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Water activity (aw)=
Vapor pressure of a substance or solution
Vapor pressure of water at the same temperature
Raoult’s law:
n

aw N  n
where N: is the number of
moles of solute and n: the number of moles
of solvent (water).
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w/ m

aw w / m  W / M
w: is the weight of solvent , W: the weight of solute, m:
the molar mass of solvent and M: the molar mass of
solute.
What is the water activity of a sucrose solution
containing 5 g in 100 ml water?
w/ m

aw w / m  W / M
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100 / 18

 0.997
aw 100 / 18  5 / 342
Oxygen Classes of Microorganisms
Microorganisms vary in their need for, or tolerance of,
oxygen. In fact, microorganisms can be divided into
several groups depending on how oxygen affects them:
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Aerobes and
obligate
(strict)
aerobes
Are species capable of growth at full
oxygen tensions (air is 21% O2) and
respire oxygen in their metabolism. Many
aerobes can even tolerate elevated
concentrations of oxygen (hyperbaric
oxygen>21% O2). The process of energy
production involves glycolysis, the Krebs’
cycle and the electron transport system for
which O2 acts as a terminal electron
acceptor. Energy is generated by the
complete oxidation of the organic substrate
to carbon dioxide and water. When glucose
is the substrate, 38 molecules of ATP are
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generated from ADP for each molecule of
glucose oxidized.
Examples: Pseudomonas fluorescens, mold
fungi (penicillium spp.) and some yeasts.
Microaerophiles Are aerobes that can use oxygen only when
it is present at levels reduced from that in
air (less than 21%) (microoxic conditions).
This is because of their limited capacity to
respire or because they contain some
oxygen-sensitive molecule such as an
oxygen-labile enzyme. Example:
Campylobacter spp. grow at oxygen
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concentrations between 1% - 10% with an
optimum at 6%. Oxygen concentrations
above 10% are toxic and will kill the
microorganisms.
Facultative
anaerobes or
facultative
aerobes
These microorganisms can grow in the
presence of oxygen (oxic) or absence of
oxygen (anoxic). When oxygen is absent,
organic electron acceptors are used in the
process of energy production
(fermentation). Less energy is generated
compared with aerobic respiration and
organic byproducts are released from the
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cell.
Example: Enterobacteriaceae (E. coli,
Salmonella spp.) and yeast (Saccharomyces
cerevisiae) and Staphylococcus aureus.
Anaerobes
1- Aerotolerant
anaerobes
2- obligate
(strict)
anaerobes
Can tolerate oxygen and grow in its
presence even though they cannot use it.
Are inhibited or even killed by oxygen.
The reason is mainly because they are
unable to detoxify some of the products of
oxygen metabolism (H2O2) (they lack
catalase enzyme needed for H2O2
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detoxification). Occur in only three groups
of microorganisms, a wide variety of
prokaryotes, a few fungi, and a few
protozoa. The best known group of
obligatory anaerobic Bacteria belongs to
the genus Clostridium, a group of Gram (+)
endospore forming rods (responsible for
poisoning and spoilage of canned foods).
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Condition
Aerobic
respiration
Anaerobic
respiration
Fermentation
properties
1- Oxygen acts as the terminal (final)
electron acceptor (oxidizing agent).
2- Produces large amount of ATP.
1- Using alternative terminal electron
acceptor (NO3- by denitrifiers, SO42by sulfur reducers, Fe3+ by metal
reducers and CO2 by methanogens.
2- There are specific ETS like
fumarate reduction system which
converts fumarate to succinate.
3- Produces less amount of ATP
1- Organic substrates act as the
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terminal electron acceptors.
2- Classified by the end products
(lactic acid and ethanol fermentation).
3- Can occur in aerobic and in
anaerobic conditions.
4- Produces the least amount of ATP.
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