The chemistry of cutting
It depends. Bread has moth small molecules (like 2-Acetyl-1-pyrroline). These molecules will just pushed aside by the blade. You have hydrogen bond network between water molecules. You break those. Bread has large polymers like starch. Knife breaks those.   

A cleaner system to study is a polyethylene. You can freeze it in N2, break under inert atmosphere and study by EPR spectroscopy. You will find free radicals at the breaking point. This is because it costs less energy to break a bond then to move one molecule relative to others.