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The Homebrew Dad yeast starter calculator (which gives you more numbers than the Mr. Malty one does) promises a yeast growth factor of about 45 times for 10 billion cells pitched into 3 litres of 1.040 gravity wort for a stir plate starter. In other words, 10 billion cells will produce about 450 billion new ones.

I know that stir plate starters can achieve high growth rates, and 10 billion cells in a 3 litre stir plate starter will have plenty of wort, oxygen and nutrients to work with, but still this seems a little too good to be true. However, I have no experience with stir plate starters yet, so I don't really know.

Is such a high growth rate (assuming that everything is optimal) actually possible, or is it indeed to good to be true?

  • I can simply relate my experiences using a stir plate. I don't have a way to measure the yeast it grows but after using a stir plate and anything from 1 to 5 liter starters, the fermentation takes off like nuts with any kind of starter. I usually make 1 liter per 5 gallons. So a 15 gallon brew day, 3 liter starter. I often see a lot of activity just a few hours after pitching. – sniperd Oct 2 '18 at 13:01
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    Have to realize cells would only have to bud 6 times to get from 10b to 640b. – Evil Zymurgist Oct 2 '18 at 20:18
  • So that would suggest that, given sufficient wort, the stir plate oxygenating the heck out of everything, and a healthy starter population, a growth factor of 45 is indeed practically possible? – Frank van Wensveen Oct 3 '18 at 7:57
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'Is such a high growth rate (assuming that everything is optimal) actually possible[...]?'

It sure is. Yeast will pretty much grow indefinitely as long as there are no nutrient limitations or inhibitory levels of ethanol. The major limiting nutrients are:

  • Fermentable sugar
  • Nitrogen (amino acids, basically)
  • Oxygen (more accurately, the lipids synthesized in the presence of oxygen)
  • Vitamins and other micronutrients.

In a regular batch fermentation, the limiting factor will usually be sugar, but may also be oxygen (a 'stuck' fermentation). Nitrogen generally isn't limiting in all-malt worts, though it can be when high levels of adjuncts are used, or for very high gravity worts. Zinc is the only micronutrient limitation you'll likely encounter.

In a stirred starter, it will most likely be sugar (again) but could be nitrogen, especially if you use simple sugars to fortify the starter wort, or, again, zinc.

As reported in Brewing Yeast and Fermentation:

'Typically, brewing yeast growing on high-gravity (15-18°Plato) wort under continuously aerobic conditions [...] yield a terminal yeast count of 200-300 x 106 cells ml-1'

In contrast, given your situation, 450 x 109 cells per 3 liters (or 3 x 103 ml.) leaves you with 150 x 106 cells/ml., only as little as half as a typical number (as quoted). Of course, this is easily accounted for by the fact that you are using a 10°Plato wort, but by extrapolation you might expect to be able to get the same cell count from a wort as low as a 7.5°Plato.

So, if all you cared about were cell count, you could hypothetically push this pretty far, as the extent of overall yeast growth will generally be only limited by wort strength. However, as I mentioned above, alcohol levels become damaging to yeast at a certain point, so you generally don't want to go too high (even 18 Plato seems a bit high to me).

But, in essence, yes this is an entirely feasible level of growth, given the right conditions.

  • I have actually seen a claim to the opposite effect. Diegobonatto suggests that "yeast cells reach a maximum concentration of 1 to 3x10^8 cellls/mL of culture medium" in post 49 of this thread: homebrewtalk.com/forum/threads/… – Frazbro Oct 4 '18 at 5:44
  • I have also seen claims that cell concentration above a certain threshold promotes flocculation. If both claims are true, the cell concentration in suspension could max out while cell growth continues (i.e. cell growth and flocculation balance each other out. Whether this is the case or not I have no idea, but it's food for thought. :) – Frank van Wensveen Oct 4 '18 at 9:54
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    @Frazbro I think that is definitely true. I should be careful not to imply that final yeast concentration is directly correlated to wort strength in a starter, because there are so many other factors. My guess is that such situations are limited by availability of nitrogen (in the form of amino acids, etc). As that poster notes, there are ways around this (bioreactors and other special equipment) but those are generally not widely available to homebrewers. – Franklin P Combs Oct 4 '18 at 10:37

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