Calculation of Heat Sterilization Times for Fermentation Media

Sterilization of nutrient media is an operation essential to all industrial fermentation processes re- quiring pure culture maintenance.AMethods for steri- lizing nutrient media include 1) heating, 2) filtration, 3) irradiations, 4) sonic vibration, and 5) exposure to chemical agents.Because heating the medium, usually by steam, is the most reliable and on a large scale the easiest to control, it is the method of choice throughout the fermentation industry.The increasing availability of radioactive isotopes undoubtedly will stimulate further study of the use of gamma radiation for sterili- zation.However, a great deal of work is needed before commerically feasible gamma ray applications will be used in the fermentation industry.The other methods, although successful on a laboratory scale, present too many operational drawbacks for large-scale use in fermentation processes.How long should a nutrient medium be exposed at high temperatures to achieve sterile conditions?This question arises often and usually is answered in bench- scale or pilot plant tests.Improper translation of these test results with large equipment may subject the medium to unnecessary overheating.A method by which minimum exposure time to achieve sterile condi- tions can be calculated from easily obtainable thermal- death relationships and the temperature conditions in a heat sterilization process is presented in this paper.The method can be used to correlate sterilization conditions among various sized fermentation vessels.It also per- mits evaluation of the temperature and retention time relationship in continuous sterilizers.It need not be restricted to fermentation processes, but should be applicable to any process involving a heat sterilization or pasteurization operation.Thermal Resistance of Microorganisms The ability of microorganisms to withstand heat is much greater than that of other forms of life.Thermo- philes capable of even tolerating common heat sterili- zation conditions, such as exposure to steam at 250 F for 20 to 30 min, exist.Their occurrence in nutrient media, however, is rare.Relative resistances of several microorganisms to moist heat are shown in table 1.