Just installed the most recent update of your plugin (version 188.8.131.52).
As soon as it was installed, W3Total Cache stopped minifying and a code of DONOTMINIFY constant is defined appeared in the page source.
After deactivating WordPress SEO by Yoast, the DONOTMINIFY wasn't in the page source any longer. I reactivated WordPress SEO by Yoast, and now W3Total Cache is minifying again.
Man... wish you warn us.... :)