Check my pitch math/pitching questions

[thirstyutahn]

Hey guys,

So I have a nice amber/red ale brewing as we speak and I'd like to reuse the yeast cake after I'm done to ferment the founders breakfast stout clone. I used White Labs San Diego super yeast and I think it would work great for the breakfast stout. I figure I can save a nice bit of green. I've been reading a lot of stuff and it seems that pitching onto the yeast cake is just not the greatest of habits. For a 1.086 beer it might not be so bad but I'd rather get it right and not chance it.

I was reading this thread https://www.homebrewtalk.com/f163/why-not-pitch-your-yeast-cake-166221/ and was trying to figure out how much harvested yeast I should pitch into my breakfast stout. Running the numbers he has in that thread I'm coming up with about 440 billion cells(for 5.5 gallons), which should work out to roughly 2 cups of harvested yeast slurry(2 cups would be about 472 ml). If the breakfast stout has an OG of about 1.086 that would be correct right?

I ran the stuff through mr. malty and using the repitching from slurry option it's coming up with only 322 billion and only 156 ml of slurry (using the defaults). I'm trying to figure out why the huge difference in numbers from my calculations to the mymalty numbers.

Also, I always just primary ferment and don't secondary so does the length of time the yeast has been sitting matter? For instance if I leave my beer in the primary for 1 month is it going to effect my ability to reuse or effect my pitch rate? Would it be a better idea to transfer to a secondary after say a week and harvest the slurry?

Any help would be awesome, but please no "oooohhh its fine just pitch it onto the yeast cake"...I don't learn anything by doing that.

Thanks!

 

[thirstyutahn]

Anyone? I know there's some serious knowledge around here and I'd love to learn more about this.

 

[Sulli]

I'll take a stab at this.
First off, the discrepancy in pitch rates between Mr. Malty and the why not to pitch on your yeast cake thread, is [I think] coming from the fact that Mr. Malty calculates pitching rates using the 0.75 million cells/ml of wort/degree Plato formula, and I believe Bob [the guy who started that thread] uses the 1 million cells/ml of wort/degree Plato formula. Either number [IMO] would be fine to use.
Second, if you leave your yeast sitting in primary for 4 weeks, you will have to adjust the viability setting on Mr. Malty to match the age of the yeast, I would set the date to a day or two after flocculation occurs.
Third, I would set the non-yeast percentage slider all the way to the right, because your going to have a ton of trub and hop material in there;
and I would leave the Yeast Concentration slider at it's default position, as you are going to need a fairly thin slurry if you want to pour it out of a carboy, unless of course your using a bucket, in which case you can siphon the majority of the wort out and leave a thick slurry, which you can just scoop out with a measuring cup, and then adjust the Yeast Concentration slider as you see fit. This slider can be tricky to set properly, as judging slurry thickness can be pretty subjective. Thick slurry = almost no wort in the slurry; Thin slurry = solution is not visibly cloudy.....and then you have everything in between.
Anyway hope this helps, and good luck.

Edit: If you want to use the 1 million cells/ml wort/degree Plato formula, set the fermentation type on Mr. Malty to Hybrid, otherwise leave it on Ale.

 

[thirstyutahn]

Awesome that's very helpful! I do ferment in buckets so that should make it a bit easier. Thank you very much for the help.