Fundamental Food Microbiology, Third Edition - Fuad Fathir

FACTORS INFLUENCING MICROBIAL GROWTH IN FOOD 75
proton motive force as well as the ability of the cells to generate energy. The
information on the influence of pH on growth and viability of microbial cells is
important to develop methods to prevent the growth of undesirable microorganisms
in food (e.g., in acidified foods; see Chapter 35), used to produce some fermented
foods (e.g., sequential growth of lactic acid bacteria in sauerkraut fermentation), and
to selectively isolate aciduric microorganisms from food (e.g., yeasts and molds in
a medium with pH 3.5). 8,9 Acquired acid tolerance by pathogens and spoilage bacteria
can impose problems in their control in low-pH foods; this aspect is discussed in
Chapter 9.
E. Redox Potential, Oxygen, and Growth
1. Principle
The redox or oxidation–reduction (O–R) potential measures the potential difference
in a system generated by a coupled reaction in which one substance is oxidized and
a second substance is 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 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, presence of oxygen is not a requirement of O–R reactions. 10
2. 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 –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.
3. 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
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