I'm having a problem with all my beers regardless of style getting over attenuated. I've been raising my mash temps up to 4° above each beers style (up to 158° or so) to compensate with no luck. I've verified my thermometers are accurate with ice water also. I've also tried using some lower attenuating yeast strains with no luck also. Most of my beers whether they start with an OG of .50 or even .90 drop to 1.010-1.000 by the end of primary. I've thought about adding caraPils/dextrin malt to my mash, but was hoping I wouldn't have to. Any ideas to try or something I'm totally missing here?

Some details about my setup:

  • Converted cooler as mash tun
    • 60-70 minute mash stirring every 20min
    • batch sparge with 170° water for 15min
  • Converted Keg as kettle
  • Glass 6.5 Gallon carboy with blow off tube
    • usually switch to a stopper with airlock after first three days
  • Appropriate yeast starters based on mrmalty calculations
  • Oxygenate wort before pitching with with pure O2
  • Two week primary fermentations
  • Fermenting in a chest freezer with temperature controller

EDIT: After all the good answers regarding the accuracy of the hydrometer and thermometer, I went ahead and tested them. My digital thermometer matched the calibrated one at freezing and boiling and boiling was as expected 201° for my altitude/pressure. The hydrometer read 1.000 at 60°.

Even though I feel like I'm extremely thorough and sanitary when I brew I'm settling on it being contamination. I bought a stopper and air lock for my starter flasks so I can rule out foil covered starters being the time of contamination, and doing a major cleaning of all my brew equipment, hoses, freezers and whatever else... Thanks.

EDIT #2: Figured it out, I am really good about cleaning everything and sanitizing after after brew, but I wasn't cleaning the ball valve on my kettle. Took it apart and there was a bunch of black gunk in there! My next beer was spot on with the final gravity I was expecting.

  • A long shot, but have you calibrated your hydrometer? No off-flavors from possible infections?
    – JoeFish
    Jun 20, 2012 at 22:29
  • I haven't calibrated it, but it's been happening over 15 brews and two hydrometers. I can't imagine two different hydrometers being off or bad sanitization that many times would be it... possible. Jun 20, 2012 at 22:56
  • I have had some off flavors, but I thought they were just too dry, thin and harsh because of the final gravity. Jun 21, 2012 at 0:33

5 Answers 5


If you are truly being that diligent about your mash temps and hydrometers including knowing they are accurate, the ONLY thing left is a contaminating wild yeast/bacteria driving down the residual gravity.

Take a bottle of beer and put it in a warm attic for a week. Chill it down overnight and open it up in the sink as a precaution. If the beer is super foamy and thin infection is the problem.

Re-examine your sanitation practices. I hope this is not the case for your sake, but its hard to find a different explanation.

  • So I went ahead and checked my hydrometer also and it read 1.000 in 60 degree water. So, the bottle of beer in the attic, is that after it's been primed and carbonated or just straight out of primary? And would another gravity reading of that attic beer tell me also? Jun 21, 2012 at 4:40
  • Use an already bottled beer one that seems fine is just thin in taste. THe point of putting it someplace warm is to accelerate the growth of any potential contaminating microbe. If after 4 days its really foamy and super thin its contaminated. You'll probably be able to taste it too. Might be clovey or phenolic. You certainly could take a gravity reading, but you already know its pretty low. If you want to check it, degas the beer by pouring between two glasses 10-12 time then read the gravity. Good luck.
    – brewchez
    Jun 22, 2012 at 0:55

Just because your thermometer is accurate at freezing temps doesn't mean it's accurate at mash temps. You need to get a certified calibration thermometer and check it at mash temps, about 150F. It might not be your thermometer at all, but until you verify it at mash temp you just don't know.

  • Yeah the test at the opposite end of the temperature range isn't terrible useful. And getting an accurate 32F/0C sample is harder to create than say a boiling sample. ALthough boiling can be effected by altitude some too.
    – brewchez
    Jun 21, 2012 at 21:33
  • So how do I test if it's accurate at the high end, and yeah I'm in denver so boiling is 10degrees lower? Also can you recommend a certified calibration thermometer, they seem to have a crazy price range? Jun 21, 2012 at 21:43
  • Mine is allafrance and was $8. Right on at both ice and boiling, if used properly. Here's a video I did: youtube.com/watch?v=y3Megyi5Q_4
    – Dale
    Jun 21, 2012 at 22:17
  • Get a certified calibrated thermometer like this to test your brewing thermometer. Do not use the calibration thermometer for brewing. amazon.com/gp/product/B00551O8V6/ref=oh_details_o00_s00_i01
    – Denny Conn
    Jun 22, 2012 at 0:05
  • So I went ahead and got one from my homebrew store today and I was actually pretty impressed with my cheap digital thermometer matching the calibrated one exactly at freezing and boiling. I even did the altitude/barometric pressure adjustment and the boil temp was as expected. Jun 22, 2012 at 4:26

Are you fermenting at high ambient temperatures?

I've definitely noticed much more attenuation during summer months, when my basement is running at about 74F, compared to winter, when it's 68F.

I've adjusted my recipes somewhat, but more my expectations and target styles. I now brew with Nottingham and saison yeasts in the summer, with an expectation of high attenuation.

  • Good points about fermentation temps.
    – brewchez
    Jun 21, 2012 at 21:31
  • I use a chest freezer with temperature controller, and usually for the first couple days of fermentations I set it 5 degrees lower than I want it to ferment at. I only have the sticker thermometers on the carboys, but I can usually keep those showing within a degree or two of where I want. Jun 21, 2012 at 21:37

To rule out mash variables, you could double batch on your next brew day. One all-grain and one extract. If they both turn out thin, then you have a sanitization issue. If the extract is ok, but the all-grain is thin, there's a mash issue. But this is a harder solution than the others. Could be more fun, though, brewing more is always good!

  • good idea, I might try a mini extract batch to see what I get. Though I think it would be sanitization issue not a sanitation issue :) Jun 24, 2012 at 17:54
  • Fixed splelling error, hehe.
    – Dale
    Jun 26, 2012 at 0:08

What about a mash out step? Do you heat the mash to ~170 when you sparge? Do you boil the wort immediately as it comes out of the mash tun?

It might be that your sparge is taking a long time, and the enzymes are continuing to convert your wort. This would leave you with a very fermentable wort, despite the mash temperature that you used.

I have often suspected this on my own system, as my beers also tend to turn out drier than expected. Up until very recently, I didn't have a separate HLT, so the wort would sit around in buckets waiting to be transferred to the boil pot (doubling as HLT).

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