Poor Fermentation
Notice how many off flavors are caused by poor fermentation and yeast health. For the extra cost of DME and nutrient, a starter can skip over a slew of possible problems. Don't disappoint the brew gods, make a starter. Follow the All-Purpose Easy Starter.
Even with a starter, things can go wrong. Your vial of yeast may be too old, or heat may have weakened it during summer shipping. If you suspect this happened to your pack, start with a cup of starter and give it a few days. Hopefully, any remaining healthy population will replicate and get up to speed in the small starter before hitting a full-sized starter.
Extract-based worts are short of FAN and other nutrients. Yeast cells still need to replicate in the primary. Insufficient nutrient levels hamper reproduction and enzyme production. This is true of inadequate aeration. At lower gravities, this won't be as much of a problem, but higher gravities require aeration.
Another fermentation cause of off flavors is a lack of temperature control. Don't develop the bad habit of thinking your interior closet is cold enough to ferment ales safely. Fermenting too warm, as new brewers do, promotes negative esters and fusels. Conversely, fermenting too cool suppresses desired aromas and flavors, and causes the ferment to stall out and fail to finish. The solutions discussed in Chapter 11 can help keep the temperature in the appropriate range.
Caught early enough, it may be possible to fix a problematic fermentation. Here's how:
Fermentation Fixes
Aeration — Within twenty-four hours of fermentation activity, it is safe to lightly aerate the beer. Make sure there's still enough food for the yeast.
Nutrient — For stalled fermentations, boil a teaspoon or two of yeast nutrient and add to your fermenter. Gently swirl to mix and watch for renewed activity within several hours.
Repitching — The single most used technique for a stalled ferment, repitching can fix more than residual gravity. Off flavors, like acetaldehyde or diacetyl, are consumed by yeast. Active yeast added to the ferment can finish the cleanup.
Rousing — A cheap, effective way to get your yeast back to work. Swirl the carboy or bucket long enough to stir the sediment back up to the surface.
Warming — Another cause of a stalled ferment is the temperature dropping too low, even for a short period of time. Move the fermenter to a warmer location for a day and wait for signs of renewed activity.
