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very quick question: making a kit brew (Coopers Real Ale), which says a typical OG is 1.046 and FG is 1.006. Unfortunately, my OG was 1.038, and my current reading (after about 17 days) is 1.013, and holding (for the past 4 days). Been fermenting at a pretty consistent 20C (with one drop to about 17C for a few hours) and once or twice popping to 22C. I was pretty vigorous with how I mixed the wort initially - I shook and shook the fermenter. If anything, I'm worried I overdid it, because there was a lot of foam etc. (Is this normal? Did I mess up?)

Anyway, how concerned should I be by the unexpected gravity readings? With a higher FG like this, should I wait longer, or bottle? (Concerned about it continuing to ferment, and then bottling, causing bottle bombs... am I mistaken?)

EDIT: OG taken at 26C, latest gravity at 19C. (I did adjustments using online calculators, and they don't seem to make much difference...?)

EDIT #2: thanks for all the help everyone! I can only conclude now that
a) I've been using the hydrometer correction calculation incorrectly (not knowing what the hydrometer is calibrated to)
b) I somehow added more water??? (this one worries me though, because I was very precise according to the markings on the carboy - I'm now wondering whether the carboy, which I got from a local DIY brew shop, was incorrectly marked?)
c) I stalled the yeast a little (although again, the temperature drop happened in the middle of the fermentation, and there was definitely further activity and SG drop for a few days afterwards?)

So, I'm going to go ahead and bottle now this morning. Holding thumbs! :)

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    Shaking the fermentor at the start of the ferment is good. The foam does no harm, and helps absorb oxygen which the yeast use to multiply.
    – mdma
    Commented Apr 22, 2013 at 15:51
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    1.006 sounds very low for an ale yeast.
    – mdma
    Commented Apr 22, 2013 at 16:01
  • Assuming your hydrometer is calibrated for 60 F., you need to correct the OG reading for temperature. This calculator says that your OG is actually 1.040.
    – FishesCycle
    Commented Apr 22, 2013 at 23:36

2 Answers 2

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Typically with a kit if you used the proper amounts of water/sugar/extract ratio your OG will be right on.

Incomplete mixing of the wort doesn't seem like a problem considering you shook the fermenter well. In this video at 25:25 Owen Lingley from Wyeast Laboratories says that only 45 seconds vigorously shaking a carboy will oxoginate the wort to acceptable levels.

If your FG is holding steady after 17 days fermenting, I would go ahead and bottle. Coopers ale yeast has 20°C as the low end of recommended temperature so its possible that your drop to 17°C stopped your fermentation. You hit 65.8% attenuation and Coopers ale yeast typically hits 72-77% attenuation.

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  • thanks Jared. Is attenuation just the difference between OG and FG? Also, is oxygenation of the wort a good thing? (I had a vague feeling what I'd done was bad, and was considering stirring only instead of shaking, on my next brew...)
    – Kevin Versfeld
    Commented Apr 22, 2013 at 16:05
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    Yeast go through two stages, aerobic (with oxygen) and anaerobic (without oxygen). The aerobic stage happens first and is where the yeast consume the oxygen available in the wort. This stage is where the yeast do the majority of their multiplying and getting ready for the anaerobic stage. During the anerobic stage the yeast go into "survival mode" and consume the sugars present in the wort. The yeast during this stage don't multiply as much, so its very important to have a good starting supply of oxygen to ensure your yeast are healthy and multiplied enough for the job.
    – Jared Meyering
    Commented Apr 22, 2013 at 16:10
  • And yes Attenuation is basically the difference between SG and FG, the formula is - ((OG-1)-(FG-1)) / (OG-1) x 100%
    – Jared Meyering
    Commented Apr 22, 2013 at 16:11
  • +1 for "Typically with a kit if you used the proper amounts of water/sugar/extract ratio your OG will be right on." I see a lot of extract brewers posting questions about OG being off, and I virtually always attribute it to (a) incomplete mixing, or (b) too much/little water.
    – GHP
    Commented Apr 22, 2013 at 16:57
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    In my experience, shaking the fermenter does not provide sufficient mixing to get an accurate OG reading after topping up. It may provide enough O2, but that's a different matter.
    – Denny Conn
    Commented Apr 22, 2013 at 18:25
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It's hard to get a low original gravity when brewing from an extract kit. As long as you added all the malt extract and sugar provided with the kit, and you added the correct amount of water, there's really no way for the starting gravity to be low. I can think of two possible reasons that your OG was lower than expected:

  1. you added more water than specified by the recipe

  2. you didn't completely mix the wort before taking a reading, and ended up with a sample that contained a lower proportion of fermentables.

As for the finishing gravity, if it's been steady for four days that's probably all the attenuation you're going to get. As mentioned in another answer, it could be that the yeast dropped out when the temperature hit 17 C. In this case, you could try raising the temperature to 20 C. and gently rousing the yeast. Use a sterilized, long-handled spoon to stir the yeast up from the bottom of the fermenter. The critical thing here is to introduce as little air as possible. At this stage in the process, oxygen is bad for your beer. It will accelerate staling and may add off-flavors. So, stir gently.

The beer is probably perfectly drinkable as it is, so don't worry too much if you can't get any more attenuation out of the yeast. Prime, bottle, wait, then enjoy.

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