OK, but I think the leaven can be strained or boiled away from the juice, preventing the juice from fermenting. In Is. 25:6 "well refined" means to strain, extract, purge or Purify. I think that means to remove whatever causes it to ferment, namely the albuminous yeast. Notice the Strong definition of "wine well refined":
zaqaq, zaw-kak'; a primitive root; to strain, (figurative) extract, clarify :- fine, pour down, purge, purify, refine.
zaqaq, zaw-kak'; a primitive root; to strain, (figurative) extract, clarify :- fine, pour down, purge, purify, refine.