While the curd is being turned and piled, its physical properties are changing. The acid develops. When the cheddaring process is completed, the curd should be elastic, smooth and fibrous. The curd should have the close meaty texture desired in the cheese. If this step in the process is neglected, defects may appear later in the body, texture and flavor of the cheese. Attempts to pile the curd too fast result in a soft, mushy, open-textured product. Such cheese has mechanical holes, in which moisture collects, and so is likely to cause rot while curing. If gas is detected either before or during the cheddaring process, the curd should be piled until the gas holes are no longer round but flat. If the gas holes are not flattened or obliterated during this process, the cheeses will be very liable to puff on the shelves in the curing-room. The curd should be handled until the gas holes flatten out evenly, although this may require considerable time. At best, gassy curd will never produce the highest grade of cheese. Cheddaring or piling the curd is not thoroughly understood by most cheese-makers. Because the moisture contains the milk-sugar, there is danger of having so much moisture present in the cheese that it will become sour from the action of the lactic acid-forming bacteria on the milk-sugar. A cheese may be sweet when made and later become sour because it contains too much moisture or milk-sugar. This is known as "shelf souring." For the proper cheddaring of a curd, it is necessary that it be properly firmed in the whey. If the moisture is not evenly incorporated, the cheese will have a mottled color. The pieces that have the more moisture will be lighter colored. If the proper amount of moisture is not incorporated, the cheese will be dry and hard, and if too much, soft and pasty. +203. Milling the curd.+--The large pieces formed by the cheddaring process must be cut into small ones before the curd can be easily put into the hoop. This is called "milling." Properly milled curd can be salted evenly, cools more quickly and uniformly and can be distributed evenly in the hoops.