Oxidation Reduction Potential

The tendency of a substrate to accept or donate electrons, is termed its redox
potential (Eh). The O/R potential of a substrate may be defined generally as
the ease with which the substrate loses or gains electrons. When a substrate
loses electrons, the substrate is oxidized while a substrate that gains electrons
becomes reduced.

Oxidation Reduction Potential

Therefore, a substance that readily gives up electrons is a
good reducing agent, while one that readily takes up electrons is a good
oxidizing agent. In the equation below, this is represented in its most general
form to include the many redox reactions, which also involve protons and
have the overall effect of transferring hydrogen atoms.

+ ne [Reductant]

Where n is the number of electrons, e, transferred. 

The tendency of an atom or molecule to accept or donate electrons is
expressed as its standard redox potential, Eo’. When electrons are transferred
from one compound to another, a potential difference is created between the
two compounds. This difference may be measured by use of an appropriate
instrument and expressed as millivolts (mv). 

It can be measured against an
external reference by an inert metal electrode, usually platinum. The more
highly oxidized a substance, the more positive will be its electrical potential,
and the more highly reduced a substance, the more negative will be its
electrical potential.

Redox Couples in Food

Pair of oxidizing and reducing agents present in food are known as redox
couples. A large positive Eo’ of food indicates that the oxidized species of the
couple is a strong oxidizing agent and the reduced form only weakly reducing. 
A large negative Eo’ of food indicates the reverse. When the concentration of
oxidant and reductant is equal, a zero electrical potential exists. The relative
proportions of oxidized and reduced species present will also influence the
measured Eh. 
If the balances of the various redox couples present favours the
oxidized state then there will be a tendency to accept electrons from the
electrode creating a positive potential, which signifies an oxidizingenvironment. If the balance is reversed, the sample will tend to donate
electrons to the electrode, which will then register a negative potential – a
reducing environment. 
With the notable exception of oxygen, most of the couples present in foods,
e.g. glutathione and cysteine in meats and ascorbic acid and reducing sugars in
plant products, would on their own tend to establish reducing conditions. 
Oxygen, which is present in the air at a level of around 21 %, is usually the
most influential redox couple in food systems. It has a high Eo’ and is a
powerful oxidizing agent. If sufficient air is present in food, a high positive
potential will result and most other redox couples present will, if allowed to
equilibrate, be largely in the oxidized state. 
Hence, increasing the access of air
to food material by chopping, grinding or mincing will increase its Eh.
Similarly, exclusion of air as in modified vacuum packing or canning will
reduce the Eh.

Effect of Microbial Growth on Redox Potential of Food

Microbial growth in food reduces its Eh. This is usually because during their
growth, microorganisms consume oxygen and produce reducing compounds
such as hydrogen. Oxygen is the most important terminal electron accepter in
the electron transport chain, especially in case of aerobes. 
During passage of
electrons through the electron transport chain, microorganisms generate
energy and thereby oxygen is depleted. As the oxygen content of the medium
decreases, so the redox potential declines from a positive potential to a
negative potential. 
The decrease in Eh as a result of microbial activity is the basis of some rapid
tests for determination of microbial load of food, particularly dairy products.
Redox dyes such as methylene blue or resazurin are used to indicate changes
in Eh, which are correlated with microbial levels. 
These dyes become
colourless when these are reduced. The time taken for reduction of the dyes
will be inversely proportional to the microbial load of food i.e. more the
microorganisms in food, less is the time taken for dye to be reduced and vice
versa. The factors influencing redox potential of foods are summarized in
Table 2.3 given below:

Factors affecting redox potential of foods

1. Redox couples present 
2. Ratio of oxidizing species to reducing species 
3. pH
4. Poising capacity 
4. Availability of oxygen 
5. Microbial activity

Effect on Microbial Growth and Ecology

Redox potential exerts an important selective effect on the microflora of a
food since it will decide the type of microorganism which can grow in that
food. Microbial growth can occur over a wide spectrum of redox potential. 
However, individual microorganisms have their own redox ranges over which
they can grow. They are classified into one of several physiological groups on
the basis of the redox range over which they can grow and their response to
oxygen. Based on their ability to use free oxygen, microorganisms have been
classified as
1. Aerobic when they require free oxygen. 
2. Anaerobic when they grow best in the absence of free oxygen. 
3. Facultative when they grow well either aerobically or anaerobically.
Molds are aerobic, most yeast grow best aerobically and bacteria may be
aerobic, anaerobic, or facultative. A high (oxidizing) potential favours aerobes
but will permit the growth of facultative organisms also, and a low (reducing)
potential favours anaerobic or facultative organisms. 
Growth of an organism
may alter the O-R potential of a food enough to inhibit other organisms.
Anaerobes, for example, may lower the O-R potential to a level which is
inhibitory to aerobes.
Obligate aerobes are those organisms that generate their energy from oxidative
phosphorylation using oxygen as the terminal electron acceptor. 
Consequently, they have a requirement for oxygen and a high Eh and will
predominate at food surfaces exposed to air or where air is readily available,
for example, pseudomonads, such as Pseudomonas fluorescens, which grows at an Eh of +100 to +500 mv, and other oxidative Gram-negative rods. These
grow on meat surfaces and produce slime and off-odours. 
Bacillus subtilis (Eh
-100 to +135 mv) produces ropiness in the open texture of bread and
Acetobacter species growing on the surface of alcoholic beverages oxidize
ethanol to acetic acid to produce vinegar or spoil the alcoholic beverage.
Plant juices, tend to have Eh values of +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. Minced meats have Eh values of around +200 mv while
in solid meats the Eh is generally around -200 mv. Cheeses have Eh values on
the negative side from −20 to around −200 mv.
Obligate anaerobes grow only at low or negative redox potentials and require
absence of oxygen. Anaerobic metabolism gives the organism a lower yield of
utilizable energy than aerobic respiration. A reducing environment minimizes
the loss of reducing power from the microbial cell and thus, is favoured. 
Hence, presence of oxygen, which provides an oxidizing environment to the
microbes is not favoured. However, for many anaerobes, oxygen itself exerts a
specific toxic effect. For example, Clostridium acetobutylicum can grow at an
Eh as high as +370 mv maintained by ferricyanide, but would not grow at
+110mv in an aerated culture. 
This effect is due to the inability of obligate
anaerobes to scavenge and destroy toxic products of molecular oxygen such as
hydrogen peroxide and superoxide anion radical (02

) produced by one
electron reduction of molecular oxygen. They lack the enzymes catalase and
superoxide dismutase, which catalyse the breakdown of these radicals. 
Thus, in a highly oxidized food, there will be a predominance of aerobic
organisms especially at food surfaces exposed to air. Whereas, in food with
negative Eh, the anaerobic microflora requiring reduced conditions will be
For example, anaerobic bacteria do not multiply until the onset of
rigor mortis (stiffening of body after death) of muscles of horse because of the
high Eh (+250 mv) in prerigor meat. At 30 h postmortem (after death), the Eh
falls to about -130 mv in the absence of bacterial growth and this low Eh
values favour the growth of obligate anaerobes like Clostridium. 
anaerobes, such as clostridia, have the potential to grow wherever conditions
are anaerobic such as deep in meat tissues and stews, in vacuum packs and
canned foods causing spoilage and C. botulinum is of major public health
concern, since it causes botulism.
Aerotolerant anaerobes are incapable of aerobic respiration, but can
nevertheless grow in the presence of air. Many lactic acid bacteria fall into this
category. They can only generate energy by fermentation and lack both
catalase and superoxide dismutase, but are able to grow in the presence of
oxygen because they have a mechanism for destroying superoxide. 
Microorganisms affect the Eh of their environment during growth. This is true
especially of aerobes, which can lower the Eh of their environment while
anaerobes cannot. As aerobes grow, oxygen in the medium is depleted,
resulting in the lowering of Eh. Growth is not slowed, however, due to the
ability of cells to make use of oxygen donating or hydrogen-accepting
substances in the medium. 
The result of this is that the medium becomes
poorer in oxidizing and richer in reducing substances. Microorganisms can
reduce the Eh of a medium by their production of certain metabolic byproducts such as hydrogen sulphide, which has the capacity to lower Eh to
−300 mv. Since hydrogen sulphide reacts readily with oxygen, it will
accumulate only in anaerobic environments.

Poising Capacity of Food

As redox conditions change, there will be some resistance to change in a
food’s redox potential. This is known as poising capacity of food. This
capacity is dependent on the concentration of the redox couple. Poising is
greatest when the two components of a redox couple are present in equal
Most fresh plant or animal foods have a low and well-poised O-R potential in
their interior: the plants because of reducing substances such as ascorbic acid
and reducing sugars and the animal tissues because of SH (sulfhydryl) and
other reducing groups. 
As long as the plant or animal cells respire and remain
active, they tend to poise the O-R system at a low level, resisting the effect of
oxygen diffusing from the outside. Therefore, a piece of fresh meat or a fresh
whole fruit would have aerobic conditions only at and near the surface. 
meat could support aerobic growth of slime-forming or souring bacteria at the
surface at the same time as anaerobic putrefaction could be proceeding in the
Processing procedures may alter this situation. 
For example, heating may
reduce the poising power of the food by destroying or altering the reducing
and oxidizing substances present and also allow more rapid diffusion of
oxygen inward, either because of the destruction of poising substances or
because of changes in the physical structure of the food. 
Processing also may
remove oxidizing or reducing substances. For example, clear fruit juices loose
reducing substances by their removal during extraction and filtration and
therefore become more favourable to the growth of yeasts than the original
juice containing the pulp.
In the presence of limited amounts of oxygen the same aerobic or facultative
organisms may produce incompletely oxidized products, such as organic
acids, from carbohydrates, while with plenty of oxygen available, complete
oxidation to carbon dioxide and water might result. 
Protein decomposition
under anaerobic conditions may result in putrefaction, whereas under aerobic
conditions, the products are likely to be less obnoxious. Thus, the redox
potential of the food would decide the course of spoilage and the type of end
products being produced due to microbial activities.

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