Biochemical changes in Muscle.pdf

Biochemical Changes in Muscle  Muscle is the main element in most meat and meat products.  Other parts include connective tissue, fat (adipose tissue), nerves, and blood vessels around and within the muscles.  Post-slaughter, muscles undergo various bi...

Biochemical Changes in Muscle  Muscle is the main element in most meat and meat products.  Other parts include connective tissue, fat (adipose tissue), nerves, and blood vessels around and within the muscles.  Post-slaughter, muscles undergo various biochemical changes that impact meat quality.  The structural and biochemical attributes of muscle are crucial in determining: - How animals are handled before, during, and after slaughter. - The overall quality of the meat produced. Conversion of muscle to meat  After an animal dies, life-sustaining processes gradually stop.  This cessation leads to notable changes in postmortem muscle.  These changes mark the transformation of muscle into meat. Post Mortem Biochemical Changes in Muscle  After a food animal is slaughtered, muscle ceases its living functions and turns into meat.  Various physical and chemical changes occur over time.  Meat largely reflects the chemical and structural properties of muscles but differs due to biochemical and biophysical changes starting at death.  The conversion process is gradual and degradative process. Homeostasis  'Homeostasis' refers to maintaining a physiologically balanced internal environment.  During muscle-to-meat conversion, many reactions and changes result directly from homeostasis.  The nervous system and endocrine glands oversee the homeostatic mechanism.  They coordinate adjustments in function. 1 Exsanguination  The first step in conventional animal slaughter is to remove as much blood as possible from the body.  Exsanguination starts a series of postmortem changes in muscle.  Sheep and goats bleed better when in a vertical position; cattle bleed better in a horizontal position.  As blood pressure drops, the heart pumps harder and peripheral vessels constrict to maintain pressure and keep blood in vital organs.  Only about 50% of the blood can be removed; the rest remains in vital organs.  Excess blood in meat facilitates the growth of spoilage organisms and is unappealing to consumers.  Thorough bleeding is an essential first step in the slaughter process. Circulatory Failure to the Muscles  Blood acts as a transport medium, and exsanguination cuts off communication between muscle and its external environment.  Once stored oxygen is depleted after exsanguination, the aerobic metabolic pathway stops functioning.  Energy metabolism shifts to the anaerobic pathway, producing less ATP.  Without the circulatory system, lactic acid from anaerobic metabolism remains in the muscle, not going to the liver.  Lactic acid concentration increases, continuing to accumulate until glycogen in the muscle is depleted or low pH stops anaerobic glycolysis. Glycolysis  After slaughter, muscles convert glycogen to lactic acid due to the lack of oxygen.  Post mortem glycogenolysis significantly affects meat quality. 2  With no oxygen, the breakdown of muscle glycogen produces lactic acid, changing meat pH.  Two enzymes responsible for glycogen degradation are glycogen phosphorylase and debranching enzyme.  Glycogenolysis can be accelerated by hormonal mechanisms (stress) and allosteric activation by calcium ions and/or AMP.  Stress before slaughter, like transportation and manipulation, or adrenalin injection depletes glycogen and raises the ultimate pH.  A rapid pH fall from accelerated glycogenolysis and ATP breakdown causes a PSE (Pale, Soft, Exudative) defect.  The stress susceptibility gene is primarily responsible for PSE defects. In the muscle, glycogen is used as source of glucose for energy (ATP) production. Anaerobic glycolysis in the muscle produces lactate and ATP. Anaerobic glycolysis of muscle 3 pH Decline  Lactic acid accumulation lowers pH in the muscle.  The ultimate pH of meat depends on the glycogen level in the muscle at the time of exsanguination.  Lowering pH is a key post-mortem change in muscle during meat conversion.  Protein denaturation relies on high temperature and low pH levels.  Temperature significantly influences muscle protein denaturation.  Denatured proteins lose solubility, water-holding capacity, and pigment intensity.  Rapid, extensive pH decline makes muscle pale with low water-holding capacity, resulting in a wet cut surface.  Muscles maintaining high pH become dark and dry on the cut surface due to tightly bound water.  These conditions are called 'pale soft and exudative’ (PSE) and ‘dark firm and dry’ (DFD). 4 Pale, Soft, Exudative (PSE) Meat  In stress-susceptible animals, pH quickly falls to 5.8-5.6 while the carcass is still warm.  This results in meat that is pale, soft, almost mushy, and has a very wet surface.  Such meat has lower binding properties and loses water rapidly during cooking, decreasing yield.  PSE meat can be used in meat products where water loss is desirable, like dry-fermented sausages. Dark, Firm, Dry (DFD) Meat  Animals not fed before slaughter or excessively fatigued during transportation and lairage have muscle pH that doesn’t fall below 6.0.  This causes muscle proteins to retain most of their bound water, keeping the muscle swollen.  These muscles absorb most light striking the meat surface, giving it a dark appearance.  The meat remains tough and tasteless due to lack of acidity.  It is useful for raw-cooked products requiring high water binding, like frankfurters. Rigor Mortis  After an animal dies, its muscles stiffen due to rigor mortis. 5   Lactic acid from muscle glycogen causes muscle proteins to coagulate, leading to this stiffness.  Rigor mortis occurs because of permanent crossbridges forming between actin and myosin filaments in the muscles.  This change improves meat quality, making it more tender and flavorful.  Meat darkens and loses elasticity during rigor mortis.  Different animals experience rigor mortis differently.  Cooking meat before rigor mortis makes it tough and tasteless.  Acid formed during postmortem glycolysis softens connective tissues and loosens muscle fibers, enhancing tenderness and flavor. Changes in pH Post-mortem  The timing of rigor mortis stages varies by species: - Poultry: 1-2 hours - Pork: 4-6 hours - Beef and Lamb: 7-15 hours 6   Once ATP drops to half (½) its original level at higher temperatures (or) one third (⅓) at less than 15° C, muscles begin to stiffen.  Pre-rigor processing can start with minimal risk of toughening due to shorter sarcomeres.  At the onset of rigor mortis, ATP declines to about 1/3 to 1/2 of original levels.  Calcium leaks from the sarcoplasmic reticulum (SR), stimulating muscle contraction and shortening sarcomeres.  pH declines gradually due to increased lactic acid.  Creatine phosphate (CP) helps regenerate ATP in rested animals.  Many animals lack CP at slaughter because it is used up during transport and lairage.  ATP must be generated from alternative energy sources. Rigor Mortis as a Function of Meat Chemistry and Time  Glycogen is the main energy storage in muscles and is used to produce ATP, keeping meat in the pre-rigor state. 7   Animals with less muscle glycogen at slaughter enter rigor mortis faster than normal (DFD meat).  Rapid pH decline (PSE meat) causes quick ATP use and lactic acid buildup, speeding up rigor onset (1 to 4 hours).  Often, meat temperature remains high during this time, causing more protein denaturation than usual. Loss of protection from bacterial invasion  In living animals, muscles are protected from microorganisms by several defenses.  The first line of defense includes the epidermal tissues covering the body and internal organs.  During meat processing, membrane properties change.  This change makes the muscle more susceptible to bacterial invasion. Loss of structural integrity  Acute degradative changes, like altered membrane properties, start soon after exsanguination.  Z line structure disintegration in some species starts post rigor mortis, causing muscle stiffness loss.  Progressive disruption of myofibrillar structure occurs.  Degradation rates vary across different animals. Enzymatic degradation - Proteolysis  In muscle cells, proteolytic enzymes called cathepsins are inactive within lysosomes.  When muscle pH drops, cathepsins are released and degrade muscle protein structure.  Postmortem meat tenderization may result from collagen breakdown by cathepsins.  Meat quality aspects like tenderness, juiciness, flavor, color, emulsifying capacity, binding properties, and cooking losses can be affected during muscle-to-meat conversion.  Factors influencing this conversion are crucial for controlling and improving meat quality. 8 Cold Shortening  Cold shortening occurs when carcasses chill rapidly post-slaughter, before muscle glycogen converts to lactic acid.  Rapid chilling with glycogen present induces irreversible muscle contraction, tightening actin and myosin filaments.  Cold shortening makes meat up to five times tougher, common in lean beef and lamb with high red muscle fibers and little fat.  Without fat insulation, muscles cool too quickly pre-rigor mortis.  Electrical stimulation postmortem can reduce/eliminate cold shortening by inducing muscle contractions and depleting glycogen.  Thaw rigor is similar; freezing pre-rigor mortis meat, then thawing, allows glycogen-based muscle contraction, making meat extremely tough. Colour of Meat  Myoglobin is oxygen-binding protein responsible for 80-90% of the meat pigment in well- bled animals.  Meat color differences are influenced by myoglobin levels in muscle fibers.  The chemical state of iron in myoglobin also affects meat color. Myoglobin content  Various factors affect myoglobin content in skeletal muscles.  Muscles contain a mix of fast-twitch and slow-twitch muscle fibers.  Fast-twitch fibers (white fibers) have low myoglobin and rely on anaerobic glycolysis.  Slow-twitch fibers (red fibers) have high myoglobin and greater oxidative metabolism.  Dark meat color results from high slow-twitch fiber concentration in the muscle. 9  Slow twitch fiber Fast twitch fiber Muscle fiber types  Age of the animal influences myoglobin content.  Older animals have higher myoglobin concentrations in muscles.  This results in darker meat color in beef compared to veal.  Animal size affects myoglobin content due to metabolic rate differences.  Larger animals have lower metabolisms and higher myoglobin levels.  Smaller animals (e.g., rabbits) have lower myoglobin and lighter meat.  Larger animals (e.g., horses) and deep-diving animals (e.g., whales) have higher myoglobin and darker meat.  Intact males have greater myoglobin concentrations.  Muscles closer to bones and in more active animals (e.g., game) have higher myoglobin. Oxidation state of iron  The oxidation state of myoglobin's iron atom of myoglobin also plays a significant role in meat color.  Freshly cut beef is purple due to water binding to reduced iron atom of the myoglobin molecule (in this state the molecule is called deoxymyoglobin). 10  Beef turns bright cherry-red within 30 minutes of air exposure due to oxygen binding called as blooming.  Blooming is the result of oxygen binding to the iron atom (in this state the myoglobin molecule is called oxymyoglobin).  After several days, iron oxidizes, loses oxygen-binding ability, turning meat brown (the myoglobin molecule is now called metmyoglobin).  Brown color indicates the meat is no longer fresh, but it's not harmful. 11 Tenderness of Meat  The tenderness of meat is influenced by a number of factors: - the grain of the meat - the amount of connective tissue - the amount of fat Meat grain  Meat grain is based on muscle bundle size.  Finer-grained meats are more tender and have smaller bundles.  While coarser-grained meats are tougher and have larger bundles.  Muscle grain differs within the same animal and the same muscle in different animals.  More frequently used muscles have more myofibrils per fiber.  This leads to thicker bundles and tougher proteins.  Older animals and muscles used for physical work typically have coarser-grained meat. 12 Connective tissue  Connective tissue influences meat tenderness.  Collagen is the main component and has a tough structure.  Younger animals have more connective tissue but tender meat.  Aging and cooking break down collagen into a gelatin-like substance, making meat tender.  Collagen becomes more rigid with age.  Therefore, meat from older animals is tougher. Meat connective tissue Fat  High fat content in adipose tissue and marbling sites contributes to meat tenderness.  During cooking, fat melts into a lubricant-like substance.  This melted fat spreads throughout the meat.  The result is increased tenderness of the final product. 13  Meat fat Ripening or Aging:  After passing of rigor mortis, meat becomes progressively more tender, juicier, and more flavorful.  The speed with which this ripening or aging occurs depends on the time the carcass is kept and the temperature.  Changes occur quite rapidly at room temperature but more slowly at refrigerator temperatures.  A recent study shows that aging beef at elevated temperatures with high humidity, with air velocity 5 to 20 lineal ft/min and with ultraviolet radiation to control microbes for 2 or 3 days produces beef equal in quality to that aged in a refrigerator 12 to 14 days.  Aging caused an increase in the free amino acid nitrogen.  The nitrogen in drippings of freshly slaughtered meat was present mostly as non protein nitrogen with a large proportion of it as amino nitrogen. 14   After the beef had aged for two weeks, drippings showed an increase in the amount of NPN.  The amino acids histidine, leucine, tyrosine, glutamic acid, and lysine were present in a bound form.  Histidine accounted for as much as 26% of the NPN in the extracts.  It was concluded that part of the bound histidine is present as carnosine. Changes in Meat on Cooking The chemical and physical changes occur on cooking are numerous and although some studies have been made on this problem the reactions are not all known or understood. They are: 1. Denaturation of the protein, 2. Hydrolysis of collagen to gelatin, 3. Color change, 4. Formation of grid, 5. Development of brown flavor, 6. Rupture of fat cells and dispersion of fat through the meat, and 7. Decrease in vitamins and possibly decrease in the nutritive value of the protein. 1. Denaturation of the protein  Heat denaturation of proteins is common in cooking.  It doesn't always toughen the protein.  Denaturation alters the protein by breaking hydrogen bonds and sulfur-sulfur links.  These changes maintain the 3D shape of the molecule initially.  Molecular weight isn't significantly affected in early steps.  Denaturation alters properties like solubility and density. 15 2. The Hydrolysis of Collagen  Collagen was higher in raw commercial-grade meat compared to prime.  No significant difference in collagen between top round and bottom round.  Collagen loss occurs with all cooking methods.  Collagen loss increases with higher internal temperatures.  Pressure cooking at 10 lbs pressure leads to greater collagen loss than at 5 lbs pressure. 3. Color change  Cooking meat denatures oxyhemoglobin, oxymyoglobin (red), and hemoglobin (purplish red).  Ferrous iron in free porphyrin oxidizes to ferric iron of hemin (brown).  Oxyhemoglobin, oxymyoglobin, and myoglobin can transform to met hemoglobin and metmyoglobin (brown). 4. Drip Formation  Weight change in meat during cooking is partly due to the drip.  Drip contains water, soluble compounds, coagulable protein, and fat.  Heat denatures coagulable protein, forming curd in the pan gravy.  Cut of meat, fattiness, cooking method, and cooking extent affect drip fat content.  Fat degradation products may also be in the drip.  The increased soluble nitrogen compounds in drippings during cooking.  No increase in free amino nitrogen was observed. 5. Meat Aroma  Cooking meat enhances its flavor and aroma.  Cooked meat has a slightly salty, slightly sweet taste and a distinct aroma.  Aroma consists of low molecular weight, volatile compounds: amines, ammonia, hydrogen sulfide, and organic acids. 16   These compounds result from the cracking of amino acids during heating.  Reactions include decarboxylation, deamination, or desulfuring of free amino acids or polypeptides.  Flavor compounds vary by species, making lamb taste different from beef. 6. Dispersion of fat  Cooking causes fat cells to rupture.  Fat disperses through the meat during cooking.  Proteins in fat cells undergo denaturation like other meat proteins.  Cell wall permeability changes, allowing fat to flow out. 7. Decrease in vitamins  B complex vitamins are sensitive to heat, especially thiamin and pantothenic acid.  Prolonged cooking or high temperatures can lead to significant vitamin loss.  Some loss of vitamins always occurs.  Niacin and riboflavin are more heat-stable and degrade more slowly.  Folic acid group is highly sensitive to heat, with up to 90% loss.  Ascorbic acid decreases after slaughter and further diminishes during cooking.  Cooked meat supplies only small amounts of ascorbic acid. 17