A piece of bread held in the mouth begins to taste sweet as large carbohydrate molecules are broken down into smaller sugars. these reactions are examples of

The answer to this question would be hydrolysis

There is amylase enzyme inside saliva that will break down starch into maltose sugar. Maltose is the sugar that causing the sweet sensation in the tongue. Since it will take for the reaction to happen, the sweet taste only appears after a few moments after you chewing the bread.

The reaction that was catalyzed by amylase is hydrolysis.