What's the deal with yeast starters?

[kombat]

I've done quite a bit of reading, and I still don't feel like I fully understand the math behind preparing a yeast starter.

I know how to calculate how much yeast I need. That part is easy. 4 billion cells per gravity point per 5 gallon batch, and double it for lagers. Easy-peasy. The areas that are still unclear to me, however, are:

• Say I've collected some washed yeast from a batch, and stored it in a jar in the fridge for 2 months. From what I've read, the yeast cake at the bottom of the jar could vary in cell density anywhere from 1 billion cells per milliliter to 4 billion cells per milliliter, depending on age (viability), and dilution with non-yeast matter (trub). So I just guess 2 billion cells per milliliter and go from there. How can I improve this estimate without buying a microscope and a bunch of petri dishes?
• Say I need 320 billion cells, and I'm starting with a smack-pack. I've read that a 2L starter will double the cell count. But is that true for any given starting cell count? Or will 2L of starter simply grow 200 billion cells, regardless of how many I started with? I mean, if I pitch 100 mL of my washed yeast slurry (at 2 billion cells/mL) into a 2L starter, then that should already be 200 billion cells, right? Will they double to 400 billion in a 2L starter? Or would I need a 4L starter for that? Or what if my 100 mL of washed yeast actually has a lot of dead cells, and actually only has 50 billion viable yeast cells? Will they still "double" to 100 billion in a 2L starter? Or will they grow to 200 billion, because they'll always replicate enough to fill the space they're given (which happens to be 100 billion per liter)?

Even after all this reading, and having actually done a few starters myself, I still feel like this is very much a guessing-and-hoping game. Am I alone in this confusion? Am I missing some fundamental truisms that would clear this whole mess up?

 

[AnOldUR]

Unless you're counting cells and measuring out the starter slurry, it is a "guessing-and-hoping game."

But sometime an educated guess is better than winging it. :cross:

 

[LLBeanJ]

Have you checked out yeastcalc.com and mrmalty.com? Those are the standard calculators used by most folks around here to figure out how big of a starter is needed. I'm partial to yeastcalc, myself, but they are both great tools.

 

[jflongo]

If you look at MrMalty for example, and let's say you have 70% viability, and are doing a 1065 beer, 5.25 gallons. I put thick yeast around 4/4.5 and non yeast % around 20/25. It's saying you would need about 101 ML of slurry / 236 billion cells. So if you had 100 ML of slurry already, then you are probably good. It's still a good idea to do a 1L starter for a shorter time, just to wake them back up before pitching.

Does that help at all?

 

[kombat]

Does that help at all?

Yes, and I know all my answers can be found somewhere on MrMalty's site, but it's a lot of info to go through, and I guess ultimately, it comes down to one question:

Does a 2L starter double the yeast count, or does it grow to 200 billion cells?

If I pitched 25/50/100 billion cells into a 2L starter, will I always end up with 200 billion? Or will it simply double the original count?

 

[jflongo]

Yes, and I know all my answers can be found somewhere on MrMalty's site, but it's a lot of info to go through, and I guess ultimately, it comes down to one question:

Does a 2L starter double the yeast count, or does it grow to 200 billion cells?

If I pitched 25/50/100 billion cells into a 2L starter, will I always end up with 200 billion? Or will it simply double the original count?

It will depend on whether you use a stir plate, or just shake it around as you walk by.

You can use http://www.yeastcalc.com/ and play around with numbers.

Let's say your yeast was 2 months old, you started with 100 billion cells, did a 2L starter on a stir plate, it estimates growing 281 billion cells, so you end up with 355, due to viability and losing some at the start because of the age.

Now let's say you just used the walk by method and swirl it by hand. It would grow 147 billion cells and you would end up with 221 billion.

To get a rough idea of how many you will end up with, use these calculators.

 

[AnOldUR]

If I pitched 25/50/100 billion cells into a 2L starter, will I always end up with 200 billion? Or will it simply double the original count?

Neither. When the inoculation rate is less it does allow for more doubling of cells, but new cell creation depends on the density of the cell population and the food available. If you put 50 billion cells (instead of 100) in the same 2 liter starter you’d end up with around 138 billion cells.