first you need to define "sweet" biochemically
then you need to define and chat about the characteristics of the receptor on the tastebuds which results in the incident molecule tasting "sweet"
then you need to talk about the characteristics of an incident molecule which will conjugate with the receptor to form a receptor/agonist complex
then you have to define polysaccharide and discuss their general characteristics, and then say why they have a very low probability of complexing with the sweeness receptor on the tastebud
the short answer is of course that they are too big and dont complex because of this main pharmacochemical property, but if you want marks then do what i said above with a bit more structure
cheers