I recently decided to try catching wild yeast based on this thread. I only had a very little amount of DME on hand, which gave me a very low OG. After 1 week though I've seen growth, and the jar smells yeasty. I'll let it sit another week, but what's the best way to help the yeast grow. Should I make a starter after the 2 weeks? It's only about a cup of wort, what size starter should I make? If I don't plan on using this yeast right away is there a way to save it? Washing it?
Any help would be greatly appreciated.
Sure, go for it, you can move forward if it smells yeasty and seems to be fermenting. You could bump it to 2 liters, and then later go to 5 gallon or so. Adding a little bit of hops during the DME boil creates a slight anti-bacterial effect, in your 2 L. Also keeping the SG low is good for keeping the bacteria out in the beginning, but it would be good to give the yeast something to chew on so they can outcompete the rest of the bugs in there, maybe bump it to 1020-30 or so for the 2L. Rich yeast media in the lab is 1020.
For storage I would suggest putting it in an sterile (wash with boiling water, let cool) container and set it in the fridge airtight. Note, this is a live culture, so it will keep on doing its thing. Some of the yeast might be less active at this temp, and something what you dont want might be more active. dont know how long it keeps, but its probably similar to a barn for bread. Use a plastic container if you can, just so it does not pop. There is probably people here with more experience, please chime in if you read this and have a better idea.
For storage, you could also add glycerol (also called glycerin in stores) to ~25% concentration and freeze it, the glycerin keeps the cells intact by inhibiting ice crystal formation. I do this on a regular basis with pure yeast cultures, but have never done this with a mix. Should work as well.
A difference between bacteria and yeast is that yeast cells are much larger. One trick I do to quickly gage if I have no bacteria is the following,
I take a little culture out, mix it up so its a suspension, put it in a little container and set it in the fridge (say shotglass or so.) Due to the size,
the yeast will fall out faster and due to the smaller size and "swimming" the bacteria will stay much longer in suspension. if you see a clear area of liquid at the top of the glass after an hour , and whitish cells on the bottom of the glass, its a good sign (sign, not proof. Also if it does not clear it might still be good)
Hope this helps,
Jaapie