It is because of the starch present in it.
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if the bread is chewed for sometimes it tastes sweet because foods like bread and potatoes are foods that are mostly made of starch. Starch is a molecule that plants make in order to store sugar. It consists of long chains of glucose (some of them branch). In your saliva you have an enzyme called amylase, this will cut up the large starch molecules at random until you are left with sugars made of 1,2 or 3 glucose molecules long (glucose, maltose or dextrin). Starch doesn t taste sweet, but glucose and maltose do, so as you chew the bread it will slowly become slightly sickly sweet
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