Poor fermentation and yeast health can cause many off flavors in beer. Preparing a starter can help avoid fermentation problems. If the yeast doesn’t grow, it provides early warning that can save ruining a batch of beer with old yeast. Even with a starter, things can go wrong. Inadequate aeration can result in insufficient nutrient levels and cause poor enzyme production. Brewers who adequately understand the fermentation process can correct any problems that arise.
It is safe to aerate the beer within 24 hours of fermentation activity. Providing enough food for the yeast can assure the process is accomplished properly. If the fermentation has stalled, a teaspoon or two of yeast nutrient can be boiled and added to the fermenter. The yeast nutrient should be mixed in gently. The brewer should observe the fermenter for renewed activity. If successful, this should occur within several hours. Repitching is the most successful technique used for a stalled ferment. The addition of active yeast can eliminate any problems with the specific gravity as well as consume acetaldehyde or diacetyl that can produce off flavors. Rousing is also an effective way to get yeast to work again. Simply swirl the contents long enough to get the sediment back to the surface.