New answers tagged yeast-starters
In many cases, the starter beer is simply going to be diluting the wort, and not necessarily in a compatible way. The starter beer is probably not the same color, malt make up, hop profile, &c. as related to the beer. Plus, as ChinoBrews mentions, if properly aerated, the starter beer is likely to be oxidized. For the requisite starter volume for most ...
It is recommended to decant and dispose of the starter beer because the starter beer is nasty and oxidized, nasty and devoid of fermentable sugar by the time the yeast have reproduced to pitching levels. "You would not brew a beer with this level of oxidation, so why would blend it into your beer?", goes the thinking.
I don't know why people recommend against using airlocks. When the wort is fermenting, there is a positive pressure of CO2 OUT of the vessel. Oxygen can't get in anyway because of the positive pressure in the flask, so tin foil and foam stoppers aren't doing what they are purported to do anyway.
The starter is still good in that time. Very roughly, a 2L starter, presumably from a vial or smack pack, probably netted you between 200bn and 400bn cells. If you used 3/4 of it, you probably have 50-100bn cells left … about the amount in a smack pack/vial. As such, you should probably make a new starter from the remains of the previous one.
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