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Low FG

Hey everyone, had a quick question. I did a pale ale with a OG of 1.056 and when transferring to secondary I took a sample for FG (which tasted great) and it was 1.006. I was pretty surprised. This was all grain, with US-05 yeast. I will see typically between 1.012 and 1.016 but this was a little out of the ordinary. I’m not concerned but just curious. What, other than very effective fermentation, could cause this? My hydrometer is calibrated correctly too.

Lots of things can do this.

  1. Low mash temperature. If you mash at 148 F, you’ll get a lower final gravity than if you mash at 155 F. If you intended a high mash temperature, consider whether your thermometer needs to be calibrated, or whether you should have measured mash temperature in more than one spot. The temperature in the mash tun can vary from spot to spot.

  2. Long mash time. If you mashed for 90 minutes or more, the final gravity will be lower than if you only mashed for 40 to 60 minutes. Me? I’ve run a lot of experiments and determined that for my system, 40 minutes is good enough. I only mash longer when I purposely want a lower final gravity.

  3. Contamination. Gosh I hope it’s not that, but it’s happened to me about a dozen times over a dozen years.

Thanks for the reply. I mashed at 153 and it held steady at that for the whole 60 minutes. I don’t move the thermometer just so I don’t lose heat, but I do check it in a few places before closing the lid on my mash tun. I have a Taylor digital probe thermometer and tested it with boiling water to see if it was accurate. I then sparged at 168 for 15 minutes. I guess the only possibility would be contamination, but when my wife and I tasted the sample I had, we both thought it tasted great…better than most of the beers I’ve made at that stage. If it is contamination, would it mess with the flavor?

I suppose there is some remote possibility that you got rogue yeast in there that fermented the sugars way down without adversely impacting flavor. It is possible. But extremely rare. Otherwise, I guess I too am at a loss for what might be causing your low gravity.

Maybe your hydrometer is off? Also, I just did a batch with US-05 and got 84% attenuation. So, depending on your recipe, it’s possible.

Posters on another board have reported great than usual attenuation with 05 lately.

Well, regardless of what the reason is, I think the results will be great, judging by the sample I tried. I did test the hydrometer in water and it was calibrated correctly. I poured Caribou Slobber on the yeast cake which then almost blew up my fermenter just a few hours later, so I will see how this one works out to be too. If it is again lower than the expected outcome, then perhaps it will confirm what Denny said…I’ll post when I transfer or bottle. Thanks!

According to my calculation, with an OG of 1.056 and FG of 1.006 that is 89% attenuation. I used 10 lb 2 row and 2 lb 60L. Simple APA recipe.

Mash temp plays a key role in fermentability of the wort. Mashing at lower temps results in higher yeast attenuation. If your Slobber comes in low FG, it’s possible that it’s mashing temps and not over-attenuating yeast. Do you have another thermometer to test the accuracy of your mash temps?

Yeah, I have a couple digital thermometers as well as the standard mercury type. I guess I could check with all three. However, the thermometer I have been using seems to be very accurate. I tested it with boiling water and it was right at 212.

Unless you mash at boiling temps, that may not mean much! I’ve had thermometers that were on at boiling and freezing, but off by as much as 10F at mash temps.

Yea i did the St. Paul Porter a couple weeks ago with US-05 and it finished at 1.008 even though mash was a good one.

Well, I guess I’ll just have to test them out vs mercury…but they’ll need to be close to the same spot. I make sure to mix everything well in the mash tun before I set in the thermometer, and set it in a few spots too to make sure it’s consistent. I can’t really test two digital vs each other…how would I know which one is right?

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