The Control of Microbial Growth |
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| Objectives: 5. List and describe six physical
methods of microbial control.
8. Describe how to sterilize
heat labile solutions.
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Heat is frequently used to eliminate microorganisms.
Moist heat kills microbes by denaturing enzymes.
Thermal death point (TDP) is the lowest temperature at which all the microbes in a liquid culture will be killed in 10 minutes.
Thermal death time (TDT) is the length of time required to kill all bacteria in a liquid culture at a given temperature.
Decimal reduction time (DRT) is the length of time in which 90% of a bacterial population will be killed at a given temperature.
Boiling (100º C) kills many vegetative cells and viruses within ten minutes.
Autoclaving (steam under pressure) is the most effective method of moist heat sterilization. The steam must directly contact the material to be sterilized.
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Pasteurization
A high temperature is used for a short time to destroy pathogens and food spoilage organisms without altering the flavor of the food.
Classic pasteurization was done at 63° C for thirty minutes.
HTST pasteurization is done at 72º C for fifteen seconds.
Ultra-high-temperature (UHT) treatment (140º C for three seconds) is used for dairy products.
Dry Heat Sterilization
Methods of dry heat sterilization include direct flaming, incineration, and hot -air sterilization (170° C for two hours).
Dry heat kills by oxidation.
Different methods that produce the same effect (reduction in microbial growth) are called equivalent treatments.
Filtration is the passage of a liquid or gas through a filter with pores small enough to remain microbes.
Microbes can be removed from air by high -efficiency particulate air filters.
Membrane filters composed of nitrocellulose or cellulose acetate are commonly used to filter out bacteria, viruses, and even large proteins. (Figure 7.4)
The effectiveness of low temperatures depends on the particular microorganism and the intensity of the application.
Most microorganisms do not reproduce at ordinary refrigerator temperatures (0-7º C).
Many microbes survive (but do not grow) at subzero temperatures used to store foods.
High pressure denatures proteins in vegtetative cells.
Endospores aren’t usually harmed, but can be allowed to grow out and the vegetative form killed with high pressure.
Used in Japan and the U.S. to preserve fruit juices – preserves the flavor, color, and nutrient values.
In the absence of water, microorganisms can grow but cannot remain viable.
Viruses and endospores can resist desiccation.
Mycobacterium tuberculosis is fairly resistant to desiccation but the gonorrhea bacterium is very sensitive.
Environmental conditions effect efficiency of desiccation – pus, feces, and mucus are protective
Microorganisms in high concentrations of salts and sugars undergo plasmolysis.
Molds and yeasts are more capable than bacteria of growing in materials with low moisture or high osmotic pressure.
The effects of radiation depend on its wavelength, intensity and duration.
Ionizing radiation (gamma rays, X-rays and high-energy electron beams) has a high degree of penetration and exerts its effect primarily by ionizing water and forming highly reactive hydroxyl radicals.
Non-ionizing radiation causes damage to DNA.
