Producing high-quality wines can be difficult even when Mother Nature cooperates and grapes arrive at the winery in pristine condition. However, in some years events occur that thwart the efforts of even the most vigilant and skilled vineyard manger in producing ripe and undamaged fruit.

Rain close to harvest can produce conditions perfect for the growth of Botrytis, while damage from heavy rain or hail 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 (below 10² cfu per mL) on healthy berries, while their populations can reach more than 10⁵ cfu per mL on damaged or Botrytis-infected grapes. Gluconobacter species are commonly isolated from grapes and must but disappear as alcoholic fermentation begins, while Acetobacter are more ethanol-tolerant and may survive through alcoholic fermentation.

The main spoilage issue associated with these bacteria is excessive production of acetic acid, although Gluconobacter may also produce high quantities of gluconic acid. Although we usually encounter acetic acid production during wine storage and aging, you may actually detect an acetic note on damaged berries. This is due to yeast growth on damaged berries converting grape sugars to ethanol, which the bacteria then convert to acetic acid.

The major yeast found on grape berries will be Kloeckera apiculata. This yeast is capable of producing large amounts of ethyl acetate (nail polish remover smell), acetic acid and acetaldehyde. To make matters worse, the acetic acid and ethyl acetate produced by these spoilage microorganisms attract fruit flies to the damaged fruit, resulting in greater spread of the bacteria and yeast to surrounding fruit.

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 and perhaps most important step is rigorous sorting, both in the vineyard and at the winery. While the degree to which you sort and discard fruit may ultimately be ruled by economic decisions, removal of visibly damaged fruit will significantly reduce the risk of subsequent wine spoilage issues.

For both red and white grapes, higher levels of SO₂ should also be added at the crusher or press. This will help reduce the number of spoilage yeast and bacteria and aid in preventing oxidation of color compounds by the enzyme laccase. Laccases are a class of enzyme that can be produced by Botrytis, and while SO₂ does not inhibit lacasse activity directly, it can scavenge O₂, which is needed to cause oxidative spoilage.

For red wine, minimize the time between crushing and fermentation. This may mean reducing or eliminating a cold soak. If cold soaking does occur, then limit exposure of the grapes to air by blanketing with inert gas and/or applying a dry ice addition to the surface of the fermentor. Once fermentation begins, the CO₂ produced will set up an anaerobic environment that will inhibit the growth of Acetobacter and Kloeckera.

For white wine, whole-cluster press lightly to minimize the impact of Botrytis. In addition, clarify quickly (cold temperature) and consider adding settling enzymes. This will allow rapid removal of the juice from the heavy lees and reduce the risk of enzymatic oxidation.

If you have heavily Botrytis-infected grapes, consider fining the juice with bentonite, casein, PVPP or combinations. Many commercial products are available that are combinations of these fining agents. Determine product and rate using bench-top trials before making additions to a larger volume.

When it comes to dealing with compromised fruit during fermentation, the two main things to consider are nutrient additions and creating a large and healthy yeast population to perform a robust fermentation. Microbial growth on the grapes prior to harvest and during the cold soak and early fermentation can lead to depletion of yeast assimilable nitrogen (YAN) and vitamins.

In addition, a large addition of SO₂ can lead to thiamine deficiency, as SO₂ reacts with this vitamin and splits it irreversibly into two inactive ingredients. Therefore, monitor YAN and make appropriate additions of organic and inorganic nitrogen (not just DAP as this will not replace vitamins). Add a larger yeast inoculum to promote a vigorous and healthy fermentation, as increased additions of SO2 to the fruit may initially cause some loss in yeast viability.

After alcoholic fermentation is complete, you need to protect the wine with SO₂ as quickly as possible. Acetobacter will not grow during the alcoholic fermentation, but it may still be present in the wine; under conditions of low SO₂ 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 SO₂ level. Acetobacter is inhibited at low pH, and the effectiveness of your free SO₂ is also increased. One issue with controlling Acetobacter with SO₂ is that they produce spoilage products such as acetaldehyde that bind free SO₂ and minimize its effectiveness. This makes early control important because these bacteria can be difficult to deal with at high populations.

Conduct a rapid malolactic fermentation by following best practices for this procedure to minimize the time the wine spends without SO₂. During élevage wines should be protected from air by filling tanks and barrels as completely as possible and ensuring barrel-topping over time. In addition, minimize air pick-up when wines with a high count of Acetobacter are being moved during racking, pumping, fining and bottling operations.

Summary
Consider the following tactics in dealing with damaged fruit:
• Sort fruit prior to processing.
• Increase SO₂ addition during fruit processing.
• Minimize exposure to air during cold soak (inert gas or dry ice) or do not perform a cold soak.
• Lightly press whites with excessive Botrytis damage, settle juice quickly and remove from gross juice lees.
• Assess YAN and make adjustments—microbial rot will reduce YAN and vitamins, and SO₂ addition will reduce thiamine content.
• Increase vigilance in the cellar post-fermentation to prevent Acetobacter growth: Maintain a low pH and optimum SO₂ management, protect wine from air.

Bibliography

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