Yeah rather than decanting the first 64 oz starter and then putting more DME into the same 64 oz growler to further grow the yeast colony I think you would be better off decanting the liquid, then splitting the yeast slurry between the two 64 oz containers and adding another 6 oz or so DME to both. I think as another poster mentioned you limit the colony size by adding more DME to the same size vessel as you already started in the first time around. In re-using the same 64 oz vessel, the yeast would effectively evaluate the food supply from the new DME and pretty much figure the colony was about the right size to handle it and go about making beer.
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