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Dry-aged beef 
Dry-aged beef is beef that has been hung or placed on a rack to dry for several weeks. After the animal is slaughtered and cleaned, either the entire or half will be hung. Primal (large distinct sections) or sub primal cuts strip loins, rib eyes and sirloin are placed in a refrigerator unit, also known as a "hot box". This process involves considerable expense, as the beef must be stored near freezing temperatures. Subprimal cuts can be dry aged on racks either in specially climate-controlled coolers or within a moisture-permeable drybag. Moreover, only the higher grades of meat can be dry aged, as the process requires meat with a large, evenly distributed fat content. Because of this, dry-aged beef is seldom available outside of steak restaurants and upscale butcher shops or groceries. The key effect of dry aging is the concentration and saturation of the natural flavour, as well as the tenderisation of the meat texture.
The process changes beef by two means. Firstly, moisture is evaporated from the muscle. This creates a greater concentration of beef flavour and taste. Secondly, the beef’s natural enzymes break down the connective tissue in the muscle, which leads to more tender beef.
The process of dry-aging usually also promotes growth of certain fungal (mold) species on the external surface of the meat. This does not cause spoilage, but actually forms an external "crust" on the meat's surface, which is trimmed off when the meat is prepared for cooking. These fungal species complement the natural enzymes in the beef by helping to tenderize and increase the flavor of the meat. The genus Thamnidium, in particular, is known to produce collagenolytic enzymes which greatly contribute to the tenderness and flavor of dry-aged meat.
Dry-aged beef is typically not sold by most supermarkets in the U.S. today, because it takes time and there is a significant loss of weight during the aging process. Dry-aging can take from 15 to 28 days, and typically up to a third or more of the weight is lost as moisture This type of beef is served in higher-priced steakhouses and by select restaurants. Dry-aging can be done at home under refrigeration by three means: open air, with the presence of salt blocks, and with the use of a moisture permeable drybag to protect the meat while it is aging.
When dry aging using a moisture permeable material, surface mold growth is not present, flavor and scent exchange within the refrigerated environment is not a concern, and trim loss of the outer hardened surface is measurably reduced. The flavor and texture profile of the beef is similar on all dimensions to the traditional open air dry-aged results.
Wet-aged beef 
Wet-aged beef is beef that has typically been aged in a vacuum-sealed bag to retain its moisture. This is the dominant mode of aging beef in the U.S. today. Wet-aging is popular because it takes less time: typically only a few days. Moisture accumulates while in the vacuum bag and its amount depends on the timing of aging; thus, there is little weight loss.
- DeGeer, S.L.; Hunt, M.C.; Bratcher, C.L.; Crozier-Dodson, B.A.; Johnson, D..E.; Stika, J.F. (6 March 2009). "Spotlight on dry aging beef: effects of loin type, aging methods, and aging time". Publisher: Kansas State University. Agricultural Experiment Station and Cooperative Extension Service. Retrieved 14 March 2013.
See also 
- Information on aged beef
- University of Minnesota Extension Service - Aging Beef
- Dry aging of beef in a bag highly permeable to water vapour
- Meat Science - Effects of dry aging of bone-in and boneless strip loins using two aging processes for two aging times