What is the function of a stir plate when making a yeast starter?
Does it drastically improve the starter process?
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The point of a stirplate is to help the yeast propagate by aerating the wort. Yeast Propagation and Maintainance claim stirring can increase yeast cell count by 10-15 times, compared to simply using an airlock (non-aerated) or 2.5 times the cell count of the traditionally aerated starter (aquarium pump).
It's important not to use an airlock, since the primary function of the stir-plate is to help oxygen enter the wort all the while the yeast are propagating, since oxygen is needed for yeast growth. It does this by continually agitating the wort, and so increases the rate of absorbtion of oxygen into the wort.
You can buy stirplates, but they're quite expensive, or build your own.
Basically it's a rotating magnet that forces a iron stir bar inside the starter to rotate. The rotating stir bar stirs the starter, keeping it in continuous motion.
It helps the yeast starter process by exposing more surface area of the starter to oxygen. Yeast then use that oxygen to reproduce. Since the yeast are exposed to more oxygen than they would be without a stir plate, they reproduce more.
Since the yeast reproduce more, you get a larger amount of healthy yeast using a stir plate than you would without.
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