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Step Mash. Stuck Fermentation

Roadrocket

Grandmaster Brewer
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I brewed a Pilsner using Weyermanns floor malted Bohemian Pilsner malt. I did a step mash using the following recommended mash profile I got from the Weyermanns Web site which is similar to the BeerSmith BIAB pilsner step mash profile.

20 min  50?C  (122?F)
40 min  63?C  (145?F)
5 min  68?C  (154?F)
20 min  72?C (162?F)
mash-out  76?C  (169?F)

I made a two litre starter of WLP800 Urquell Pilsner yeast which took off after 6 days.
It has stalled at 1.020 and I cannot get it to shift. I've tried raising the temperature from 11C to 16C with no effect. It's been stalled now for two weeks. I'm thinking I may have a lot of unfermentable sugars caused by the mash profile.
A friend has experienced similar problems with this mash profile on more than one occasion.
Has anybody else experienced this?
 
Did you do a starch test on your wort after you finished your mash?  That would be the way to know for certain if there were unconverted materials in your wort that would not ferment well.

Other than that, it seems to me that going from 40 minutes at 63C to only 5 minutes at 68C is a little short.  Typically, the step mash profiles have a shorter rest at 63C (about 10 to 20 minutes) and much longer at 68C (about 40 minutes) from my notes.  I have also found that it takes longer for conversion as the mash temperature drops below 65C.

I use the floor malted Pilsner from Weyermanns and haven't had a problem with conversion even with a single infusion, but it takes longer than their standard pilsner malt (I do a full 60 minute mash at around 65.5C) and it has turned out very fermentable.


 
+1 oginnme

I brew only 4 step pilsner mash  beer for all my styles.  I have had the final step take over one hour depending on percentage of specialty malts (like victory and biscuit) in the mash. Last batch took 90 minutes for sacc.step alone. These recipe step times always seem too low for sacc.conversion.

Only when  the iodine don't turn blue can you  commence with the next step of the brew!

I suspect there maybe other causes such as  insufficient oxygenation of wort, or fermentation temperature too low. Transferring to a carboy or other secondary fermentation vessel has been known to help me finish off the brew and bring it down to the proper s.g. when this happens. CHEERS!

 
You probably already know this, but just to be sure: If you used a refractometer to get the 1.020 final gravity, it will need to be corrected to give the actual gravity.
 
Greetings - I recently read an article in a book written by the publishers of BYO who suggest a lack of yeast nutrients as a possible cause for a stuck fermentation.  They suggest adding a small amount of nutrients to the fermenter to kick start it.  I don't have any experience with this in particular, but it might be worth a try.  It certainly can't hurt.
 
Oginme, I didn't do a starch test but I should have. Good point. You're right about the 5 minutes at 68 being too short. I think it should have been 35 minutes. The Weyermann web site definately states 5 but I guess it's a misprint.  I should have questioned the number when I programmed it in.
The OG was spot on target so I can only guess I have a lot of unfermentable sugar. I will try another Hydrometer. It has dropped about two points over the last couple of days so I'll leave it a while to see what happens.
 
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