Course:FNH200/Lessons/Lesson 06/Page 06.2
6.2 How are heat treatment selected?
The intensity of the heat treatment employed for a particular food preservation application depends upon a number of factors. The main considerations in selecting the required temperature-time conditions for thermal processing are:
- What is the objective or purpose? (blanching or pasteurization or commercial sterilization)
- Are there additional preservation steps? (is it combined with other preservation methods?)
- What are the physical, chemical properties of the food? (Type of food)
- What is the heat resistance of microorganisms in the food?
Here are some examples to illustrate these points:
- Foods that will be consumed within a short period of time after processing can have storage life extended by a combination of pasteurization and refrigerated storage (used for pasteurized milk and for pasteurized, vacuum packaged, cured meats).
- Longer storage times at ambient temperatures in evacuated sealed containers requires the use of commercial sterilization.
- The time-temperature combination required for pasteurization and commercial sterilization is determined by the most heat-resistant disease-causing and spoilage-causing microorganisms in the particular food commodity.
- For a particular food commodity, the type of thermal processing operation and the rate of heat penetration into the slowest heating portion of the food within a particular container are governed by the food's physical properties (solid vs. liquid, or solid particles suspended in a liquid) and chemical properties (pH, fat content, presence or absence of heat-inducible thickening agents, food components that have protective or antagonistic effects on the thermal resistance of microorganisms).
- It is imperative that thermal preservation processes be designed so that the slowest heating portion of the food commodity receives the specified time-temperature thermal treatment to minimize risks of illness and/or post-processing spoilage.
- The thermal processes applied to foods are governed by the heat resistance of the microorganisms in the food.
In low acid foods which are to be thermally processed and vacuum sealed within gas-tight containers, the microorganism of most concern is Clostridium botulinum.
The habitat of Clostridium botulinum can be soil (agricultural and forest), water (fresh, brackish and marine) and mud (fresh water and salt water). As a consequence, all foods of agricultural and fisheries origin must be considered as being potentially contaminated with Clostridium botulinum spores.
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Low acid foods which are to be packaged and stored under anaerobic condition, require a specifically designed thermal processing treatment to ensure the destruction of any Clostridium botulinum spores. This will provide a large margin of safety. Actively growing vegetative Clostridium botulinum cells produce a very potent neurotoxin.
How can we determine if C. botulinum spores have been destroyed?
- To determine the thermal resistance of heat-resistant spores in foods, "Inoculated pack studies" are carried out using a non-pathogenic spore-forming bacterium, Clostridium sporogenes PA3679 (a putrefactive anaerobe).
- Since PA3679 spores are more heat resistant than those of Clostridium botulinum spores, a process designed to kill PA3679 spores will definitely kill Clostridium botulinum spores with a wide margin of safety.
- The concept of "margin of safety" is later described in this lesson.
- The concept of "margin of safety" is later described in this lesson.