Control of Microbial Growth

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CONTROL OF MICROBIAL GROWTH

  • killing microorganisms/inhibiting microbial growth 
  • physical/chemical agents - kill/prevent growth 
  • level of resistance - descending order
    • endospores (bacterial+blue-green algae)+protozoal cysts
    • mycobacteria
    • fungal sexual spores
    • small non enveloped viruses - polio, rotavirus, rabies, norwalk virus
    • vegetative fungal spores
    • enveloped viruses - herpes, hepatitis B+C, HIV
    • vegetative bacteria
  • prevention - control growth, prevent infection
  • sterilization - complete destruction. heat, radiation, chemicals, pysical removal 
  • disinfection - reducing growth nonliving surfaces. (organic matter interferes - heat treatments+chemical control agents)
  • antisepsis - reducing growth living tissue
  • bacteria die - constant rate. log plot=straight line
  • time needed - kill - population size
  • physical methods
    • heat 
    • filtration
    • low temperatures
    • desiccation
    • osmotic pressure
    • radiation
  • heat 
    • moist+heat (boiling/autoclaves) - denatures enzymes, kills most vegetative cells+inactivates viruses - 10 mins
    • 121 C, 30 mins kills nearly all microbes
    • hospital method 134 C, 3 mins - doesn't kill prions
    • thermal death point (TDP)=lowest temperature, kills all bacteria - broth 10 mins
    • thermal death time (TDT)=time span - kill all bacteria, given temperature
    • decline reduction time (DRT)=length time - 90% bacterial population killed - given temperature
  • bacterial agents
    • bacteriostatic - bacteriostasis=growth inhibition, no destruction
    • bacteriocidal=kill/destroy
    • bacteriolytic - bacteriolysis=dissolution bacterial walls
  • autoclaving
    • steam under pressure
    • 121 C, 30 mins, 15lb/in2
    • heat labile sunstances=denatured
    • steam must contact material
  • dry heat sterilization 
    • direct flaming. bunsen burner flame=1300-1600 C
    • incineration
    • hot air sterilization (oven)
  • pasteurisation
    • high temperature - short time
    • batch method 63 C - 30 mins
    • flash method 72 C - 15 seconds
    • ultra high temperature (UHT) 140 C - 3 seconds
    • long shelf life
    • some nutritional loss - folate, B vitamins, vitamin C - UHT milk
  • filtration
    • passage liquid/gas through filter - small pores retain microbes
    • important sterilize solutions - denatured by heat - antibiotics, injectable drugs, amino acids, vitamins
  • low temperatures
    • decreasing temperature, decreases chemical activity 
    • bacteriostatic
    • freezing may kill microorganisms - ice crystal size
  • drying (desiccation, dehydration)
    • disrupts microbial metabolism
    • stops growth - organisms still viable
    • viruses+endospores - resist desiccation
    • freeze-drying foods
  • freeze drying
    • fresh/cooked foods=flash frozen 
    • vacuum chamber - food moisture drawn off
    • foods sealed - moisture+oxygen free packaging
    • water replaced, food regains original flavour, texture+appearance - theory
  • drying (osmotic pressure)
    • food preservation
    • plasmolysis
    • sugar

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