Dummy Operator - 3 Piece Air Lock Mistake

So I went through the "trouble" of making a yeast starter for my latest batch (brewed on Saturday night). I pitched it at the proper temp and all that.

Well when I got home tonight there was still no activity in the airlock besides some bubbles. Then I realize I forgot to put the middle part of the airlock in. I found it, sanitized it, and tossed it on and there was immediate activity.

I'm guessing my batch isn't tainted but what *COULD* happen if you omit that part of the airlock and allow outside air to get in (even if the only air would come through the tiny holes on the air lock lid). I'm guessing enough air was being pushed out where not much could really get in - just my horribly unscientific theory.

On starters I wouldn't recommend using an airlock. You want oxygen to get in there to encourage the yeast to multiply. To do this without risking the yeast a whole lot I would use a piece of aluminum foil around the opening or a foam stopper. I prefer the foam stoppers now since an incident where I fruit fly found its way around the aluminum foil. If the foil is all you have on hand then make sure you seal it tightly to the sides of the starter container.

As for the risk to your starter; I wouldn't worry about it. Like you said CO2 was pushing out and the cap on the airlock kept any large object falling in.

+1, sanitized aluminum foil. (Actually it's probably sanitary enough right off the roll, but I always have a spray bottle of Starsan on hand anyway.)

Edit: I'm gonna have to get some of those foam stoppers...

I misspoke. The airlock mistake happened on the primary fermenter, not the yeast starter.

It's probably fine. There was activity before you put the airlock together properly, whether you saw it or not. You just noticed it when the CO2 was forced to bubble through the liquid.