**Temperature**

52 Salmonella – A Dangerous Foodborne Pathogen

Foods are generally considered ecosystems made up of a habitat and a community of living organisms (biocenosis) that can influence each other and, in turn, be influenced by the habitat itself. Given the minute size of the microorganisms, the environment that affects them, at least in a solid matrix, is very small – of the order of a few millimeters or centimeters – so very heterogeneous physical and chemical conditions can exist in food (different conditions between the surface and the inside). In addition, a succession of microbial communities can be observed in food. The original microbial load, largely depending on the initial sources of contamination, is then replaced by new microbial communities that depend on the set of factors that appear during production and conservation processes. The factors that influence the development and survival of contaminating microorganisms are: (i) intrinsic factors, i.e. the characteristics of the food, arising from its composition or structure, pH levels, water activity (Aw) and redox potential (OR-value); (ii) environmental, extrinsic factors which come into action during the processing or storage of the food (storage or treatment temperature; relative humidity, light, storage environment); they may affect the intrinsic factors; (iii) implicit factors derive from the interaction between populations during manufacturing or storage. They can either be positive, such as mutualism and commensalism, or negative, such as competition,

Although it is possible to control the growth or survival of an individual or a group of microorganisms acting on only one of these factors, this is not always desirable, because it could have excessive consequences on the sensorial and nutritional qualities of food. As modern consumers are increasingly in demand of foods that look "natural" or "fresh", that are safe and have a relatively long shelf life, it is often necessary to act using a combination of factors, each of which is present at sublethal levels, but that, together, ensure the desired level of control. Therefore, instead of using a single barrier, combinations of barriers are used (the so called "hurdle effect") which cause, if the exposure to these conditions is prolonged, such damage that the microorganisms irreparably lose the ability to multiply, reaching their inactivation. *Vice versa*, if the microorganisms are subjected to lower levels of stress (sublethal conditions), they can adapt by activating a number of protection mechanisms, synthesizing proteins and other substances that improve their resistance to the stress in question or different stress. In recent decades, specific mathematical models have been implemented to describe the phenomena such as the growth, the production of metabolites and the death of the microorganisms found in various conditions, useful both for the conservation and for the hygienic safety of food. Predictive models, in particular, see to formulate mathematical models using adequate experimental designs which should provide information about the danger or conservation of food and about the possibility of growth, death or production of a toxin from pathogenic microorganisms. So, in view of the previous observations, we can predict microbial behaviour in similar environmental conditions (Ross & McMeekin, 1994). These approaches, however, are not disadvantage free, such as the variation of strains and the biomolecular knowledge for understanding which factors are responsible for pathogenicity. As regards *Salmonella* the ranges of the factors that favour their growth, death or survival

**3. Dynamics of the** *Salmonella* **population: Ecological factors** 

antagonism and parasitism.

are shown in **Table 2**.

In particular, we can say that the minimum temperature for the growth of *Salmonella* is 7 °C (at 8 °C generation time is 22-35 hours); under 15 °C its development is still low. The storage of food at temperatures below 5 °C therefore prevents the multiplication of all serotypes; the only one able to develop up to 5.3 °C is *S*. Heidelberg (Matches & Liston, 1968). The highest mortality occurs during the slow freezing phase (0 to -10 °C), while in the deep-freezing one, for reaching temperatures below -17 °C rapidly, its survival is more likely, as damage to the cell membrane is minor. However, freezing does not guarantee the destruction of *Salmonella*: they have been found in frozen foods stored for years (ICMSF, 1996; Farkas, 1997), due to the changes and the production of cold shock and cold acclimation proteins (Scherer & Neuhaus, 2002). Maximum development temperature is 49.3 °C, beyond which *Salmonella* begin to die due to the denaturation of cell wall components and to the inactivation of heat-sensible enzymes. Although a temperature of 55 °C is sufficient to kill them, the legal limit for the storage of cooked foods meant to be eaten hot is normally 63 °C. *Salmonella* is not particularly resistant, so a pasteurization process is more than enough to destroy it. Several authors agree that the most heat-resistant serotype is *S*. Senftenberg 775W which registers D65= 0.29-2.0 and D60=1.0-9.0 when the substrate is in normal conditions, but the D value decreases if you move away from the optimal range for growth. Finally, its z value is 5,6-6,4 (°C). Resistance to heat is influenced by other factors, such as:

