Whey is removed from the curds by allowing it to drain out of the vat. In general, a gate is present to prevent curds from escaping. When most of the whey is gone, the curds are raked to either side of the vat, allowing whey to drain down the middle of the two piles. Cheddaring is a unique process in making Cheddar cheese that involves stacking "loaves" of curd on top of one another in order to squeeze additional whey out of the loaves below. It is a multi-step process that reduces whey content, adjusts acidity, adds characteristic flavour, and results in a denser and sometimes crumbly texture. The curds are allowed to set until they reach a pH of about 6.4. "Loaves" of curds are cut about wide along each side of the vat. After ten minutes, the loaves are turned over and stacking begins. Every ten minutes when the loaves must be turned over, they are stacked. This step uses the weight of the loaves upon each other to help expel additional moisture. The first time this occurs, two loaves are stacked together. The next time, the loaves are turned, and two stacks of two are put together. When the stacks get large enough (in general, 4 high), stacking stops but the loaves are still turned every ten minutes. This process is complete when the acidity of the whey is between 5.1 and 5.3, so it is checked constantly. When the turning process is complete, the loaves must be cut down into a size that fits in the mill. The mill will cut the matted curd into about pieces. During this process, the milled curds are constantly stirred to avoid re-matting. When all of the curd is milled, salt must be added. The amount of salt varies, but it will be between 1% and 3% by weight. The salt must be mixed thoroughly. Salt helps remove some of the whey from the cheese, which lowers moisture content, adds to the flavour of the cheese, and will also stop the cheese from becoming too acidic, which imparts a bitter taste. The curds are placed into moulds that will be used to press the curds and form the blocks of Cheddar. After this, the cheddar cheese will be aged. Aging time depends on the type of Cheddar. In general, mild Cheddar is aged for two to three months, while mature (sharp) and extra mature Cheddar is aged 12 to 18 months.