Agriculture Reference
In-Depth Information
during handling, transportation and processing can be
regarded as a form of spoilage, as can the presence of
insects, other pests and chemicals. Such physically
damaged foods are more susceptible to spoilage by
microbial action.
Fresh meats may be initially contaminated from many
different sources - soil, dust, faeces, water and equipment -
as well as from the hands and clothing of personnel. In
addition, although it was originally thought that the flesh
of healthy animals at slaughter was sterile, it is now known
that it can harbour organisms, mostly Gram-positive mes-
ophiles. Depending on the types of bacteria present, meat-
borne disease or spoilage or both may result, especially if
substandard handling methods are adopted.
The main types of bacteria involved in the spoilage
of meat belong to genera listed below with their
characteristics.
Psychrophilic, for example,
Cl. estertheticum
and
Cl.
gasigenes
. Limit of
a
w
value for growth is about 0.95
with no growth at a pH of less than 4.5. An important
member is
Cl. botulinum
. Clostridia work with
Lactobacillus and Leuconostoc (lactic acid bacteria) to
produce large amounts of gas (H
2
and CO
2
).
10
Arcanobacterium (Actinomyces) - formally Coryne-
bacterium
. Fine, non-sporing rods. Limit of
a
w
value
for growth, 0.98-0.95. No growth below pH of 4.5.
Some strains sensitive to reduced
p
CO
2
.
11
Microbacterium
. Limit of
a
w
value for growth, 0.98-
0.95. No growth below pH 4.5. Psychrotrophic, insen-
sitive to reduced
a
w
and able to spoil meat stored at
chilling temperature with reduced relative humidity.
Many of the organisms tested earlier are able to grow
under reduced and elevated
p
O
2
(partial pressure of
oxygen). The former situation is made use of in vacuum
packaging; the latter along with a reduced initial load
of spoiling organisms, lowered temperature and
a
w
and
increased
p
CO
2
has enabled chilled meat to achieve a
shelf life of over 6 months within modified-atmosphere
packaging.
Gram-positive organisms
1
Brochothrix
. Grows under both aerobic and anaerobic
conditions - optimum temperature 20-25°C - but can
grow at temperatures as low as 0°C and optimum pH 7.0.
2
Carnobacterium
. Grow anaerobically at high CO
2
. Some
can produce antimicrobial peptides and bacteriocin.
3
Micrococcus
. Some are salt tolerant, some thermoduric
and some psychrophilic. Cause spoilage of salted and
chilled meats. Optimal growth temperature 25-30°C.
4
Staphylococcus. Staphylococcus albus
is responsible for
spoilage and
Staph. aureus
for food poisoning. Salt
tolerant. Optimum temperature 37°C but can grow
below this temperature.
5
Streptococcus
, for example,
Str. faecalis
,
Str. faecium
and Str. durans
. Wide temperature range for growth,
10-45°C. Some degree of salt tolerance.
6
Lactobacillus
. Mainly mesophilic with some thermo-
duric and psychrophilic strains.
a
w
limit (water activity)
value for growth −0.91. Can grow at pH of less than 4.5.
7
Leuconostoc
. One of the lactic acid bacteria. Can pro-
duce slimes especially in high-sugar foods. Some are
salt tolerant and some can elaborate flavours due to
diacetyl production. Produced H
2
O
2
which gives
spoiled meat its green discolouration.
8
Bacillus
, for example,
B. subtilis
(mesophilic),
B. ther-
mophilus
and
B. coagulans
(thermoduric). Very active
biochemically with strains that are saccharolytic (able
to split carbohydrates), proteolytic and lipolytic. Some
forms can cause flat sours in canned meats. Limit of
a
w
value for growth is 0.95.
9
Clostridium.
Originate from soil and animal intestine.
Proteolytic and putrefactive, for example,
Clostridium
sporogenes
and
Cl. histolyticum
. Saccharolytic, for
example,
Gram-negative organisms
1
Pseudomonas
. The predominant bacteria associated
with spoiled meat. Widely distributed in soil, freshwa-
ter and seawater and decomposing organic matter.
Grow well in protein foods with the production of
slime, pigments and odours. Preference is for a high
a
w
.
Many are psychrophilic, but temperature range is wide.
2
Flavobacterium
. Pigmented colonies (orange and yellow)
causing discolouration of meat and other foods such as
eggs, butter and milk. Some types are psychrophilic.
3
Acinetobacter
. Able to oxidise ethanol to acetic acid.
4
Achromobacter
. Similar in action to
Pseudomonas
. Forms
slime. Includes
Alcaligenaceae
and
Burkholderiales.
5
Halobacterium
. Obligate halophiles spoiling meats high
in salt content.
6
Moraxella
. Sometimes classified as acinetobacter, for
example,
M. liquefaciens
.
7
Enterobacteria
. Abundant in the soil and intestines of
man and animals. Commonly found in large numbers
in raw foods of animal origin and also in cooked foods
that have been contaminated in various ways.
E. coli is
indicative of faecal or sewage pollution. Spoilage of
meat by fermentation of carbohydrates to acid and gas
causing 'off ' odours. The verotoxic strain
E. coli
O157:H7 is an important food-poisoning bacterium.
Includes
Klebsiella
. Non-motile, non-sporing rods and
the pathogenic
Salmonella
,
Shigella
and
Proteus
belong
to the same group.
Cl. perfringens
and
Cl. butyricum
.
