MICROBIOLOGY OF MEAT
• Meat is easily spoiled by microorganisms because it contains a lot
of nutrient, growth factors etc. the pH for meat is the range of
• Microorganism for soil, water and manure make up the dominant
flora of meat. During slaughter, the external surface of the animal
may contaminate the meat by direct contact through the above
sources and equipment, personnel and slaughtering area.
• Bacteria generally found in meat:
a. pathogenic microorganism
E.g. Brucella, Salmonella, Streptococcus, Mycobacterium
b. spoilage microorganisms
E.g. Acromobacter, Pseudomonas, Bacillus and
E.g. Aspergillus, Mucor, Penicillium
• factors affecting the spoilage of meat
a) water content
Meat has high water content with dissolved substances
such as glycogen, lactic acids and amino acids. All these
substance can cause microbial growth which can lead to
b) Redox potential
Has a big influence on microbes. Tissue respiration
continues after death and uses up oxygen present and
produce CO2 . Finally oxygen tension will fall (Eh
negative). The inner side of meat becomes anaerobic;
together with the production of lactic acids. The bulk of
meat becomes anaerobic except on the surface. At the
surface, aerobic flora and inside meat, anaerobic flora
predominates. But, these microorganisms do not grow
readily at low temperature, therefore little growth may be
c) pH value
pH value of the meat is between 5.5-7.4. Acidic pH (5.5)
not suitable for growth of many types of
microorganisms. pH of meat depends on amount of lactic
acid produced by glycolysis in muscle after death. The
more acid produced the lower the pH. But, the amount of
acid depends on the amount of glycogen in the muscle at
Glycogen ↑ Lactic Acid ↑ pH ↓
i. If animal is exercised before slaughter, the
glycogen is low, little lactic acid produced and
the pH of carcass is relatively high. Muscle will
be dry and firm in texture and dark in color
ii. If non-exercised animal is stimulated before
slaughter such as stress, glycogen will rapidly
turn to lactic acid and low pH is approached
before tissue has time to cool. This cause
denaturation of some, muscle protein. Meat will
become pale, soft and exudative (PSE
• The type and spoilage of meat is determined by:
i. Initial numbers present
ii. pH value of meat – if pH > 6.0, amino acid is attacked.
Microorganisms can grow fast. Meat will putrefy mainly due
to Pseudomonas and Aeromonas type.
• Two types of spoilage: Aerobic condition
• Aerobic condition
a) Surface slime
Accumulation of microorganism on meat surface cause by
gram-negative rods such as Pseudomonas and Achromobacter
b) Colour change
Occur more quickly if stored in oxygen. Red coloured meat
(oxidized myoglobin) can change to brown due to production
of oxidizing compound such as peroxides or H2S from bacteria
e.g. Leuconostoc spp.
• Red spots on meat – pigments of Serratia
• Yellow spots – pigments of Flavobacterium
• Green spots – pigments of Pennicilium
• White spots – pigments of Rhizopus
c) Off flavors and odours
Due to formation of fatty acids on the meat surface such as
formic, butyric and propionic acid.
It can also due to production of compounds such as amines,
ammonia caused by degradation of amino acid by proteolytic
bacteria such as Pseudomonas spp whereas Actinomycetes will
give an earthy odour.
Caused by some lipolytic bacteria and molds
• Anaerobic condition
Breakdown of protein with the formation of foul smelling
compounds such as mercaptans, H2S, indol, amines and
others. This occurs inside the meat such as Proteus,
Clostridium and Bacillus
Can be microbial and biochemical. Microbial souring is due
to the formation of fatty acids and lactic acids, with no
putrefaction. E.g. Clostridium and Bacillus
MICROBIOLOGY OF MILK
• Milk and its product contain a large amount of nutrient. Excellent
media for growth of spoilage microorganisms.
• Before milking, milk is sterile. As soon as it comes out of the
udder, it becomes contaminated. Contamination comes from
milking utensils, air, personnel etc.
• The number of microorganisms present depends on how the milk
is handled. Aseptic handling is important in reducing
• Milk collected should be immediately put in tank, then put in cold
storage, pasteurize and refrigerate. Make sure aseptic condition
after pasteurization otherwise post pasteurization contamination
can be occur.
• Examples of microflora in milk:
a. Pathogenic – Campylobacter fetus, Listeria
monocytogenes, Clostridium perfringens
b. Spoilage – Pseudomonas spp, Aeromonas spp.,
Streptococci, Acinetobacter and Flavobacterium
• Preservation is commonly done by pasteurization and
• Milk spoilage
Milk is an excellent media for microbial growth. If the
preservation methods are inadequate, different type of spoilage
1) Souring of milk
Milk considered spoilt if it curdles or sours. This is due to
the formation of acid caused by lactic acid bacteria.
At room temp. : Streptococcus lactis
2) Gas production
During fermentation, acid and gas produced, formation of
foam on surface can be seen. If too much gas, it cause
stormy fermentation. The gas ripped the curd apart, which
produces gas. Example: Clostridium, Bacillus, yeast and
heterofermentative lactic acid bacteria.
Hydrolysis of milk protein with the production of peptides
causing bitter flavor. Proteolysis can occur if:
Milk store at low temperature
When acid forming bacteria are destroyed due to
heating, leaving only the proteolytic ones (no
Types of proteolysis can be divided into 4 categories:
a. Acid proteolysis-acid production occurs together
b. Sweet curdling-early stage curdling caused by
rennin like enzymes produced by bacteria.
c. Slow proteolysis-caused by endoenzymes of
bacteria which undergo autolysis. Important in
cheese making and maturation.
d. Residual proteolysis-caused by heat stable
Proteolytic microorganisms are: Micrococcus,
Proteus and Pseudomonas spp.
There are two types of bacterial ropiness:
a. Surface – slimy material on surface of food. Develop
best at refrigeration temperature.
Examples: Alcaligenes viscolactis
b. Throughout – the whole milk becomes ropy. Caused
by Alcaligenes spp. and Strep. Lactis. These
microorganisms reproduce within capsule and form
Ropiness can be reduced if acidity is increased. Acid
usually produced at mesophilic temperature.
5) Alkali production
Due to formation of ammonia from urea, carbonates or
organic acids. Caused by Pseud. fluorescens, Pseu. trifolil,
Alcaligenes faecalis, Alcaligenes viscolactis
6) Colour change
May be due to surface growth of pigmented
MICROBIOLOGY OF FRUITS AND VEGETABLES
• 25% of fruits and vegetables are spoilt before harvesting because
of infection and disease.
• Spoilage after harvesting is due to mishandling, over-ripening etc.
Skin becomes soft, makes it easier to be infected by
• Fruits are covered with skin/shell for:
a. Natural protection from microbial infection.
b. Minimize moisture loss to environment.
c. Stabilize enzymatic changes.
• Healthy fresh fruits are free from microorganisms. Once skin is
broken e.g. insect sting, it will become infected especially if put
in place where sanitary condition is not satisfactory.
• The kind of microorganisms in fruits depend on various factors:
a. Characteristics of fruit – soft ones more prone to infection.
b. Nutrient content – sugar and acid.
c. Climatic condition.
• Bacteria do not play important role in fruit spoilage due to acidity
• Mold spoilage usually more significant.
• Yeast usually initiate spoilage then later taken over by molds.
Spoilage by yeast is not extensive.
• Spoilage of fruits:
a. Blue mold rot: Penicillium expansum
b. Black mold spoilage: Aspergillus spp
c. Green mold spoilage: Cladosporium spp.
d. Bacteria soft rot: Xanthomonas, Erwinia and Pseudomonas
• Vegetables are easily spoilt, just like fruits.
• Examples: 40%tomatoes are already spoilt before consumption.
• Through genetic engineering, new hybrid of tomato is produced
to reduce spoilage. Example: flav sav tomato.
• Vegetable contain more available water and less carbohydrate or
fat content compared to fruits. High pH and the less acidic
content are favoured by bacteria.
• Spoilage of vegetables:
a. Bacteria soft rot – the vegetables part becomes soft and moist.
These bacteria produce pectinase which can hydrolyze pectin
such as in tomato, garlic, celery etc.
b. Mold spoilage to vegetables:
Grey mold rot – Botrytis spp.
Sour rot – cause formation of acid. Geotricum candidum.