The Control of Microbial Growth •
Physical and Chemical Control of
Microbes
Chapter 7
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Control of microbes
Physical and chemical methods
to destroy or reduce microbes
in a given area
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STERILIZATION
• All killed
• non-selective
Microbial death
Involves permanent loss of reproductive •
capability, even under optimum growth
conditions
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Mode of action of Microbial control agents
Alteration of membrane permeability •
( lipid & proteins)
Damage to proteins and Nucleic acids •
(cellular proteins
replication)
&
DNA •
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Physical Methods of Control
Heat •
Filtration •
Radiation •
Physical Methods of Control
HEAT
Mode of action: Denaturing proteins.
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HEAT
Mode of action: Denaturing proteins
Moist heat
Denaturation of proteins
Dry heat
Oxidation or coagulation of proteins
Boiling = 100C
Flaming
Pasteurization less than 100C
Hot air oven
Autoclaving more than 100C
Physical Methods of Control
HEAT
Moist heat (Kills by denaturation of proteins) •
= boiling •
= pasteurization •
= autoclaving (Steam under •
pressure)
Dry heat
=
flaming •
= hot air oven •
•
Pasteurization
Eliminate pathogenic microbes 
Lowering microbial count and 
prolong shelf life. without altering
the flavor
(HTST) High temperature at short 
time
140 oC for 3 second (dairy
products) store several month
without refrigeration
63  C for 30 minutes ordinary 
72  C for 15sec
HTST 
UHT Ultra high temperature, 
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Steam under pressure
Autoclaving
The higher the pressure the higher the •
temperature15 min at 121 C at 15 psi
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Dry heat using higher temperatures than
moist heat, can also sterilize
Flaming ( oxidation) 
Dry ovens – 150-180oC- 2 hours 
(coagulate proteins & oxidation)
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Using Heat to Control Microbes
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Filtration
Is the passage of liquid •
or gas through filters with pores small
enough
to retain bacteria
Membrane filters
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Membrane filters
pores
bacteria
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Radiation
Effect on cells varies according to its wavelength,
duration and intensity.
Ionizing and non ionizing radiation. •
Ionizing radiation:
Gamma rays: x-rays & high –energy electron •
beams = has a high degree of penetration
•
used to sterilize medical supplies & food –
products
Non-ionizing radiation •
- little penetrating power to sterilize air, water & solid
surfaces
UV light which interfere with DNA replication –
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RADIATION EFFECT
Oxidation of DNA •
effect of ionizing
radiation
Interferes with DNA •
replication
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Chemical control
Chemical agents that used on living
tissues as antiseptics
And on non living objects as
disinfectants
Halogens .1
Phenolics .2
Chlorhexidine .3
Alcohols .4
Hydrogen peroxide .5
Detergents & soaps .6
Heavy metals .7
Aldehydes .8
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