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Scott,
        Modified food starch may or may not be GF, depending on what was
used to make it. Sometimes corn, potatoes, rice, tapioca, oats or wheat
may be used or even some combination of the above. Unless you check with
the manufacturor, you have no idea whether or not it is Gf and it's best
to avoid it. Also, the formulation may change. So products that once used
GF modified food starch may suddenly contain gluten modified food starch
and there is no way that you will be able to tell from the label. That's
why it would be so wonderful if manufacturors were required to be more
specific in their labeling.
        As far as masa is concerned, I believe that your are right in
that is is made from corn.
        Ellen Eagan