I have had recent brews over-attenuate. I changed thermometers, from a “dial” thermometer to an old-fashioned lab glass one. On my most recent IPA I compared the two, and there was a 4 deg difference, with the dial thermometer reading high. I felt confident in trusting the glass lab reading, and was pretty sure I had solved the problem, but apparently not. Below is the grain bill.

I mashed at 152 (according to the glass thermometer) for 60 min.
I fermented with 1056 from a 3L starter

OG 1.067

2-Row 73.9%
Maris Otter 11.4%
CaraPils 4.5%
Dextrose 4.5%
Wheat 3.4%
Crystal 20L 2.3%

My FG is 1.007. That is an attenuation of 91%

It’s not infected.

Perhaps I mildly over-pitched the yeast, but I would be surprised if that is it.

Does anyone have any suggestion on what might be going on? I feel like a 152 deg mash, even with the use of the dextrose in the boil, should not attenuate this high.

In my limited experience I’ve been more successful controlling fermentability with ingredients as opposed to mash temp. Everything in your grain bill with the exception of the C20 is quite fermentable. The dextrose is only going to make it more so.

Maybe next time skip the dextrose and bump the caramel malts up to 5-7%. That should probably help to increase some of that unfermentable sugar.

Try a different yeast and/or try mashing for just 35-40 minutes. No need to go to 60 with today’s well modified malts full of enzymes.

Thanks guys.

Well if I include the carapils and C20 together, that is almost 7% “crystal”. I could certainly increase the carapils. Or maybe add some flaked barley.

Decreasing mash time would be interesting. I have always done a 60 minute mash. Are there many folks out there routinely going shorter? And if so with any experience to support reproducible changes in fermentability? I wonder what the relative benefit (besides time) is between changing mash temp vs changing mash time.

With regards to changing yeast, my thought is that the yeast is just going to eat up whatever is presented to it, within a given strain’s profile. So I would rather figure this out on the wort-production side. Instead of limiting my yeast options.

So, just to be clear, with regard to my original post, I assume it is a safe conclusion to make that the glass thermometer is accurate. Agree??

There is no way to tell for sure how accurate your thermometer is without testing it against a known temperature (like boiling water).

Yeah, I completely missed the carapils in there.

Thing I would try next time:

  • Remove the simple sugar
  • increase caramel malts or switch to C40
  • mash a bit warmer at 156

some combination of those should help get your attenuation down.

OTOH, I always go for at least 60 and more often than not 90. There are advantages for me doing that.

OTOH, I always go for at least 60 and more often than not 90. There are advantages for me doing that.[/quote]

Same here…SOP is usually 90 min for me.

I did some quick calcs and with the grainbill, ratios, mash temps, and yeast listed the OP listed, that would generally give me an FG of around 1.011 (even with the simple sugars in there).

Assuming the mash has been adequately stirred to minimize the hot spots in the vessel, I’d definitely test the thermometer to see how accurate it is…once you know if it’s off and by how much, you can easily compensate.

I’m still puzzled. I re-brewed very similar recipe to the original post. Only change is this one is without the dextrose. And I verified that my thermometer is accurate.

OG 1.066
FG 1.005

60 min mash at 152.

I’m wondering if this could be the “watery beer” phenomenon from higher mash efficiency, as I am routinely at around 84%, and although the issue seems to be more pronounced with these last 2 batches, I have noticed it as a trend.

I really thought the carapils without dextrose would have made some difference though.


How are you measuring specific gravity? My guess is your hydrometer is out of whack. And if you’re using a refractometer, those aren’t designed to measure specific gravity when alcohol is present. If you’re confident that your thermometer is accurate, then my guess is it’s one of these two things.

It’s extremely unlikely that you are getting high attenuation out of every single batch unless perhaps you are re-pitching yeast for many batches and it has mutated to become more highly attenuative. Are you re-pitching, or using fresh yeast every time?

I had a string of batches over attenuate. At first it was minor a little high but not insane. The beer was drinkable no blatant signs of infection. I thought it was process/recipe related. Eventually after several batches attenuating 85+ % then dropping enough after bottling to have gushers I realized I had something lurking in my gear. I replaced everything plastic and It stopped. May not be your problem just thought I’d share.

Thanks guys.

My hydrometer is off by 0.002 so that is a small part of it.

I’ve been playing with refractometer calculations online, and using both those calcs and my hydro readings. The calcs seem to be fairly close.

I did take another hydro sample with the cylinder completely full when the hydrometer was submerged, and that gave me a little better reading of 1.009 (maybe the hydrometer was sticking to the side?), with a correction to 1.011 now. So this batch might not be as bad off as the previous.

I am sticking my head in the sand with regard to contamination, for now. But in the end you may be right about that.

I brewed an American Pale Ale recently with an OG of 1.050, and after 3 weeks I got a reading of 1.008. I was using WLP001 that I had created a starter with. That yeast was washed, so it was “second generation” yeast. I know that if yeast is harvested too often, over-attenuation can occur due to the yeast “mutating” but I wouldn’t expect that to happen after just one time.

I know that WLP001 typically does attenuate well, but this is not the first time I’ve had FG’s that are below 1.010. All of the beer has tasted great, so I don’t believe there was an infection of any kind.