Does the sour taste that emerges from the teeth while awake affect the fast, and is it considered something that mixes with the saliva and is excused?

Swallowing saliva when there is a sour taste emanating from between the teeth does not invalidate the fast, because a mere taste does not break the fast, and whatever remains between the teeth is secondary to the saliva. It is advisable to ignore intrusive thoughts (waswas) in such matters.