Kenneth wrote:

of that particular protein. The gas generated from the yeasts used in
rye breads is held by chemicals called pentosans. (I believe, but am
not certain, that these are also proteins.)

As Steve said, they're carbohydrates. I _think_ they're sugars, but
please don't quote me on that. Unlike gluten, they are in the flour
from the start. If the dough is mixed without enough sour and left
for too long the pentosans will break down, ruining the bread.
Therefore, all good leavened 100% rye bread is sourdough.

David