Control of Microbial Growth

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
  • …