Intrinsic and Extrinsic Parameters of Foods That Affect Microbial Growth
51
bacteria such as some members of the genus Bacillus. Some aerobic bacteria actually grow better
under slightly reduced conditions, and these organisms are referred to as microaerophiles. Examples
of microaerophilic bacteria are lactobacilli and campylobacters. Some bacteria have the capacity to
grow under either aerobic or anaerobic conditions. Such types are referred to as facultative anaerobes.
Most molds and yeasts encountered in and on foods are aerobic, although a few tend to be facultative
anaerobes.
With regard to the Eh of foods, plant foods, especially plant juices, tend to have Eh values of from
+300 to 400 mV. It is not surprising to find that aerobic bacteria and molds are the common cause of
spoilage of products of this type. Solid meats have Eh values of around −200 mV; in minced meats,
the Eh is generally around 200 mV. Cheeses of various types have been reported to have Eh values on
the negative side, from −20 to around −200 mV.
With respect to the Eh of pre-rigor as opposed to post-rigor muscles, Barnes and Ingram
2,3 undertook
a study of the measurement of Eh in muscle over periods of up to 30 hours postmortem and its effect
on the growth of anaerobic bacteria. These authors found that the Eh of the sternocephalicus muscle
of the horse immediately after death was +250 mV, at which time clostridia failed to multiply.
At 30 hours postmortem, the Eh had fallen to about 30 mV in the absence of bacterial growth.
When bacterial growth was allowed to occur, the Eh fell to about 250 mV. Growth of clostridia was
observed at Eh values of 36 mV and below. These authors confirmed for horse meat the finding
for whale meat: that anaerobic bacteria do not multiply until the onset of rigor mortis because of
the high Eh in pre-rigor meat. The same is undoubtedly true for beef, pork, and other meats of this
type.
Eh Effects
Microorganisms affect the Eh of their environments during growth just as they do pH. This is true
especially of aerobes, which can lower the Eh of their environment while anaerobes cannot. As aerobes grow, O 2 in the medium is depleted, resulting in the lowering of Eh. Growth is not slowed,
however, as much as might be expected due to the ability of cells to make use of O 2 -donating or
hydrogen-accepting substances in the medium. The result is that the medium becomes poorer in oxidizing and richer in reducing substances.
32 The Eh of a medium can be reduced by microorganisms
by their production of certain metabolic byproducts such as H 2 S, which has the capacity to lower
Eh to −300 mV. Because H 2 S reacts readily with O 2 , it will accumulate only in anaerobic environments.
Eh is dependent on the pH of the substrate, and the direct relationship between these two factors is
the rH value defined in the following way:
Eh = 2.303
RT
F
(rH − 2pH)
where R = 8.315 joules, F = 96,500 coulombs, and T is the absolute temperature.
34 Therefore, the
pH of a substrate should be stated when Eh is given. Normally Eh is taken at pH 7.0 (expressed
Eh
). When taken at pH 7.0, 25
◦ C, and with all concentrations at 1.0 M, Eh = Eh
o (simplified Nernst
equation). In nature, Eh tends to be more negative under progressively alkaline conditions.
Among naturally occurring nutrients, ascorbic acid and reducing sugars in plants and fruits and
–SH groups in meats are of primary importance. The presence or absence of appropriate quantities
of oxidizing—reducing agents in a medium is of obvious value to the growth and activity of all
microorganisms.
51
bacteria such as some members of the genus Bacillus. Some aerobic bacteria actually grow better
under slightly reduced conditions, and these organisms are referred to as microaerophiles. Examples
of microaerophilic bacteria are lactobacilli and campylobacters. Some bacteria have the capacity to
grow under either aerobic or anaerobic conditions. Such types are referred to as facultative anaerobes.
Most molds and yeasts encountered in and on foods are aerobic, although a few tend to be facultative
anaerobes.
With regard to the Eh of foods, plant foods, especially plant juices, tend to have Eh values of from
+300 to 400 mV. It is not surprising to find that aerobic bacteria and molds are the common cause of
spoilage of products of this type. Solid meats have Eh values of around −200 mV; in minced meats,
the Eh is generally around 200 mV. Cheeses of various types have been reported to have Eh values on
the negative side, from −20 to around −200 mV.
With respect to the Eh of pre-rigor as opposed to post-rigor muscles, Barnes and Ingram
2,3 undertook
a study of the measurement of Eh in muscle over periods of up to 30 hours postmortem and its effect
on the growth of anaerobic bacteria. These authors found that the Eh of the sternocephalicus muscle
of the horse immediately after death was +250 mV, at which time clostridia failed to multiply.
At 30 hours postmortem, the Eh had fallen to about 30 mV in the absence of bacterial growth.
When bacterial growth was allowed to occur, the Eh fell to about 250 mV. Growth of clostridia was
observed at Eh values of 36 mV and below. These authors confirmed for horse meat the finding
for whale meat: that anaerobic bacteria do not multiply until the onset of rigor mortis because of
the high Eh in pre-rigor meat. The same is undoubtedly true for beef, pork, and other meats of this
type.
Eh Effects
Microorganisms affect the Eh of their environments during growth just as they do pH. This is true
especially of aerobes, which can lower the Eh of their environment while anaerobes cannot. As aerobes grow, O 2 in the medium is depleted, resulting in the lowering of Eh. Growth is not slowed,
however, as much as might be expected due to the ability of cells to make use of O 2 -donating or
hydrogen-accepting substances in the medium. The result is that the medium becomes poorer in oxidizing and richer in reducing substances.
32 The Eh of a medium can be reduced by microorganisms
by their production of certain metabolic byproducts such as H 2 S, which has the capacity to lower
Eh to −300 mV. Because H 2 S reacts readily with O 2 , it will accumulate only in anaerobic environments.
Eh is dependent on the pH of the substrate, and the direct relationship between these two factors is
the rH value defined in the following way:
Eh = 2.303
RT
F
(rH − 2pH)
where R = 8.315 joules, F = 96,500 coulombs, and T is the absolute temperature.
34 Therefore, the
pH of a substrate should be stated when Eh is given. Normally Eh is taken at pH 7.0 (expressed
Eh
). When taken at pH 7.0, 25
◦ C, and with all concentrations at 1.0 M, Eh = Eh
o (simplified Nernst
equation). In nature, Eh tends to be more negative under progressively alkaline conditions.
Among naturally occurring nutrients, ascorbic acid and reducing sugars in plants and fruits and
–SH groups in meats are of primary importance. The presence or absence of appropriate quantities
of oxidizing—reducing agents in a medium is of obvious value to the growth and activity of all
microorganisms.