Transcript Lecture 7
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 oventemperature 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 highefficiency 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