Re: What happens when mixing sucrose and tartaric/citric acid?

Sucrose, or table sugar, is a single molecule that consists of a glucose
and a fructose molecule chemically bound to one another.  Sucrose does not
have the same properties as either fructose or glucose, in the same way
that sodium chloride has none of the properties of sodium or chlorine.  If
it did, the seas would be far more lively than they are.

Pure compounds tend to form crystals very easily, which is why sugar is
almost always sold as a nice crystalline material.  The less pure a
compound, however, the harder it is for the molecules to line up in the
proper orientation to form a crystal.  Too many impurities, and the
molecules will just cling together in an amorphous mass.

If you heat pure sugar up and then cool it down, it will crystallize, and
you will get rock candy.  Lower the purity, though, and you are more likely
to get the amorphous blob that you are looking for.  Here is where the
tartaric acid comes in.  The acid is not forming any esters, or if it does
it is forming them in very small quantities.  Instead, it is added as a
catalyst to break the bond between the glucose and the fructose.  At high
temperatures you no longer have sugar.  You instead have a practically
50/50 mixture of glucose and fructose, which is about as impure a mixture
as you can get.  As it cools, these molecules are attracted to one another
by powerful hydrogen bonds, but instead of aligning into crystals, they
simply stick any old way.  That produces the goop that can be shaped into
sugar glass. As it cools the mass will harden, but it will not crystallize,
so it remains nice and clear.

The yellow color is caused by tiny amounts of degradation products.  Any
time a chemical reaction occurs there are almost always some side reactions
happening.  Very small quantities of degraded material will be present
after the glucose-fructose bond is broken, and these are responsible for
the yellow color.  Heat the mixture longer or at a higher temperature and
you will form more byproducts, and the mixture will darken to brown, and
eventually to black.