*Checking Your Gravity*
While your specific gravity will begin to drop during the fermentation period, this is an aspect you should trust and not necessarily test. You want your beer to be exposed to as little possible contamination as possible. Every time you unseal the airlock, this exposure occurs. A good rule of thumb, measure your original gravity prior to pitching your yeast, then anytime you might transfer your beer, either to another container or, finally, during bottling/kegging.
Within a day or two of brew day, fermentation begins. As the yeast convert malt sugars into CO2 and alcohol you will see bubbles come through the airlock. The specific gravity will steadily drop and a cap of thick tannish foam called krausen forms above the beer.
You may want to store your brew in an area that is easily cleaned, a particularly violent fermentation could cause a bit of a mess. Violent? Yes, violent. As with anything that builds with increasing gas levels, explosions can occur. Explosions are most common if your airlock fills with gunk essentially stopping the flow of gas out of the carboy. If your krausen starts filling your airlock. You may want to initiate a blow off set up.
Roughly one to two weeks from brew day, fermentation ends. Bubbles coming through the airlock become very slow or stop entirely, the specific gravity is stable and the cap of foam starts to subside.
During our fermentation process, we see a layer of krausen form atop our beer, where does it go? That krausen normally dissipates over time and any remaining grain particles, hop particles and dead yeast cells will accumulate instead at the bottom of your fermenter in a mass known as "trub".
While sitting on this trub for a short while can impart flavors we want to see in a beer, letting our brew sit atop this trub for too long can create flavors we don’t want. To avoid these flavors setting in, we will "rack" (siphon) the brew out of the first fermenter, careful to leave the trub behind and into a new, clear and clean fermenter. Doing so allows the brew to settle out and condition in flavor. It also gives the brewer an opportunity to clear out the beer, after racking the beer into a secondary fermenter, more trub may form, but when racked into bottles during the final stage the beer should be less hazy and more clear than it started off.
Remember, when racking into a new, secondary fermenter, it is equally important that this vessel is clean and sanitary. Be sure to sanitize your auto-siphon, your carboy, your airlock and stopper and any tubing that may come in contact with the brew.
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