Re: What makes sugars sticky when they get wet?

Hi Dave,

I'm not exactly sure why sugars get sticky when they're wet, but here are 
some ideas:
 
Soap makes your hands slimy and oil (composed mostly of non-polar 
hydrocarbon chains) is greasy because neither substance interacts with the  
surface of your skin (soap doesn't interact, it strips skin cells away). On 
that logic, the sugar seems to interact more with the surface of your skin 
thereby resulting in stickyness. 

Sugars are hygroscopic, which means that they'll readily take up and retain 
moisture.  When water is hygroscopically bound to the sugar, the sugar will 
stick to surfaces until the water evaporates.  After evaporation of the 
water, the sugar crystals can easily be brushed off the surface.  This 
itself doesn't explain why sugars are sticky, though. 

Sugars will also retain their chemical formulas when dissolved in water, 
whereas a substance like salt will dissociate into its ions. When sugar 
dissolves in water, the molecule is polar and will "stick to" or interact 
with other polar molecules (itself, or molecules on the surface of your 
skin cells).  Therefore, sugar is both adhesive and cohesive when exposed 
to water.  Adhesion occurs when particles of substance bind to a surface 
versus cohesion, which is when particles of a substance bind to themselves.  
This would explain why sugar is sticky when it gets wet.

If you're interested in food chemistry involving sugars or other things, 
you might want to try Harold McGee's books titled On Food and Cooking and 
The Curious Cook.  I hope my answer helped.

Sarah Earley
CU Boulder