As we homebrewers get geekier about our hobby we want to imitate the pro brewers and use their fancy techniques that set craft beer apart from "homebrew." One big way to do this is to improve our yeast handling. As a rule of thumb, most beers under 1.055 or so will do just fine with a pitched vial or smack pack, provided you oxygenate the wort well and keep the fermentation temps where you want them. If you are not doing those things, then I recommend you do so, but also not a bad idea to use a starter to improve your yeast activity and health. I usually use a starter when going above the 1.055 gravity beers, and even do a stepped starter for lagers.
|Don't forget your tin-foil hat to prevent alien mind probes...|
Throw some tin foil on top of the flask and immerse in cold or ice water bath. Ideally you want it around room temp before adding yeast. Too hot or too cold can stress out the yeast and make for a less successful starter. I usually dip my yeast packet or vial in sanitizer and then dump it on in.
The whole idea of a starter is to do a few things. The first is to rouse the dormant yeast and get it alert and active--like having your morning coffee. The second is to get the yeast to multiply and give you more active yeast ready to ferment your wort. Small starters (1 L) will usually get the yeast going, but doesn't have the volume to really increase the amount of yeast. 2L and above will result in more actual yeast proliferation which is why that is my standard. In addition, in order to get to a point where the yeast wants to multiply and to ferment wort, it needs oxygen. This step is very important for starters as well as your batch of beer. I use a stir plate for my starters, and use a foam stopper in the top to keep out fruit flies and dust. This will allow air/oxygen entry to the starter, as well as keep the yeast swirling in suspension and active. Do not use an airlock, as this will prevent air entry. If you don't have a stir plate, you can periodically swirl the starter, but you aren't going to get as good of a result. I recommend using an oxygen stone to oxygenate the starter if you can't stir it. You can make your own stir plate out of computer motors, magnets and cigar boxes--you-tube it! I have a 1940's looking 30 pound behemoth that Kent gave me about 5 years back--this thing will probably function longer than I will. For really big beers and lagers you may want to step up your starter and do a second larger batch--I'll write that up next time I do one. You will also need a stir magnet for a stir plate--get more than one just to be safe.
Using a stir plate, my starters usually are done fermenting within 24-48 hours, so you do need to plan ahead for these. If you do a last minute starter (previous night) you will still have a healthier and active yeast than not using one at all. I also like doing the starter as a way to tell if my yeast is still functional before adding it to my beer. I have had a couple of vials/packs that were DOA, and the starter never got going--but I had enough time to go buy new yeast before my brew day. I also keep a couple of dry yeast packets in the fridge as back-up--no starters needed for those.
If your starter is still active and foaming at the time you are adding to your wort, then just add the whole thing. If you have reached the point where it isn't foaming anymore, I would stop the stir plate and give it 5-8 hours to settle out, then carefully decant 3/4 of the clearer top "beer" down your drain. I usually use a butane creme brulee torch to quickly sterilize the lip of the flask before doing this and adding to final wort. Great for creme brulee as well! A trick I recently learned is to hold a magnet to the outside of the flask while decanting/adding to the wort--this will hold your stir magnet inside the flask instead of dropping down the sink drain (done that) or into your fermenter (and also done that.)
With a good starter and properly aerated wort, you should see activity within 12-24 hours. By cutting down lag time, you have a quicker production of alcohol that keeps down any spoilage bacteria, as well as a large number of good yeast to overwhelm/compete with any wild yeasts that may have made it into your batch. Be aware that this process can result in a more vigorous fermentation than you may be used to seeing...watch carefully and use a blow-off tube. Your wife will do bad things to you, (or nix your budding homebrew hobby,) if you spray fermenting beer all over the clothes in her closet. I'm not speaking from experience here. Really. I mean it....ahhh, you got me!
For the pictures here, I was actually doing two separate starters with different yeasts for a split 10 gallon batch (see here for details on that experiment.) I used a stir bar in each and just switched them off on the stir plate every time I went downstairs to my mad-scientist lab. This worked very well and both were done in under 48 hours and ready to pitch into my beer.
Good luck and feel free to comment!