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Home Brew Forums > Home Brewing Beer > All Grain & Partial Mash Brewing > Higher than expected FG - is mash temp to blame?
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Old 04-20-2012, 05:39 AM   #1
Rubes
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Default Higher than expected FG - is mash temp to blame?

The last couple of beers I did have stopped fermenting sooner than expected. One was an Irish Red that went from 1.051 to 1.016. The other was an Oktoberfast ale that went from 1.058 to 1.020. They taste great so that's not really an issue, but I am curious why they stopped short of where I thought they would.

I mashed the Irish Red at 149F for 90 minutes. I also raised the temperature to 170 for the last 10 minutes (I BIAB). I fermented with a washed Wyeast Irish Ale yeast.
I mashed the Oktoberfest at 156F for 90 minutes and raised the temperature to 170 for 10 minutes on that also. I used S-04.
I also did Yooper's Pale Ale a couple of months ago and it stopped at 1.020. It was fantastic still, but I didn't know why. I don't remember what I mashed at, I didn't take as detailed notes back then. It would have been in the area of 154 though. I fermented with S-05.

I've read that higher mash temps = a less fermentable wort and will leave sugars behind that can't be fermented. I assume this is the reason for the Oktoberfest stopping where it did. That doesn't explain why the Irish Red stopped early though, unless the 10 minutes at 170 was enough to leave a lot of unfermentable sugars in the wort? I don't know if that's possible, the science of it still confuses me. Or maybe it's simply that the washed yeast couldn't do anymore.

Anyway, should I try keeping my mash temp lower? Should I get rid of my little mashout? I always thought that it helped with my effiency, but if it's affecting the FG then maybe I need to stop. Thanks

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Old 04-20-2012, 07:13 AM   #2
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Default Mash Temps are not excessively high

Your mash temps seem to be in the range that one would expect. The 149 temp is toward the lower end of the scale, and is certainly not too high. Similarly, the 156 temp is toward the upper middle part of the scale, but not excessively high. Also, I don't think that the mash out would cause the problem. The other variables would be yeast selection -- but I have had real good luck with US 05. I do not know anything about US 04. What is your grain bill like? Like you, I would like to find out more about what may be the cause.

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Old 04-20-2012, 01:07 PM   #3
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Did you use a starter or just pitch the yeast? The problem with just pitching the yeast is it uses up some of the oxygen for converting sugar for multiplying. I'm not sure how much that would help your FG but it couldn't hurt.

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Old 04-20-2012, 01:33 PM   #4
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Did you check the accuracy of your thermometer and hydrometer?

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Old 04-20-2012, 01:47 PM   #5
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Quote:
Originally Posted by Rubes View Post
One was an Irish Red that went from 1.051 to 1.016. The other was an Oktoberfast ale that went from 1.058 to 1.020.
1.016 seems about right to me. What were you expecting? Remember, your expected gravity and your actual gravity won't always work out.

Also, there may be a difference between your system efficiency and the efficiency of the system of the author of the recipe.

If the person that wrote the recipe has an efficiency of 75% and you have 70%, there will be a difference.
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Bobby_M>>I flood the keg with CO2 for one minute with the lid off, rack the beer in to the bottom gently, seal it, flood it, vent it. If there's still O2 in there after that, F it.

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Old 04-20-2012, 01:59 PM   #6
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Quote:
Originally Posted by jcdouglas View Post
Did you check the accuracy of your thermometer and hydrometer?
I just went through this same issue. Come to find out my thermometer was 5 degrees off! So When I mashed my Oatmeal Stout at 156, it was really 161!! Got a new thermometer and I am now getting lower FG's (The range they should be in for the particular style)
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Old 04-20-2012, 02:05 PM   #7
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Not only check your thermometer's calibration but also the hydrometer and with the hydrometer be sure you are making temperature corrections as they are calibrated to 60F.

+1 to all above the above suggestions as well.

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Old 04-20-2012, 06:11 PM   #8
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Thanks for the responses. My hydrometer is calibrated correctly, I have never calibrated my thermometer. I just assumed it was right. Maybe I should check that. I suppose 1.016 is pretty average, my OG was also higher than the recipe's OG so I guess it works out in the end.

I was more concerned over the other two, as both the S-05 and S-04 I thought stopped around 1.020. The Oktoberfest, which I fermented with S-04, started at 1.058 so maybe the yeast just got too tired to continue? The ABV is plenty and a little sweetness in an Oktoberfest is fine, but I'm more concerned about future brews stopping around 1.020 when I may want them drier.

Maybe I'm just overreacting and nothing is wrong at all. I'll be monitoring my mash temps a bit more closely from now on though.
Also, the only one I made a starter for was the washed Irish Ale yeast, as it was a few months old.

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Old 04-21-2012, 04:45 AM   #9
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Some grains give you points but no fermentables. Meaning they add gravity numbers that will not go away. They tend to add malty flavors in the end.

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Old 04-21-2012, 11:55 AM   #10
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Quote:
Originally Posted by Rubes View Post
Thanks for the responses. My hydrometer is calibrated correctly, I have never calibrated my thermometer. I just assumed it was right. Maybe I should check that. I suppose 1.016 is pretty average, my OG was also higher than the recipe's OG so I guess it works out in the end.

I was more concerned over the other two, as both the S-05 and S-04 I thought stopped around 1.020. The Oktoberfest, which I fermented with S-04, started at 1.058 so maybe the yeast just got too tired to continue? The ABV is plenty and a little sweetness in an Oktoberfest is fine, but I'm more concerned about future brews stopping around 1.020 when I may want them drier.

Maybe I'm just overreacting and nothing is wrong at all. I'll be monitoring my mash temps a bit more closely from now on though.
Also, the only one I made a starter for was the washed Irish Ale yeast, as it was a few months old.
This could be your problem. I had a thermometer (dial with long stem) that I had checked and was right on until I dropped it. Nothing broke so I thought I had gotten by just fine until a couple of my beers ended up with a quite low FG (1.002 for one that should have been 1.016). I bought a lab grade thermometer to check it against and found that it was reading 4 degrees high and would stick a little as it approached mash temp. I still use it but when I get close to my mash temp, I use the lab grade for the final few degrees and now my beers come out much closer to the expected FG.
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