The following article will guide you about how to describe a heat process.
Heating processes are neither uniform nor instantaneous. To be able to compare the lethal effect of different processes it is necessary for us to have some common currency to describe them. For appertization processes this is known as the F value; a parameter which expresses the integrated lethal effect of a heat process in terms of minutes at a given temperature indicated by a subscript.
A process may have an F121 value of say 4, which means that its particular combination of times and temperatures is equivalent to instantaneous heating to 121 °C, holding at that temperature for four minutes and then cooling instantly, it does not even necessarily imply that the product ever reaches 121 °C.
The F value will depend on the £ value of the organism of concern; if z = 10 °C then 1 minute at 111 °C has an F121 = 0.1, if z = 5 °C then the F121 value of the same conditions will be 0.01. It is therefore necessary to specify both the z value and the temperature when stating F. For spores z is commonly about 10 °C and the F121 determined using this value is designated F0.
To determine the F0 value required in a particular process one needs to know the D121 of the target organism and the number of decimal reductions considered necessary.
F = D121 (log N0— log N) (4.7)
In this exercise, the canner will have two objectives, a safe product and a stable product. From the point of view of safety in low acid canned foods (defined as those with a pH > 4.5) Clostridium botulinum is the principle concern. The widely accepted minimum lethality for a heat process applied to low-acid canned foods is that it should produce 12 decimal reductions in the number of surviving C. botulinum spores (log N0 — log N – 12).
This is known as the 12D or botulinum cook. If D121 of C. botulinum is 0.21 minutes then a botulinum cook will have an F0 of 12 x 0.21 = 2.52 min. The effect of applying a process with this F0 to a product in which every can contains one spore of C. botulinum (N0 = 1) will be that a spore will survive in one can out of every 1012.
The canner also has the objective of producing a product which will not spoil at an unacceptably high rate. Since spoilage is a more acceptable form of process failure than survival of C. botulinum, the process lethality requirements with respect to spoilage organisms do not need to be so severe. In deciding the heat process to be applied, a number of factors have to be weighed up.
(1) What would be the economic costs of a given rate of spoilage?
(2) What would be the cost of additional processing to reduce the rate of spoilage?
(3) Would this additional processing result in significant losses in product quality?
Most canners would regard an acceptable spoilage rate due to under-processing as something around 1 in 105-106 cans and this is normally achievable through 5-6 decimal reductions in the number of spores with spoilage potential (the USFDA use 6D as their yardstick). PA3679, Clostridium sporogenes is frequently used as an indicator for process spoilage and typically has a D121 of about 1 min.
This will translate into a process with an F0 value of 5-6; sufficient to produce about 24-30 decimal reductions in viable C. botulinum spores – well in excess of the minimal requirements of the botulinum cook. Some typical F0 values used in commercial canning are presented in Table 4.4.
Having decided the F value required, it is necessary to ensure that the F0 value actually delivered by a particular heating regime achieves this target value. To do this, the thermal history of the product during processing is determined using special cans fitted with thermocouples to monitor the product temperature.
These must be situated at the slowest heating point in the pack where the F0 value will be at a minimum. The precise location of the slowest heating point and the rate at which its temperature increases depend on the physical characteristics of the can contents. Heat transfer in solid foods such as meats is largely by conduction which is a slow process and the slowest heating point is the geometric centre of the can (Figure 4.5).
When fluid movement is possible in the can, heating is more rapid because convection currents are set up which transfer heat more effectively. In this case the slowest heating point lies on the can’s central axis but nearer the base.
The slowest heating point is not always easy to predict. It may change during processing as in products which undergo a sol-gel transition during heating, producing a broken heating curve which shows a phase of convection heating followed by one of conduction heating. In most cases heating is by conduction but some can contents show neither pure convection nor pure conduction heating and the slowest heating point must be determined experimentally.
Movement of material within the can improves heat transfer and will reduce the process time. This is exploited in some types of canning retort which agitate the cans during processing to promote turbulence in the product. The F value can be computed from the thermal history of a product by assigning a lethal rate to each temperature on the heating curve. The lethal rate, Z,R at a particular temperature is the ratio of the microbial death rate at that temperature to the death rate at the lethal rate reference temperature.
For example, using 121 °C as the reference temperature:
LR = D121/DT (4.8)
where LR is the lethal rate at 121 °C. Since
Z= (T1— T2)/ (logD2– logD1) (4.9)
and substituting T1, = 121 °C
LR = l/10 (121–T)z (4.10)
Lethal rates calculated in this way can be obtained from published tables where the LR can be read off for each temperature (from about 90 0C and above) and for a number of different z values (Table 4.5). Nowadays though this is unnecessary since the whole process of F value calculation tends to be computerized.
Total lethality is the sum of the individual lethal rates over the whole process; for example 2 minutes at a temperature whose LR is 0.1 contributes 0.2 to the F0 value, 2 minutes at a LR of 0.2 contributes a further 0.4, and so on. Another way of expressing this is that the area under a curve describing a plot of lethal rate against time gives the overall process lethality, F0 (Figure 4.6).
This procedure has safeguards built into it. If the slowest heating point receives an appropriate treatment then the lethality of the process elsewhere in the product will be in excess of this. A further safety margin is introduced by only considering the heating phase of the process; the cooling phase, although short, will also have some lethal effect.
Process confirmation can also be achieved by microbiological testing in which inoculated packs are put through the heat process ,and the spoilage/survival rate determined. Heat penetration studies though give much more precise and useable information since inoculated packs are subject to culture variations which can affect resistance and also recovery patterns.
A change in any aspect of the product or its preparation will require the heat process to be re-validated and failure to do this could have serious consequences. An early example of this was the scandal in the mid-nineteenth Century when huge quantities of canned meat supplied- to the Royal Navy putrefied leading to the accusation that the meat had been bad before canning.
It transpired that the problem arose because cans with a capacity of 9-14 lb were being used instead of the original 2-6 lb cans. In these larger cans the centre of the pack took longer to heat and did not reach a temperature sufficient to kill all the bacteria. More recently, replacement of sugar with an artificial sweetener in hazelnut puree meant that spores of C. botulinum surviving the mild heat process given to the product were no longer prevented from growing by the reduced aw.