Though numerous species are used as a source of meat around the
world, ranging from flying foxes to frogs and from kangaroos to croc-
odiles, the meat animals of principal importance in economic terms are
cattle, pigs, sheep, goats and poultry.
5.3.1 Structure and Composition
Edible animal flesh comprises principally the muscular tissues but also
includes organs such as the heart, liver, and kidneys. Most microbio-
logical studies on meat have been conducted with muscular tissues and it
is on these that the information presented here is based. Though in many
respects the microbiology will be broadly similar for other tissues, it
should be remembered that differences may arise from particular aspects
of their composition and microflora.
Structurally muscle is made up of muscle fibres; long, thin, multinu-
cleate cells bound together in bundles by connective tissue. Each muscle
fibre is surrounded by a cell membrane, the sarcolemma, within which
are contained the myofibrils, complexes of the two major muscle pro-
teins, myosin and actin, surrounded by the sarcoplasm. The approximate
chemical composition of typical adult mammalian muscle afterrigor
mortisis presented in Table 5.5. Its high water activity and abundant
nutrients make meat an excellent medium to support microbial growth.
Though many of the micro-organisms that grow on meat are proteolytic,
they grow initially at the expense of the most readily utilized substrates—
the water soluble pool of carbohydrates and non-protein nitrogen.
Extensive proteolysis only occurs in the later stages of decomposition
when the meat is usually already well spoiled from a sensory point of
view.
The carbohydrate content of muscle has a particularly important
bearing on its microbiology. Glycogen is a polymer of glucose held in
the liver and muscles as an energy store for the body. During life, oxygen
is supplied to muscle cells in the animal by the circulatory system and
glycogen can be broken down to provide energy by the glycolytic and
respiratory pathways to yield carbon dioxide and water. After death the
supply of oxygen to the muscles is cut off, the redox potential falls and
respiration ceases, but the glycolytic breakdown of glycogen continues
leading to an accumulation of lactic acid and a decrease in muscle pH.
Provided sufficient glycogen is present, this process will continue until the
glycolytic enzymes are inactivated by the low pH developed. In a typical
mammalian muscle the pH will drop from an initial value of around 7 to
5.4–5.5 with the accumulation of about 1% lactic acid. Where there is a
limited supply of glycogen in the muscle, acidification will continue only
until the glycogen runs out and the muscle will have a higher ultimate
pH. This can happen if the muscle has been exercised before slaughter
132 Microbiology of Primary Food Commodities