In the Winery Dealing with

mouldy grapes When dealingwith unwanted flavours in wine, yeast can be a winemaker’s best friend. By Gary Strachan


hen I see black mottled patches on green grape shoots, my heart sinks. It’s like an evil omen that the harvest may have to deal with an unpleasant flavour. It’s not a guarantee, just a probability. About 40 years ago I had the time and opportunity to make wine on purpose from a series of grapes that were badly infected with mildew. It had been a wet, nasty autumn, and John Vielvoye, the BC Ministry of Agriculture and Food grape specialist, asked if I had a use for a bin of badly-infected Okanagan Riesling grapes. Fortunately I also had a bin of clean grapes. We blended clean and infected grapes in 5 per cent increments and produced 10 small batches of wine with increasing amounts of mouldy grapes.

I was surprised at how much mould could be present before our taste panel detected the off flavour. Perhaps the off flavour was concealed by the strong varietal character of Okanagan Riesling. Perhaps some of the flavour was removed by the standardized winemaking method we used. Our panel didn’t flag mouldy character until the wine was made from grapes containing 20 per cent infected clusters. This was well beyond the level at which a typical winemaker would reject the shipment.

When dealing with unwanted flavours in wine, yeast can be a winemaker’s best friend. The cell membrane of yeast is composed of fats and sterols, all oriented in a specific configuration that enables some compounds to enter the cell yet excludes others. Many unwanted flavours are soluble in fats and can be selectively taken up by yeast cells.

If you have an off flavour in wine, a 26

fining with yeast can easily be bench tested for effectiveness. It might require more than one treatment to overcome the problem. A series of small

extractions is more effective than one large extraction. A typical treatment would be to use 200 mg/L of freshly hydrated

yeast or (say) one g/L of fresh, clean lees. Maintain a control and check the nose after the yeast has settled. There are many effective commercial treatments for wine made with mouldy grapes. Most of them are quite effective but if you overtreat, they are also capable of removing varietal character. In extreme cases, a stripped, neutral wine for blending may be the only alternative to distilling or dumping the wine.

The usual commercial formulations advised as treatments for mouldy wine are activated carbon, potassium caseinate, and bentonite. Activated carbon is a really good adsorbent for flavours. In small amounts it can be used on its own for flavour and colour removal. As always, don’t ever treat a tank until you have determined the lowest effective dose with a bench test. If the first test still leaves a flavour, it is almost always more effective to do two smaller treatments than to administer a single large treatment.

Potassium caseinate is really effective at removing oxidized notes that could have resulted from mould laccase. Again, potassium caseinate can be used alone, and should be tested on the bench before a tank is treated. An old treatment to “freshen” oxidized wine was to add skim milk or skim milk powder to wine. This would also introduce small amounts of lactose along with a few other milk components

British Columbia FRUIT GROWER • Spring 2018

such a globulin and albumin proteins. If used properly, most of the milk components will precipitate and be filtered out.

Caseinate treatment will remove phenolic compounds and anthocyanins. Overtreatment will remove a lot of wine character and colour.

The final member of this trio is the familiar fining agent bentonite. Bentonite occurs as tiny platelets with a negative charge around the perimeter. The negative charge attracts positively charged proteins and assists their precipitation.

A jungle of enzymes can be present in wine made from mouldy grapes. Bentonite may take most of them out if used properly.

If you have the time to test each of the potential treatments for an off flavoured wine, it may be the closest targeted treatment for the wine while minimizing damage to it. If not, then a commercial product may be a better answer for you.

The commercial blends have been tested on typical wines and offer a single treatment that will be effective in most cases. Again, don’t treat the tank until you have done the test on the bench. Also, two small treatments are more effective and do less damage to the wine.

Treatment is a contingency. The best treatment is to avoid infection in the vineyard. That’s where the best wines are made.

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