Proposed yeast starter

Question

Within the next few days I'll be 'brewing' 100 litres of ale from slightly concentrated sterile wort. I've used these kits before with decent results, but this time I want to try a liquid yeast instead of the packaged dry (Cooper's) yeast that's included in the box. So I bought a Wyeast 'Direct Pitch' smack pack (British Ale II) that allegedly contains 100 billion cells.

Not enough for this much wort, obviously, so I propose to boil up half a pound of dry malt extract to make a 1/2 gallon starter. When that's rolling nicely I'll use it to pitch this next mega-batch. Good idea? Bad idea? Any reason that wouldn't be satisfactory?

Much of my homebrewing knowledge, if you can call it that, dates back to Neolithic (i.e. pre-Internet) times, so until I can lay my hands on some good recent books on the subject, I thought I'd ask the question here.

Answer 1

1/2 gallon (~1900mL) will probably net you about 350bn cells (if you have a stir plate, yeast isn't too old, &c.).

For 100L of ~1.040 wort, you want closer to 750bn cells (depending on gravity, assuming an ale).

Use a pitch rate and starter calculator to get a better sense of your requirements for yeast and the the sizes/steps of starter needed to get to those pitch rates.

Answer 2

Obviously you know now that you are under-pitching with the 1/2 gallon starter idea.

For 100L you'd need several stir plates going really. IMO, the best bet would be to brew a standard 20L batch, then use that entire cake as the pitch for the 100L batch.