R' Gershon Dubin wrote:
> Making a dough without water is chayav in chala (Y"D 329:9).
> However, it causes another problem. We burn chala nowadays
> because it's tamei, since we are all teme'ei mesim. In order
> for it to become tamei, it must have hechsher mashkeh, which
> it does NOT get if no water is in the recipe. You are then
> stuck with chala tehora which one is not allowed to burn,...

B'shaas had'chak, the Mechaber 453:4 allows one to make matza with 
ordinary store-bought flour. However, Mishne Brurah 453:24 writes 
that nowadays, the common practice is to wash and soak the wheat, so 
it is assur to use such flour even b'shaas had'chak. Rav Shimon Eider 
(page 213) confirms that this is the practice nowadays, and 
that "commercial flour should be considered as actual chometz."

Based on this, I suspect that our flour is already muchshar l'kabel 
tumah when we buy it, and the act of taking the chalah is more than 
sufficient to insure that it is indeed tamei, solving RGD's concerns. 
But I won't pretend to be overly familiar with hilchos tumah 
v'tahara, and would appreciate hearing comments from the rest of the 
chevra.

Akiva Miller