Heat influence on microorganisms T3

• Preservative effect of heat processing


  Denaturation of proteins, which destroys enzyme activity and enzyme-controlled metabolism in micro-organisms
• Microbial destruction by heat


  1. First-order reaction

  2. Same percentage die in a given time interval regardless of the numbers present initially

  3. Logarithmic order of death

  4. Described by a death rate curve

  
• Decimal reduction time (D value)


  Time needed to destroy 90% of the micro-organisms (to reduce their numbers by a factor of 10)
• D values differ for different microbial species, and a higher D value indicates greater heat resistance
• Higher number of micro-organisms present in raw material


  Longer it takes to reduce the numbers to a specified level
• A specific temperature–time combination is used to process every batch of a particular product, and adequate preparation procedures are used to ensure that the raw material has a satisfactory and uniform microbiological quality
• Microbial destruction takes place logarithmically, it is theoretically possible to destroy all cells only after heating for an infinite time

  
• Commercial sterility


  The vast majority of containers are sterile but there is a probability that non-pathogenic cells survive the heat-treatment in a pre-determined number of containers

  
• The level of survival is determined by the type of micro-organism that is expected to contaminate the raw material
• A 12D process is used when Clostridium botulinum is likely to be present
• Z value (thermal resistance constant)


  The number of degrees Celsius required to bring about a ten-fold change in thermal death time (TDT) or decimal reduction time (D)

  
• F0 value (unit of sterilization)


  The inactivating effect of 1 min at 121.1°C on Clostridium botulinum spores
• For total inactivation (at 121.1°C and other temperatures!) according to the 12D concept a value of 2.45 F0 is necessary (= 2.45 for C. botulinum)

  
• Calculating sterilizing effect at a certain temperature


  F0 = L(T) x t, where L = the lethal value at temperature T, t = time (min)