Heat sterilization

Temperature affects all chemical reactions. The Arrhenius equation is:

where k = reaction rate coefficient
A = some constant
e = basis of Napierian logarithms
E = activation energy
R = universal gas constant
T = absolute temperature

We can plot logarithm of k versus 1/T to get a straight line and calculate E from the slope. Among the several factors that influence killing are temperature, pH, osmotic pressure, shear, mass transport, and concentrations of extraneous substances that also react with the killing agent. These factors operate synergistically, and temperature plays roles other than simply affecting the kinetics of a reaction.

Two waste treatment processes raise the temperature well above the normal boiling point and hold the material long enough for complete killing; these are wet oxidation and heat syneresis of sludge. Neither process has sterilization as its main objective, but it is a definite plus when considering these options. Environmental engineers seldom select heat sterilization except for special situations such as hospital wastes. Pasteurization of milk aims at killing pathogenic organisms and reducing the levels of spoilage organisms; environmental engineers engaged in public health tasks may well need to study this process.

Various life forms differ widely in their resistance to heat. Most pathogenic organisms thrive only within a very narrow temperature range, with the usual optimum at the body temperature of their host. Bacterial endospores are seedlike structures that endure through hard times such as desiccation. While some spores are not particularly resistant to heat, many can survive prolonged exposure to elevated temperature. Once in a while, a strain of organisms may contaminate a laboratory because its spores have abnormally high heat resistance and are not killed by the autoclaving procedures that eliminate its competitors.

Batch sterilization uses steam or direct firing to elevate the temperature, and then cooling water stops the process and brings the material back toward room temperature. Both the heat and the cooling water are spent with no opportunity for energy recovery. Large volumes should be passed continuously through heat exchangers for energy economy with the hot, treated fluid heating the cold, incoming feed. High temperatures for short times are used in preparing nutrient media for industrial fermentations and in pasteurizing milk, because this causes less damage to biochemicals than more prolonged times at lower temperatures. This exploits the temperature effects on activation energies because bacterial killing is affected by a temperature change more than is heat destruction of biochemicals.