2. Factors Influencing Growth of
Microorganisms in Food
ï‚® Food Microbiologists must have through
understanding of the factors which influence
microbial growth
ï‚® 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:
ï‚® 1- INTRINSIC
• Nutrient Content
• pH and organic acids
• Water Activity
• Biological Structures
5. Intrinsic Parameters
ï‚® 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. ï‚® 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. ï‚® 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. ï‚® 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
ï‚® 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.
ï‚® 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.
ï‚® Gaseous Environment
 Oxygen comprises 21% of earth & is the most
important gas in contact with food.