1. Physical and Chemical Control of Microorganisms
Lecture 10
Physical and Chemical Control of Microorganisms

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

3. 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

4. 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

6. Physical Methods of Microbial Control

7. Physical methods of microbial control
Heat
Filtration
Refrigeration
Radiation
Dessication
Osmotic Pressure

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

9. 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

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

12. Dry Heat Sterilization
Flaming- heat wire to red glow
Hot-air sterilization- place items in oven- temperature 170oC for 2 hours ensures sterilization

13. 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

15. 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

16. 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

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

18. 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

19. Chemical Methods of Microbial Control

20. 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

21. Antiseptics and Disinfectants
Several parameters should be considered:
Temperature pH
Stability

22. Halogens
Oxidize proteins
Examples: Chlorine and Iodine

23. Phenol and Phenolics Denature Proteins
Phenol- strong odor; caustic to skin
Phenolics used more often

24. Heavy Metals Interfere with microbial metabolism
Believed to bind protein molecules, making them unusable

25. Alcohols Denature proteins and disrupt membranes
Preferred alcohol- ethyl alcohol

26. Soaps and Detergents
Act as surface active agents
Not bacteriocidal

27. Hydrogen Peroxide Breaks down into toxic form of oxygen
Not good on open wounds

28. Antiseptics in your pantry
Cinnamon
Garlic
Honey
Wasabi

30. Figure 7.11