The sterilizing value, usually referred to as F0, describes the level of microbial destruction obtained by thermal treatment. The results are expressed in minutes or, to be more precise, in equivalent-minutes of exposure to 121.1°C. In practice, the F0 is the result of a calculation made by measuring the temperature at the cold spot of a product. The term “F0 measurement” used in the trade is therefore a misuse of language. <br>
The values Tref =121.1°C and Z=10°C are mandatory for calculation of the F0; they correspond to a reference germ: Clostridium Botulinum. By extension, we could say that:
1 minute at 121.1 °C gives 1 F0 point.
Each product to be treated has a specific microbial load, which varies in type and in number. It is therefore necessary to apply an appropriate minimum F0 in order to guarantee industrial sterility (F0=3 is used as the minimum value).
On the other hand, an excess of heat and therefore of F0 often detracts from the organoleptic qualities of a product. Quality control of raw materials is therefore crucial to guarantee the stability of products sensitive to cooking phenomena.
The precision of F0 calculation is directly related to that of temperature measurement, from which it arises. There are three main causes of temperature measurement errors:
• sensor positioning (error in tens of °C)
• sensor geometry/dimensions (error in °C)
• sensor precision (error in tenths of °C)
It can be seen that the impact of the metrological quality of the sensor is practically negligible compared with the impact of its dimensions or its positioning. However, the trade recommends the use of sensors that are calibrated (at least once a year), with a maximum dispersion of ±0.25°C.
Notion of minimum F0 and target F0
The minimum F0 is the result of the calculation of F0 obtained by combining the different factors unfavourable to the sterilization process, as follows:
• conditions of product filling in the packaging: temperature, weight, pressure, gaseous volume
• basket packing conditions: compacted layers, autoclave cold spot
• autoclaving conditions: critical load, critical maintenance of each autoclave and critical operation of the bank of autoclaves
The minimum F0 calculated in this way must meet the required destruction level, known as the target F0. This guarantees product stability in line with the term “preserved/canned food”. If the minimum F0 is lower than the target F0, the treatment time or temperature is generally increased. However, it is often possible to obtain the same result by reducing F0 dispersion (and therefore increasing the minimum F0 value) by adapting the sterilization scale. This method avoids lengthening the cycle time and increasing deterioration due to overcooking.