I follow BYO on FB. Today “Mr. Wizard” answered a question and included the following info on yeast starters which I found interesting for a couple reasons.
- Who has ever stepped up a starter 10x in volume?
- Who doesn’t cold crash between steps of a multi stage starter? What else could you do?
"There is another rule of thumb that I am going to recall here and that
is the 10x rule when propagating yeast. The 10x rule means that yeast is
moved up in volume by 10-fold increases. Assume that you grow up yeast
in a 100 mL starter. At the end of a fairly
normal growth cycle the cell density usually grows to about 100 million
cells/mL. This assumes the culture is exposed to air through a cotton
plug or the growing yeast is intermittently aerated during growth. The
10x rule means that the culture density is
diluted to 10 million cells/mL after the transfer. When using this rule
the batch size is usually 10 times larger than the last jump in the
step when the gravity of the wort is around 1.040 (10 °Plato). In the
case of high gravity beers the final pitch volume
may change, but the propagation steps are about the same.
So what does this rule have to do with the price of doppelbock in Ozark?
When yeast is put into a growth media and the idea is to grow cells, it
is key that there is sufficient cellular building blocks for cells to
actually reproduce. If yeast is grossly overpitched,
reproduction may never occur and if cells do divide the daughter cells
may not be healthy. To put this into animal terms, it’s a similar
concept to underfeeding a large population of deer. Too many deer on a
plot of land may very effectively devour all of
the edible food in sight, but the individual health of the population
may well be very poor.
Your starting point was a smack pack of yeast. Dilute this yeast slurry
in 2 liters (2 qts.) of wort and you have a good starting point for
healthy yeast growth. Using the 10x rule your next step should have been
20 liters (5.3 gallons) of wort, or a normal
batch size of average strength wort. Instead you fed this slurry with
another 2 liters (2 qts.) of wort and likely put your healthy culture
into an environment that did more harm than good.
On top of that, you crashed your culture to help separate yeast from
beer produced in the first propagation step. Unfortunately most lager
yeast strains are really not very flocculent, and this is a dicey method
to use. After your first step of propagation
you had yeast that was healthy, vital and ready to munch on maltose . .
. and then you stuck it in the cooler, putting the yeast in a quiet
place. When you pitched this yeast into 2 liters (2 qts.) of wort it
probably perked up very quickly because of the
high cell density, metabolized all of the nutrients in the wort,
increased in cell number to some degree by producing daughter cells that
were most likely not very healthy. And when you pitched this culture
into your delicious sounding doppelbock wort the
stage was set for a stalled fermentation."
I just did a 4L starter and stepped it up to a 4gal. By his calculations I’d have needed a 10 gal starter. Give me a break…