Seek and Ye Shall Find

Ah, yes. It's good to have access to expertise.

Called up Weekend Brewer, the shop where I picked up my brewing equipment, and asked whether six days was a long time for the particular recipe I was given to undergo primary fermentation. Turns out that yes, my process is going a tad slowly, but they reminded me of a way to check to see how far along I'd gotten.

First, a lesson in the chemistry of fermentation. Wort (the goop to which I added my yeast) is a sugar solution created by soaking malted barley grains and then adding prepackaged barley-malt extract to water. Water has a specific gravity (a measurement of density) of 1.000 at 60° F. Adding stuff to water, as I did in making my wort, increases its specific gravity. Mine was measured (with a little floating gauge called a hydrometer) at 1.050, which is on target according to the recipe, when I dumped it into the fermenting bucket at 78° F.

During the process of fermentation, yeast converts the sugars in the wort to alcohol and CO₂, and the lion's share of the CO₂ is vented via the airlock during primary fermentation. This reduces the fledgling beer's specific gravity again, and the recipe for my ale calls for a finishing room-temperature gravity of approximately 1.010 (because, of course, we still have lots of malt proteins and hop oils and other stuff in there that won't ferment but will taste great).

So, using the fermenter's spigot, I poured about a quarter cup of my beer and performed a hydrometer reading: 1.020. So it looks like the fermentation process is only about 75% done. Well, that would certainly explain the lack of dropoff in bubble times; there's still work to do!

Oh, and I tasted a bit. Sort of sweet and slightly cidery (and flat, of course - carbonation doesn't come until it's bottled), but since there's still a decent amount of malt sugar left to convert I imagine that's normal.

-Rich

PS. Caveat: as I'm sure Matt will point out, specific gravity is very dependent upon temperature and altitude, and none of my readings were done at exactly sea level, or 60° F. So take the readings with a grain of salt, but the range of readings looks good.