Toasting bread lowers levels of certain B vitamins in bread. It also lowers the glycemic index and increases acrylamide. Toasting bread is a chemical change, which means the change cannot be reversed.
The more severely a piece of bread is toasted, the more changes occur within the bread. When the sugars and amino acids in bread are heated up, they create acrylamide. This is a chemical linked to cancer in animals, though no human link has been proven. Burnt toast contains small amounts of polycylic aromatic hydrocarbons (PAHs). PAHs are also known to be carcinogenic, though no direct link in humans has been found.
Experts suggest trying to toast bread as little as possible to reduce the presence of these chemicals. Even if the bread is not burnt, a piece of toasted bread has 1 percent less of the daily value of folate and thiamine. Heating the bread turns the starch on its surface into dextrin, which is slightly sweet in taste and may make toast more appealing than regular bread.
The changes that occur by toasting bread are chemical changes, which means the original matter of the bread is changed by breaking apart the molecules in the bread and reforming them in new ways. This chemical transformation explains why toast has so many different chemical properties than raw bread.