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:– # of microbes– Environmental influences– Time of exposure– Microbial characteristics

Physical Methods of Microbial Control

Physical methods of microbial control

• 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 oven-

temperature 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 high-efficiency 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

• Cinnamon• Garlic• Honey• Wasabi

Figure 7.11