Thermal sterilization moist heat

Relative thermal resistance for the different types of microorganisms encountered in typical environments associated with fermentation broths is shown in Table 24-3. Bacterial spores are far more resistant to moist heat than are any other type oi microbial contaminants thus, a sterilization cycle based on the destruction of bacterial spores should destroy all life. 

Moist heat offers the advantage of greater effectiveness at low temperatures. The thermal capacity of steam is much greater than that of hot air. Spores and vegetative forms of bacteria may be effectively destroyed in an autoclave employing steam (121°C) under pressure (ISpsig) for 20 min or (27psig at 132°C), for 3 min. The lag time to complete exposure of the material to be sterilized is important. 

For heat treatment, a D-value only refers to the resistance of a microorganism at a particular temperature. In order to assess the influence of temperature changes on thermal resistance a relationship between temperature and log D-value can be developed, leading to the expression of a z-value, which represents the increase in temperature needed to reduce the D-value of an organism by 90% (i.e. 1 log cycle reduction Fig. 20.2B). For bacterial spores used as biological indicators for moist heat (B. stearotbermopbilus) and dry heat (B. subtilis) sterilization processes, mean z-values are given as 10°C and 22°C, respectively. The z-value is not truly independent of temperature but may be considered essentially constant over the temperature ranges used in heat sterilization processes. 

As with sterilization by saturated steam, thermal damage to biological systems as a result of dry heat sterilization processes is a function of absorbtion of heat energy. Inactivation of microorganisms is by oxidation. The kinetics of oxidation and population death approximate to first-order reactions, but they are significantly different from the processes of coagulation of cellular proteins found with moist heat sterilization in that they require far higher temperatures and proceed more slowly. 

---