The cooks are not allowed to decide the amount of salt to be included in the dishes, as the proportion is fixed to maintain the quality of the flavor.

As is a conjunction, so the entire sentence that follows "as" is an adverb clause that modifies the main clause " The cooks ... included in the dishes".

Is my knowledge correct?

And "to maintain the quality of the flavor" is an infinitive used as an adverb to modify the phrase "is fixed" or is the infinitive used as a complement for the passive verb "fixed"?