Expert: Ed Westemeier - 4/18/2009

Question
QUESTION: I have a problem that is similar to some I have read on your site. We made an all grain, higher gravity version of an Amber ale and we pitched a small starter into it.  It bubbled up and actually almost blew the lid off at one point. After a week, we started to transfer to a secondary before realizing that we should have checked the gravity first.  We checked it after the transfer and it was not even close to being finished. It was in the 45% attenuation range (1.037 I believe).  We made a new starter and re-pitched yeast to try to help it along, and it dropped another couple points or so and stopped.  We use a brew belt and it keeps a temp of about 72 degrees so I don't think temperature is a problem. According to our measurements during the mash it stayed a fairly consistent 155 degrees. We also aerated with a paint stirrer before pitching the yeast and it was quite frothy.  We are stumped as to why the fermentation isn't finishing.  Any ideas?

ANSWER: There are quite a few things to consider. Here are a few:

The fermentation might actually be finished, despite the high gravity reading. You don't say what the starting gravity was, but most yeasts will ferment to 70-75% under ideal conditions, and yours were not ideal.

A constant 155 degrees in the mash would be too high for complete conversion.

Proper aeration is a tricky thing, and you may not have actually achieved enough of it.

Keeping the fermenter at 72 degrees is definitely high for that style beer, and may contribute to the production of unpleasant esters, fusels, and higher alcohols, which could inhibit the yeast.

Regardless of expectations, if your gravity hasn't dropped any further for a week, despite repitching, I would think the fermentation is truly finished, and you just have a fairly sweet beer. Remember that unless your mash was done properly, a high percentage of the wort will not actually be fermentable (unless maybe by a distiller's yeast, which won't let it taste very good).

---------- FOLLOW-UP ----------

QUESTION: Thanks for your help.  I have always thought that 155 degrees Fahrenheit was a good target temp and I thought anywhere between 154 and 162 was at least "safe".  What would you suggest for temperatures? Is there a good list of mash temps for different beer styles?

Answer
Whoever told you that didn't give you the whole story.
There are two groups of enzymes that have to work in the mash in order to give you full conversion. They're called alpha and beta amylases, and they work at different temperatures.
The beta enzymes work at lower temperatures, and are inactivated pretty rapidly above about 150 degrees. The alphas work at higher temperatures, and don't do much of anything down where the betas are active. So you generally want to start lower and ramp up higher on a gradual basis. Sometimes a brewer will start high (as you did) in order specifically to limit the activity of the beta amylase. That gives you more unfermentable sugars (called dextrins). Since they're not fermentable by normal beer yeast, they give you more body in your beer.

As you can see, mashing is a bit more complicated than you might have thought.

Go over to howtobrew.com and read more about mashing. That should help you get the picture.