Smoking and cooking of food is a necessary part of the processing. Why, how does this work best and what are the underlying calculations? Here we explain what C and F values mean, how cooking has changed over the years and how we are developing it further.
How the cooking process determines shelf life
In the manufacture of preserved foods, the use of high temperatures reduces the number of microorganisms responsible for the spoilage of the product.
Because of this method of preservation, it is also known that it is not only the height of the temperature that is important. It is much more important to know how long a temperature has an effect on the product. The efficiency of these two influences, the level of the temperature and the exposure time, are expressed in the so-called F value.
The importance of this value has been recognised in the sterilisation of canned food. Modern production methods cannot be imagined without the F-value. The same applies to the normal cooking process, which takes place below 95 degrees Celsius. However, this is pasteurisation: the microorganisms to be killed are not spores, but D-streptococci. These are the most resistant in this temperature range.
Research work in this field of pasteurisation makes it possible to use the correct F-value here as well and to cook for a correspondingly long time on the basis of experience.
… is composed of a ratio of two numbers: On the one hand, the height of the temperature in the chamber. On the other hand, the duration (time) of exposure to the product of this temperature.
(F = D (\log N_0 – \log N))
What happens with the F-value?
The microorganisms on which the calculations are based, in particular the D-Streptococci, are killed from a temperature of 55 degrees Celsius. This means that the total number of germs is reduced from a temperature above 55 degrees Celsius. Depending on the level of this temperature achieved, a corresponding F value is achieved per unit of time of one minute. The following values are important for the F-value of pasteurisation:
|Anfangsmesswert||55 Grad Celsius|
|Bezugstemperatur||70 Grad Celsius|
|Wert||1 (Wert für die Zeit-Temperatur-Abhängigkeit der Keimabtötung)
With this basis and the formulas generally known for calculation (F = D (\log N_0 – \log N)), the F values listed below are used to determine the total value.
|Grad Celsius||F-Wert||Grad Celsius||F-Wert|
We therefore find that with every temperature increase of 10 degrees Celsius, the corresponding F-value also increases tenfold. The relationship between temperature and time is also evident. For example, if a product had a temperature of 63 degrees Celsius for five minutes, this would give an F-value of 1,000. A temperature of 70 degrees Celsius needs only one minute for the same F-value.
What is the final F-value to be achieved by the cooking process?
A recommendation about the F-value to be achieved indicates that it should be 40.
This recommendation is based on the following values:
beginning bacteria count before cooking:10E7 total bacteria per gram
End bacterial count after cooking:10E-3 Total bacteria per gram
In practice, this reduction in the germ count means that 99 out of 100 products are germ-free and that one piece still has one germ.
How to calculate the F-value
Two methods allow you to obtain an end F value. The first option requires that the core temperature be plotted as a curve using a temperature recorder. This curve is subdivided into minute intervals in the range above 55 degrees Celsius. The entire temperature curve, which is over 55 degrees, is decisive. This also includes the time until the temperature falls below 55 degrees again when cooling down. Each time period is now assigned the F-value corresponding to the temperature value and added continuously. The addition results in the final F-value. This method is only used if you want to check a cooking process in its sequence.
A second possibility arises with the use of microprocessors. These devices register the temperature curve via a core temperature sensor during the cooking process and independently calculate the achieved F-value. In addition, with these controls it is also possible to control a final F-value and abort the cooking process on the basis of this criterion. This type of F-value cooking is standard with our controllers.
Why by F-value and not by core temperature?
Since the cooking according to the core temperature is already known and it seems much more complicated to record the F-value, this question is understandable.
In our opinion, the determination of a certain final core temperature value, as is done in many places today, does not make sense throughout. Due to the wide range of products, the different calibres, the different product dimensions, the varying product weights and not least the different thermal conductivity values, the same core temperatures produce different results with regard to shelf life and the killing of microorganisms.
With the experience of canning it is obvious that the F-value is the decisive factor. The problem is that different final core temperature values would have to be used for each calibre, each dimension, each weight, etc. in order to achieve the same F-values. This is almost impossible even with today’s controls.
… describes the surface boiling damage of a product. If the temperature at the surface of a product is higher than the core temperature, this has a negative effect on the natural quality such as jelly heel or vitamin degradation. This C-value is calculated on the basis of a temperature curve.
Why do people talk more and more about cooking damage during the cooking process?
The increase in core temperature is much slower compared to room temperature. This can be explained by the different thermal conductivity values of the room air and the product. It is therefore rather suboptimal to run a cooking process with a single temperature value for the room temperature if the core temperature shows a very slow increase.
In other words, it has disadvantages if an ambient temperature of 75 degrees is maintained at an initial core temperature of 12 degrees Celsius, if no drastically faster increase in the core temperature is achieved than with clearly lower values. On the one hand, this is energy-technically inefficient, and the harmful influence of the high ambient temperatures on the food to be cooked can be just as problematic. These influences affect, among other things, the natural quality, such as jelly sales or vitamin degradation. These effects, summarized under the term “cooking damage”, are expressed in a specially created comparative value, the C value.
This C-value is also calculated based on a temperature curve. The room temperature is used for this purpose. The calculation is done analogously to the F-value, also from and up to 55 degrees Celsius, but with other individual values, as shown in the table below.
Each time after this temperature has been reached, a corresponding C value is obtained per unit time of one minute. The C value for the cooking damage is based on the following values:
|Anfangsmesswert||55 Grad Celsius|
|Bezugstemperatur||100 Grad Celsius|
|Wert||38 (Wert für die Zeit-Temperatur-Abhängigkeit der Garung)|
|Grad Celsius||C-Wert||Grad Celsius||C-Wert|
The C values, unlike the F values, only increase tenfold every 38 degrees.
Ebenso, the table shows that it is not ideal to work with unnecessarily high temperatures with regard to cooking damage. If we assume, for example, that a product is to be brought to 65 degrees Celsius and this is achieved in 10 hours, whether we work at 78 degrees or 70 degrees room temperature, the following two C-values result:
At 10 hours with 70 degrees Celsius = 600 x 0,162 = C 97,2
At 10 hours with 78 degrees Celsius = 600 x 0,264 = C 158,4
Which final C-value is the best?
The optimal C-value cannot be quantified directly like the F-value. Rather, the cooking process should be designed so that the C-value is as low as possible. Based on the various considerations, it can be stated that a cooking process fulfils the following criteria:
|Erreichen eines idealen End-F-Wertes|
|Energietechnisch muss die Raumtemperatur laufend auf die absolut minimal notwendige Höhe angepasst werden.|
|Mit einem möglichst tiefen C-Wert Kochschädigungen vermeiden.|