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What we in the U.S. call Parmesan cheese is an imitation of Parmigiano Reggiano cheese from the Parma region of Italy. Production of the real Parmesan cheese is highly regulated in Europe. For instance, it may contain only three ingredients: milk that is pure and unadulterated, salt, and rennet.
From SeriousEats:
“Domestic [e.g., U.S.] rules allow for, among other things, the addition of calcium chloride and artificial coloring. The milk (which may be whole, skim, reconstituted from dry, cream, et cetera) can be bleached, in which case vitamin A can be added back “to compensate for the vitamin A or its precursors destroyed in the bleaching process.” And, though the FDA’s definition of Parmesan is a cheese “characterized by a granular texture and a hard and brittle rind,” many supermarket wedges of domestic “Parmesan” have no crystals whatsoever and look remarkably uniform, like a triangle of cheddar.
To make matters worse, a significant amount of our so-called Parmesan cheese is pre-grated, with cellulose added to keep it from clumping. The industry norm for achieving this goal would be to add cellulose equal to between 2 and 4% of the finished product, but widely publicized tests by Bloomberg and Inside Edition earlier this year (later dubbed ParmesanGate) showed that these percentages are routinely doubled or tripled. In one notable case, a product marketed as Parmesan was found to be more than 20% cellulose. While naturally occurring, the ingredient, a plant-based fiber, is neither milk-based nor naturally present in cheese at all, leading to Bloomberg’s memorable headline: “The Parmesan Cheese You Sprinkle on Your Penne Could Be Wood.”
Source: What You’re Really Getting When You Buy Parmesan Cheese in America | Serious Eats
Advanced Mediterranean Diet 