As far as I can tell that is the work of the enzyme amylase in your mouth breaking down starch and converting it into maltose. Maltose is a sugar, and so if you keep the starch in your mouth long enough for amylase to do its work, eventually enough of the starch will be converted into sugar that you will be able to taste the newly formed maltose. (You can do the same thing with anything that contains starch, it seems easier to do the experiment with things that contain a lot of starch into it to begin with like bread or potatoes...never tried it with potato chips before ^^).