Cedar Rapids Brewing Society

I learned today what the proper pitching rates of healthy yeast can do for your lag time. A few weeks ago, I brewed 5.5 gallons of a Dunkel (around 1.056). I made a 1.5 L starter, pitched in two vials of WLP833, and set it in the fermentation chamber next to the conical at 50F. 12 hrs later, I aerated the main wort to 14 ppm and pitched the starter into the main fermentor. The lag time was approximately 36 hrs from pitching until I noticed bubbles in the airlock. If I had to guess, I'd say the starter didn't do much in that time b/c it was run at a much lower temp than normal, because I did not aerate the starter wort, and because I did not use a stir plate. As a result, I probably pitched 220B cells into the fermentor (400B+ was recommended by the Brewer's Friend calculator).

A few days ago, I harvested a bunch of thick yeast slurry from the dump valve on the conical in which I fermented the Dunkel. Based on the pitching rate calculator at Brewer's Friend and some measurements provided by Kai Troester, I weighed out 200 gms of slurry, around 400 Billion cells. Late last night, I aerated 5.5 gallons of a 1.050 Helles to 14 ppm O2 and pitched the 200 gms of slurry into the fermentor. This time the lag was only 7 hrs.

To summarize:
1. Underpitching by over 50% - 36 hr lag
2. Not underpitching - 7 hrs
3. Both worts were of a similar gravity, both had similar O2 levels, and both were pitched/maintained at 50F.

This may not be useful to any of you, but I found it interesting.

I ask this out of ignorance, but could the lag time be due to the state (or age) the yeast is in when pitched vs pitch rate?

would it seems that if the yeast in the vial for a couple months it could is still 'at rest' after 12 hrs in the starter and would take longer to 'wake up' then the yeast that was freshly harvested from the conical.

kai mentioned something in his blog about the potential older yeast taking extra nutrients before reproducing, but he had not characterized this.

on another note, I have read a couple blogs about yeast concentration vs thickness of slurry and don't quite understand how this can be correlated based on the vague descriptions given (thick vs thin slurry), compounded with the inaccuracy of measurement for highly flocculant yeast strains. I've been tempted to do some yeast count experiments...this could be another tech meeting... but I have not attempted to make measurements, yet.

The yeast was very fresh (around 3 weeks old), so I don't think it would take long to "wake up" in this case. It's entirely possible that it hadn't oriented itself yet, but I'm not sure that would explain the difference between a 7 hr lag time and what is effectively a 48-hr lag time (36 hrs after pitching plus 12 hrs in the starter). The starter was active when I pitched it. I didn't measure the gravity b/c I didn't want to wait any longer and it seemed like enough time had passed, based on previous starters I'd made.

I've measured slurry volume in the past, but this time I decided to follow Kai's advice and measure the weight of the yeast cake after it had setted in the fridge and been decanted. In a blog post (linked below), Kai estimated that harvested yeast slurry, after being decanted, contained about 2B cells per gram for WLP830. In my case, some of the harvested slurry might have been trub, but it looked like pure yeast to me so I decided to stick with his given density value. I like the idea of weighing out yeast, since I can easily do so more accurately than I can measure volume and I don't have to guess as much about the relative thickness of the slurry.

http://braukaiser.com/blog/blog/2012/08 ... by-weight/

http://braukaiser.com/blog/blog/2012/08 ... er-for-me/