A company response posted on Delphi-Forums - Avoiding Corn, proves to us that their beers are not entirely corn-free or safe for a corn allergy.
"Sierra Nevada Pale Ale is one of our bottle conditioned beers. Bottle conditioning is unique to our style of beer. We dose back a small amount of yeast in the bottle. The yeast ferments the priming sugar and creates the finish carbonation and flavors unique to our beer. Currently, we are using very small amounts (less than 1/2 of 1% by weight) of potentially four different GMO free dextrose's in our priming’s, some of the starch may have been derived from corn but is converted to nearly 100% fermentable sugar and is completely converted to alcohol and CO2.
Sierra Nevada’s “primings” (bottle conditioning) specifically- We do not use cane sugar. Dextrose, is going to be made from wheat, corn, or potato starch. Presently, we are using Dextrose from corn and potato starch."
Do use extreme caution with their beers.