Re: what is the chemical change in bread dough rising

Is it a chemical change, or a biological one?

One thing that a baker adds to bread dough is yeast.  Yeast looks like a 
powder, but believe it or not, it is actually a living thing very similar 
to a mushroom.  Yeast lives on sugar, which it converts into carbon dioxide 
gas.  That gas gets trapped in tiny bubbles in the dough, which causes it 
to rise just like a balloon inflating.  When the baker kneads the dough he 
releases the carbon dioxide.  This prevents the dough from becoming too 
light.

Yeast also releases ethyl alcohol as well as carbon dioxide.  That is why 
when yeast is mixed with a bit of sugar and some hops and barley, after a 
while it will give us...beer!

Dan Berger adds:
The conversion of sugars to carbon dioxide is a chemical change. The CO2 
bubbles stretch the gluten in the flour, a rather springy, stretchy protein,
and this stretching is a physical change.

The reason raised bread stays that way after baking is that the heat changes
the detailed structure of the gluten protein, making it lose its elasticity
so that it stays in its expanded form. The process is called "denaturing" and
is similar to the way egg whites become hard and white during cooking.

But is denaturing a physical or a chemical change? That depends on whether 
you think the chemical change comes in the structural change in the protein 
(making it change many of its chemical properties) or you reserve it for 
actually breaking and forming chemical bonds...