Heat Sterilisation 1

Heat Sterilisation Microbial Micr obial Heat Resistance & Survivor Sur vivor Curves;

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Thermal Death times & Spoilage probability;

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Process Calculations

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Processing Systems

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Preservation Pr Processes ocesses 

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Definition: Processing steps required to reduce or eliminate the potential for food borne illnesses and spoilage. Pasteurisation (traditional): ◦

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Increase in product temperature to inactivate specific pathogenic micro-organisms; Is product Shelf stable? Typically a few weeks with refrigeration (< 5 oC);

Commercial Sterilisation: ◦

More intense thermal process to reduce population of all microorganisms in product;

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Is product shelf stable?

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Absolutelyy, for 12 months or more at room temperature Absolutel

(does not require refrigeration).

Microbial Survival Curves 

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The main issue relating to food preservation is the microbial population; An external agent (heat) is used to reduce the population of microbes present; Vegetative cells (eg. E. coli, Salmonella, Listeria monocytogenes) will decrease; Microbial spores spores will decrease in a similar manner –  manner  –  after  after a lag period;

Effect of Heat on Micro-organisms Micro-organisms 

Preservative Effect due to denaturation of Proteins: ◦

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Destroys enzyme activity & enzyme controlled metabolism in microbes;

Rate of destruction is a first order reaction reaction ◦

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When food is heated to a given temperature, the same % die in a given time interval –  interval –  regardless  regardless of the initial numbers present. This gives rise to a logarithmic order order of death (death curve);

 survival

curve

dN/dt = - k.N n , n = 1 for first order; order ; N = microbial population, t = time, k = first order rate constant;

Microbial Micr obial Survivo Sur vivorr Curves Cur ves