A necessary ingredient of making bread is yeast (a member of the fungus kingdom). In the presence of oxygen, yeast feeding on sugar produce carbon dioxide, just as we do. Here is a video of making bread. Note the role of yeast.https://www.youtube.com/watch?v=ZXYZYKfjNBg
The following video descries a simple lab experiment that shows the production of carbon dioxide by yeast. https://www.youtube.com/watch?v=FYClCHVT00M