What is the as in the sentence called, preposition or adverb clause?

Question

But yogurt tasted with that spoon was also rated as less sweet than when eaten with heavier or larger spoons.

I have been always wondering what this kind of as is called grammatically, e.g. adverb, adverb clause, preposition etc. If it is a preposition, does that mean there is being omitted between as and less since there should be a noun to come after the preposition as ? Or, is it an adverb clause ? How to clarify it?

Answer 1

According to oxforddictionaries.com "as" in comparisons is called an adverb and refers to the extent or the degree of something.

It tasted like grape juice but not as sweet