I'm really late to this discussion, so I don't want to side-track it, but if you are truly in the space filling mixture design world (which means you completely understand short and long-term Noise effects and the process is consistent) the analysis should be less statistical and more graphical. Analysis of such designs should be contour plots and surface response plots. You should already know the statistical significance through previous experiments.
Also, you might want to read books/papers regarding mixture designs and the reason those are typically analyzed without the intercept. See Cornell's book.
Lack of fit is a result of removing terms from the model (vs. pure error which is the result of randomized replicates).
"All models are wrong, some are useful" G.E.P. Box