Physical and Chemical Control of Microorganisms

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Lecture 10

• Physical and Chemical Control of Microorganisms

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History

• First microbial control practices developed by
  Ignatz Semmelweis in 1800s
• Some techniques developed
• Hand washing with chloride of lime
• Aseptic technique during surgery

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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

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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
• of microbes
• Environmental influences
• Time of exposure
• Microbial characteristics

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Physical Methods of Microbial Control
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Physical methods of microbial control

• Heat
• Filtration
• Refrigeration
• Radiation
• Dessication
• Osmotic Pressure

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Heat

• Kills microbes by denaturing their enzymes
• Thermal Death Point
• Thermal Death Time
• Three types
• Moist heat, Pasteurization, Dry Heat

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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

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Pasteurization

• High temperature used for a short period of time
  to destroy pathogens without altering the flavor
  of the food

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Dry Heat Sterilization

• Flaming- heat wire to red glow
• Hot-air sterilization- place items in oven-
  temperature 170oC for 2 hours ensures
  sterilization

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Filtration

• Passage of a liquid or gas through a filter with
  pores small enough to retain microbes
• Microbes can be removed from air by
  high-efficiency particulate air filters
• Membrane filters are commonly used to filter out
  bacteria, viruses, even large proteins from
  liquids

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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

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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

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Dessication

• Dessication absence of water
• Microorganisms cannot grow or reproduce but can
  remain viable for years
• Viruses and endospores can generally resist
  desiccation

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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

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Chemical Methods of Microbial Control
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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

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Antiseptics and Disinfectants

• Several parameters should be considered
• Temperature
• pH
• Stability

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Halogens

• Oxidize proteins
• Examples Chlorine and Iodine

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Phenol and Phenolics

• Denature Proteins
• Phenol- strong odor caustic to skin
• Phenolics used more often

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Heavy Metals

• Interfere with microbial metabolism
• Believed to bind protein molecules, making them
  unusable

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Alcohols

• Denature proteins and disrupt membranes
• Preferred alcohol- ethyl alcohol

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Soaps and Detergents

• Act as surface active agents
• Not bacteriocidal

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Hydrogen Peroxide

• Breaks down into toxic form of oxygen
• Not good on open wounds

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Antiseptics in your pantry

• Cinnamon
• Garlic
• Honey
• Wasabi

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Figure 7.11