Ok... so what's the difference between "parmesan" (lower case P), pre-grated, packaged cheese from the supermarket versus the real Parmigiano-Reggiano (Upper case P) sold by a wedge taken from a large wheel of cheese and grated at home? Keep in mind, that in the United States, the word "parmesan" is not regulated. A cheese labeled as parmesan in the United States is not genuine Parmigiano-Reggiano, but is really a poor imitation. In essence, the large American cheese companies are lying to their customers. Period. In Italy, Parmigiano-Reggiano is strictly controlled with its D.O.P. protected status. It has to be made adhering to strict guidelines and processes. Years ago, in the U.S. the word "parmesan" was used to label real Parmigiano-Reggiano cheese, but nowadays is used on labels of store bought factory made pre-grated cheese, none of which bare any resemblance to the real thing.
These bottle, pre-grated cheeses are usually factory made, contain fillers, and are fluffed up (to fill jars with less product by weight) by the addition of cellulose powder (some may also use corn starch), they say on the label "to prevent caking". Thanks for doing us such a favor! In fact, when you grate an aged imported cheese like Parmigiano-Reggiano (called "the King" of Italian cheeses), clumping or caking happens when you store it in a jar or bowl. This is not a deficit at all. Trust me, the only reason they add cellulose is as a filler... fluffing up the cheese so less by weight fits into each jar. Besides, who wants to eat more fiber in their cheese in the form of wood or plant cellulose?
Lately at home, we've been buying the more expensive, real stuff because it just plain tastes better with no additives. It's real cheese.
Interestingly, right now, Kraft brand grated "parmesan" in a jar sells for $1.37 an ounce, where a 1 pound wedge of 3 year aged Parmigiano-Reggiano (ordered on Amazon) costs only $1.34 an ounce. Buy a 3 pound wedge and the price comes down to 89 cents an ounce. The bog box store BJs has wedges for as low as 81 cents an ounce! When we have a wedge of imported Parmigiano-Reggiano in our fridge, it usually lasts 3-4 weeks or so... mostly used for a grated topping or ingredient. Personally, I wouldn't dream of ruining my Cacio e Pepe alla Romano pasta recipe by throwing in handfuls of jarred parmesan. Disgustoso!
Sure, you have to grate it yourself, but then again, that's an advantage. When you buy pre-grated "cheese", you can only use it as a grated cheese. With a block cheese you can cut slices to have with apples or pear, shave off slivers to put on top of soups or salads, grate into little curls to top off pasta, larger gratings to top pizza, or very fine to put into your grated cheese bowl or shaker. Grated cheese also goes bad faster (my Mom used to leave hers out, refrigerated--ugh). When you grate your own, you grate only what you need for a few days ahead.
I also buy imported caciocavallo cheese in a 5 pound ball and hang it to age and dry in my cellar for a couple of months. After it's well aged, I start using it... cutting off small pieces at a time and using it past and other recipes. At the very end there is always a super dry, very aged "piccante" caciocavallo that I loves using as grated cheese.... it even blows Parmigiano-Reggiano away with it's intense flavor on top of pasta dishes.
Here is a link to an article from the Huffington Post that gives a balanced report on the differences between "parmesan" and "Parmigiano-Reggiano" and the controversy about the fillers and cellulose used in off the shelf parmesan pre-grated cheeses... CLICK HERE.
UPDATE 2/18/16: I checked the ingredients on all the brands in the supermarket this morning. Virtually all of the brands sold include cellulose to "prevent caking" and some even include corn starch. Kraft brand had even more things in it, while the 4C brand contained no fillers, cellulose or other ingredients other than cheese. It even looked different--fresher--than the other brands.