2. Factors Influencing Growth of
Microorganisms in Food
Food Microbiologists must have through
understanding of the factors which influence
microbial growth
The ability of microorganisms to grow or
multiply in a food is determined by the food
environment as well as the environment
where the food is stored.
3. • Many parameters can affect the growth
and survival of microorganisms. These
parameters are divided into:
1- INTRINSIC
• Nutrient Content
• pH and organic acids
• Water Activity
• Biological Structures
5. Intrinsic Parameters
Nutrient Content
The chemical composition of a food influences
the type of microorganisms that will grow and the
products that they will produce during growth.
They use organic compounds as energy and
carbon sources.
Food is rich in nutrients.
Meat are rich source of vit. B and fruits are low
but fruits are rich in ascorbic acid.
Egg white contain biotin but also contains Avidin
which ties up biotin making it unavailable to
micro-organisms & eliminating possible spoilage
organism those which must have biotin supply.
6. pH and organic acids
Yeast & molds are acid tolerant than bacteria.
Some foods have low pH because of inherent
acidity. For ex. Fermented product because
of lactic acid during fermentation.
Vegetable juices have low buffering power so
decrease in pH with production of only small
amount of acid by Lactic acid bacteria.
Milk is high in protein & permits growth
& acid production in LAB in the
manufacture of fermented milk.
7. Water Activity
All microorganisms require water in an
available form to grow and metabolize.
Availability of water is measured by water
activity (aw).
aw= Vapor pressure of food substrate/ Vapor
pressure of pure water at the same
temperature.
aw of pure water is 1.0.
The aw of a food can be reduced by increasing
the concentration of solutes in the aqueous
phase of the food.
8. Yeasts and molds can tolerate lower aw than
bacteria.
Gram-negative bacteria require higher a w than
Gram-positive bacteria.
Some of the aw value
o 0.98 & above – fresh meat, fish, fruits,
vegetables
o 0.93-0.98 - tomato paste, bread, cheese
o 0.85-0.93 – beef, condensed milk
o 0.60-0.85 - nuts, jam, jellies
o Below 0.60 – chocolate, honey, potato chips,
biscuits.
9. Biological Structures
Outer barriers against the invasion of microorganisms
(E.g the skin of fruits and vegetables form a
protective layer to invasion by microorganisms).
Damages during harvesting pr processing (peeling,
skinning, chopping) expose tissues and increase
microbial loads throughout the product.
Milk has no protective barrier.
Ground meat spoils faster than whole meat
cuts(grinding distributes surface microorganisms
throughout).
Eggs are usually sterile inside but heavily
contaminated on the shell, crack in the shell allows
microbes to enter.
10. Extrinsic Parameters
Relative humidity
When the food commodity have low water activity, they
are stored in atm. of high relative humidity.
Storage of fresh vegetables require control of relative
humidity & if it too low than many of the vegetables will
loose water & become flaccid.
Temperature
Microorganisms grow over a wide range of
Temperatures.
For ex. Psychrotrophs – grow well below 7 C, optimum
at 20-30 C
• Mesophiles – grow well 20-45 C, optimum at 30-40 C
11. • Thermophiles – grow well at and above 45 C,
optimum at 55-65 C.
Gaseous Environment
Oxygen comprises 21% of earth & is the most
important gas in contact with food.
