64
Modern Food Microbiology
external carcass biota to become normalized among carcasses, although a few days may be required.
The practical effect of this is the predictability of the biota of such products at the retail level.
BIOCHEMICAL EVENTS THAT LEAD TO RIGOR MORTIS
Upon the slaughter of a well-rested beef animal, a series of events takes place that leads to the
production of meat. Lawrie
106 discussed these events in great detail, and they are presented here only
in outline form. The following are stages of an animal’s slaughter:
1. Its circulation ceases: the ability to resynthesize ATP (adenosine triphosphate) is lost; lack of
ATP causes actin and myosin to combine to form actomyosin, which leads to a stiffening of
muscles.
2. The oxygen supply falls, resulting in a reduction of the O/R (oxidation–reduction) potential.
3. The supply of vitamins and antioxidants ceases, resulting in a slow development of rancidity.
4. Nervous and hormonal regulations cease, thereby causing the temperature of the animal to fall
and fat to solidify.
5. Respiration ceases, which stops ATP synthesis.
6. Glycolysis begins, resulting in the conversion of most glycogen to lactic acid, which depresses
pH from about 7.4 to its ultimate level of about 5.6. This pH depression also initiates protein
denaturation, liberates and activates cathepsins, and completes rigor mortis. Protein denaturation
is accompanied by an exchange of divalent and monovalent cations on the muscle proteins.
7. The reticuloendothelial system ceases to scavenge, thus allowing microorganisms to grow
unchecked.
8. Various metabolites accumulate that also aid protein denaturation.
These events require between 24 and 36 hours at the usual temperatures of holding freshly slaughtered beef (2–5
◦ C). Meanwhile, part of the normal biota of this meat has come from the animal’s own
lymph nodes,
109 the stick knife used for exsanguination, the hide of the animal, intestinal tract, dust,
hands of handlers, cutting knives, storage bins, and the like. Upon prolonged storage at refrigerator
temperatures, microbial spoilage begins. In the event that the internal temperatures are not reduced
to the refrigerator range, the spoilage that is likely to occur is caused by bacteria of internal sources.
Chief among these are Clostridium perfringens and genera in the Enterobacteriaceae family.
90 On
the other hand, bacterial spoilage of refrigerator-stored meats is, by and large, a surface phenomenon
reflective of external sources of the spoilage biota.
90
THE BIOTA OF MEATS AND POULTRY
The term “biota” is used throughout this text in lieu of “flora” as a general reference to bacteria.
Flora refers to plant life. “Bacterial flora” dates back to the time when it was believed that bacteria were
primitive plants. Since bacteria are not plants, “bacterial biota or microbiota” is preferred to flora. The
major genera of bacteria, yeasts, and molds that are found in these products before spoilage are listed in
Tables 4–1 and 4–2. In general, the biota is reflective of the slaughtering and processing environments as
noted above, with Gram-negative bacteria being predominant. Among Gram-positives, the enterococci
are the biota most often found along with lactobacilli. Because of their ubiquity in meat-processing
environments, a rather large number of mold genera may be expected, including Penicillium, Mucor,
Modern Food Microbiology
external carcass biota to become normalized among carcasses, although a few days may be required.
The practical effect of this is the predictability of the biota of such products at the retail level.
BIOCHEMICAL EVENTS THAT LEAD TO RIGOR MORTIS
Upon the slaughter of a well-rested beef animal, a series of events takes place that leads to the
production of meat. Lawrie
106 discussed these events in great detail, and they are presented here only
in outline form. The following are stages of an animal’s slaughter:
1. Its circulation ceases: the ability to resynthesize ATP (adenosine triphosphate) is lost; lack of
ATP causes actin and myosin to combine to form actomyosin, which leads to a stiffening of
muscles.
2. The oxygen supply falls, resulting in a reduction of the O/R (oxidation–reduction) potential.
3. The supply of vitamins and antioxidants ceases, resulting in a slow development of rancidity.
4. Nervous and hormonal regulations cease, thereby causing the temperature of the animal to fall
and fat to solidify.
5. Respiration ceases, which stops ATP synthesis.
6. Glycolysis begins, resulting in the conversion of most glycogen to lactic acid, which depresses
pH from about 7.4 to its ultimate level of about 5.6. This pH depression also initiates protein
denaturation, liberates and activates cathepsins, and completes rigor mortis. Protein denaturation
is accompanied by an exchange of divalent and monovalent cations on the muscle proteins.
7. The reticuloendothelial system ceases to scavenge, thus allowing microorganisms to grow
unchecked.
8. Various metabolites accumulate that also aid protein denaturation.
These events require between 24 and 36 hours at the usual temperatures of holding freshly slaughtered beef (2–5
◦ C). Meanwhile, part of the normal biota of this meat has come from the animal’s own
lymph nodes,
109 the stick knife used for exsanguination, the hide of the animal, intestinal tract, dust,
hands of handlers, cutting knives, storage bins, and the like. Upon prolonged storage at refrigerator
temperatures, microbial spoilage begins. In the event that the internal temperatures are not reduced
to the refrigerator range, the spoilage that is likely to occur is caused by bacteria of internal sources.
Chief among these are Clostridium perfringens and genera in the Enterobacteriaceae family.
90 On
the other hand, bacterial spoilage of refrigerator-stored meats is, by and large, a surface phenomenon
reflective of external sources of the spoilage biota.
90
THE BIOTA OF MEATS AND POULTRY
The term “biota” is used throughout this text in lieu of “flora” as a general reference to bacteria.
Flora refers to plant life. “Bacterial flora” dates back to the time when it was believed that bacteria were
primitive plants. Since bacteria are not plants, “bacterial biota or microbiota” is preferred to flora. The
major genera of bacteria, yeasts, and molds that are found in these products before spoilage are listed in
Tables 4–1 and 4–2. In general, the biota is reflective of the slaughtering and processing environments as
noted above, with Gram-negative bacteria being predominant. Among Gram-positives, the enterococci
are the biota most often found along with lactobacilli. Because of their ubiquity in meat-processing
environments, a rather large number of mold genera may be expected, including Penicillium, Mucor,