Fermentation temperature control is the most crucial factor in achieving significant improvements in your beer. It can be particularly challenging when brewing in the Deep South, where summer brewing in temperatures exceeding 100°F can be brutal. Without proper temperature control, it is nearly impossible to brew beers correctly. Unfortunately, many homebrewers underestimate its importance and end up producing mediocre beers.
What Happens if Your Fermentation Temperature is Too Warm
The main issue that arises from high temperatures during fermentation is the production of off flavors from esters and fusel alcohols by the yeast. Sometimes these flavors are not necessarily “off,” but they may be inappropriate for the beer style being brewed.
In some cases, the yeast can ferment aggressively at the start, consuming all available nutrients, only to run out before finishing the sugar. This often results in incomplete fermentation.
Poor temperature control during fermentation frequently leads to excessively high temperatures, causing the yeast to become more susceptible to the toxic effects of alcohol. As a result, the yeast may die off before reaching their usual alcohol tolerance level.
Heat stress caused by high temperatures can cause yeast to die off, leaving the remaining yeast to handle all the work. This ultimately leads to a situation where you have under-pitched yeast, resulting in off flavors.
Since yeast metabolism generates heat, starting fermentation at excessively high temperatures will quickly lead to problems as the temperature continues to rise above 80°F, causing yeast death.
What Happens if Your Fermentation Temperature is Too Cold
When brewing in colder climates or during the winter without temperature control, yeast can also experience problems due to stress. Winter is traditionally a favorable time to brew because there is less risk of spoilage bacteria and wild yeast contamination.
Inadequate temperature control may prevent fermentation from starting altogether.
Alternatively, fermentation may proceed at a sluggish pace and continue for weeks before becoming stuck.
If brewing ales that require fruity flavors from esters, fermenting at too cold temperatures can result in a beer that lacks character or complexity, which may not align with the intended style. Judges will expect flavors and aromas that may be absent due to cold fermentation.
If there is any contamination in your beer, it becomes a race between the desired yeast and the contaminating bacteria to dominate the fermentation process. Poor temperature control at the beginning provides an opportunity for these bacteria to take over and ruin the entire batch of beer.
When fermentation starts in an environment that is too cold, carbon dioxide (CO2) becomes trapped in the cold beer. Flavors that are usually expelled during fermentation remain dissolved and may persist until the final product. This is particularly noticeable in lager fermentations, where “sulfur” aromas and flavors can develop.