Prof. Dr. Mehmet Ali Kaşlıoğlu
(Expert in Food and Nutrition Science)
Meat is generally defined as the tissue of an animal used as food. Accordingly, it includes not only muscles but also non-muscular organs and fat tissues. Liver, tongue, brain, and kidneys can be used as meat. Red meat refers to the meat group obtained from cattle and small ruminants. Meat is a frequently consumed food source worldwide because it is rich in proteins and suits taste preferences. Meat quality depends on healthy fattening. Additionally, the animals to be slaughtered must have been raised under veterinary supervision.
Red meat is an important source of animal protein necessary for healthy human nutrition. In Muslim societies, pork is not consumed due to religious beliefs, so the demand for red meat is mainly met by beef cattle. Despite rising meat prices, beef remains one of the most important food sources. The quality of beef varies after slaughter due to many factors. Moreover, factors such as pre-slaughter stress, inappropriate slaughter techniques, and genetic characteristics influence meat quality. Therefore, understanding the changes that occur after slaughter is crucial to improving the quality of meat and meat products. Meat quality is closely related to changes in the structure of meat proteins as well as the physical, chemical, and microbiological properties of the meat.
Color Changes in Meat
The freshly slaughtered meat of an animal gets its red color from the protein myoglobin (Mb.Fe+2) and can also be purplish or violet. Myoglobin is not found in circulating blood but is stored in tissues and has a purplish color. Upon contact with oxygen, it transforms into oxymyoglobin and takes on a red color. When meat is sold, myoglobin combines with the oxygen in the air and produces the bright cherry-red color seen on the surface of the purchased meat. After several days, the surface of fresh meat may take on an unpleasant, brownish hue.
The color of red meat is related to the age, sex, breed, feeding method, and season of the slaughtered animal. However, the most important factor is the relationship with pre-slaughter stress. Pre-slaughter stress refers to transport and the conditions during transport. Long or short transports, road conditions, weather conditions during transport, the mixing of unfamiliar animals, hunger, horned calves, fear, unfamiliar noises, calves jumping on each other, and the driver's skills are factors that can cause stress.
Feeding conditions and feed also influence the color of the meat. Meat from fattening farms has a different color than meat from animals from pastures and meadows. Meat from calves slaughtered under poor feeding conditions can be dark.
Beef is pale red, and the fat is lemon yellow. Bull meat is dark red, and the fat is whitish. Veal is pale red, and ox meat is reddish, with white-yellow fat. Additionally, the color of the meat can vary depending on the part of the animal it comes from, even in animals of the same breed. Consumers should ensure that the meat they buy is in various shades of light to dark red.
There is a widespread belief that the darker red meat is, the less fresh it is. However, this view is not entirely correct. The redness of meat varies depending on the animal species, breed, age, region, and feeding method. A color change alone is not an indicator of spoilage. Color changes are normal in fresh products. In spoilage, the color change usually appears as fading or darkening.
The color of meat can turn green due to the growth of microorganisms that break down myoglobin. With excessive growth of microorganisms, the meat turns green under the influence of hydrogen sulfide (H2S) and oxygen. Under the same conditions, myoglobin can also turn green under the influence of oxygen in the presence of reducing agents such as hydrogen peroxide (H2O2) and ascorbic acid. The green caused by the growth of microorganisms can turn brown or even yellow in later stages due to oxidation, indicating that the meat is completely spoiled.
Changes in Meat After Slaughter
As muscles transform into meat, the energy stored in the muscles is gradually depleted. Through aerobic and anaerobic metabolism, the pH level drops to 5.4–5.8 due to the accumulation of lactic acid in the muscles. The development of meat tenderness is closely related to changes in myofibrillar proteins. A large part of these changes is caused by proteolytic enzymes. The enzymes in the muscles are inactive. After slaughter, the decreasing pH level in the muscles activates these enzymes, causing changes in the structure of myofibrillar proteins and promoting the maturation of the meat.
Meat consists of body muscles. After slaughter, the muscles undergo a series of biochemical reactions to transform into meat. The conversion of muscles into meat occurs through the conversion of muscle sugar (glycogen) into lactic acid. After slaughter, metabolic reactions in the muscles continue. With the cessation of circulation, the muscles attempt to replace the required energy by consuming the glycogen stored in the muscles. If the muscles do not contain enough glycogen or if energy cannot be provided after death for other reasons, sufficient lactic acid may not accumulate in the muscles. Therefore, the pH of the meat (5.4–5.8) does not drop to the desired level, and the meat quality does not reach the desired standard.
Aging of Meat
After slaughter, the animal's muscles harden. Therefore, it should not be consumed immediately after slaughter. The potential quality characteristics of meat are determined by production, processing, species, age, breed, sex, feeding, and the management of pre-slaughter conditions. The maturity of meat is related to the connective tissue of the muscles and the protein components of the muscle fibers. One of the biggest problems faced by meat consumers is the maturity of the meat. Meat must be aged. Freshly slaughtered meat is very sinewy and tough. To soften this toughness, the meat must rest and mature. To ensure good eating quality, meat should not be cooked immediately after slaughter but after a resting period. Aged meat develops purple rings. If meat is rested for at least 24 hours after slaughter, it becomes tastier and more tender.
Aging after slaughter also has a significant impact on color formation. The pH of the muscles of live slaughter animals is 7.0–7.3. After slaughter, the pH level drops depending on the amount and breakdown of glycogen in the muscles, and depending on the extent of the pH drop, different meat colors are formed due to "rigor mortis (RM)." The pH of the muscles drops to 5.8 and below (5.6–5.3) within 6–24 hours. At low pH, the ability of myoglobin to bind oxygen increases. In this case, beef has a bright cherry-red color and is ready to eat. This color is the normal meat color that consumers desire and that it should be. After the animal is slaughtered, the meat comes into contact with the oxygen in the air during cutting and chopping and takes on a bright pink-red color. RM is a term used to describe the hardness that occurs in the muscles and represents the greatest physical change in the muscles. The duration until completion varies depending on the species and the energy level in the muscles. In cattle, it takes 12–24 hours, and in sheep, 8–12 hours.
