Download Factors affecting the growth of microorganisms

Factors affecting the growth
of microorganisms
Factors affecting the growth of M.O
divided to
A) Intrinsic factors
Factors inherent to food itself
1-Water activity
2- pH
3- Oxidation reduction potential
4- Nutrient content
5- Antimicrobial constituents
6-Biological structure
Factors affecting the growth of M.O
divided to
B) Extrinsic factors
Factors inherent to the environment
1- Temperature
2- Gaseous environment
3- Relative humidity
4- Microbial interaction.
Water activity (aw)
Vapor pressure of water divided by that of pure
water at the same temperature; therefore, pure
distilled water has a water activity of exactly
one.
aw = vapor pressure of food substrate (solution)
vapor pressure of pure water
Related to relative humidity: RH= 100 x aw
Substance
Distilled Water
Tap water
Raw meats
Milk
Juice
Saturated NaCl solution
Point at which cereal loses crunch
Honey
Dried fruit
aw
1
0.99
0.97 - 0.99
0.97
0.97
0.75
0.65
0.5 - 0.7
0.5 - 0.6
Total moisture of a food product =
free moisture (available for bacterial use) +
bound moisture (moisture that is unavailable
for bacterial use).
Bound moisture in foods can be increased by
(Reducing the aw):
1. Formation of gels or gums. Add flour,
pectin, etc.
2. Addition of solutes (salt, CHO)
aw requirements for microbial growth

Each microorganism has its own minimum
water activity value below which it will not
grow.
bacteria > yeast > mold
Microorganism Inhibited
aw
Clostridium botulinum E
Pseudomonas fluorescens
Escherichia coli
0.97
0.97
0.95
Salmonella
Vibrio cholerae
Bacillus cereus
Listeria monocytogenes
0.95
0.95
0.93
0.92
Staphylococcus aureus
Most Fungi
No microbial proliferation
0.87
0.70
0.60
Equilibrium relative humidity (ERH)

At constant temperature the moisture
content of a food changes until it comes
into equilibrium with the water vapor in
the surrounding air.

Proper storage (packaging) of these foods
is required in order to prevent mold
growth.
Microorganisms growing at marginal
aw (high osmotic pressure)
1. Halophiles:
 Tolerate or require salt ( Na+, K+) for
growth.
 Many originate from marine (hypersaline)
environments.
2. Osmophiles:
• Grow in foods with high osmotic
pressure
• Sugars syrups, honey, soft drinks, candy,
jams jellies etc.
• Most are yeasts.
3. Xerophiles:
• Dry loving. Grow to an (aw = 0.65).
• Many are fungi particularly storage
fungi.
Intermediate moisture foods
(IMF)
–Have
moisture content 15-50% (aw 0.6-0.85)
–Shelf stable at room temp 20-22C.
–Includes jams, sausages, soft centered
candies, dried fruit
–Moisture has been withdrawn via drying
and/or addition of ( humectants) solutes
including sugars, salts ( glycerol, sorbitol,
sucrose).
Intermediate moisture foods
(IMF)
–Drying
food down to a level where both M.O
and enzymatic activity is halted or reduced.
–Most important spoilage organisms are yeast
and molds
–IMF contain fungistats: sorbate and benzoic
acids
Proper storage (packaging) of these foods is
required in order to prevent mold growth.
Preservation By Drying
Microbial growth and enzymatic activity
depend on free water
 M.O need water for nutrient transport, nutrient
metabolism and removal of cellular waste
 Drying food down to a level where both MO
and enzymatic activity is halted or reduced
 For all microbial growth (aw< 0.60)

Water is Removed:
Under natural field conditions
– grains, raisins, seeds
 Cooking/baking
 Controlled dehydration processes

treatment of foods prior to drying
includes:
1- Peeling, cutting, trimming of fruits and
vegetables: may reduce microbial load
2- Light colored fruits and vegetables may be
treated with SO2 (1,000-3,000 ppm).
Reduces enzymatic browning by
polyphenol oxidase; also reduces MO on
surface. Preserves vitamin C
3-Blanching or scalding ( 80 -100C) of
vegetables is performed to inactivate some
of the enzymes that could be problematic
during drying/storage of the product
Most meats are cooked prior to
dehydration ( chicken cubes used in
dried soup mixes)
Eggs (albumen) are usually desugared
prior to drying in order to prevent
browning
Dehydration Principle:
Apply Heat / Remove Water
 Enhanced by:
– Increased surface area
– Increased temperature
– Increased air velocity
– Reduced humidity
– Reduced pressure (vacuum)

Spoilage Of Dried Foods
dried foods do not undergo microbial
spoilage
 chemical changes occur in foods containing
fat and oxygen (oxidative rancidity)
 reducing sugars ( carbonyl group) in dried
foods react with the amino group of amino
acids and proteins  Maillard reaction
( non enzymatic browning) bitterness


loss of vitamins ( C), discoloration and
structural changes can occur during
drying/storage
Freeze-Drying



It is a combined method of preservation
based on the dehydration of food in the
frozen state through vacuum sublimation
of its ice content.
The moisture content is reduced to a level
2-8% or aw 0.10-0.25
It should be packed in an inert atmosphere
in vapor-impermeable packaging