The Control of Microbial Growth
The Terminology of Microbial Control
• The control of microbial growth can prevent infections and food spoilage.
• Sterilization is the process of destroying all microbial life on an object.
• Commercial sterilization is heat treatment of canned foods to destroy C. botulinum endospores.
• C. botulinum endospores will germinate at food storage temperatures
• Resistant thermophilic endospores will cause food spoilage, but don’t germinate at food storage temperatures – problems from these organisms comes at temperatures above 45° C.
• Disinfection is the process of reducing or inhibiting microbial growth on a nonliving surface.
The Control of Microbial Growth
The Rate of Microbial Death
Bacterial populations subjected to heat or antimicrobial
chemicals usually die
at a constant rate .
The Rate of Microbial Death
Such a death curve, when plotted
logarithmically, shows this constant death rate as a straight line.
The Rate of Microbial Death
The effectiveness of antimicrobial treatments depends on four factors:
•
Number of Organisms
– The number of organisms in the starting population determines how long it takes to reduce the number of organisms to a given level at a logarithmic death rate.
•
Environmental Influences
– Organic matter may interfere with heat treatments and chemical control agents.
– Heat is more effective at lower pH.
•
Time of Exposure
– Chemical agents require longer exposures to control more resistant organisms.
Physical Methods of Microbial Control
Heat
•
Heat is frequently used to eliminate microorganisms.
•
Moist heat kills microbes by denaturing enzymes.
•
Heat works better at low pH.
•
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.
•
Fifteen psi, 121° C for fifteen minutes kills all vegetative bacteria and endospores
Physical Methods of Microbial Control
Physical Methods of Microbial Control
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
Physical Methods of Microbial Control
Filtration
• 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.
• HEPA – 0.3 um pore size
• Decreases numbers
• Membrane filters composed of nitrocellulose or cellulose acetate are commonly used to filter out
bacteria, viruses, and even large proteins. (Figure 7.4)
Physical Methods of Microbial Control
Physical Methods of Microbial Control
Low Temperatures
• 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
• 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.
Desiccation
• 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 Osmotic Pressure
• Microorganisms in high concentrations of salts and sugars undergo plasmolysis.
Radiation
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.
Destroys DNA.
Non-ionizing radiation causes damage to DNA.
Ultraviolet causes formation of thymine dimers.
Microwaves heat water but it is possible to isolate vegetative forms from microwave ovens after usage and uneven heating in solid foods may leave live organisms in the food after microwave cooking.
