9.1 Principles of thermal processing

Thermal processing is a crucial food preservation method that uses heat to destroy harmful microorganisms and extend shelf life. From pasteurization to sterilization, these techniques ensure food safety while maintaining quality and nutritional value.

Understanding thermal processing calculations is key to designing effective heat treatments. D-values, Z-values, and F-values help determine the time and temperature needed to achieve desired microbial reduction levels in various food products.

Heat Treatment Methods

Pasteurization and Sterilization

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• Pasteurization involves heating food to a specific temperature for a set time to destroy pathogenic microorganisms and inactivate enzymes, extending shelf life (milk, beer, fruit juices)
• Sterilization heats food to a higher temperature than pasteurization to destroy all microorganisms and their spores, resulting in a shelf-stable product (canned vegetables, soups, sauces)
• Both pasteurization and sterilization aim to reduce microbial growth and spoilage while maintaining food safety and quality
• The specific time-temperature combinations for pasteurization and sterilization depend on the food's pH, water activity, and other factors

Blanching, Canning, and Retort Processing

• Blanching involves briefly immersing fruits or vegetables in boiling water or steam to inactivate enzymes, remove air from plant tissues, and soften the food (pre-treatment for freezing or drying)
• Canning is a method of preserving food by sealing it in airtight containers and heating it to a specific temperature to destroy microorganisms and create a vacuum seal (jams, pickles, tomato sauce)
• Retort processing uses a combination of high temperature and pressure to sterilize low-acid foods packed in sealed containers, ensuring commercial sterility (canned meats, fish, vegetables)
• These methods help extend the shelf life of foods while maintaining their quality, nutritional value, and safety

Thermal Processing Calculations

D-value and Z-value

• D-value (decimal reduction time) represents the time required at a specific temperature to destroy 90% (1 log cycle) of a target microorganism population
  • Determined by heating the microorganism at various temperatures and plotting the survival curve
  • Used to calculate the time needed to achieve a desired level of microbial reduction
• Z-value is the temperature change required to change the D-value by a factor of 10
  • Represents the microorganism's resistance to temperature changes
  • Determined by plotting the log of D-values against their corresponding temperatures
  • Used to adjust processing times for different temperatures

F-value, Heat Penetration, and Thermal Death Time

• F-value (process lethality) is the time required to achieve a specified level of microbial inactivation at a reference temperature (usually 121.1°C or 250°F)
  • Calculated using the D-value and Z-value of the target microorganism
  • Ensures that the thermal process is sufficient to produce a safe and stable product
• Heat penetration refers to the rate at which heat is transferred from the heating medium to the coldest point within the food product (often the geometric center)
  • Influenced by factors such as food composition, container size and shape, and processing conditions
  • Determined using thermocouples or other temperature-measuring devices
• Thermal death time (TDT) is the time required to kill a specific number of microorganisms at a given temperature
  • Depends on the type of microorganism, initial population, and desired level of inactivation
  • Used to design and validate thermal processes to ensure food safety