Having created a yeast starter for all but my earliest beers—like many others—I am (or at least, used to be) a frequent user of the Mr. Malty calculator. Within the past year, I stumbled upon the YeastCalc calculator, and began using it only because it was quicker to load; the parameters, options, and numbers calculated between the two tools were virtually identical. That is, until in early 2013 (to the best of my knowledge), YeastCalc suddenly included a second stir plate option from Kai Troester, in addition to the the “classic” stir plate calculation originated by Jamil Zainasheff. As Kai’s stir plate option calculates a yeast production value significantly greater than Jamil’s option, I was faced with a question: “which should be chosen when creating a yeast starter?”
After having stumbled upon the 2013 National Homebrewers Conference presentation by Kai Troester, entitled “Step Up Your Starters” [PDF], I now feel confident in using Kai’s stir plate option in YeastCalc (or also available in Brewer’s Friend) as my primary calculator. To boil down the origin of why their calculators are different: Jamil’s growth factor for stir plates is supposedly a scale-up from Chris White’s results for a non-stirred starter, whereas Kai’s growth factor is a result of the research he performed.
Having read Kai’s presentation, the take-aways for stir-plate starters I found most valuable (which include my interpretations or translations of his presentation) are these:
- No statistical difference in growth rates was observed between a “foil-covered” and an completely “uncovered” starter (though a statistical difference was observed between a “foil-covered” starter and a starter into which air was actively injected), meaning: foil is going to be just as good as a foam plug.
- Faster stir speed resulted in faster growth (due to increased oxygen availability), meaning: don’t settle for a gentle stir; a bigger vortex is better!
- Higher-gravity starters will result in both significantly lower growth rate and modestly lower viability, when compared to lower-gravity starters, with greatly diminishing returns below 1.040 SG, meaning: target a 1.040 SG, but a few points higher or lower is okay. As a side note, I love that the Brewer’s Friend calculator actually includes an entry field for the starter gravity, though this seems somewhat moot as smart choices for this value won’t differ greatly.
So, to answer the question posed in the title, could I be overpitching by using the Mr. Malty stir plate calculator? I believe the answer is yes, with a qualification: for very fresh yeast (i.e., approaching 100% viability) used for a relatively mid-gravity beer (e.g., 1.060 SG), the overpitched amount may be as little as 10 billion cells (about 4% over the desired amount). For a more likely scenario involving moderate viability (e.g., 70%) of the same 1.060 SG beer, the overpitched amount may be 50 billion cells (about 20% over), and as the gravity increases, this overpitched amount may swell to over 150 billion cells (about 50% over).