What the balanced chemical equation for the conversion of starch to dextrin by dry heating?
Let's take a look at the structure of both molecules. First, starch:



Now, dextrin:



Do you see the difference? They are the same molecule (both are glucose polymers), the only difference is the molecular weight, or the length of the polymer chain.

Starch, by convention, is 300-600 monomers long. Dextrin is something less than that.

Since the number of dextrin molecules you get will depend on the polydispersity (size distribution), average chain length, and extent of the reaction, there is no way to write a balanced reaction equation for a real reaction.

You could assume that a starch molecule of a given length breaks into dextrin molecules of a given length, and write an equation. It would look like this:

$\ce{C6H11O5[C6H10O5]_{m}C6H11O6 + (\frac{$m-n$}{2})H2O-&gt;} \frac{(m+2)}{(n+2)}\ce{C6H11O5[C6H10O5]_{n}C6H11O6}$

This would only work for one length of starch and dextrin (in reality there could be 10 - 1000) and would also only be true if $m &gt; n$, and $m$ is evenly divisible by $n$.

Also, in the question you specify dry heat, but there is no way to balance the equation without including water (unless you use hydrogen and oxygen gas). I suspect that even under dry heat conditions, there is moisture on the bread, and it is consumed in the reaction. This could be part of the reason that toasting drys bread.