure to air, these populations may rapidly increase. Aside from minimizing exposure of wine to air, the principal defense against Acetobacter is to maintain a low pH and an adequate free sulfur dioxide level.
Management Strategies
Although it may be difficult or impossible to control events that produce damaged or compromised fruit, there are a number of steps that you can take to minimize the impact this damage will have on wine quality. The first step is selective harvesting and sorting of healthy and infected fruit where possible. The potentially most damaging fruit are the brown, broken, sour rot berries that are full of spoilage yeast and bacteria and acetic acid and other off-flavors. If possible, this fruit should be excluded from the must, both in the vineyard and at the winery.
Click on the following topics for more information on crush pad operations.
