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The Fo value for the process is calculated by summing all the L values and multiplying this value by the standard time interval between readings.

The trapezoidal method also allows simple calculation of the contribution to total process lethality of the heating and cooling portions of the process.

In Table 2 are shown L values and in Table 3 is shown a worked example in which temperature was recorded at 5 minute intervals for a process of 60 minutes at 121.1 °C.

To calculate Fo for the process: Summing the L values gives 2.925 which when multiplied by 5 (the time interval between readings) gives an Fo value of 14.6 min.

To calculate Fo for the heating phase: The sum of L values at time 25 and 60 min (0 and 0.776) is divided by 2 and this value (0.388) is added to the sum of L values from time 30 to 55 min. This gives 1.730 which when multiplied by 5 yields on Fo of 8.6 min for the process lethality at the stage when the steam was turned off.

The Improved General Method which relies on a temperature-time plot. for the entire process is the most accurate of all methods for calculating Fo value and for this reason is frequently quoted as the "reference method". Like the Trapezoidal Method there are no assumptions made regarding product heating and cooling characteristics, however the benefits of accuracy have to be balanced against the lack of versatility. Data from one set of trials cannot easily be used to calculate Fo values when product temperature and/or retort temperature are (is) altered. This means that once process conditions are altered new temperature-time data must be collected under the new experimental conditions.

A plot of temperature versus time is made on specially constructed lethal rate paper which has on its left-hand vertical axis product temperature (on a log scale) while on the other vertical axis is drawn lethal rate (on a linear scale). Thus for each temperature can be shown the corresponding lethal rate. Time is plotted along the horizontal axis, using a convenient scale.

The area under the graph which represents the product of exposure time at all lethal rates throughout the process, is then divided by the area equivalent to that of an Fo value of unity. This yields the total sterilising effect, or the Fo value, for the process. In Figure 3 is shown a hypothetical heat penetration curve for a semi-solid product processed for 40 min at 120 ºC.

Figure 3. Heat penetration lethal rate curve

The temperature profile shown is that of the slowest heating point. By counting squares or using a planimeter the area under the graph is found to be 71 cm², while the area corresponding to one unit of lethality (Fo = 1) is 4 cm². Therefore the total process lethality can be calculated,

 
This means the total sterilising effect of the process is equivalent to 17.5 minutes at 121.1 ºC, assuming instantaneous heating and cooling. We have now expressed the severity of sterilisation, as experienced at the slowest heating point of the can.

In the worked example, the retort was not operating at the reference temperature (121.1 ºC) nor did the product reach retort temperature. It is important not to confuse the specification for the process (40 min/120 ºC) with Fo for the process. A process specification alone indicates little about the total process lethality. It would be possible to have a process specification of 60 min at 121.1 ºC and Fo values of, say, 6.2 min and 11.5 min for 450-g and 225-g cans respectively, the different process severity in this case reflecting can size. Similar mode of heating (convection/conduction), pack weight and fill temperature can all affect the Fo value even though retorting conditions may be constant.

The measure Fo value heat penetration studies are conducted for representative packs of the canned fish filled to the maximum fill weight likely to be encountered. These cans are then fitted with thermocouple probes which must be located so as to measure the temperature at the SHP. (As can-to-can variation in the rate of heat penetration can be significant, it is recommended that at least twelve replicates are tested before data from the slowest heating of all the test cans are used to compute the Fo value for the process).The thermocouples are connected to digital or graphical recorders, some of which indicate the product temperature during the thermal process, while others can be purchased which automatically compute Fo value. Where automatic computation is not possible, the temperature-time data can be used in a number of ways to calculate Fo value.

Clostridium botulinum

Although by convention the sterilising effect of a process is expressed in standard units of minutes at 121.1 ºC (the symbol used is Fo), the product inside a can does not instantaneously reach processing temperature and in some cases of conduction heating, the temperature at the thermal centre of the can never reaches that of the heating medium (which need not be at 121.1 ºC).

This paradox is resolved by making use of a relationship which shows that the rate of change in the thermal destruction of bacteria (i.e. the rate of change in their D values) is logarithmic around temperatures commonly used in heat sterilisation. This means that the lethal rate of destruction at any temperature can be related to that at a reference temperature. This relationship is graphically represented .in Figure 2  which shows a thermal death time curve passing through 1 min at 121.1 ºC. This "phantom" curve shows that relative to the lethal rate of unity at 121.1°C the lethal rates at 91.1, 101.1, 111.1, 131.1, 141.1 and 151.1 ºC are 0.001, 0.01, 0.1, 10, 100 and 1 000, respectively.

The sterilising effect of a thermal process (the process Fo value) can therefore be computed by integrating the combined lethal effect of exposure at all time/temperature combinations throughout the process. This means that a process that delivers an Fo value of 2.8 min (the so called 12D process for Clostridium botulinum) is equivalent in . sterilising effect to heating the contents of the can to 121.1 ºC instantly, holding it at that temperature for 2.8 min, and then cooling it instantly. Similarly, a process for solid style canned tuna packed in 84 x 46.5 mm cans may have a target Fo value of 10 min, which can be achieved by processing for 74 min at 116 ºC or 50 min at 121.1 ºC. With each process, however, the sterilising effect is the same as, and equivalent to, holding the can of tuna at 121.1 ºC for 10 min under conditions of instantaneous heating and cooling.

It is desirable that the test organism in comparative thermal death time determinations be one which grows readily on the ordinary culture medium, with the production of an abundant yield of spores in a minimum time.

It should, furthermore, be an organism which has a characteristic type of growth, or should possess some readily determinable which will serve to differentiate it from contaminants likely to be encountered.

The heat resistance of bacterial spores is specified by the time required to kill 90 per cent of the population at constant temperature; this enables a comparison of heat resistance of spores of many different bacteria. For most spores of importance in canned food spoilage their heat resistance is measured at 121.1 ºC (250 ºF), a common retorting temperature, and is expressed as the D value. A typical plot of the number of survivors against heating time is shown in Figure 1. It can be seen that the time to reduce the population from 1 000 000 to 100 000 is the same as that required to reduce it from 100 to 10. That is, the D value is constant for specific bacterial spores when they are subjected to heat at constant temperature. In Table 1 are summarised the D values of bacterial spores important in canned foods.

Destruction of all spores of Clostridium botulinum is the minimum safety requirement , when thermally processing low-acid canned foods. Canners aim to reduce the probability of one spore surviving the thermal process to such a low level that, for all practical purposes, the contents of the container pose no health risk due to survival of Clostridium botulinum (spores). Experience has shown that a process equivalent in sterilising effect to twelve decimal reductions of the population of Clostridium botulinum is sufficient to protect consumer safety. Such a process is referred to as a "12 D" process and it is equivalent to holding the contents of the container at 121.1 ºC for 2.8 min (12 D= 12 x 0.23 = 2.8 min). A process as severe as this will satisfy requirements (under conditions of good manufacturing practice); however, it will be insufficient to reduce to a commercially acceptable level, the probability of survival for the extremely heat resistant spores (with D values of 2.0 to 5.0 min) of non-pathogenic bacteria. This is why canned fish manufacturers select a thermal process which goes beyond the safety requirements of destruction of Clostridium botulinum.

* Formerly C. nigrificans

The data from these trials (or from suitable reference sources) are .used by fish canning technologists to determine the processing temperatures and times necessary to render the canned product commercially sterile. Manufacturers of canned fish (and all low-acid canned foods) can specify their thermal processes in terms of target Fo values, where the Fo value is a measure of thermal processing severity.

Having selected an appropriate Fo value (which may be far in excess of that required to reduce to an acceptably low level, the probability of survival of Clostridium botulinum spores as may be the case when the process is designed to bring about bone softening) the canner then adopts a time and a temperature for the thermal process which will ensure its delivery at the SHP of the container.

What is Canning?

Canned fish

Canning is a method of preserving food in which the food is processed and sealed in an airtight container, providing a typical shelf life ranging from 1 year to 5 years and under specific circumstances a freeze dried canned product can last as long as 30 years and can still be safely consumed. The process was first developed as a French military discovery by Nicolas Appert in 1810. The packaging prevents microorganisms from entering and proliferating inside.

The Principles of Canning
The purpose of thermal processing during manufacture of canned fishery products is the destruction of bacteria by application of moist heat. Only having satisfied the safety requirements of protecting consumer health, and the commercial requirements of preventing non-pathogenic spoilage, does the canner set about choosing a thermal process schedule that will optimise the sensory quality of the finished product.

Of the bacteria contaminating fishery products, some (the pathogenic bacteria) cause food poisoning while others only spoil the food. Of particular concern to fish canners is the possibility of there being contamination by Clostridium botulinum which, if present, can form heat resistant spores capable of withstanding a mild thermal process. As this micro organism can grow at the pH of fish flesh it is important that the processor ensure that all his cans have received a process that is sufficiently severe to kill spores and vegetative forms of the bacterium. Survival of Clostridium botulinum, after the thermal process, is an extreme health risk as low-acid canned foods (pH > 4.5) support growth of the organism, and under certain conditions will also favour formation of the neurotoxin responsible for outbreaks of botulism.

Sterilization is a heat treatment given foods capable of supporting the growth of heat resistant spore forming bacteria. Sterilization processes destroy all pathogenic contaminants and all other micro organisms capable of growing under normal storage conditions; survivors of the process will be extremely heat resistant spores which pose no health risk and only grow at elevated temperatures (= 40 ºC). Rather than make canned foods absolutely sterile, canners aim for "commercial sterility" which means that the contents are safe (as all pathogenic microorganisms have been destroyed) and shelf-stable at normal storage temperatures. Were the thermal process designed to make all cans absolutely sterile, there would be unnecessary loss of sensory and nutritional quality without there being any increase in the safety of the product.

The higher the temperature of sterilization the greater is the rate of thermal destruction, which is why canners process their canned fish in steam under pressure rather than in water at atmospheric pressure. The rate of thermal destruction is also affected by the nature of the product (liquids heat faster than solids) and the container size (large cans of fish packed in brine take longer to reach lethal temperatures, than do small cans containing the same product). The total sterilization effect of a thermal process can be expressed as the sum of all the sterilization effects achieved by all the time-temperature combinations throughout the entire thermal process. By convention, sterilizing effect is expressed in standard units of minutes at 121.1 ºC, so that. an entire processing cycle is expressed as being equivalent, to holding the product at 121.l ºC for a given time. The unit of sterilization is the Fo unit, where an Fo value of one minute is equivalent to holding the product at 121.1 ºC for one minute and then cooling it instantly.