Paper No. : 08 Technology of Meat, Poultry, Fish and and Seafood Module : 06 Post Mortem muscle chemistry-1: Loss of homeostasis and post- mortem glycolysis Development Team Principal Investigator Prof. (Mrs.) Vijaya Khader, Ph.D Former Dean, Acharya N G Ranga Agricultural University Paper Coordinator Prof. Dharmesh Chandra Saxena SLIET, Longowal Content Writer Miss Romee Jan & Prof. Dharmesh Chandra Saxena SLIET, Longowal Content Reviewer Prof. (Mrs.) Vijaya Khader, Ph.D Former Dean, Acharya N G Ranga Agricultural University Dr. MC Varadaraj, Chief Scientist CSIR-CFTRI, Mysore 1
Description of Module Subject Name Paper Name Module Name/Title 08 Post Mortem muscle chemistry-1: Loss of homeostasis and post- mortem glycolysis Module Id Pre-requisites Objectives Keywords 2
1.1 Introduction: Slaughter of animals is followed by series of physical and chemical changes over a period of several hours or may take even days. This is determinant stage of type of meat produce from muscle i.e. from muscle to meat conversion. The process starts with Exsanguination of animal where bleeding takes place. Myoglobin is thus decrease in oxygen and as a result inhibition of aerobic pathway by both citrate cycle and cytochrome system. This type of conditions thus leads to anaerobic pathway for energy production and results in breakdown of glycogen to lactic acid. This process continues till stored glycogen in muscle totally exhausted. This type of ATP production pathway is not enough to maintain required ATP level. Another major change is formation actomyosin complex resulting in onset of Rigor Mortis. Muscle contains small amount of buffers that neutralize the lactic acid formed as a product of anaerobic cycle. But as the more and more lactic acid is formed, the ph of tissue begins to fall. The amount of lactic acid formed is directly proportional to the amount of glycogen present in the muscle. A well nourished muscle has as much as 1% of the glycogen theta can be converted into 1.1% lactic acid and drops to a ph of 5.6, whereas ph never falls below 5.3 as enzymes becomes inactive at this low ph. Feeding of animal before slaughtering also affects the amount of muscle glycogen and hence affects the resultant ph. Well fed animal contains maximum glycogen and ph drop is hence possible. Another factor that affects the ph drop is amount of work done or exercise that utilizes the glycogen. An animal that is chased before slaughtering has low possibility for the the development of low ph. The ph attained by meat has important effects in the quality of meat and its properties. Meat that develops the high ph is difficult to cure as salts do not penetrate the meat at normal rate; ph between 5.8 to 6.0 there is a rapid increase in resistance because of its direct relation with electrical resistance which in turn affects the ion exchange. 1.2 Loss of homeostasis: Homeostasis is the process of adaption of body s internal environment with the external environment to cope up the extreme variation conditions like change in temperature, daylight, oxygen deficiency, energy supply and other conditions that stimulates the internal body changes. Homeostasis is controlled by nervous system. The common example of homeostasis is the adaptation of sight when coming from dark to light. In case of animal slaughtering the homeostasis ceases within 4-6 min. after bleeding, which is accompanied by loss in body heat and decrease in temperature takes place. The series of chemical reaction takes place which produce enough heat 3
to rise temperature. Average body temperature of meat is 99.7 F but shortly after death, internal body temperature rises to 103 to 105 F. Fresh meat cools very slowly even in refrigerator condition. This heat produced is called Animal Heat. As per the old age farmers and butcher, mast should not be eaten until animal heat is escaped. A short period hanging and occurrence of rigor mortis improves meat before cooking. After slaughter, aerobic metabolism begins to fail due to the stored oxygen supply being depleted. To maintain homeostasis, anaerobic energy metabolism starts producing lactic acid, consuming the glycogen stored in the muscles. Thus, the structural integrity of the cells is maintained for a period of time although less energy in the form of ATP is produced. In the exsanguinated animal, the circulatory system is not capable of removing metabolites so that lactic acid remains in the muscle and increases its concentration during the post-mortem period until the glycogen stored in muscle is consumed. 1.3 Post mortem Glycolysis: As discussed above glycogen is served as raw material which is being utilized by body through aerobic pathway for the production of ATPs which is achieved in living animals by aerobic Glycolysis. Due to depletion of oxygen supply there is occurrence of anaerobic glycolysis which leads to formation of lactic acid from glycogen reserves. Production of lactic acid lowers the muscle ph, which is an important postmortem change during conversion of muscle to meat. After an animal is slaughtered, blood circulation stops, and muscles exhaust their oxygen supply. Muscle can no longer use oxygen to generate ATP and turn to anaerobic glycolysis, a process that breaks down sugar without oxygen, to generate ATP from glycogen, a sugar stored in muscle. Within 24 hours after death the changes in muscle ph and color is as shown below. Glycogen -------> Lactic acid Muscle ph: 7.0 -------> 5.6 (because of lactic acid) Muscle color: purple changes to bright red or pink (ph 7.0 -------> 5.6) The breakdown of glycogen produces enough energy to contract the muscles, and also produces lactic acid. With no blood flow to carry the lactic acid away, the acid builds up in the muscle tissue. If the acid content is too high, the meat loses its water-binding ability and becomes pale and watery. If the acid is too low, the meat will be tough and dry. After slaughter, aerobic metabolism begins to fail due to the stored oxygen supply being 4
depleted. To maintain homeostasis, anaerobic energy metabolism starts producing lactic acid, consuming the glycogen stored in the muscles. Thus, the structural integrity of the cells is maintained for a period of time although less energy in the form of ATP is produced. In the exsanguinated animal, the circulatory system is not capable of removing metabolites so that lactic acid remains in the muscle and increases its concentration during the postmortem period until the glycogen stored in muscle is consumed. Figure1. Difference of aerobic and anaerobic pathway Lactic acid buildup also releases calcium, which causes muscle contraction. As glycogen supplies are depleted, ATP regeneration stops, and the actin and myosin remain locked in a permanent contraction called Rigor mortis. Freezing the carcass too soon after death keeps the proteins all bunched together, resulting in very tough meat. Ageing allows enzymes in the muscle cells to break down the overlapping proteins, which makes the meat tender. ph measurements are important for detecting meat quality characteristics. Meat quality greatly depends on the type and extent of glycolysis. Glycolitic muscles produce higher amounts of lactic acid than oxidative ones because they use the glycolitic pathway to produce energy rather than the oxidative pathway. 1.4 (Pale, Soft, Exudative) PSE meat A sharp decline in postmortem ph even before the dissipation of body heat through carcass chilling may cause denaturation of muscle proteins. This kind of meat exhibit Pale, Soft, Exudative (PSE) condition. The low ph prevents or retards microbial growth. The rate of ph change post mortem also influences meat quality. Development of a low ph (acid) in muscle causes denaturisation of muscle proteins. This denaturisation causes loss of protein solubility, loss of water- and protein-binding capacity, loss in intensity of muscle pigment coloration. All these changes are undesirable, whether the muscle is going to be used as fresh meat or subjected to further processing. The gene that is responsible for PSE meat is the Halothane sensitivity gene (HAL), which is associated with a fast rate of post-mortem decline in ph and which is more likely in stress-susceptible animals. Halothane positive animals are homozygous, and they can be detected by exposure to halothane gas, which induces 5
the Porcine Stress Syndrome (PSS). However, a more accurate identification of three genotypes is now possible using a direct marker test (DNA Test) developed after the identification of the gene (Ryadonine receptor gene, RYR1) and the discovery of the specific mutation. 6.5 Dark, Firm, Dry (DFD) Meat Contrary to PSE condition, muscles which maintain a consistently high ph during post mortem changes or conversion of muscle to meat depicts dark, firm, dry (DFD) meat. In such cases the ph values affect many meat properties including colour and drip loss. Muscles that maintain a high ph during conversion of muscle to meat may be very dark in colour, and very dry on the exposed cut surface, because the water is kept tightly bound to proteins. The high ph also enhances microbial meat spoilage. The presence of DFD meat develops mainly in the forequarter, LD and glycolitic muscles of ham for all the species, but the frequency is different among species. The pig is not the only animal that can be affected by this kind of defect. Pale meat or exudative meat can be also detected in beef and poultry, but are not necessarily undesirable attributes in these species. Although it is not frequent, exudative meat has been found in carcasses of young bulls. The deviation in quality is generally much less pronounced than that of PSE pork, so that PSE in beef has been little investigated. In poultry, particularly turkey, it is common to find pale meat but it is not Exudative. Usually it is related to a low haem pigment content of the muscle. 6.6 Importance of ph change in Muscle to meat conversion. 6
ph also is important in determining the water-holding capacity of meat, the ability of meat to retain its water during application of external forces such as cutting, heating, grinding, or pressing. There are three locations of water found in meat 1. Bound (charged hydrophilic groups on the muscle proteins attract water, forming a tightly bound layer) 2. Immobilized (has less orderly molecular orientation toward the charged group), 3. Free (held only by capillary forces, and their orientation is independent of the charged group). The following graph depicts the relationship between ph and water-holding capacity. Where water-holding capacity is the lowest is the isoelectric point, where the number of positively and negatively charged groups of the myofibrillar proteins is equal. Thus, the charges cancel out and no charge is available to hold the bound and immobilized water. 7