"more sour" is correct and usable in written English.
You can use it to describe something that has a stronger flavor than something else, for instance "This lemonade is much more sour than the last one."
The scene in Phoenix was decidedly more sour.
Each bite is different, sometimes more sweet, sometimes more sour".
Add more sour cream as needed to loosen.
In addition, petroleum feedstocks are becoming heavier and more sour with time.
That double lack of accountability, both before and after, makes the situation all the more sour.
It was his friend, Mr. Darcy, who had made a rather more sour impression.
They're different ages; some are more manic, some sweeter or more sour.
"The betrayals become more sour and more sorry," he says softly.
No more sour notes about world debate or a referendum were heard in the unification chorus.
More sour notes arrive in scenes with two proverbial "good Germans".
His position turns more sour as we learn the bosses are trying to weed out employees.
