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Methods for determining the thermal resistance characteristics of microorganisms

Department of Food Preservation and Meat Technology, FPBT, VŠCHT Prague. Methods for determining the thermal resistance characteristics of microorganisms. Petra Klodnerová Supervisor: Dr. Ing. Miroslav Čeřovský. Heat Preservation. Aim of heat process:

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Methods for determining the thermal resistance characteristics of microorganisms

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  1. Department of Food Preservation and Meat Technology, FPBT, VŠCHT Prague Methods for determining the thermal resistance characteristics of microorganisms Petra Klodnerová Supervisor: Dr. Ing. Miroslav Čeřovský

  2. Heat Preservation • Aim of heat process: • elimination of key pathogens and spoilage microorganisms • maximum retention of nutritional and sensory characteristic of food products • performance of legislative requirements

  3. Determination of the thermal resistance characteristics • When? • to calculate of heat treatment for a new product • to determine of the influence of different environmental factors in heat resistance of microorganisms • to study of mechanisms of the heat resistance

  4. Technique of determination of thermal resistance Choise of target microorganism. Determination of D- and z- value. Heating of inoculated medium for varying lenghts of time at a series of different temperatures. Determination of the number of surviving microorganisms. Construction of microbial survival curve.

  5. Criteria for selection of the method • to attain very fast heating rates of inoculated media • to take into account the effect on the heat resistance of any influencing factor • composition of medium • food product • model medium (e.g. phosphate buffer) • pH • concentration of NaCl • to be precise, accurate and easy to perform • not to be time consuming, so as to obtain an appropriate number of data for statistical analysis

  6. Heat resistance determination methods • TDT (Thermal death time) tube method • TDT can method • “Tank” method • 3–neck flask method • Unsealed TDT tube method • Capillary tube method • Thermoresistometer method

  7. Thermoresistometer • Heating medium is heated and agitated in a stainless steel vessel(8,5 x 12 cm). • Automatic injection of inoculum to heating medium. • Automatic sampling to revolving fractions collector. • High speed of operation (52 samples/s). Thermoresistometer. (Dyrlæge H. Riemann, 1954)

  8. Advantages Heat resistance determination at temperatures from pasteurization to UHT (57-150°C). Rapid heating and cooling of samples. Constant monitoring of pH and precise control of temperature. Determination of very low D-values. Labor saving, simplicity of operation, low upkeep. Disadvantages High initial cost. Using only for liquid suspension and homogenates. Thermoresistometer

  9. Capillary method • Stern a Proctor (1953) • inoculated medium was heated in sealed thin-walled glass capillary tube. • Today – stainless steel capillary → labor saving at sealing and opening of glass capillary.

  10. Capillary method essentially lower initial cost higher time consumption not used for too viscous products Thermoresistometer obtaining more data during short time higher precision determination of very low D-value Capillary method vs. Thermoresistometer ▲thermoresistometer ○ capillary tube

  11. Conclusion • Capillary tube method should be sufficiently accurate and reasonable at temperature below 120 °C. • This method is tested in our laboratory. • This method means improvement in comparison with methods used in our laboratory till now.

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