Brewers yeast seems to be the protein of choice for bees. I am sure that the milk products have solids that are difficult for the bees to ingest...but that is only a guess. You can pick up Brewers yeast at your local CO-OP as it is used as a feed supplement for cattle and such. You can also get it from your local brew pub as excess in liquid form (usually for a few jars of honey), just be sure to de-activate it before using it.
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