Microorganisms heat sterilization

The lethal effects of dry heat on microorganisms are due largely to oxidative processes which are less effective than the hydrolytic damage which results firm exposure to steam. Thus, dry heat sterilization usually employs higher temperatures in the range 160-180°C and requires exposure times of up to 2 horns depending upon the temperatrrre employed (section 10). 

Dry-heat processes kill microorganisms primarily through oxidation. The amount of moisture available to assist sterilization in dry-heat units varies considerably at different locations within the chamber and at different time intervals within the cycle. Also, the amount of heat available, its diffusion, and the environment at the spore/air interface all influence the microorganism kill rate. Consequently, cycles tend to be longer and hotter than would be expected from calculations to ensure that varying conditions do not invalidate a run. In general, convection dry-heat sterilization cycles are run above 160°C [37],... 

The heat sterilization of microorganisms and heat inactivation of enzymes are examples of first-order reactions. In the case of an enzyme being irreversibly heat-inactivated as follows ... 

A culture medium that is contaminated with 10 ° rn microbial spores of microorganisms will be heat-sterilized with steam of 121 °C. At 121 °C, the specific death rate of the spores can be assumed to be 3.2 mm [1]. When the contamination must be reduced to 1 in 1000 fermentations, estimate the required sterihzation time. 

Reduction of microorganisms in milk before cheese production must be achieved in such a way that the functionality of the milk proteins is not affected. Heat-resistant spores, such as Clostridium tyrobutyricum and C. sporogenes can cause severe spoilage of the cheese by late fermentation that can result in the production of H2 and CO2 gases, and unpleasant smelling fermentation products [102]. Although heat sterilization reduces these spores, because of the heat-induced complexation between 3-Lg and K-casein, UHT milk normally does not form a rennet gel and consequently could not be used efficiently for cheesemaking [87]. 

Ethylene Oxide. Ethylene oxide, C2H4O. is a colorless flammable gas that liquefies at I2°C. It has been used to sterilize temperature-sensitive medical equipment and certain pharmaceuticals that cannot be heat sterilized in an auuxrlavc. Ethylene oxide diffuses readily through porous materials and very effectively de.stroys all forms of microorganisms at ambient temperatures."... 

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. 

Whenever possible, products should be sterilized in the final container, preferably by heat sterilization. Certain solutions and liquids that cannot be sterilized in the final container can be filtered through a sterile fitter of nominal pore size 0.22pm (or less), or with at least equivalent microorganism-retaining properties, into a previously sterilized container. Such filters can remove bacteria and moulds, but not all viruses or mycoplasmas. Consideration should be given to complementing the filtration process with some degree of heat treatment. 

A new field of radiation application is the radiation sterilization of drugs. The requirement for sterility of drugs depends on type of administration. It concerns mostly ophthalmic preparations, sterile topical products, and injectable solutions. In some cases, radiation sterilization appears to be an attractive alternative to other types of sterilization, e.g., heat sterilization. For drugs the usual requirement (six orders of magnitude decrease in the microorganism level) is achieved by applying an absorbed dose of 25 kGy. 

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