Under-attenuation/stuck fermentation thought?

[kugeman]

Does anyone have thoughts on what could lead a beer to under-attenuate? The situation is this, I recently brewed a batch of our smoked porter (7bbl system) which had a starting gravity of 15.0 degrees plato. The mash was normal (single temp infusion at 154 degrees F). We use the whitelabs california ale yeast, and I pitched about 3-4 gallons for a 7bbl batch. Nothing out of the ordinary, fermentation seemed to start fine and the beer I pulled the yeast from had attenuated to 72%. However this batch of smoked porter got stuck at 7.3 degrees plato.

My first thought was that I must have under-pitched the beer, and so I added more healthy yeast, and have managed to get the beer down to 6.7 degrees plato (or about 55 % attenuation). Obviously I'm not comfortable serving the beer as is and I'd hate to have to dump it. If anyone has thoughts on what might have caused the problem in the first place, or if anybody has a trick up there sleeve for restarting a stuck fermentation I'd be forever grateful.

If you're ever in Lake Placid, NY stop in and I'll buy you a brew.

thanks...

[dick murton]

Possibly poor mixing / temperature control on mashing in - though I suspect this is the least likely cause.

More likely is lack of yeast food, in particular zinc, often added as zinc sulphate or zinc chloride to the wort just prior to pitching, to achieve 0.1 to 0.2 ppm zinc, or lack of oxygen.

If you added more yeast, you needed to have aerated it at the same time. However, I would be tempted to ditch it now and start a fresh batch. If you added even more yeast + yeast food and aerated now, it will almost certainly taste very different from what you expect - and not pleasantly so.

I don't know what oxygen requirement this particular yeast requires, partly because it varies according to flavour required and other fermentation conditions, but most yeasts require in the region of 8 to 18 ppm oxygen added, either as air (up to about 8ppm only) or oxygen (if 8ppm + required)

Cheers

[bstreety]

To mirror something Dick said, a place I worked had continual problems with attenuation. We changed from a sterile air pump to medical grade oxygen, and improved our attenuation overnight.

[kugeman]

I appreciate the advice. I actually am using a yeast nutrient added during the boil and aeration with pure oxygen during the transfer to the fermenter, so I'm thinking there must be another problem...

[MIKE STIMPSON]

Perhaps your temp prob is fualty in your mash tunn & the temp is reading a lot higher than expected & you have alot more dextrins than desired?
I had the same problem at one stage, once I changed the prob problem solved.
Also I had malt that did'nt have a great deal of enzymatic activity & my attenuation was not good at all, once the maltsters sent what I needed that problem went right away.
I would try some yeast that has been pitch into fresh wort for 12 hrs or so so when it goes into the beer it goes in fighting.

[MikeJordan]

One additional thought is the health of your yeast. What generation did you pitch with? Did the yeast pitched come from another "big" beer? You say you pitched 3-4 gallons of yeast but this can vary widely depending upon the thickness of the slurry. Continuous bottom harvesting could select dormant non-viable yeast. If your yeast was somewhat aged and thinner than normal it is very possible it was under-pitched. I recommend pitching yeast based on cell count and viability. A forced attenuation/quick ferment can really help out in this scenario as well. I would consider brewing another batch if the flavor is off due to under-attenuation. Maybe blending the 2 batches would be an option?

[Trub-Man]

Could you do an attenuation limit test on the stuck brew and see if it ferments any further.
If it does not attenuate any further you have a mashing problem.
If it does move and gets down to a more respectable final gravity you have yeast / aeration problems.

Adding more yeast on a brew that has already fermented a bit usually does not work a you are adding yeast in a non-aerated environment, therefore does not cell divide and multiply.

[Valleybrew]

You may want to try the old trick of hooking up a CO2 line to the bottom of the fermenter and giving it a good 1 minute blast to remix the settled yeast back into the beer/wort.

Steve