of the surface tissue. With high respira-
tion rates, reduction to myoglobin
occurs, correlating with similar changes
under controlled oxygen atmospheres.- Pigment oxidation and reduction are
controlled by adjustment of oxygen
level in the storage atmosphere with a
light load of microorganisms. - Agents inhibiting high oxygen uptake
rates in exposed tissues preserve color
under atmospheric conditions but are
ineffective under low oxygen pressures.
These observations result in the conclu-
sion that the reduction of oxygen in muscle
tissue by microbial growth or by physical
effects can produce an increase in reduced
myoglobin through oxidation by metabolic
hydrogen peroxide produced by muscle tis-
sue or by bacteria. With oxygen tension
reduced to a low enough level, hydrogen per-
oxide formation is nil, and no oxidation will
occur. This condition indicates that the dis-
sociation of the oxy compound increases as
oxygen tension decreases. Fresh meat pig-
ments are more vulnerable to discoloration
at oxygen tensions below that of air at
atmospheric pressure.
Clearly, the growth of bacteria from poor
sanitation contributes to muscle color degra-
dation through reduced oxygen concentra-
tion and ultimate discoloration. Various
genera and species of microorganisms differ
in their effect on pigment alteration; how-
ever, improved cleanliness can delay the
development of high numbers of microbes.
Those who handle meat should strive to min-
imize the initial microbial load.
Meat and Poultry Contamination
During the slaughter, processing, distribu-
tion, and foodservice cycle, food items are
handled frequently-often as many as 18 to 20
times. Because almost anything contracting
meat and poultry can serve as a contamina-
tion source, the risk of this condition occur-
ring rises each time these products are
handled.
When alive, a healthy animal possesses
defense mechanisms that counteract the
entrance and growth of bacteria in the mus-
cle tissue. After slaughter, the natural
defenses break down, and there is a race
between humans and microbes to determine
the ultimate consumer. If the handling is
careless and ineffective, the microbes win.
Those involved with sanitation must create a
less favorable environment for the microor-
ganisms. (Chapter 5 discusses contamination
sources during slaughter and processing.)
Approximately 1 billion microorganisms
are contained in a gram of soil attached to
the hide of a live animal. A gram of manure
contains approximately 220 million micro-
bes. Sticking knives contaminated with bac-
teria introduce contamination through the
wound. An animal's heart may beat for 2 to
9 minutes after sticking, thereby permitting
thorough distribution of microbes. Unwas-
hed animals have approximately 155 million
microorganisms/cm^2 of skin where the jugu-
lar vein is cut.
Although the temperature of a scalding
vat is approximately 60ºC, the microbial load
is approximately 1 million bacteria per liter
of water. The dehairing operation for hogs is
responsible for microorganisms being beaten
into the surface skin.
Contamination during evisceration of ani-
mals is increased because the stomach and
intestinal contents are loaded with microor-
ganisms. A major contamination source for
meats in the slaughterhouse is rumen fluid,
which averages 1.3 billion microorganisms
per milliliter.
Carcass surface counts of microorgan-
isms average 300 to 3,000/cm^2. Beef and
pork trimmings contain 10,000 to 500,000
bacteria per gram, depending on contami-
nation and sanitation practices, cutting