Chapter 2 THERMAL PROCESSING PRINCIPLES . INFLUENCE OF ELEVATED TEMPERATURES ON MICROBIAL POPULATIONS ESTABLISHMENT OF PRODUCT SHELF LIFE AND SAFETY INFLUENCE OF THERMAL PROCESS ON PRODUCT QUALITY INTRODUCTION TO PROCESS CALCULATIONS. Vocabulary.
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microbial population elevated temperature viability vegetative Salmonella spp., Listeria or Escherichiacoli-anyClostridium botulinum function of time logarithmic microbial spores abruptly semilogarithmic coordinates decimal reduction time, or D--value food spoilage microorganisms decimal reduction curve
thermal resistance curve thermal resistance constant or Z--value, thermal death time (F) survivor curve linear relationship commercial sterilization yeast and molds acidity, or pH low--acid anaerobic Low Acid Canned Food pathogen Spoilage probability quality attributes heat-sensitive nutrients flavor, texture, and color over-processing lethal rate lethality
When the microbial populations as a function of time are presented on semilogarithmic coordinates, a linear decrease in microbial population with time is observed. It is the survivor curve. It is emphasized that these relationships occur when the environment around the population is at a constant temperature throughout the period of exposure.
A plot of decimal reduction time as a function of temperature on semilog coordinates results in a linear relationship. This linear relationship is the thermal resistance curve for a given microbial population.
Each log cycle reduction in the microbial population on the thermal death curve represents a decimal reduction time, or D-value F--values may be expressed as multiples of D--values. The most common of these relationships is F = 12D for Clostridium botulinum in commercial sterilization.