Question

Is yeast causing bread to rise a chemical reaction?

Answer 1

This is not an easy one to say "yes" or "no" to.......

Explanation:

Flour in bread dough contains enzymes that are able to break down starches present in the flour converting them to sugars such as maltose which is subsequently converted to glucose. The yeast is able to feed off the sugars and convert them into carbon dioxide, and small amounts of alcohol. The alcohol is what gives the bread its typical baked bread taste. The carbon dioxide expands as the dough is warmed, and causes the dough to rise.

So yes, there is a chemical reaction taking place, whereby the starches are converted to sugars and subsequently metabolised to carbon dioxide etc. However, the actual rising of the bread is simply caused by the expansion of carbon dioxide which is a physical process.

So whilst you may think it is being overly pedantic, if you're going to think about this sort of thing scientifically it is essential to be precise. Accordingly I would say, yeast generating carbon dioxide is a chemical reaction. The rising of the bread is a physical change caused by gas expansion.