I'd make a guess that the carbon atoms are bonded to each other, and other atoms (eg hydrogen, oxygen) in a different way in the bread, potatoes, rice and sugar compared to the carbon powder, which probably contains solely carbon. This different bonding will produce a different substance; an example of this could be the polymerization of ethene. Ethene is an alkene, of formula C2H4, with the carbons bonded by a double bond. It will be a clear liquid that burns with a smoky flame. When polymerized, the carbon-carbon double bond is broken into a single bond along with the addition of hydrogen, and depending on the conditions, can form either high density polyethene or low density polyethene, producing low-grade plastics (eg shopping bags) or higher-grade plastics (such as a plastic basin). A very different substance from the liquid ethene. Simplified example, but I think it works.The difference lies in the bonding and the atoms making up the molecule. There are probably other atoms making up the food substances than simply carbon, that also help to change its structure, appearance and function as well. As far as I know, that could be an answer - although, I'm only going on AS-Level Chemistry and Biology knowledge here, just to warn you.

Carbon powder is just carbon whereas bread isn’t, it is carbon bonded with other elements (mostly hydrogen and oxygen). It is possible to remove these other elements and return to black carbon eg:1. Burning toast sends it black.2. Adding concentrated sulphuric acid to sugar sends it black by extraction of water (the hydrogen and oxygen).