Hot Pressing In A Cool Climate

Great news for fungi, bad news for grapes

The cool climate, here in the Puget Sound Region, keeps growers on the edge. We want to grow noble varieties like Pinot Noir and Chardonnay, which will ripen here but just barely. We also want reliable grapes that ripen even in bad years like this one. Grapes like Madeleine Angevine and Seigerrebe shrugged off the cold wet season and delivered good, if somewhat smaller, crops. A lot of Pinot Noir and Chardonnay is still braving the wet weather, and the fungal diseases that thrive in it, to ripen. Some wont be harvested at all, and many will be harvested with the beginnings of botrytis.

Heat the grapes to repair some of the damage

How do you salvage barley ripe grapes beset by bunch rot? Thermovinification, or “hot pressing,” is the process of heating red wine grapes to kill spoilage organisms, improve color, and reduce unripe flavors. Heating to 150F (65C) for 20 minutes, 180F (90C) for 2 minutes, will do the trick. At this point you can cool the must and ferment normally or press the grapes and ferment like a white wine. It’s best not to leave under ripe grape skins in contact with the fermenting wine for very long.

Adjust the acidity

If you had to harvest before your grapes are ripe, you will have to deal with their acid profile. It isn’t just a matter of reducing high acid levels with something like potassium bicarbonate (KHCO3), the mix of acids may be unfavorable. As grapes ripen, the amount of malic acid falls, while tartaric acid rises. So in under ripe grapes, there will be a disproportionately more malic. That’s when a yeast like Lavlin’s 71B, which consumes malic acid, can come in handy. If, after fermentation, there is still too much acid, then it’s time for the KHCO3.

These steps can make the difference between a lost cause and a drinkable wine, but they won’t conjure up a fine wine from mediocre grapes. Do your best, look forward to next year, and sip some heat treated wine 🙂

Further reading

In his 10/3/2003 Letter to NY winemakers, Thomas Henick-Kling writes about making wine after a difficult harvest. My thanks to Gerard Bentryn of Bainbridge Island Vineyards for recommending that paper.

Jancis Robinson has a great entry on thermovinification in her The Oxford Companion to Wine.

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