It should be noted that the proportions of saturated, monounsaturated, and polyunsaturated fatty acids (PUFA) found in animal tissues are species dependent (table 3.1), and, in monogastric species, may be influenced by diet. In general, fish muscle contains the greatest concentration of polyunsaturated fatty acids followed by poultry and pork
The double bonds located within PUFAs are sites of chemical reactivity. Oxygen is a necessary ingredient for lipid oxidation and may react with these site to form peroxides, which lead to rancidity. Polyunsaturated fatty acids are especially susceptible to oxidative rancidity because of their high number of reactive double bonds. The formation of lipid breakdown products leads to development of undesirable flavors and odors. Those muscle foods with high concentrations of PUFAs (e.g.,fish) typically develop rancid flavors and odors faster than foods with less PUFA. The oxidative rancidity process cannot occur in the absence of oxygen. Thus, vaccum-packaging of meat products provides longer shelf-life by excluding oxygen from the packaging environment. The interaction of oygen with PUFA to cause rancidity is a nonenzymatic process. Lipid oxidation and rancidity may also be caused by enzymatic processes occurring within the muscle food.
Enzymic-based lipid oxidation occurs in muscle foods and has also been termed “microsornal lipid oxidation.” “Microsomes” do not constitute a specific cellular organelle but refer to membrane fractions of these (e.g., sarcoplasmic reticulum). This process requires certain biochemical cofactors or activity including reduced forms of nicotine adenine dinucleolide phosphate(NADPH) or nicotine adenine dinucleotide (NADH), adenosine diphosphate (ADP), and iron ions. The enzymatic nature of the process implies involvement of membrane-bound proteins. Cooking of meat provides sufficient heat to denature enzymes, and thus enzymic microsornal lipid oxidation will not occur in cooked meats. During normal physiological functioning, the enzymes found in these subcellular organelle membranes produce chemically reactive substances known as radicals. These are a necessary part of normal cell functioning, and in the “living state” the cell has a variety of mechanisms for protecting itself against the undesirable actions of radicals. In postmortem muscle tissue, many of these protections are lost and radicals may hasten lipid oxidation and cause rancidity
Metal Ions and Warmed-Oven Flavor
All metal ions are potent catalysis of nonenaymic lipid oxidation. Meat is an excellent source of iro and, although this is a nutritional benefit, the iron can also serve to enhance lipid oxidation in meats. Iron in meat is bound in the heme (heme iron) portion of myoglovin or hemoglobin, or is present as nonheme iron(Nill). Nonheme iron is considered to be the more potent lipid oxidation catalysis. The concentration of nonheme iron can be increased by simply grinding meat through a cast-iron meat grinder, or be cooking. The cooking process provide sufficient heat to denature myogiobin, allowing iron which is bound within the heme molecule to be liberated.
Warmed-over flavor (WOF) is a flavor defect that occurs in reheated meat products. The initial cooking of meat increases the NHI concentration within meat. During the time period between initial cooking and reheating , the iron acts to catalyze lipid oxidation (I.e., liftovers); the warm temperatures of the reheating process may also accelerate lipid oxidation. The outcome is that rancid flavors develop, resulting in WOF. The degree to which WOF is detected is dependent on the individual consumer. Warmed-over flavor is a special concern for manufacturers of precooked meat products.
Post-Rigor Changes for Predicting Freshness: K value and Nucleotide Catabolism
During early postmortem metabolism. The supply of ATP within muscle is kept at a high level by regeneration via creatine phosphate, and by glycolysis. As the postmortem condition progress, creatine phosphate reserves become depleted and, as glycolysis slow, ATP production is reduced. The muscle responds physiologically by using an enayme, myokinase, to covert 2 moles of ADP to ATP plus AMP (Fig.3.6). AMP then enters a catabolic (breakdown) pathway in which it is sequentially converted to other compounds including inosine and hypoxanthine. The catabolism continues and both inosine and hyposanthine accumulante during storage of muscle foods. The concentrations of inosine and hypoxanthine, expressed as percentage of total ATP-related compounds present, can be used as an indicator of freshness in muscle foods. The longer a piece of meat has been stored, the greater the relative concentration of hypoxanthine and inosine. This concept has been most extensively applied in fish where a K value is calculated as shown in Fig.3.6. Longer storage times are correlated with higher K values and reduced freshness. The K value at which fish are considered no longer fresh is species dependent. K values have been less thoroughly studied in traditional red meats but theoretically should be just as applicalble as freshness indicators.
It should be noted that the proportions of saturated, monounsaturated, and polyunsaturated fatty acids (PUFA) found in animal tissues are species dependent (table 3.1), and, in monogastric species, may be influenced by diet. In general, fish muscle contains the greatest concentration of polyunsaturated fatty acids followed by poultry and pork The double bonds located within PUFAs are sites of chemical reactivity. Oxygen is a necessary ingredient for lipid oxidation and may react with these site to form peroxides, which lead to rancidity. Polyunsaturated fatty acids are especially susceptible to oxidative rancidity because of their high number of reactive double bonds. The formation of lipid breakdown products leads to development of undesirable flavors and odors. Those muscle foods with high concentrations of PUFAs (e.g.,fish) typically develop rancid flavors and odors faster than foods with less PUFA. The oxidative rancidity process cannot occur in the absence of oxygen. Thus, vaccum-packaging of meat products provides longer shelf-life by excluding oxygen from the packaging environment. The interaction of oygen with PUFA to cause rancidity is a nonenzymatic process. Lipid oxidation and rancidity may also be caused by enzymatic processes occurring within the muscle food. Enzymic-based lipid oxidation occurs in muscle foods and has also been termed “microsornal lipid oxidation.” “Microsomes” do not constitute a specific cellular organelle but refer to membrane fractions of these (e.g., sarcoplasmic reticulum). This process requires certain biochemical cofactors or activity including reduced forms of nicotine adenine dinucleolide phosphate(NADPH) or nicotine adenine dinucleotide (NADH), adenosine diphosphate (ADP), and iron ions. The enzymatic nature of the process implies involvement of membrane-bound proteins. Cooking of meat provides sufficient heat to denature enzymes, and thus enzymic microsornal lipid oxidation will not occur in cooked meats. During normal physiological functioning, the enzymes found in these subcellular organelle membranes produce chemically reactive substances known as radicals. These are a necessary part of normal cell functioning, and in the “living state” the cell has a variety of mechanisms for protecting itself against the undesirable actions of radicals. In postmortem muscle tissue, many of these protections are lost and radicals may hasten lipid oxidation and cause rancidity Metal Ions and Warmed-Oven Flavor All metal ions are potent catalysis of nonenaymic lipid oxidation. Meat is an excellent source of iro and, although this is a nutritional benefit, the iron can also serve to enhance lipid oxidation in meats. Iron in meat is bound in the heme (heme iron) portion of myoglovin or hemoglobin, or is present as nonheme iron(Nill). Nonheme iron is considered to be the more potent lipid oxidation catalysis. The concentration of nonheme iron can be increased by simply grinding meat through a cast-iron meat grinder, or be cooking. The cooking process provide sufficient heat to denature myogiobin, allowing iron which is bound within the heme molecule to be liberated. Warmed-over flavor (WOF) is a flavor defect that occurs in reheated meat products. The initial cooking of meat increases the NHI concentration within meat. During the time period between initial cooking and reheating , the iron acts to catalyze lipid oxidation (I.e., liftovers); the warm temperatures of the reheating process may also accelerate lipid oxidation. The outcome is that rancid flavors develop, resulting in WOF. The degree to which WOF is detected is dependent on the individual consumer. Warmed-over flavor is a special concern for manufacturers of precooked meat products.Post-Rigor Changes for Predicting Freshness: K value and Nucleotide CatabolismDuring early postmortem metabolism. The supply of ATP within muscle is kept at a high level by regeneration via creatine phosphate, and by glycolysis. As the postmortem condition progress, creatine phosphate reserves become depleted and, as glycolysis slow, ATP production is reduced. The muscle responds physiologically by using an enayme, myokinase, to covert 2 moles of ADP to ATP plus AMP (Fig.3.6). AMP then enters a catabolic (breakdown) pathway in which it is sequentially converted to other compounds including inosine and hypoxanthine. The catabolism continues and both inosine and hyposanthine accumulante during storage of muscle foods. The concentrations of inosine and hypoxanthine, expressed as percentage of total ATP-related compounds present, can be used as an indicator of freshness in muscle foods. The longer a piece of meat has been stored, the greater the relative concentration of hypoxanthine and inosine. This concept has been most extensively applied in fish where a K value is calculated as shown in Fig.3.6. Longer storage times are correlated with higher K values and reduced freshness. The K value at which fish are considered no longer fresh is species dependent. K values have been less thoroughly studied in traditional red meats but theoretically should be just as applicalble as freshness indicators.
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