During preservation processes for foods, an external agent is used to reduce the population of microorganisms present in the food. The population of vegetativecells such as E. coli, Salmonella, or Listeria monocytogenes will decrease in a pattern as shown in Figure 5.8 .
The population of microbial spores will decrease in a similar manner, butafter an initial lag period. These curves are referred to as microbial survivor curves. Although the shape of these curves is often described by a ﬁrst order model, there is increasing evidence thatalternate models are more appropriate when the application is the design of a preservation process.
A general model for description of the microbial curve would be:
22.214.171.124Resistencia térmica de los microorganismos. Valor D.
Decimal reduction time is usually employed in calculations of thermal treatment problems and can be deﬁned as the treatment time required to reduce thenumber of microorganisms to the tenth part, represented as DT. In thermal treatment calculations it is assumed that this time is independent of the initial concentration of microorganisms, but that itdepends on temperature, type of microorganism, and culture or food media in which microorganisms grow.
The following expression is obtained from Equation 15.2:
and since N = 0.1 N0, thedecimal reduction time is expressed as a function of the rate constant of thermal death as:
and treatment time is expressed according to Equation 15.4:
126.96.36.199 Efecto de latemperatura sobre el tiempo de muerte térmica. Valor Z
Thermal treatments are carried out at different temperatures, depending in each case on the needs or facilities of each industry. For this reason,thermal treatments are not necessarily performed at the temperatures at which thermal death data were obtained. Therefore, a relationship between thermal death time and temperature should be found. This...