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Lecture 10
Physical and Chemical Control
of Microorganisms
History
• First microbial control practices developed by
Ignatz Semmelweis in 1800s
• Some techniques developed:
– Hand washing with chloride of lime
– Aseptic technique during surgery
Microbial Control
• Sterilization: removal or destruction of all forms of
microbial life
• Disinfection: aimed at destroying harmful
microorganisms
• Antisepsis: disinfection directed at living tissue
• Degerming: mechanical removal of most of the
microbes in an area
• Sanitation: intended to lower microbe counts to safe
public health levels
Rate of Microbial Death
• When bacterial populations are heated or treated
with antimicrobial agents they die at a constant rate
• Factors influencing the effectiveness of microbial
treatments:
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# of microbes
Environmental influences
Time of exposure
Microbial characteristics
Physical Methods of Microbial Control
Physical methods of microbial control
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Heat
Filtration
Refrigeration
Radiation
Dessication
Osmotic Pressure
Heat
• Kills microbes by denaturing their enzymes
• Thermal Death Point:
• Thermal Death Time:
• Three types:
– Moist heat, Pasteurization, Dry Heat
Moist Heat
• Denatures proteins
• Boiling: kills bacteria, viruses, fungi and their
spores within 10 minutes
– Endospores and some viruses are not easily killed
• Autoclave: uses steam under pressure
Pasteurization
• High temperature used for a short period of
time to destroy pathogens without altering
the flavor of the food
Dry Heat Sterilization
• Flaming- heat wire to red glow
• Hot-air sterilization- place items in oventemperature 170oC for 2 hours ensures
sterilization
Filtration
• Passage of a liquid or gas through a filter with
pores small enough to retain microbes
• Microbes can be removed from air by highefficiency particulate air filters
• Membrane filters are commonly used to filter
out bacteria, viruses, even large proteins from
liquids
Refrigeration
• Different types of bacteria have optimum
growth temperatures
• Those that are infectious to humans generally
grow best at human body temperature
• By putting something in fridge you are slowing
the growth of these organisms
Radiation
• Effects of radiation depend on wavelength,
intensity, and duration
• Ionizing Radiation- ionizes water, forms highly
reactive hydroxyl radicals
• Non-ionizing Radiation- UV light damages DNA
• Microwaves- do not have much effect on
microorganisms
Dessication
• Dessication: absence of water
• Microorganisms cannot grow or reproduce but
can remain viable for years
• Viruses and endospores can generally resist
desiccation
Osmotic Pressure
• Microorganisms at high concentrations of salt
and sugars undergo plasmolysis
• Molds and yeasts are more capable than
bacteria of growing in materials with low or
high osmotic pressure
Chemical Methods of Microbial
Control
Antiseptics and Disinfectants
• The agent should be able to:
– Kill or slow growth of microbe
– Non-toxic to humans and animals
– Soluble in water, good shelf life
– Useful in diluted form
– Able to perform job in short time
Antiseptics and Disinfectants
• Several parameters should be considered:
– Temperature
– pH
– Stability
Halogens
• Oxidize proteins
• Examples: Chlorine and Iodine
Phenol and Phenolics
• Denature Proteins
• Phenol- strong odor; caustic to skin
• Phenolics used more often
Heavy Metals
• Interfere with microbial metabolism
• Believed to bind protein molecules, making
them unusable
Alcohols
• Denature proteins and disrupt membranes
• Preferred alcohol- ethyl alcohol
Soaps and Detergents
• Act as surface active agents
• Not bacteriocidal
Hydrogen Peroxide
• Breaks down into toxic form of oxygen
• Not good on open wounds
Antiseptics in your pantry
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Cinnamon
Garlic
Honey
Wasabi
Figure 7.11