A Typical Ferment
Fermentations follow a standard script with the yeast providing the accent and flavor. You manage the successful translation of story to glass. Along the way, there are many pitfalls, but fermentation happens no matter what you do.
Aeration and Oxygenation
The fermentation story begins as the cooled wort transfers to the fermenter. As the beer splashes its way into the vessel, it picks up oxygen. Smart brewers use this opportunity to add more oxygen to the mix. Some aerate the wort mechanically, shaking the fermenter for 20 minutes, using a sanitized whisk or drill-mounted paint mixer to whip air into the wort.
Others choose an aquarium pump to push air through a sterile filter and diffusion stone, letting small bubbles rise through the wort. The final choice scraps the pump and attaches the stone to an oxygen tank. This is the last time oxygen is added to the brew. Done right, the yeast grow stronger and more capable of tackling the fermentation task.
Pitching
Pitching yeast isn't a complicated task, but to ensure a successful pitch, make a starter. Follow the All-Purpose Easy Starter instructions a day or two before brewing. Starters increase the chances of success. Clean and sanitize the mouth of the starter vessel (or yeast packet) and fermentation vessel before pouring. Use a spray of isopropyl alcohol or Star-San. Pour the yeast and seal up the fermenter. If you're feeling mix happy after the aeration, give it a swirl and walk away.
Phases of Primary Fermentation
Yeast on hitting the wort begins a five-stage fermentation life cycle. They go to work immediately, adjusting to their new environment. During this lag phase, yeast takes in the dissolved oxygen and nutrients, gearing up to convert sugar to ethanol and CO2. Within a few hours, they enter the accelerating growth phase, a period of reproduction and energy storage.
A few hours after pitching, growth becomes exponential and fermentation apparent. During this time the yeast cells divide to saturate the wort. This exponential phase is when many yeast flavors and aromas are generated. Yeast cells treated to subpar conditions, lack of nutrients, underpitching, or too much heat release stressor chemicals altering the beer's flavor.
Ten hours later, the fermentation hits the decelerating growth phase. The rocky krausen goes mad and alcohol production ramps up. When the airlock blurps crazily, you're here. By this point, you should have your beer at or below fermentation temperature. Fermentation generates enough heat that you can't cool the wort down.
In several days, the messy krausen falls away and the yeast enters the final stage, the stationary phase. By this point no nutrition remains and almost all of the sugar is converted. The yeast and krausen drops and the beer clears. The yeast consumes fermentation byproducts, like buttery diacetyl.
Once the yeast hits the stationary phase, the bulk of fermentation is done. Little activity occurs beyond cleanup work. Less flocculant yeast cells lag behind, consuming additional sugars before giving up the ghost.
With a good starter, you can shorten the lag and accelerating growth phases and jump quickly to the exponential phase. The head start means less energy spent reproducing to reach saturation. The yeast remains less stressed and retains more energy for the ferment ahead.
Autolysis
Some brewers monitor primary fermentation's end with vigilance. The moment the primary is done, they move to secondary to age and clarify. What they hope to avoid is autolysis. Yeast, along with other unicellular creatures, has a built in suicide mechanism. When food runs out, cells go dormant, using their glycogen reserves. Left too long, the cells become damaged. The cell releases digestive enzymes, destroying the cell. The cell's “guts” release into the surrounding beer. The resulting flavor contribution has been described as burnt, rubbery, vegetal, and funky.
Older texts stressed quickly moving the beer to avoid this problem. Conventional wisdom set a week's deadline, but award-winning brewers leave beer in primary for a month with no ill effects when using healthy yeast.
Secondary Fermentation
“Secondary fermentation” is viewed as a magical period. Very little fermentation occurs when you rack the beer to a second smaller vessel to age for several weeks. Transfer-roused yeast may kick off a brief fermentation. New brewers panic seeing “little white islands” appear on their beer. Don't worry. It's just yeast.
Is a secondary fermentation absolutely necessary?
No. Many ale styles can go straight from primary to bottling without worry. Dark beers work well since you worry less about clarity. Neutral and flocculant strains of yeast clear in a weeklong primary. Some brewers favor skipping the secondary to avoid the risk of infection and oxidation.
To perform a secondary, sanitize a five-gallon carboy and gently siphon the beer from the primary. Glass is preferred because it is impervious to oxygen. The gentle siphon avoids stirring air into the beer.
Allow the beer to age and clarify in the carboy for two to four weeks. Dry hopping is best done in the secondary since, as the ferment is complete, CO2 won't scrub out the fresh hop aromas. The same goes for flavoring of any sort. Sugary additions, like fruit, require a large six-gallon carboy. Once the yeast are done attacking the new sugar source, rack over to the smaller carboy.
Beer can be held in secondary indefinitely, as long as the airlock is topped up. One brewer's batch of date mead sat in the secondary for nine years before finally being served!
