Most meats are judged on their tenderness and the

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Unformatted text preview: rily of bundles of tiny muscle fibers bundled together inside long strings of connective tissues that hold it together. The tenderness of a certain cut of beef depends on the location of the cut, the age, and the activity level of the animal. Cuts from the relatively inactive mid-backbone section of the animal such as short loins, sirloin, and prime ribs are more tender than the cuts from the active parts such as the legs and the neck (Fig. 4–38). The more active the animal, the more the connective tissue, and the tougher the meat. The meat of an older animal is more flavorful, however, and is preferred for stewing since the toughness of the meat does not pose a problem for moist-heat cooking such as boiling. The FIGURE 4–37 Typical cooling curve of a beef carcass in the chilling and holding rooms at an average temperature of 0°C (from ASHRAE, Handbook: Refrigeration, Ref. 3, Chap. 11, Fig. 2). Chuck Rib Short loin Brisket Flank Foreshank Short plate Sirloin Round FIGURE 4–38 Various cuts of beef (from National Livestock and Meat Board). cen58933_ch04.qxd 9/10/2002 9:13 AM Page 244 244 HEAT TRANSFER Tenderness scale 10 5 0 5 Time in days 10 FIGURE 4–39 Variation of tenderness of meat stored at 2°C with time after slaughter. Meat freezer Air – 40 to –30°C 2.5 to 5 m/s Meat FIGURE 4–40 The freezing time of meat can be reduced considerably by using low temperature air at high velocity. TABLE 4–6 Storage life of frozen meat products at different storage temperatures (from ASHRAE Handbook: Refrigeration, Chap. 10, Table 7) Storage Life, Months Temperature Product Beef Lamb Veal Pork Chopped beef Cooked foods 12°C 4–12 3–8 3–4 2–6 3–4 2–3 18°C 23°C 6–18 12–24 6–16 12–18 4–14 8 4–12 8–15 4–6 8 2–4 protein collagen, which is the main component of the connective tissue, softens and dissolves in hot and moist environments and gradually transforms into gelatin, and tenderizes the meat. The old saying “one should either cook an animal immediately after slaughter or wait at least two days” has a lot of truth in it. The biomechanical reactions in the muscle continue after the slaughter until the energy supplied to the muscle to do work diminishes. The muscle then stiffens and goes into rigor mortis. This process begins several hours after the animal is slaughtered and continues for 12 to 36 h until an enzymatic action sets in and tenderizes the connective tissue, as shown in Figure 4–39. It takes about seven days to complete tenderization naturally in storage facilities maintained at 2°C. Electrical stimulation also causes the meat to be tender. To avoid toughness, fresh meat should not be frozen before rigor mortis has passed. You have probably noticed that steaks are tender and rather tasty when they are hot but toughen as they cool. This is because the gelatin that formed during cooking thickens as it cools, and meat loses its tenderness. So it is no surprise that first-class restaurants serve their steak on hot thick plates that keep the steaks warm for a long time. Also, cooking softens the connective tissue but toughens the tender muscle fibers. Therefore, barbecuing on low heat for a long time results in a tough steak. Variety meats intended for long-term storage must be frozen rapidly to reduce spoilage and preserve quality. Perhaps the first thought that comes to mind to freeze meat is to place the meat packages into the freezer and wait. But the freezing time is too long in this case, especially for large boxes. For example, the core temperature of a 4–cm-deep box containing 32 kg of variety meat can be as high as 16°C 24 h after it is placed into a 30°C freezer. The freezing time of large boxes can be shortened considerably by adding some dry ice into it. A more effective method of freezing, called quick chilling, involves the use of lower air temperatures, 40 to 30°C, with higher velocities of 2.5 m/s to 5 m/s over the product (Fig. 4–40). The internal temperature should be lowered to 4°C for products to be transferred to a storage freezer and to 18°C for products to be shipped immediately. The rate of freezing depends on the package material and its insulating properties, the thickness of the largest box, the type of meat, and the capacity of the refrigeration system. Note that the air temperature will rise excessively during initial stages of freezing and increase the freezing time if the capacity of the system is inadequate. A smaller refrigeration system will be adequate if dry ice is to be used in packages. Shrinkage during freezing varies from about 0.5 to 1 percent. Although the average freezing point of lean meat can be taken to be 2°C with a latent heat of 249 kJ/kg, it should be remembered that freezing occurs over a temperature range, with most freezing occurring between 1 and 4°C. Therefore, cooling the meat through this temperature range and removing the latent heat takes the most time during freezing. Meat can be kept at an internal temperature of 2 to 1°C for local use and storage for...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.

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