[Yardstick]

I did a little Googling to try to answer my own question and the nub of it is this: unlike hydrocarbons, which consist only of hydrogen and carbon, carbohydrates also contain oxygen. That's the key. They're partially oxidized, which means they have less chemical energy -- and it also means that you converting them into a hydrocarbon is not a trivial thing.

[P-40]

and it also means that you converting them into a hydrocarbon is not a trivial thing.

The oxygen is beneficial in the right quantity; it helps with combustion.

From: http://www.biodiesel.com/theFuel.htm

The biodiesel molecules are very simple hydrocarbon chains, containing no sulfur, ring molecules or aromatics associated with fossil fuels. Biodiesel is made up of almost 10% oxygen, making it a naturally "oxygenated" fuel.