68 ◾ Fundamental Food Microbiology
reduced simultaneously. The process involves the loss of electrons from a reduced substance (thus
it is oxidized) and the gain of electrons by an oxidized substance (thus it is reduced). The electron
donor, because it reduces an oxidized substance, is also called a reducing agent. Similarly, the electron recipient is called an oxidizing agent. The redox potential, designated as Eh, is measured in
electrical units of millivolts (mV). In the oxidized range, it is expressed in +mV and in the reduced
range in –mV. In biological systems, the oxidation and reduction of substances are the primary
means of generating energy. If free oxygen is present in the system, then it can act as an electron
acceptor. In the absence of free oxygen, oxygen bound to some other compound, such as NO 3 and
SO 4 , can accept the electron. In a system where no oxygen is present, other compounds can accept
the electrons. Thus, the presence of oxygen is not a requirement of O-R reactions. 10
Redox Potential in Food
The redox potential of a food is influenced by its chemical composition, specific processing treatment given, and its storage condition (in relation to air). Fresh foods of plant and animal origin are
in a reduced state because of the presence of reducing substances, such as ascorbic acid, reducing
sugars, and the –SH group of proteins. Following stoppage of respiration of the cells in a food,
oxygen diffuses inside and changes the redox potential. Processing, such as heating, can increase
or decrease reducing compounds and alter the Eh. A food stored in air will have a higher Eh (+mV)
than when it is stored under vacuum or in modified gas (such as CO 2 or N 2 ). Oxygen can be present in a food in the gaseous state (on the surface, trapped inside) or in dissolved form.
Redox Potential and Microbial Growth
On the basis of their growth in the presence and absence of free oxygen, microorganisms have
been grouped as aerobes, anaerobes, facultative anaerobes, or microaerophiles. Aerobes need free
oxygen for energy generation as the free oxygen acts as the final electron acceptor through aerobic
respiration (see Chapter 8). Facultative anaerobes can generate energy if free oxygen is available, or
they can use bound oxygen in compounds such as NO 3 or SO 4 as final electron acceptors through
anaerobic respiration. If oxygen is not available, then other compounds are used to accept the
electron (or hydrogen) through (anaerobic) fermentation. An example of this is the acceptance
of hydrogen from NADH 2 by pyruvate to produce lactate. Anaerobic and facultative anaerobic
microorganisms can only transfer electrons through fermentation. Many anaerobes (obligate or
strict anaerobes) cannot grow in the presence of even small amounts of free oxygen as they lack
the superoxide dismutase necessary to scavenge the toxic oxygen free radicals. Addition of scavengers, such as thiols (e.g., thiolglycolate), helps overcome the sensitivity to these free radicals.
Microaerophiles grow better in the presence of less oxygen.
Growth of microorganisms and their ability to generate energy by the specific metabolic
reactions depend on the redox potential of foods. The Eh range at which different groups of
microorganisms can grow are as follows: aerobes, +500 to +300 mV; facultative anaerobes, +300
to +100 mV; and anaerobes, +100 to –250 mV or lower. However, this varies greatly with concentrations of reducing components in a food and the presence of oxygen. Molds, yeasts, and
Bacillus, Pseudomonas, Moraxella, and Micrococcus genera are some examples that have aerobic
species. Some examples of facultative anaerobes are the lactic acid bacteria and those in the family Enterobacteriaceae, such as Salmonella, Escherichia coli, and Yersinia spp. The most important
anaerobe in food is Clostridium. An example of a microaerophile is Campylobacter spp. The Eh
range indicates that in each group some species are stricter in their Eh need than others. Although
reduced simultaneously. The process involves the loss of electrons from a reduced substance (thus
it is oxidized) and the gain of electrons by an oxidized substance (thus it is reduced). The electron
donor, because it reduces an oxidized substance, is also called a reducing agent. Similarly, the electron recipient is called an oxidizing agent. The redox potential, designated as Eh, is measured in
electrical units of millivolts (mV). In the oxidized range, it is expressed in +mV and in the reduced
range in –mV. In biological systems, the oxidation and reduction of substances are the primary
means of generating energy. If free oxygen is present in the system, then it can act as an electron
acceptor. In the absence of free oxygen, oxygen bound to some other compound, such as NO 3 and
SO 4 , can accept the electron. In a system where no oxygen is present, other compounds can accept
the electrons. Thus, the presence of oxygen is not a requirement of O-R reactions. 10
Redox Potential in Food
The redox potential of a food is influenced by its chemical composition, specific processing treatment given, and its storage condition (in relation to air). Fresh foods of plant and animal origin are
in a reduced state because of the presence of reducing substances, such as ascorbic acid, reducing
sugars, and the –SH group of proteins. Following stoppage of respiration of the cells in a food,
oxygen diffuses inside and changes the redox potential. Processing, such as heating, can increase
or decrease reducing compounds and alter the Eh. A food stored in air will have a higher Eh (+mV)
than when it is stored under vacuum or in modified gas (such as CO 2 or N 2 ). Oxygen can be present in a food in the gaseous state (on the surface, trapped inside) or in dissolved form.
Redox Potential and Microbial Growth
On the basis of their growth in the presence and absence of free oxygen, microorganisms have
been grouped as aerobes, anaerobes, facultative anaerobes, or microaerophiles. Aerobes need free
oxygen for energy generation as the free oxygen acts as the final electron acceptor through aerobic
respiration (see Chapter 8). Facultative anaerobes can generate energy if free oxygen is available, or
they can use bound oxygen in compounds such as NO 3 or SO 4 as final electron acceptors through
anaerobic respiration. If oxygen is not available, then other compounds are used to accept the
electron (or hydrogen) through (anaerobic) fermentation. An example of this is the acceptance
of hydrogen from NADH 2 by pyruvate to produce lactate. Anaerobic and facultative anaerobic
microorganisms can only transfer electrons through fermentation. Many anaerobes (obligate or
strict anaerobes) cannot grow in the presence of even small amounts of free oxygen as they lack
the superoxide dismutase necessary to scavenge the toxic oxygen free radicals. Addition of scavengers, such as thiols (e.g., thiolglycolate), helps overcome the sensitivity to these free radicals.
Microaerophiles grow better in the presence of less oxygen.
Growth of microorganisms and their ability to generate energy by the specific metabolic
reactions depend on the redox potential of foods. The Eh range at which different groups of
microorganisms can grow are as follows: aerobes, +500 to +300 mV; facultative anaerobes, +300
to +100 mV; and anaerobes, +100 to –250 mV or lower. However, this varies greatly with concentrations of reducing components in a food and the presence of oxygen. Molds, yeasts, and
Bacillus, Pseudomonas, Moraxella, and Micrococcus genera are some examples that have aerobic
species. Some examples of facultative anaerobes are the lactic acid bacteria and those in the family Enterobacteriaceae, such as Salmonella, Escherichia coli, and Yersinia spp. The most important
anaerobe in food is Clostridium. An example of a microaerophile is Campylobacter spp. The Eh
range indicates that in each group some species are stricter in their Eh need than others. Although