When a risotto recipe calls for Parmesan cheese, should you buy Parmesan or Parmigiano-Reggiano? Technically, you can use either. The difference is that Parmigiano-Reggiano is Parmesan, but not all Parmesan is Parmigiano-Reggiano.
Parmesan cheese is a hard, cow's-milk cheese with a nutty taste and a gritty texture. It's a staple of Italian cooking, often grated and baked into dishes or sprinkled on top. The word Parmesan is an anglicization of Parmigiano. Its basic ingredients are cow's milk, salt and rennet. (Rennet is enzymes from a cow's stomach, so Parmesan made with rennet technically is not a vegetarian cheese.)
In the U.S., the word Parmesan is commonly used as a generic term for this type of cheese, but labeling rules may be stricter in some countries (see below). Among generic Parmesan cheeses, quality can vary greatly. There are many small dairy farms throughout the world making delicious Parmesan, and there are a lot of mass-produced Parmesan wedges that are less than stellar.
Parmesan is usually bought in wedges, but it can also come pre-grated. When it comes grated, there's often a little flour or cornstarch on it to stop it from sticking together. The convenience of having it pre-shredded may be worth the little bit of flour on it, but if you want just pure Parmesan, grate it yourself.
Oh, and that shelf-stable shaker cheese in the plastic canister? Skip it. It's often not authentic Parmesan, and the FDA allows up to 4 percent cellulose (wood pulp) in it as an anti-clumping agent.
Parmigiano-Reggiano is the OG — the original, authentic Parmesan. It's what all other Parmesan cheese strives to be. Made only in certain parts of Italy, it's a protected product. It can be labeled Parmigiano-Reggiano only if it's made in the provinces of Parma, Reggio Emilia, Modena and certain parts of the provinces of Mantua and Bologna, according to the Parmigiano Reggiani consortium. (Within the European Union, this labeling restriction also extends to the word Parmesan.)
Parmigiano-Reggiano is a highly regulated product. The raw milk used must come from cows fed primarily from fodder obtained in the area of origin. The cheese must be aged for at least 12 months. There are even regulations about the size of the wheels of cheese and the color of the external crust, or the rind. There may be differences in flavor because of the different regions it's produced in, but authentic Parmigiano-Reggiano will always be a high-quality product.
You can use this cheese in any recipe that calls for Parmesan because it is Parmesan, a prized Parmesan produced in specific regions. To know if the wedge you're purchasing is authentic, look at the rind. If the rind is embossed with the name over and over (as in the photo above), that's the sign it's authentic. If the rind isn't embossed at all, or if it simply says Parmesan, the cheese may be good, but it's not Parmigiano-Reggiano.
Speaking of that rind, do not throw it out after you've eaten all the cheese. Parmesan cheese rinds have many culinary uses, including flavoring soups or infusing olive oil.