One of the key challenges any homemade winemaker faces is controlling temperature during fermentation. Fermentation is arguably the most critical phase of the winemaking process and a wine can be made or lost based on the outcome. Yeast will generally do its job during fermentation, but the winemaker also has work to do and the most important job during the primary fermentation phase is to control the temperature of must until fermentation is complete.
Overview of the fermentation process.
Primary fermentation occurs when yeast is added to either must or juice and the sugars present in the juice are converted to alcohol. Specifically, glucose, or C6H12O5 is converted to ethanol, or C2H5OH and carbon dioxide, or CO2. If done right, all of the glucose will convert to alcohol, so that the winemaker has created wine with the desired alcohol content. The CO2 that is produced is also desirable, as it will protect the wine from exposure to oxygen and the resulting oxidation that could ruin wine. Another byproduct of this reaction includes heat and this makes it even more important to manage the temperature of the process. This is the macro-level process that takes place and without it, you don’t have wine. On a micro-level, however, there’s a lot more that takes place, and temperature has a major impact on the rate and type of interactions that can impact the finished product.
Why control temperature during fermentation?
The fermentation process functions best within a certain temperature range, depending on the grape varietal and type of yeast used. Temperatures that are too low or too high will stall the reaction altogether. Excessively low temperatures are uncommon and because fermentation creates heat, so most often the winemaker must reduce temperatures during this phase, rather than increasing it. Excessively high temperatures will have the primary effect of either converting sugar to alcohol too quickly, or creating a stuck fermentation. (see chart below) This means that you will have residual sugars present and will not have the desired alcohol level in your wine. This has an effect on the overall quality of the wine. For this reason, most winemakers prefer to keep temperatures low enough to keep the process smooth and gradual. But sugar and alcohol content are not the only outcomes of the fermentation process.
What’s actually inside of a grape?
At a basic level, there are five elements found in a grape; including cellulose and water, sugars, acids and phenols. Cellulose and water are fairly neutral to the winemaking process and we’ve reviewed how fermentation converts sugars to alcohol. While secondary fermentation has a significant impact on the type and amount of acidity in wine, this is less so with the primary fermentation phase.
Phenols are compounds that give the wine most of its flavor, depth, color and finish. Phenols come from both the grape itself, the grape stems and seeds and the wine’s exposure to any oak elements. Primary fermentation temperature and rate have a major impact on the amount of phenols present and the balance and mix of these elements within the wine. Both commercial and homemade winemakers rely upon their own experience, tradition, in-house experimentation and academic research results to find the perfect temperature to maintain in order to have the best possible outcome.
What does the research tell us?
There are a number of studies on temperature and the resulting effects on wine. (see links below). While the results vary slightly in their objectives, methods and results, the overall findings group within a fairly tight range. The most desirable outcomes are typically produced when primary fermentation occurs between 10 – 18 °C for white wines, (50 – 64 °F) and between 22 – 30 °C for red wines, (72 – 86 °F). There are winemakers who have had some success with temperatures outside of these ranges, but these are the exception, rather than the rule. Varietal, yeast type, winemaking methods and local climate will dictate which range you choose to target.
As always, the best way to find the right fermentation temperature is to balance learning from others with your own experimentation. These experiments will often create a “right” temperature range for you, but it is valuable to learn from the experience of others and use that information as a starting point for your own winemaking process.