"...as the proportion is fixed..." is an adverbial phrase of reason (or cause) modifying the main clause, whose principle verb is "are". The subject is the cooks, the verb is to be, and the object is "not allowed...". Unless you think of it as a negative passive construction, in which case the principle verb is "not allowed"; either way, it parses.
So, "The cooks are not allowed to decide the amount of salt to be included in the dishes" is the (rather complex) main clause. This is modified by the adverbial of reason, "as the proportion is fixed" which in itself is modified by the adverbial of purpose "to maintain the quality...".
Thus, the verb to which "to maintain the quality..." attaches is "is" - "the proportion is fixed" - and the verb to which "as the proportion..." attaches is either "are" or "allowed", depending on how you prefer to parse it.
I've known grammarians who insist that the principle verb in a passive is the one which could be considered auxiliary, rather than the one that really tells you what is happening, and those who insist on the opposite. I suspect it's a difference in philosophy or something. I know that from a machine-parsing perspective, the "are" would be the principle verb, because that's where the clause first decomposes. Some people would also treat "allowed" as an adjective rather than a verb here, simply because it's so rare to see it in active form, as compared to passive form.
