Define Steps To determine thermal resistance of a microorganism?
Most food components and microorganisms obey first order reaction kinetics, which means that the destruction rate is dependent on initial concentration. We can find a heat treatment, which will take care of the target microorganism while allowing only minimal quality damage to food components. To determine thermal resistance of a microorganism at a specific temperature, the following steps are required:
- Heat at constant "known" temperature for different times.
- Recover surviving cells.
- Plot survival curve - e.g. time versus number survivors.
As line transverses one log cycle of survivor number represents a 90% reduction in number of survivors - because this is a first order reaction - this % reduction remains constant i.e. for the next log cycle another 90% reduction occurs. The time required to reduce one-log cycle of survivor at a particular temperature is known as D-value at that temperature.
The sterility index is represented by F value. This index often has a subscript representing the specific temperature and a superscript indicating the z value of the particular organism (F temperature change required to change the thermal death time by a factor of 10) thus represents the F value of C. botulinum at 121.1 (250°F). The temperature of 121°C is usually used as a reference temperature and is always indicated as "Fo". Since similar first order reactions occur for various food nutrients and qualities, a similar procedure can be used to analyze their loss as "thermal resistance curves". Another useful system for representing temperature response by biological systems is the Q value, which is the change in reaction rate for a 10°C temperature change.
The F value for C. botulinum is the time required to reduce by 12 D the number of viable spores and this has become the Standard Heat Process for foods which have the potential to have C botulinum outgrowth (i.e. pH > 4.5). For a temperature of 121°C,
Fo = 12 D = 2.45 minutes = Sterilizing Value.
This is the 12 D concept for canning operations. Thus if there were 1012 spores present in a can of food and it received a 12 D process, then there would be only 1 spore left. There are some food spoilage organisms that are more thermally resistant than C. botulinum. For foods that contain these microorganisms and for foods with pH > 4.5, processors typically process to 5 D.
This would give a probability of loss due to spoilage of less than 1 can per 1000 for normal contamination. While on the topic of sterilization, we also need to highlight that the following two methods of heat sterilizing foods are employed.
1. Foods can be heat sterilized then placed into a sterile container - aseptic processing
2. Foods can be placed into a non-sterile container then the entire container is processed -conventional canning
These steps are commonly done at the food canning establishments, where the actual processing is done. We will learn more about the canning process in the next sub-section.