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 • 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
Vocabulary • 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
survivor curve • 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.
Definition of Decimal Reduction Time • On survivor curve, the time required for a one log cycle reduction in microbial population is the decimal reduction time (D), or D--value .
thermal Resistance Curve • 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.
Definition of Thermal Resistance Constant (Z) • On Thermal Resistance Curve , the temperature increase required to cause a one log cycle reduction in the decimal reduction time is defined as the thermal resistance constant (Z), or Z—value.
Definition of Thermal Death Time (F) • Thermal death time (F) is defined as the time required to achieve a stated reduction in the microbial population at a given temperature (in minutes), or F--value.
Relations Between F & D • 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.
Establishment of Product Shelf Life • Factors influence the heat resistance of microbial. • When product safety is concerned. • When product shelf life is concerned.
Factors Influence the Heat Resistance of Microbial • Species of microorganisms. • Acidity or pH . • Oxygen (some of the spores with high thermal resistance are anaerobic). • Water activity • Food composition
When product safety is concerned • The Low Acid Canned Food (LACF) • Food and Drug Administration • l2-D process for low--acid canned foods
INTRODUCTION TO PROCESS CALCULATIONS • Lethal Rate Curve • Time/Temp. Process & LR • LR Curve for Thermal Process
Problems • What is D-value, What is decimal reduction curve or survivor curve,? • What is Z-value, What is thermal resistance curve? • What is F-value, What is thermal death time curve? • What are the relationships between them?