Primary fermentation is the step in the beer brewing process where the homebrewer gets to put his feet up and let Mother Nature take over.  After pitching the yeast the fermentation vessel is sealed with an airlock and over the course if the next several days the wort is transformed into beer.  Understanding what is going on inside the fermentation vessel, how to help insure fermentation is as successful as possible and when fermentation is complete are some of the more important things to know about this step.

How Wort Ferments to Beer

Since here at the Homebrew Garage we are beer mechanics and not chemists we’ll keep things simple!  The entire brewing process’ goal is to provide an ideal environment for the yeast to do its thing.  All of the homebrewer’s careful cleaning and sanitation is to make sure the yeast has very little competition once it is pitched into the wort.  The ingredients in the wort (especially the malt) provide food and nutrients for the yeast.  The vigorous aeration of the wort provides the oxygen necessary for the yeast to reproduce until their numbers are sufficient to begin the fermentation.  Are you seeing how important the yeast is in all of this?  The yeast metabolizes the sugars in the wort and produce alcohol and carbon dioxide as by-products.  The airlock on the fermentation vessel lets the CO2 escape (you don’t want your fermentor to blow up do you?).  The escaping CO2 also pushes any excess oxygen out of the fermentor to help keep the beer from oxidizing.  When primary fermentation is complete you have 5 gallons of delicious (but flat) beer! 

How to Insure a Successful Fermentation 

Since you have made it this far there are a few things the homebrewer can do to make sure the yeast does the best job it can.  First off it is best to put the fermentation vessel somewhere where it will not be in contact with direct sunlight.  A basement, a closet or an extra room all work well.  Unnecessary exposure to direct sunlight can effect the smell and taste of your beer.  Regulating temperature is also very important.  Every type of yeast has a recommended temperature range where it performs its best.  You want to keep your fermenting in that range with as little fluctuation as possible.  While the guys here at the Homebrew Garage do their brewing in the garage, we have to do our fermenting in the basement as the temperature in the garage fluctuates too much to be a suitable place for primary fermentation.  Lastly just leave the yeast alone and let them do their job.  It is certainly OK (and even a little fun) to watch the airlock bubble but resist the urge to pick up the fermentor or remove the airlock.  There is a lot of complex chemistry going on in that carboy or bucket and the last thing the homebrewer wants to do is risk an infection at this stage in the game.  Following these basic tips can help your fermentation be as successful as possible.

How to Know When Fermentation is Done

Knowing when the yeast have completed their job can be a little tricky the first few times you do it.  You would think that when the bubbles in the airlock slow significantly or even stop then fermentation would be complete.  However, this isn’t always the case.  Sometimes the release of CO2 has slowed dramatically but the yeast are still converting sugars and even eating up a few off flavors and odors produced during fermentation.  Typically with ales most fermentation is completed in 3-5 days and lagers in 10-14 days.  This is not a hard and fast rule unfortunately.

Probably the best way to know when primary fermentation is done is by taking a gravity reading.  When the gravity reading taken in 2 or more consecutive days is the same (don’t test for at least 4 to 5 days) then fermentation is complete.  The problem with this method is you are removing the airlock (we know we told you not to do it before, but this is different!) and have to be very careful with your sanitation or you could ruin everything at the last minute by contaminating your beer.

If you are using a glass or plastic carboy or clear plastic bucket you can generally tell when fermentation is done when the yeast begin to flocculate and fall out of suspension.  The beer will be significantly clearer than it was before and you can see the inactive yeast lying on top of any other sediment on the bottom of the fermentation vessel.

Here at the Homebrew Garage we typically give the process (with ales) at least 7 days and if the beer has significantly clear and there are no obvious signs of fermentation we take a gravity reading.  We are looking for the gravity reading to be about 75% less than it was before we began the fermentation.  If everything looks good we consider the fermentation to be complete.

Well, that’s about it.  Your handcrafted wort is now delicious beer.  From here you can either package your beer for consumption or rack it over into another fermentation vessel for more aging.  Either way, congratulate yourself on a job well done and pick a date on the calendar for the big party where you will amaze your friends and family!