Why are polysaccharides not sweet in taste? Polysaccharides are defined as polyhydroxy aldehyde or ketone which on hydrolysis yield many units of monosaccharides.
I got one answer(to my question above) as:


  On our tongue, we have things called taste receptors. These receptors are loosely categorised into sweet, sour, bitter and salty. Our sweet-receptors bind to specific types of molecules, namely monosaccharides and disaccharides. Polysaccharides are not as sweet because they do not readily bind to the sweet-receptors on our tongue, as the other smaller molecules do! 


My question is that in our body (due to the presence of water),polysaccharides and oligosaccharides are hydrolysed to simpler units of monosaccarides...

So why does the latter respond to the receptors(indicating sweetness) while the former does not(while both yield monosaccarides on hydrolysis)?
Well, while indeed polysacharides are metabolised to simpler units, even in mouth (saliva in the mouth can account for 30% of initial starch digestion), it usually doesn't happen fast enough, so full digestion follows later in you digestive system.

You can`t sense the sweetness of polysaccharides as they do not fit the receptors.