My understanding is that white corn is used for tortillas in Mexico, while yellow corn is used for animal feed and ethanol. There have apparently been lots of turmoil in the Mexican market, for various reasons, but I don't think that ethanol has much if anything to do with it.
At least that's what I recall based on a story on the radio a while back.
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[link|http://money.cnn.com/magazines/fortune/fortune_archive/2007/02/19/8400170/index.htm?postversion=2007021406|CNN reprint of a Fortune article].
FWIW.
Cheers,
Scott.