I. VUKOVIC (Belgrade University) and P. NITSCH (BAFF/Kulmbach) have developed a method of retrospective rapid determination of the germicidal effect of heating processes using the so-called F value. This makes it possible to calculate the F value of a heating process in a matter of seconds merely by observing a few measurements. Both the F70 value for fresh products and the F121.1 value for heating preserves can be determined. The method is highly accurate and superior to other methods of rapid determination described so far. Free downloads are available for each method (F121.1 or F70), consisting of three worksheets in EXCEL format readable from EXCEL 5.0 upward, and two worksheets in StarCalc format (*.SDC) for StarOffice 5 and OpenOffice (important for LINUX users).
One of the features vital for producing microbially safe and stable food preserves as well as fresh products is the intensity of heating the preserve or package necessary to achieve the desired shelf life. This can only detected and documented during quality control by determining the F value.
There are no (longer any) statutory regulations, but certain F values are accepted as necessary for the production of stable meat products.
Contrary to popular opinion, preserves are NOT sterile in the same way as medical dressings, infusions etc. This is not possible in terms of flavour in any case. Instead, preserved foods are heated depending on the intended purpose according to the motto - as much as necessary, as little as possible.
A distinction is made between four types of heating:
Heating takes place at temperatures of at most 100°C. F values of over 0.4 must be achieved. Irrespective of quality, the shelf life is limited to one year. Preserves of this kind must be kept refrigerated at temperatures of <10°C (or better <7°C), because only cryophilic spore-formers and normal (i.e. vegetative) germs are destroyed.
2. Three-quarter preservation
Heating takes place at temperatures over 100°C. (However, three-quarter preservation can also be achieved when small sizes of preserves are boiled.) The F values are between 0.6 and 0.8. Irrespective of quality, the shelf life is limited to one year. Preserves of this kind must be refrigerated at temperatures of <10°C because in addition to the above germs, only some types of bacillus are destroyed.
3. Full stable cans
Heating takes place at temperatures over 100°C. F values of over 3 must be achieved. Irrespective of quality, the shelf life is limited to four years without refrigeration, at 25°C. Everything except thermophilic spore formers (which, however, only consist of food spoilers and grow at storage temperatures significantly above 25°C) is destroyed.
4. Cans under tropical conditions
Heating takes place at temperatures over 100°C. F values of over 12 must be achieved. Irrespective of quality, the shelf life is limited to one year without refrigeration, at 40°C. (The shorter shelf-life compared with full preservation, which initially appears paradoxical, results from the much more rapid flavour deterioration of preserves at higher storage temperatures.) All hazardous germs are destroyed.
F70 values of 30 to 60 are recommended for fresh produce in order to achieve microbial stability. However, in practice these values are often traditionally far exceeded in certain products. The rule of thumb here is: the lower the germ count of the initial material, the lower the necessary F70 value.
The calculations necessary to determine the F value from measured temperatures during heating are performed automatically by special devices known as integrators or computers. However, for reasons of cost these devices are often barely used in the production of preserves by small-scale craft businesses, farm shops or even some medium-sized companies. It is also possible to determine the F value manually. The drawback here is the time and manpower input needed. The same applies when F values are calculated from available core temperature graphs, or when F value measuring records are to be checked for accuracy/plausibility. This is especially true for records of longer heating times (large packs, gentle heating etc.).
Vukovic (1985) developed a highly accurate graphical method based on Gaussian integration of the core temperature course to determine the F value of meat preserves from just three measuring points to be located on the heating curve. With the worksheets for spreadsheet programs available in the Internet as a free download an easy to use solution is now offered.
For canning (F10/121.1; target germ = C. botulinum)
Extensive experiments revealed an accuracy of +/- 1.8%. In the case of tropical preserves, which can generally only be calculated less accurately using these methods, the maximum deviations were 0.4 from the F value determined by addition (F>12) for each heating process, equivalent to a maximum deviation of +/- 3.3%. This means that a highly accurate, easy-to-use and rapid method is available for checking and calculating F values from graphs or measurement tables.
For fresh products (F70, target germ = D-streptococci)
Experiments revealed that the level of accuracy attained in rapid calculation of F values for full and tropical preserves was not quite achieved in rapid calculation of F70 values. However, a slight modification of the calculating method can reduce the deviations to +/- 3.1%, which in these experiments corresponded to an F70 value of 2.2. The method tended to produce slightly lower F70 values than obtained with the additive method, which increases the safety of the method under microbial aspects. The deviations are irrelevant for practice because due to the customary control steps at minute intervals they are no longer controllable.
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The canning of fish and meat.
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Mitteilungsblatt der Bundesanstalt f. Fleischforschung 36, 386
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Schnellbestimmung des F-Wertes mittels Gauß'scher Integration.
Fleischwirtschaft 11, 122-123
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Tehnologija Mesa, 4/Vol 26, 120-124
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Tabellarische Methode zur Bestimmung des Sterilisationswertes für Fleischkonserven.
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