530 ◾ Fundamental Food Microbiology
It is also important to recognize that at a low A w , some microorganisms may develop resistance
to the killing effect of heat.
Modified Atmosphere
In vacuum-packaged (N 2 or CO 2 ) foods, growth of aerobes is prevented and that of facultative
anaerobes can be reduced. But there is a concern that vacuum packaging can selectively facilitate
growth of anaerobic spoilage and pathogenic bacteria. Thus, suitable combinations of other factors, such as low pH, A w , or preservatives, must be used with modified atmosphere to control their
growth.
Preservatives
Some preservatives, such as NaCl and BHA, act synergistically to increase the antimicrobial action
of sorbates. Organic acids are also effective at low pH because of higher concentrations of undissociated molecules. Some preservatives may not be effective at higher pH, and some may lose
potency during storage. Bacteriocins may be destroyed by proteolytic enzymes present in the food
(raw food). The bactericidal effect of bacteriocins can be enhanced when used in combination with
acids, SDS, and EDTA.
Hydrostatic Pressure Processing (HPP)
The antibacterial effect of ultrahigh hydrostatic pressure can be increased by incorporating mild
heat, slightly low pH, bacteriocins, lysozyme, and chitosan. 4 Spores are difficult to destroy unless
very high pressure is used (~700 MPa). However, after low-pressure treatment (≥100 MPa), the
spores may be induced to germinate, which can then be killed by higher pressure, mild heat, bacteriocins, or other antibacterial agents (see Chapter 40).
Conclusion
Many of the control methods listed, such as low heat, low pH, low A w , low temperature, and
hydrostatic pressure, induce sublethal stress among the surviving cells and spores. Cells and spores
injured by one parameter become sensitive to other parameters and are killed in their presence.
Sublethal injury to microbial cells and spores and increased susceptibility of injured cells and
spores to one or more preservation methods can play important roles in controlling microorganisms by a combination of factors. 6
It will be important to study the beneficial effect of combining different parameters at different
levels to determine their antimicrobial efficiency against target microorganisms in food systems.
The hurdle concept is based on the scientific concept that when two or more methods are used
together the combinations are more effective even at a much lower treatment level. However,
because of the large number of possibilities, actual product challenge studies may not be possible
for all. Instead, at the initial stage, computer simulation by using multifactorial parameters can
be designed that take into consideration the characteristics of a food. Then a limited number of
selected parameters can be studied by product inoculation. These methods are now being developed (see Appendix A).
It is also important to recognize that at a low A w , some microorganisms may develop resistance
to the killing effect of heat.
Modified Atmosphere
In vacuum-packaged (N 2 or CO 2 ) foods, growth of aerobes is prevented and that of facultative
anaerobes can be reduced. But there is a concern that vacuum packaging can selectively facilitate
growth of anaerobic spoilage and pathogenic bacteria. Thus, suitable combinations of other factors, such as low pH, A w , or preservatives, must be used with modified atmosphere to control their
growth.
Preservatives
Some preservatives, such as NaCl and BHA, act synergistically to increase the antimicrobial action
of sorbates. Organic acids are also effective at low pH because of higher concentrations of undissociated molecules. Some preservatives may not be effective at higher pH, and some may lose
potency during storage. Bacteriocins may be destroyed by proteolytic enzymes present in the food
(raw food). The bactericidal effect of bacteriocins can be enhanced when used in combination with
acids, SDS, and EDTA.
Hydrostatic Pressure Processing (HPP)
The antibacterial effect of ultrahigh hydrostatic pressure can be increased by incorporating mild
heat, slightly low pH, bacteriocins, lysozyme, and chitosan. 4 Spores are difficult to destroy unless
very high pressure is used (~700 MPa). However, after low-pressure treatment (≥100 MPa), the
spores may be induced to germinate, which can then be killed by higher pressure, mild heat, bacteriocins, or other antibacterial agents (see Chapter 40).
Conclusion
Many of the control methods listed, such as low heat, low pH, low A w , low temperature, and
hydrostatic pressure, induce sublethal stress among the surviving cells and spores. Cells and spores
injured by one parameter become sensitive to other parameters and are killed in their presence.
Sublethal injury to microbial cells and spores and increased susceptibility of injured cells and
spores to one or more preservation methods can play important roles in controlling microorganisms by a combination of factors. 6
It will be important to study the beneficial effect of combining different parameters at different
levels to determine their antimicrobial efficiency against target microorganisms in food systems.
The hurdle concept is based on the scientific concept that when two or more methods are used
together the combinations are more effective even at a much lower treatment level. However,
because of the large number of possibilities, actual product challenge studies may not be possible
for all. Instead, at the initial stage, computer simulation by using multifactorial parameters can
be designed that take into consideration the characteristics of a food. Then a limited number of
selected parameters can be studied by product inoculation. These methods are now being developed (see Appendix A).