Physical and Chemical Control of Microorganisms 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