The Importance of Fermentation Temperature
Temperature plays a critical role in fermentation. Yeast needs to be warm enough to be healthy, but too warm will stress the yeast. Too cool and the yeast will be sluggish and sleepy. As temperature increases, fermentation rate accelerates. With increased fermentation rate, more aromatic compounds are produced because the metabolic intermediates are excreted from the yeast cells just like people getting sweaty and stinky during vigorous exercise.
Fruity is the most common aroma associated with warm fermentation. This includes aromas reminiscent of banana, pineapple and pear which belong to a class referred to as esters. Some esters, like ethyl acetate, smell like solvent or nail polish remover.
Active fermentation does generate an appreciable amount of heat. When the fermentation temperature gets too hot, desirable fruit, malt and/or hop aromatics will be blown off and the yeast can become stressed. Heat-stressed yeast can produce elevated levels of hydrogen sulfide and they can even die off due to an increased permeability of the cell wall to alcohol which is toxic to the yeast. Warm fermentation also increases the production of higher alcohols. This class of compounds is known for their spicy, vinous aroma and propensity to cause headaches.
Fermentations which are too cool are equally problematic. When too cool most yeast strains will become sluggish and some will even shut down completely as they flocculate to the bottom of the fermenter and take a nap. Cold fermentation, while good for preserving aromatic properties in white wines are not beneficial for red wines just as cold fermentation and aging creates the clean crisp properties characteristic of lagers and are not generally recommended for ales. At cooler fermentation temperatures yeast will work more slowly increasing the fermentation time.
Each yeast strain is unique in its ideal temperature range. It’s a good idea to check the recommendation of the manufacturer for ideal fermentation temperature range for the yeast you choose. Wyeast Beer and Wine product guides (available upon request at no charge) provide this information as well as characteristics, flocculation, attenuation, and more. Most yeast strains are happiest in the 60-80 degree F range. Temperatures much below the ideal range will contribute to fermentation slowdown and eventually a stuck fermentation.
Proper fermentation temperature is important to ensure a healthy complete fermentation.