The most abundant chemical in meat is water followed by protein then fat. Carbohydrates, minerals and vitamins occur on much smaller amounts but nevertheless are very important metabolically and nutritionally. The primary carbohydrate reservoir of the animal body is the liver. That organ contains about one—half of the carbohydrates found in the body. Carbohydrates are stored as glycogen in the liver and in muscles.
The remaining SO percent of carbohydrates are distributed throughout the body, largely in the muscles, but with substantial quantities in the blood (usually as glucose) and in other tissues, organs and glands. The changes that occur in energy metabolism, e.g., the conversions of glycogen to glucose and glucose to lactic acid, are complex; all such changes are controlled and mediated by enzymes and hormones. The lactic acid content of the muscles in a carcass increases during initial stages of aging or ripening, lowering the pH (muscle acidity).
The pH of muscle considered “normal” is 5.6 (pH is the negative log of the hydrogen ion concentration; the higher the pH, the less acidic is the muscle). Muscle color, texture, water-holding capacity and tenderness are influenced by pH. If an animal experiences vigorous stress or exercise of the muscles immediately prior to slaughter and has no opportunity to restore its normal glycogen levels, the glycogen content within the muscles at slaughter will be reduced substantially. Because so little glycogen is available to be converted (after death) to lactic acid, a higher ultimate pH (e.g., a final pH of 6.2) will occur in this animal's muscles after slaughter, and the muscles will be dark, firm and dry (DFD). This is a reasonably rare occurrence in beef (perhaps 2 percent of carcasses are affected) and those carcasses are termed “dark cutters’ The DFD condition also occurs in pork and lamb carcasses. It is thought that the dark color of muscles with a high pH is due to its higher water-holding capacity, which causes muscle fibers to be swollen. The swollen state of the ï¬bers causes more incidents light to be absorbed, rather than reflected, by the meat surface, and thus the color appears to be darker. “Dark cutters" are severely discounted in price by packers and retailers, due to poor consumer appeal of this meat; therefore, stress and rough handling of animals is minimized prior to slaughter.
A too—rapid postmortem (after death) drop in muscle pH (to a final pH of 5.1, for example) is associated with the pale, soft, and exudative (PSE) conditions that are somewhat common in pork muscle. PSE muscle is characterized by soft and mushy texture, low water-holding capacity and pale muscle color. The looser muscle structure of PSE muscle associated with its lower water-holding capacity is responsible for a greater reflectance of incident light and hence it has a pale color.
