<<Disclaimer: Verify this information before applying it to your situation.>>

Got some info directly from a couple of wine makers about potential for
grain products in wines, including bubbly.

The classic method for making sparkling wine is known as "Methode
Champagnois" (sp?). This involves making a pure grape wine, then adding
in a dose of sugar and yeast to get the bubbly, yeasty character.  This
is called the "dosage".  Substances called "yeast nutrients" are
sometimes added along with the yeast and sugar to improve  or insure the
results of this "dosage' step in the process.  These "yeast nutrients"
are proprietary mixes from various manufacturers and most are likely to
contain grain products .   Yeast nutrients are not always used -- only if
the wine maker thinks it might be needed.  There is no way to know
whether any given bottle of bubbly had them added.

These yeast nutrients may occasionally be added in the fermentation stage
to other still wines.  Again, this is done based on the wine makers
judgment about the needs of a given batch.