Hi Ladies,
I am the KitchenAid Lady on QVC, here is my 2 cents... no plastic, always glass or metal. Before I beat egg whites live on air I wipe down the bowl and beaters with white vinegar on a piece of paper towel so no trace of any oils- even those from your hands as you put in the beater.
This link is the perfect explanation for egg whites and the science behind them.
http://www.ochef.com/223.htm
"Beating Egg Whites into Submission" Hope this helps. Love the wonderful info on this board. Laura Weathers