Parmesan Cheese is made from raw skim milk. Calf Rennet is added to the milk, and the milk heated until it curdles. The curd is cut, heated to 125 F (51 C), stirred, then heated further up to 131 F (55 C.) The curd is packed into moulds lined with cheesecloth, then removed from the moulds and soaked in brine for a month. The whey leftover from the process is used to feed pigs, which then become meats such as Mortadella and Parma ham. 4 1/2 gallons (US) / 16 litres of milk are needed to make 2 1/4 pounds (1 kg) of the cheese.
The cheeses are aged on wooden racks for anywhere from 12 months to 3 years. Young Parmesan cheese, just over a year old, is mostly sold inside Italy; they are served shaved into thin curls. "Vecchio" (old) is aged from 1 1/2 to 2 years. "Stravecchio" (extra-old) Parmesan is aged over two years. These latter two are the cheeses sold and used for grating.
Cheese makers often leave wheels of Parmesan cheese with their bank as collateral. The banks have special vaults to store the cheese in, so that it can age properly while they are holding it.
The wheels of aged Parmesans are straw-coloured. The cheese is crumbly and has a slightly salty taste. The rinds are not eaten or grated; they are discarded.
Parmesan must be made in Parma or Emilia-Romagna. As of 2004, there were at least 900 small cheesemakers in the area joined in a consortium. Any Parmesan-type cheese produced outside that area has to call itself a "grana."
Each dairy's production is tracked and inspected in "lots", which correspond to 4 months of the year:
- January to April (inspection starts 1 December)
- May to August (inspection starts in the following year, on 1 April)
- September to December (inspection starts in the following year, on 1 September)
(Some foodies feel the best cheeses are those produced between April and November.)
Inspection is done by experts appointed by the Consortium. At least one cheese from each "lot" must be inspected by cutting a portion from it for internal inspection. Its appearance, texture and aroma are assessed. The lot is then graded first grade, medium grade or rejected. The grading is branded on the wheels of cheese that pass the testing with a hot iron. Rejected wheels have to have their Mark of Origin removed; then are then sold on for industrial processing and sale not under the official name.  Dairies have four days in which to register an appeal over rejected lots; if they do, a second, different inspection team is sent out within 15 days of the appeal being received.
Even if the cheese is only 8 months old when it passes inspection (example: produced in April, inspected in December), "cheese cannot be put on the market for consumption under the Parmigiano-Reggiano protected designation before having reached the 12th month of maturation". 
When Parmesan is cut into small wedges for retail sale, it must by law be cut so that some portion of the rind is showing. This allows consumers to see the trademark on the trade and see that it is the genuine article. Coincidently, it also means that the inedible rind becomes part of the weight that consumers have to pay for.
Parmesan Cheese Grater
© Denzil Green
There are better and worse qualities of "pre-grated deli Parmesan." The first clue to better ones is that they are sold refrigerated and must be kept refrigerated. It is the ones not requiring refrigeration that generally get dismissed as sawdust. And those that dismiss it as "sawdust" may have more of a point than they know. Cellulose is often added to those dried, grated cheese mixtures to prevent clumping. (It's also what prevents that grated cheese from melting nicely on top of hot food.) And the source of that cellulose? Cotton or wood pulp.
Generally, you need to consider Parmesan a high-sodium cheese. How much it has depends on what kind of Parmesan you are taking about. The makers of genuine Parmesan say that it has 650mg of additional sodium per 100g, which would make 87mg of sodium per tablespoon, finely grated. 
The "Grande" brand sold at Costco in Canada has 85mg of sodium per 1 tbsp (7.5g), finely granted.
Consequently, 2 tablespoons of grated parmesan, at around 175 mg, can add up to more than a 10% of the daily 1500 mg sodium intake recommended by the American Heart Foundation.
It is, however, a very low-fat cheese and so is heart-friendly in other ways.
* PointsPlus™ calculated by CooksInfo.com. Not endorsed by Weight Watchers® International, Inc, which is the owner of the PointsPlus® registered trademark.
1/2 cup grated Parmesan cheese = 2 ounces = 50g
If little patches of mould appear, the cheese is still fine to use: just scrape them off and use the cheese.
It freezes well if frozen whole; the flavour doesn't survive so well if frozen grated.
Literature & Lore
 Nutritional Features. Consortium of Parmigiano-Reggiano. Retrieved January 2013 from http://www.parmigianoreggiano.com/made/nutritional_features/default.aspx
Extra-Hard CheesesAsiago Cheese; Cotija Cheese; Extra-Hard Cheeses; Grana Padano; Manchego Viejo; Parmesan Cheese; Pecorino Cheese; Pecorino Romano Cheese; Pecorino Sardo Cheese; Pecorino Siciliano Cheese; Pecorino Toscano Cheese; Sapsago Cheese; Sbrinz Cheese
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