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CHAPTER 9
CONTROL OF MICROBIAL GROWTH
Terminology of Microbial Control
• Sepsis/ Asepsis
– Aseptic techniques
• Sterilization
• Commercial Sterilization
• Sanitization
• Antisepsis vs Disinfection
– Biocide vs Biostatic
• Degerming
• Chemotherapy
Factors that influence the effectiveness of antimicrobial procedures:
1. microbial characteristics
2. number of microbes
– Decimal reduction time (D value)
3. environmental influences
4. time of exposure
5. concentration or intensity of treatment
Physical or Mechanical Methods
1.
2.
3.
4.
5.
6.
Heat (moist & dry)
Filtration
Low temperature
High pressure
Desiccation & Osmotic pressure
Radiation
1. Heat
•
Moist Heat
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–
–
•
Boiling kills most vegetative forms in 10 min
Autoclave – steam under pressure
– Sterilization - 15 psi, 121OC for 15 min
Pasteurization
– Standard methods do not sterilize
– higher temp, shorter exposure time
Dry heat
•
•
Incineration
• Flaming
dry heat sterilization
• Requires higher temp and longer exposure
• 170°C for 2 hours is equivalent to autoclaving
2. Low Temperatures
–
–
–
Slows metabolic rate
Freezing forms ice crystals that can damage cells
Slow freezing then thawing does most damage
3. Filtration
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–
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Mechanical sterilization of heat-sensitive material
HEPA filters (high efficiency particulate air filters)
Membrane filters have pores as small as 0.01
microns
4. High Pressure
–
–
–
–
Up to 130,000 psi
Endospores are resistant
Commercially pasteurize foods
Preserves flavor, taste, appearance, and
nutritional value
5. Desiccation
–
Resistance varies by species
• can’t reproduce or grow, but may remain viable
–
Osmotic pressure
• create a hypertonic environment; dehydrates
cells
• Preserved fruits (sugar), cured meat (salt), and
pickles (salt)
6. Radiation
• depends on wavelength, intensity and duration
• Ionizing vs non-ionizing
• Sterilization of food products and disposable
medical equipment; water treatment
Evaluating Anti-microbial Compounds
•
Phenol coefficient test
•
Use-dilution test
–
Current industry standard
Disk-Diffusion Method
Types of Anti-microbial Chemicals
– Phenol (carbolic acid)
•
•
•
Rarely used, irritating qualities and bad odor
Penolic compounds more common
Disinfect for surfaces (Lysol) and antiseptic
(antimicrobial soaps and lotions)
– Halogens
•
Iodine – antiseptic or disinfectant
– May be available as a tincture or iodophore
•
Chlorine - disinfectant (water treatment)
– Alcohols
• Affect vegetative forms of bacteria, fungi, and
enveloped viruses
• ethanol and isopropanol - degermer (hand
sanitizer) and disinfectant
• 70% solution typically is most effective
– Heavy metals
• Silver; mercury; copper
• antiseptic, algicide, mildew control paints,
placed in newborn’s eyes to prevent spread of
gonorrhea
– Surfactants
• Soap
– Emulsification; degerming
– Least effective of all chemical methods
• Anionic Detergents
– Commercial sanitizers
• Cationic Detergents
– disinfectant and antiseptic
– Peroxygens
• Disinfectants
• Ozone – supplements chlorine in water
treatment
• Hydrogen peroxide – good disinfectant not
antiseptic
– Where do the bubbles come from?
– Aldehydes
• Disinfectant and preservative
• Formaldehyde – formalin
– Irritating and carcinogenic
• Glutaraldehyde
– Liquid chemical sterilant
– Biguanides
• Chlorohexidine most effect class
• antiseptic in lotions, soaps and impregnated into
surgical meshes and plastics
– Ethylene oxide
• Gaseous chemo-sterilizer
• 4 -18 hours exposure kills all microbes
• Useful for heat sensitive materials
– Food preservatives
• Organic acids – Sodium benzoate, Sorbic acid
– Inhibit mold growth in acidic foods
• Calcium propionate - fungicide used in bread
– Inhibit mold growth
• Nitrates- added to many meat products
– prevents germination and growth of botulism
endospores
– preserves the pleasing red color ????
– Antibiotics
• Nisin- added to cheese to inhibit growth of
endospore formers
• Natamycin – antifungal used in food (mostly
cheese)
• Why should medical antibiotics not be used in
food preservation?