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Control of Microbial Growth: Techniques and Chemical Methods

This chapter focuses on microbial control methods, including terminology, aseptic techniques, and sterilization processes. It covers the factors influencing the effectiveness of antimicrobial procedures such as microbial characteristics, exposure time, and concentration of treatments. Different physical methods like heat, filtration, and radiation are explained, as well as the types of antimicrobial chemicals, including alcohols, halogens, and peroxygens. Understanding these methods helps in the effective control of microbial growth in both clinical and food preservation settings.

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Control of Microbial Growth: Techniques and Chemical Methods

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  1. CHAPTER 9 CONTROL OF MICROBIAL GROWTH

  2. Terminology of Microbial Control • Sepsis/ Asepsis • Aseptic techniques • Sterilization • Commercial Sterilization • Sanitization

  3. Antisepsis vs Disinfection • Biocide vs Biostatic • Degerming • Chemotherapy

  4. Factors that influence the effectiveness of anti-microbial procedures: • microbial characteristics • number of microbes • Decimal reduction time (D value) • environmental influences • time of exposure • concentration or intensity of treatment

  5. Physical or Mechanical Methods • Heat (moist & dry) • Filtration • Low temperature • High pressure • Desiccation & Osmotic pressure • Radiation

  6. Heat • Moist Heat • Boiling kills most vegetative forms in 10 min • Autoclave – steam under pressure • Sterilization - 15 psi, 121OC for 15 min • Pasteurization • Standard methods do not sterilize • higher temp, shorter exposure time • Dry heat • Incineration • Flaming • dry heat sterilization • Requires higher temp and longer exposure • 170°C for 2 hours is equivalent to autoclaving

  7. 2. Low Temperatures • Slows metabolic rate • Freezing forms ice crystals that can damage cells • Slow freezing then thawing does most damage 3. Filtration • Mechanical sterilization of heat-sensitive material • HEPA filters (high efficiency particulate air filters) • Membrane filters have pores as small as 0.01 microns

  8. 4. High Pressure • Up to 130,000 psi • Endospores are resistant • Commercially pasteurize foods • Preserves flavor, taste, appearance, and nutritional value

  9. 5. Desiccation • Resistance varies by species • can’t reproduce or grow, but may remain viable • Osmotic pressure • create a hypertonic environment; dehydrates cells • Preserved fruits (sugar), cured meat (salt), and pickles (salt)

  10. 6. Radiation • depends on wavelength, intensity and duration • Ionizing vs non-ionizing • Sterilization of food products and disposable medical equipment; water treatment

  11. Evaluating Anti-microbial Compounds • Phenol coefficient test • Use-dilution test • Current industry standard

  12. Disk-Diffusion Method

  13. Types of Anti-microbial Chemicals • Phenol (carbolic acid) • Rarely used, irritating qualities and bad odor • Penolic compounds more common • Disinfect for surfaces (Lysol) and antiseptic (antimicrobial soaps and lotions) • Halogens • Iodine – antiseptic or disinfectant • May be available as a tincture or iodophore • Chlorine - disinfectant (water treatment)

  14. Alcohols • Affect vegetative forms of bacteria, fungi, and enveloped viruses • ethanol and isopropanol - degermer (hand sanitizer) and disinfectant • 70% solution typically is most effective • Heavy metals • Silver; mercury; copper • antiseptic, algicide, mildew control paints, placed in newborn’s eyes to prevent spread of gonorrhea

  15. Surfactants • Soap • Emulsification; degerming • Least effective of all chemical methods • Anionic Detergents • Commercial sanitizers • Cationic Detergents • disinfectant and antiseptic

  16. Peroxygens • Disinfectants • Ozone – supplements chlorine in water treatment • Hydrogen peroxide – good disinfectant not antiseptic • Where do the bubbles come from? • Aldehydes • Disinfectant and preservative • Formaldehyde – formalin • Irritating and carcinogenic • Glutaraldehyde • Liquid chemical sterilant

  17. Biguanides • Chlorohexidine most effect class • antiseptic in lotions, soaps and impregnated into surgical meshes and plastics • Ethylene oxide • Gaseous chemo-sterilizer • 4 -18 hours exposure kills all microbes • Useful for heat sensitive materials

  18. Food preservatives • Organic acids – Sodium benzoate, Sorbic acid • Inhibit mold growth in acidic foods • Calcium propionate - fungicide used in bread • Inhibit mold growth • Nitrates- added to many meat products • prevents germination and growth of botulism endospores • preserves the pleasing red color????

  19. Antibiotics • Nisin- added to cheese to inhibit growth of endospore formers • Natamycin – antifungal used in food (mostly cheese) • Why should medical antibiotics not be used in food preservation?

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