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Exercise 22: Physical Methods of Control: Heat
Terms in this set (39)
What is extreme heat widely used for?
To control the growth of microbes
Why do we test for different time points?
Different time points for us to see if different bacteria will still grow, to find the thermal death time.
Compare the bactericidal effectiveness of dry heat and moist heat. evaluate heat tolerance of microbes.
What is the objective of this experiment?
To see the length of time required to kill all bacteria in a liquid culture at a given temperature.
What does heat/cold do to microbes?
Heat kills them
Cold inhibits them
How is heat sensitivity determined?
Genetically; Partially reflected in the optimal growth ranges and by the presence of heat-resistant endospores
Who are the most heat-tolerant organisms?
What are the optimal growth ranges of Bacteria?
Psychrophilic heat range:
0-20 degrees C
Psychrotrophic heat range:
20-30 degrees C
Mesophilic heat range:
25-40 degrees C
Thermophilic heat range:
45-65 degrees C
Hyperthermophilic heat range:
About 80 C degrees or higher
What can affect the heat sensitivity of organisms?
What are the two types of heating?
Dry or Moist heat
What is Dry Heat?
Denatures enzymes, dehydrates microbes, and kills by oxidation effects.
Ex. Hot-air ovens, flaming the loop
What is the standard application of dry heat in a hot-air oven?
170 C for 2hrs
The heat of the hot air is not readily transferred to a cooler body such as a microbial cell.
What transfers heat energy to the microbial cell more efficiently than dry air?
Moisture; Resulting in the denaturation of enzymes.
What are 3 types of Moist Heat methods?
Pasteurization, boiling, and autoclaving
What happens during Pasteurization?
The temperature is maintained at 63C for 30 minutes or 72C for 15 seconds to kill designated organisms that are pathogenic or cause spoilage.
What happens during Boiling?
100C for 10 minutes will kill vegetative bacterial cells; boiling DOES NOT kill endospores.
What is the most effective/best method of moist heat sterilization?
What happens during Autoclaving?
The use of steam under pressure.
Increased pressure raises the boiling point of water and produces steam with a higher temperature.
What are the standard conditions of Autoclaving?
15 psi, at 121C for 15 minutes
The effectiveness of heat against a specific microbe can be expressed as?
The Thermal Death Time
What is The Thermal Death Time? (TDT)
The length of time required to kill all bacteria in a liquid culture at a given temperature
What is the Thermal Death Point? (TDP)
The temperature required to kill all bacteria in a liquid culture in 10 minutes.
What is the Decimal Reduction Time? (DRT, or D Value)
The time, in minutes, in which 90% of a population of bacteria at a given temperature will be killed
What kind of streaking are we using in this lab?
In the exercise, was the thermal death time or the thermal death point determined?
Thermal Death Time
Compare the heat sensitivity of fungal spores to that of bacterial endospores
Heat activates fungal spores, while bacterial endospores are heat resistant
Compare the effectiveness of autoclaving and dry heat
Dry heat denatures enzymes, dehydrates microbes and kills by oxidation effects; autoclaving uses steam under pressure and kills endospores
Give an example of an application (use) of thermal death time
Give an example of incineration
Cremation of the dead
Give an example of pasteurization
Give an example of Autoclaving
Metals to be inserted into surgical tools and equipment
Define the term pasteurization, what is the purpose?
Process of heating food, usually liquid, to specific temperatures for predefined length of time and then cooling after heat is removed. Reduce bacterial population of a liquid like milk and destroy organisms that cause spoilage and disease.
Explain why fungi and Bacillus sometimes grow better after heat treatment.
Produce endospores, heat activates this. Bacillus and fungi able to divide and replicate because heat has activated endospores.
Assume that a DRT value for autoclaving a culture is 1.5 minutes. How long would it take to kill all the cells if 10^6 cells were present? What would happen is you stopped the heating process at 9 minutes?
1.5 minutes to kill 90%, after 1.5 minutes 10% remain. After 3 minutes 1% remain.
10^6*(0.1)= 0.1 = 10.5 minutes to kill 1 million cells.
10^6* (0.1)^4= 100 cells left after 9 minutes
Indicators are used in autoclaving to ensure that sterilization is complete. One type of chemical indicator turns color when it has reached a specific temp.; the other turns color when it has reached a specified temp and been exposed to steam. Which type should be used?
Indicators that change color when temp. is over 120. This means required temp. was found, and you are unlikely to have heat resistant spores.
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