The fermentation temperature of your wine can have a tremendous effect on the flavor and aroma profile. Immediately after harvesting white grapes, they are usually crushed, destemmed, and pressed into juice to later be fermented into wine. This is very different from red wine, which is crushed, destemmed and fermented on the seeds and skins before pressing. While on the skins, red wine is generally fermented warm to maximize color and flavor extraction at the expense of fruity aromas. Most white wine is treated almost completely opposite with regards to fermentation temperature and fermented cool.
Why So Cool?
A major goal when making white wine is to retain fruit forward aromas. The fruity aromas in a grape are volatile, meaning they would love to completely escape the wine to never return. Warm temperatures encourage vigorous fermentation and evaporation of these delicate compounds. When you stick your nose into a wine, every little particle that floats into your nose has left that wine forever, so it is important to take steps to trap these aromas in. The major step during the first weeks would to keep it cool during fermentation.
How cool?
Most aromatic white wines are fermented in the range of 45°F to 70°F. Cooler is better with regards to aroma retention but there are some risks to take into account when fermenting at low temperatures. For home winemakers, 55°F to 65°F is a pretty safe zone to ferment within.
What are the Risks of Cold Fermentation?
The first risk is a stuck fermentation. Stuck fermentation occurs when the yeast fail to convert all available sugar to alcohol or when you just can't get the fermentation started in the first place. To get your fermentation started (or re-started), you may need to creep up towards 70°F, then apply cooling to reduce the temperature once the yeast becomes established. When pouring your yeast starter into the must, make sure the starter is no more than 10°F warmer than the must that you are inoculating. This will help prevent the yeast from going into shock. A stuck fermentation is not too big of a problem, but something to keep in mind if you are keeping the temperature low.
Another risk involved in fermenting cool is the possible production of hydrogen sulfide which creates a rotten egg smell. Hydrogen sulfide is a byproduct of stressed yeast cells and can be tricky to get rid of. To prevent hydrogen sulfide, make sure to choose a yeast strain that is capable of fermenting at the planned temperature and pH level of your wine. Some yeasts are available that produce little to no hydrogen sulfide. One company that specializes in these is Renaissance but their yeasts are sold in 500g bricks so consider teaming up with a buddy when purchasing. Make sure to provide adequate nutrition for the yeast that you use by using a yeast nutrient blend. Also make sure that you frequently stir up the gross lees that will build up on the bottom of your fermenter. If you do encounter the dreaded hydrogen sulfide it can usually be removed by splash racking or if absolutely necessary it can be removed with a very small dose of copper sulfate (less than 0.5 parts per million). Copper sulfate is dangerous so do not use unless you are very familiar with the proper dosage, and be sure to use only the minimum required to eliminate the sulfide aroma.
When fermenting cool and slow, you are also at higher risk of oxidation. White wines do not produce as reliable of a CO2 blanket due to the cold temperatures of fermentation. Additionally, oxygen is absorbed much easier at cold temperatures and the low tannin of a white wine does not provide much protection against it. For this reason, consider fermenting a white wine in a closed top fermenter with adequate head space for foaming. An example of a closed top fermenter would be a carboy. Topping up the fermenter at this stage will only result in a volcano, so don't worry about a little head space until after the first racking.
How do I Chill My Fermenter?
Large wineries use jacketed stainless fermenters that pump chilled propylene glycol around the inner stainless tank. This is very expensive and not something you would expect to find laying around in your basement. A trick I like to use at home is to place my carboy in a bin of water. Each morning, toss one or two frozen 2-liter bottles in the water surrounding the carboy. Swap them out with new frozen bottles in the afternoon and repeat until the fermentation is completed. Keep your eye on the temperature of the carboy and use more or less frozen bottles to dial in your temperature. Early on you will need more cooling power, because an active fermentation creates some heat. As the fermentation slows down, it will take a bit less to cool the wine.
Final Steps
When the fermentation is completed, rack off the lees into a clean carboy and top-up to the skinny neck area to protect from oxygen. Now is a good time to hit the wine with 1/4 tsp of potassium metabisulfite per 6 gallons of wine to further protect from oxygen and microbial spoilage. Try to keep the wine around or below 60°F as it is settling our and clearing up in preparation for bottling. An under sulfited white wine stored above 60° is at risk of going through malolactic fermentation which may be okay for a Chardonnay but not many other whites will respond nicely to a heavy dose of butter.
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