Immediately after slaughter, changes occur in the muscle of an animal. These changes, like the changes in milk and eggs, can be retarded by method of handling and storage. They are brought about by enzymes and microorganisms, and by chemical and physical means which alter the structure and chemical composition of the meat.

Muscle in the living animal is (1) pliant, soft, gel-like, yet somewhat viscous. After slaughter the muscles pass from this state into a stiff or rigid one (2) known as rigor mortis, or muscle rigor. After some time the muscles again become pliant. This stage (3) is known as the passing of rigor. With longer storage enzymes and chemical means bring about (4) more extensive changes which produce ripened meat. With bacterial action and still more extensive changes (5) incipient putrefaction occurs. The passage from one stage to another is gradual with no definite dividing zone and is accelerated at higher temperatures and retarded at lower ones.

Meat may be cooked during any of these stages and heat denaturation causes characteristic changes which are part of the post-mortem changes. Means of retarding these changes will be considered before these changes are discussed.

Meat cooked before the onset of rigor is said to be tender. But rigor develops quickly so that this period is short. Concerning the question of tenderness of meat before the onset of rigor, Dr. Trowbridge, who has had many years of experience in meat work, wrote the author, "I doubt if freshly killed meat is ever as tender as the same meat ripened."

Preservation of meat. Freezing. Meat may be frozen and then stored at temperatures of - 10° to - 15°F. In this way the post-mortem changes are nearly inhibited.

Curing. Common salt is the basis of all cures or pickles. Although many modifications are used, the methods may be divided into two classes, brine and dry-salt cures. Salt not only preserves but tends to dry the meat. Sugar may be added for flavor. Sugar also tends to keep the muscles softer than when salt is used alone and thus tends to increase the tenderness. When sugar is added to the brine, the process is known as "sugar cure" or "sweet pickle." Saltpeter (potassium nitrate) or Chili saltpeter (sodium nitrate) or the nitrite salts of potassium and sodium may be used in cured meat. The red color of cured meat is due to the action of nitrite on the hemoglobin of the muscle. If only nitrate salts are used in the brine, they are reduced to nitrite by bacterial action.

Cured meat may be smoked and partially dried on the surface. Actual smoke and not chemical treatment to produce a smoke flavor must be used in establishments under federal inspection.

Ham, bacon, and salt pork are probably the best known of cured pork products. The loins when cured are often known as Canadian bacon.

Dried or chipped beef and corned beef are among the more familiar cured-beef products. Beef cured in a brine is known as corned beef. Beef cuts most often cured are the plates, flanks, and rumps, though from the lower grades the chucks and rounds are often used. Dried beef is cured in a sweet pickle, then dried and smoked. Usually the rounds and sometimes the shoulder clods are used for dried beef.

Cold storage. The passing of rigor, ripening, and development of putrefaction are delayed by quickly chilling the dressed meat and keeping it at a low temperature just above the freezing point of meat. Moran states that if the dressed meat is chilled slowly more protein is denatured than if it is chilled quickly, and, because bacteria attack denaturated protein more rapidly than native protein, quick chilling is one means of increasing the storage life of meat.

Chemically conditioned cold. Moran states that with temperature control alone the storage life of chilled beef is about 35 to 40 days. If, in addition to low temperature control, what is sometimes known as "chemically conditioned cold," i.e., 10 per cent of carbon dioxide, is used in the storage atmosphere, the storage life of the meat may be extended to 60 or 70 days. The carbon dioxide retards or checks bacterial growth but if too much is used with red meats such as beef and mutton, the surface of the meat turns dark. This is because of the decreased amount of oxygen obtained by the meat when carbon dioxide is increased. For fish a higher percentage of carbon dioxide can be used. Stansby and Griffith have reported that haddock packed in ice and plus an atmosphere of 15 to 40 per cent of carbon dioxide have their storage life doubled over that when packed in ice alone.

Contamination with bacteria and storage life. Another factor affecting the storage life of meat is the initial contamination with bacteria. But with care the initial contamination should be small. Other things being equal, the smaller the initial load of bacteria the longer the storage life of the meat. Spoilage by bacteria, yeasts, and molds is largely surface, and in general does not extend to a greater depth than 1/4 to 1/2 inch. Hoagland, McBryde, and Powick in their investigations of changes in beef during cold storage above freezing found that "bacteria and molds grow on the surface of cold storage carcasses but do not penetrate to any great depth (less than 1 inch in 177 days)."

Microorganisms may enter the meat by penetration from the surface, which is a slow process, and by following the cavities in meat, that is, the blood and lymph vessels.

Empey found that bacteria grow and develop very slowly on meat during rigor, as a pH of approximately 5.3 to 5.6 is not a desirable one for their growth. Stansby and Griffith also found that bacteria do not develop rapidly on fish during rigor.

Humidity. Mueller and Richardson have reported that dry air as well as the low temperature is a factor in preventing bacterial growth. Hoag-land, McBryde, and Powick found that in a cooler with low humidity the growth of mold on the surface of the meat at 177 days was no greater than in a cooler with higher humidity at 53 days.