Explosive fermentation - doing it right, or something else?

In back-to-back beers, using different yeast, I’ve had krausen reach the top of a 6.5 gal carboy, with the beer line at the 5.25 - 5.5 gal mark. I’ve brewed 12-15 batches this year, and I can think of maybe only one other time this has happened. Is this normal? Will the same yeast strain produce totally different krausens, all else held constant? Is this a sign of pitching enough yeast with enough oxygen? Or is this a sign of something else?

First instance
The first instance was a 1.058 OG red ale with wyeast 1056, kept at 64F using a thermowell*. I actually had to use a blowoff tube for the first time ever, even though this isn’t my first time with 1056. The mug of starsan into which the blowoff tube was placed also eventually overflowed, much to my chagrin.

Second instance
The second instance was a 1.100 OG imperial stout with wyeast 1986, kept at 66F using a thermowell. I thought this one was, at first, going to be normal: I had maybe 2-3" of krausen after 18-24 hours, but sometime after the 36-hour mark, I returned to find the krausen caked so thickly right up to the neck, it’s impossible to see the liquid surface. I didn’t have to use a blowoff tube, though it looked like it was a close one.

Counter-Example
As a counter-example, I recently made two extract-batches (otherwise, I’m all AG) using Surly Furious kits I’d received. I used Wyeast 1335 for that. The second kit was pitched onto a large quantity of washed slurry from the first kit, but neither had much more than 1-1.5" krausen. If I would have guessed, repitching slurry should have resulted in explosive fermentation…

Possible causes?
I use yeast nutrient, but sporadically… not sure if this could be a cause. I also oxygenate, and for these two beers, I extended my oxygenation time using my O2 tank & stone. I bubbled in O2 for about three-four times as long as usual (90-120 seconds, with 30 seconds being the usual) for the first instance, and perhaps only two-three times as long as usual for the second instance.

*The stick-on liquid crystal thermometer on the side of the fermenter is accurate-enough to tell me that my temp controller reading is accurate.

The extra oxygen might explain it. When I harvest a slurry I count the density and viability it with a hemocytometer. Pitching from a slurry can go either way. One slurry I saved was 97% viable when harvested, another was only 60% viable. The density can vary widely as well. Mr. Malty was off by over a factor or ten last time I used it.

Here is some more information:

http://woodlandbrew.blogspot.com/2012/1 ... cells.html

[quote=“WoodlandBrew”] Pitching from a slurry can go either way. One slurry I saved was 97% viable when harvested, another was only 60% viable. The density can vary widely as well. Mr. Malty was off by over a factor or ten last time I used it.

Here is some more information:

http://woodlandbrew.blogspot.com/2012/1 ... cells.html[/quote]

Interesting, thanks for this. My take-away is that, basically, the Mr. Malty calculator (and others) are useful estimating tools, but other variables may come into play and the yeast amounts I’m pitching may be greatly under or over the ‘calculated’ amounts.

yeast can be unpredictable. the same strain can act differently. especially with different grain bills and gravity’s. It’s difficult to regulate stable conditions from batch to batch. looks like your examples are very different gravity’s with different strains. It’s a common problem for brewers to look for a problem before there is one, and even then, the vigor of fermentation isn’t a diagnosis - as far as i know.

i’d bet It’s normal. Don’t be overly concerned with fermentation appearances. even if the beer taste funny, or off, blaming that fault on the appearance of fermentation would be difficult to prove.

I wouldn’t sweat it…if you have a vigorous ferment, RDWHAHB and thank the beer gods. In my experience, a tumultuous ferment is generally a very good thing and is a sign of happy yeasties.

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