Examples and problems
1. A continuous sterilizer is to treat 50m 3 of a complex medium in 2 hours. Kd @121C is 1/min while for 140C, 61/min. Unsterilized medium has an initial microbial count of 10 4 # of spores/ L. Calculate the residence time in the sterilizer for the 2 temperatures if a 99% successful sterilization is achieved for 4 weeks of operation. 2. An autoclave malfunctions and is only able to reach temperature of up to 119.5C for 20 min. Kd for 121C is 1/min. a. Determine the probability of failure in sterilization for 119.5C, 20 min. b. Determine the probability of failure in sterilization if temperature is brought back to 121C for 15 min. as standard specification for USFDA. Compare with a. 4
3. Determine the loss of thiamine for 2 temperatures during sterilization of a vitamin-containing medium with initial spore number of 10 . Sterilization for 111C will be accomplished in 25 min. Use data for kd; k d@111C, /min k
[email protected], /min Bacillus Thermophillus Thermophillus 0.4 0.017 Thiamine 0.2 0.0125 4. Medium at a flowrate of 2 m 3/hr is to be sterilized by a heat exchanger with a wet steam in a continuous set-up. The medium contains bacterial spores spores at a concentration concentration of 5x10 5x10 12/ m3. Ed = 283 kJ/gmol and arrhenius constant is = 5.7x10 39/hr with contamination risk of 1 organism as acceptable criteria for every 60-day operation. Sterilizer cylindrical pipe has an inner diameter of 0.1 meter and the length of the holding section is 24 m. Density of medium is 1000 kg/m 3 and viscosity 3.6 Pa.s. What sterilization temperature is required? 3
5. A 15m chemostat is operated with dilution rate of 0.1/hr. A continuous sterilizer with steam injection and flash cooling delivers sterilized medium to the fermenter. Medium in the holding section of the sterilizer is maintained at 130C. The concentration of contaminants in the raw medium is 10 5/mL. An acceptable risk is 1 organism every 3 months of operation. Arrhenius constant and E d are 7.5x1039/hr and 288.5 kJ/gmole respectively. respectively. Sterilizer pipe diameter is 12 cm. Liquid density is same as water with viscosity 4 Pa.s. a. Assuming perfect plug-flow, determine the length of the holding section. b. What is then the length if axial dispersion is accounted? c. What is the rate of fermenter contamination with the length in a. but with axial dispersion?
6. Figure below shows Arrhenius plot for the deactivation of B. stearothermophilus spores and thiamine degradation. Fill up the table below to estimate loss of thiamine and determine the best operating temperature.
Sterilization temperature, 0C
Time to reach N/No=10-16(min.)
% Loss in thiamine
110 130 150 7. A tubular sterilizer (inner diameter 0.102m) with steam injector and a flash cooler is used to sterilize the culture medium with a flow rate of 3 12 3 2m /hr. The time for heating and cooling is negligible with this type of equipment. The typical spore count of the medium is 5x10 /m , whic needs to be reduced where 1 MO can survive in 2 months operation. Steam at 600kPa (gauge) is available to bring the sterilizer operating temperature to 125C. Determine the length of sterilizer.
Graphs for Peclet/ Damkohler number and Reynolds/ Peclet number
