When you eat bread, it does not taste sweet at first but after chewing for some time you can feel the sweetness because

Carbohydrates which are soluble in water and are sweet in tastes are called sugars (e.g., monosaccharides like glucose and disaccharides like maltose and sucrose). Carbohydrates which are insoluble in water and are not sweet in taste are called non-sugars (e.g., polysaccharides like starch). Bread contains high content of starch (polysaccharide; non-sugar) which does not taste sweet. Salivary amylase digests 30% of starch into maltose. Maltose is a sugar (disaccharide) and tastes sweet.