Dimensioning heat treatments and thermal processing

The information below comes from National Canners Association (NCA, 1968) and Stumbo (1973). The dimensioning of  thermal processes refers to calculating the degree of lethality (F), which is defined as the time necessary to attain a number of pre-established  decimal reductions in the population of an pre-established target microorganism at a given reference temperature. The calculation uses the Equation 1 (Log N⁰ – Log N^f = t/D ) of the survival curve, where t (time in minutes) is called F:

 t = F = D Log(N₀/N_f)  (2)

According to this equation, the time necessary to promote n decimal reductions of the target population, that is, to reach a final population = N_f = N₀/10n is: F = D Log(N₀/N₀/10ⁿ) = D Log10n = nD. Or in other words, the time required to reach n decimal reductions is simply F = nD. When the reference temperature is 121°C (250°F) F is called F0.

Definition of the intensity of the thermal process

The longer the time and the higher the temperature of a heat treatment, the more intense (the more lethal) will be the treatment and the smaller the chance of survival of the microorganisms. However, the time and the temperature may not be indiscriminately increased, as this would compromise the nutritional and sensory quality of the food.

For  low acid foods, the reference temperature is 121.1ºC (application of heat under pressure) and the microorganisms considered in the dimensioning of the process are  C. botulinum and other spore forming bacteria.

For  C. botulinum, the intensity of the process must be sufficient to promote 12 decimal reductions in the population of the most heat-resistant spores of the species (types A and B). The D value121.1ºC of these spores is 0.21 min and the greatest z value determined is 10ºC. Hence, the time equivalent to the reference temperature (in this case F₀) is calculated as: F₀  = nD  = 12  × 0.21  = 2.5 min. This means that the lethality of the actual process, at non-constant temperature, should be equivalent to 2.5 min at 121.1ºC.

If we assume an initial contamination of one spore per package (probably secure, according to Stumbo, 1973), the final contamination would be N_f = N₀/1012 = 1/1012 = 10−12. The N_f value is called the survival probability or sterility assurance level. The value of 10⁻¹² means that there is a chance that one in each 10¹² packages may contain viable spores after heat treatment.

For other facultative mesophilic and thermophilic spore forming bacteria, which do not represent a risk from the point of view of public health, manufacturers work with a survival probability of sterility assurance level of 10⁻⁵ (five decimal reductions) (Stumbo, 1973). If we assume the greatest z and D121.1ºC values of  Clostridium sporogenes (1.5 min and 10°C), which is one of the most heat-resistant spore forming spoilage bacteria, the time equivalent to five decimal reductions is F₀  = nD  = 5  × 1.5  = 7.5 min. Therefore, the heat processing of low acid foods is more intense than that required for  C. botulinum.

In the case of foods intended to be stored at temperatures above 40°C (vending machines), strictly thermophilic spore forming bacteria must be taken into consideration. According to Stumbo (1973), the degree of lethality of several thermal processes used in the United States for these foods is equivalent to 14–16 min at 121.1°C. Using the highest D121.1ºC and z values of G. stearothermophilus (5 min and 12.2°C), the number of decimal reductions of this treatment, for this bacteria, is  n  = F/D  = 14–16/5  = 2.8 to 3.2 reductions.

For acid foods, the reference temperature is lower than 100°C but is not pre-established, i.e., the temperature is choosed to be lethal for the microorganisms with the highest heat resistance among those that may grow in this kind of food.

References

Stumbo, C.R. (1973) Thermobacteriology in Food Processing. 2^nd edition. New York, Academic Press.

Silva, N.D .; Taniwaki, M.H. ; Junqueira, V.C.A.;  Silveira, N.F.A. , Nasdcimento , M.D.D. and Gomes ,R.A.R .(2013) . Microbiological examination methods of food and water a laboratory Manual. Institute of Food Technology – ITAL, Campinas, SP, Brazil .