M5: T2: Sterillisation

• 3 types of sterilisation

  1. heat
  2. radiation
  3. pasteurisation
  4. ultra high temp
• heat

  2 types

  1. dry heat: flame, incineration and hot air oven
  2. moist heat: boiling water, autoclaving
• radiation 


  2 types

  1. ionising: DNA destruction by ions/reactive molecules, x-ray/gamma rays, treats: plastic, medical and food
  2. non-ionising: form of pyrimidine dimmers in DNA, UV and treats: air, water and surfaces
• pasteruisation

  1. low temp, long time
  2. high temp, short time
• filter sterillisation

  • aim: removed microbes
  • types: membrane filters and depth filters
  • uses: heat sensitive liquids, gases and water
• how long is sterilisation

  15 minutes
• explore the effect on bulky items 


  delay in time, object needs time to get heat to the center
• what is the death per minute

  90% of the number of survivors = 1 log dcrease
• efficiency of heat sterillisation

  1. decimal reduction time “D value”: time required to kill 90% of pop at given temp
  2. thermal death pt “TDP”: lowest temp required to kill all microbes in liquid in 10 minutes
  3. thermal death time”TD”: min time required to kill all microbes keeping temp constant
• if the starting population is high

  more time is required to kill microbes
• how we know if heat sterilisation worked

  • its a prob function
  • bacteria “spore-forms“ use bio indicators to asses effectiveness
  • incubation via is clear = spores killed
  • incubation vial is cloudy = spores survived and no sterilisation occured