I made a comment to a facebook post earlier today about saving yeast from a smack pack. A previous commentator admonished the owner of said smack pack to wash the yeast for re-use once he had brewed the beer. I commented and said that there's no reason to wash the yeast, since you could just save a portion of the starter and re-cuture. I then got yelled at that you "don't save a piece of any starter, certainly not of lager." Seemed to me like a good chance to crunch some numbers.
So lets start with the basics. Why do we build a starter to begin with? Well, it's because we need a certain number of yeast cells to insure a healthy fermentation and, dang it, the yeast we get from the store just isn't enough. Let's take the case of my friend's brew as a test subject: He's planning to brew a fairly strong lager, so let's assume he's starting with an OG of 1.060. Using the handy-dandy Mr. Malty pitching rate calculator I find that I need no less than 439 Million yeast cells.
Now, given that the average yeast vial or smack pack is suppose to have about 100 Million cells in it, it would take four of them to pitch a proper pitch. But the fact is that yeast dies in transit, and over time the viability of that vial degrades. If I'm really lucky, and the vial was transported cold, and was very fresh when it was bought, and I pitched it right away, I might get about 40% viability. So of that theoretical 100 Million cells, I have about 40 Million to work with. Mr Malty doesn't tell me how big of a starter I'd need for that, but yeastcalc.com does: it would take a 12 liter starter to make enough yeast for this beer. For those following at home, we're talking about a 18.9 Liter brew.
The reason it takes so much starter wort to generate this much yeast is that growth is not liner to starter size. In other words, a 2 Liter starter will not propagate twice as many cells as a 1 Liter starter. It's the law of diminishing returns, on a cellular level.
So how do you make that much yeast? Well, you try to always pitch as much yeast as would hit the optimal growth rate for that size starter. In other words, the more starter wort you have, the more yeast you need to pitch. Or, flip it, the more yeast you have, the larger a starter you can pitch it to. This is called making a step-up starter.
So lets take our 40 Million cells that we got in the smack pack, pitch them into a 1 liter starter, and throw it on a stir plate until it ferments out. At the end of fermentation, our yeast has grown to about 139 Million cells (according to yeastcalc). Now lets do it again, take those 139 Million, and pitch them into 2 Liters (more yeast means bigger starter, right?) let it ferment and we get 377 Million cells. Getting close to the target, but not quite there. So lets do it one more time, and pitch into a 3L starter. At the end of that cycle we get...ready for this?...a whooping 779 Million cells! WAAAAY more than we need. In matter of fact, if we took that 3 Liter starter, mixed it well, and pitched only 1.75 liters of it into the wort, we will hit our target rate almost exactly. And the rest? That's enough yeast slurry there to fill a couple of vials and share with friends, and we've made it using half of the starter it would have taken in a single-step starter.
So yes, you can save a portion of the starter and re-culture it. in fact, if you do your step-up right, you probably wont need to re-culture. You'll have enough ready to use yeast that you can pitch what you need and save the rest. So get starting!