Intrinsic and Extrinsic Parameters of
Foods That Affect Microbial Growth
As our foods are of plant and/or animal origin, it is worthwhile
to consider those characteristics of plant and animal tissues
that affect the growth of microorganisms.
The plants and animals that serve as food sources have all
evolved mechanisms of defense against the invasion and
proliferation of microorganisms, and some of these remain in
effect in fresh foods.
INTRINSIC PARAMETERS
The parameters of plant and animal tissues that are an inherent
part of the tissues are referred to as intrinsic parameter. These
parameters are as follows:
1. pH
2. Moisture content
3. Oxidation–reduction potential (Eh)
4. Nutrient content
5. Antimicrobial constituents
6. Biological structures
It has been well established that most microorganisms
grow best at pH values around 7.0 (6.6–7.5), whereas
few grow below 4.0.
Sensitivity to acidity: Bacteria > yeasts> molds
pH minima and maxima of microorganisms should not be taken to
be precise boundaries, as the actual values are known to be
dependent on other growth parameters.
For example, the pH minima of certain lactobacilli have been
shown to be dependent on the type of acid used, with citric,
hydrochloric, phosphoric, and tartaric acids permitting growth at a
lower pH value than acetic or lactic acids.
With respect to temperature, the pH of the substrate becomes
more acid as the temperature increases.
Concentration of salt has a definite effect on pH growth rate
curves, where it can be seen that the addition of 0.2 M NaCl
broadened the pH growth range of Escherichia coli .
However, when the salt content exceeds this optimal level, the
pH growth range is narrowed.
Young cells are more susceptible to pH changes than older or
resting cells.
When microorganisms are grown on either side of their
optimum pH range, an increased lag phase results.
Some food such as fruits and vinegar fall below the point at
which bacteria normally grow.
It is a common observation that fruits generally undergo mold
and yeast spoilage, and this is due to the capacity of these
organisms to grow at pH values <3.5, which is considerably
below the minima for most food-spoilage and all food-
poisoning bacteria .
Most vegetables have higher pH values than fruits, and,
consequently, vegetables should be subject more to bacterial
than fungal spoilage.
Most of the meats and sea foods have a final ultimate pH of
about 5.6 and above.
This makes these products susceptible to bacteria as well as to
mold and yeast spoilage.
With respect to the keeping quality of meats, it is well
established that meat from fatigued animals spoils faster than
that from rested animals and that this is a direct consequence
of final pH attained upon completion of rigor mortis.
Upon the death of a well-rested meat animal, the usual 1%
glycogen is converted to lactic acid, which directly causes a
depression in pH values from about 7.4 to about 5.6,
depending on the type of animal.
Some foods such as fruit are characterized by “inherent
acidity” or “natural acidity”.
The natural or inherent acidity of foods, especially fruits, may
have evolved as away of protecting tissues from destruction by
microorganisms.
Others owe their acidity or pH to the actions of certain
microorganisms. This type is referred to as “biological
acidity” and is displayed by products such as fermented milks
(e.g yoghourt) and pickles.
Some foods are better able to resist changes in pH
than others.
Those that tend to resist changes in pH are said to be
“buffered”.
Meats are more highly buffered than vegetables.
Contributing to the buffering capacity of meats are
their various proteins.
Vegetables are generally low in proteins and, consequently,
lack the buffering capacity to resist changes in their pH during
the growth of microorganisms
Although acidic pH values are of greater use in inhibiting
microorganisms, alkaline values in the range of pH 12–13 are
known to be destructive, at least to some bacteria.
For example, the use of CaOH2 to produce pH values in this
range has been shown to be destructive to Listeria
monocytogenes and other foodborne pathogens.
With respect to fish, it has been known for some time that
halibut, which usually attains an ultimate pH of about 5.6, has
better keeping qualities than most other fish, whose ultimate
pH values range between 6.2 and 6.6.
Effects of adverse pH on microorganisms:
Disruption of cellular enzymes
Disruption of the transport of nutrients into the cell
Disruption of such key cellular compounds as DNA and ATP
require neutrality.
The morphology of some microorganisms can be affected by
pH.
An adverse pH makes cells much more sensitive to toxic agents.
It appears that the internal pH of almost all cells is near
neutrality.
When microorganisms are placed in environments below or
above neutrality, their ability to proliferate depends on their
ability to bring the environmental pH to a more optimum value
or range.
When most microorganisms grow in acid media, their
metabolic activity results in the medium or substrate becoming
less acidic, whereas those that grow in high pH environments
tend to effect a lowering of pH.
The amino acid decarboxylases that have optimum
activity at around pH 4.0 and almost no activity at pH 5.5
cause a spontaneous adjustment of pH toward neutrality
when cells are grown in the acid range.
When amino acids are decarboxylated, the increase in pH
occurs from the resulting amines.
Bacteria such as Clostridium acetobutylicum raise the
substrate pH by reducing butyric acid to butanol, whereas
Enterobacter aerogenes produces acetoin from pyruvic
acid to raise the pH of its growth environment.
When grown in the alkaline range, a group of amino acid
deaminases that have optimum activity at about pH 8.0
and cause the spontaneous adjustment of pH toward
neutrality as a result of the organic acids that accumulate.
2. Moisture content
The preservation of foods by drying is a direct consequence of
removal of moisture, without which microorganisms do not grow.
It is now generally accepted that the water requirements of
microorganisms should be described in terms of the water activity
(aw) in the environment.
Aw is defined by the ratio of the water vapor pressure of food
substrate to the vapor pressure of pure water at the same temperature:
aw = p/po
Where p is the vapor pressure of the solution and po is the vapor
pressure of the solvent (usually water).
This concept is related to relative humidity (RH) in the following
way:
RH = 100 × aw.
When salt is employed to control aw, an extremely high
level is necessary to achieve aw values below 0.80.
Pure water has an aw of 1.00
22% NaCl solution (w/v) has an aw of 0.86
Saturated solution of NaCl has an aw of 0.75.
•The water activity (aw) of most fresh foods is above 0.99.
In general, bacteria require higher values of aw for growth
than fungi
Gram-negative bacteria having higher requirements than
Gram positives.
Just as yeasts and molds grow over a wider pH range than
bacteria, the same is true for aw.
halophiles = “salt-loving”
Xerophilic = “dry-loving”
Osmophilic= “preferring high osmotic pressures”
Pathogens are active at higher aw.
Certain relationships have been shown to exist among aw,
temperature, and nutrition:
At any temperature, the ability of microorganisms to
grow is reduced as the aw is lowered.
Second, the range of aw over which growth occurs is
greatest at the optimum temperature for growth.
The presence of nutrients increases the range of aw over
which the organisms can survive.
Effects of Low aw
The general effect of lowering aw below optimum is:
Adverse effects on the functioning of the cell membrane,
which must be kept in a fluid state
Disruption of nutrition of microorganisms (due to disruption
of nutrient transportation through an aqueous environment)
Increase the length of the lag phase of growth
Decrease the growth rate
Decrease size of final population
Aw is influenced by other environmental parameters such as
pH and temperature:
When both the pH and temperature of incubation were
made unfavorable, the minimum aw for growth is higher.
In general, the strategy employed by microorganisms as protection
against osmotic stress is the intracellular accumulation of compatible
solutes.
The three most common compatible solutes in most bacteria are:
Carnitine
Glycine betaine
Proline
Osmophilic yeasts accumulate polyhydric alcohols to a
concentration commensurate with their extracellular aw.
3. Oxidation–reduction (O/R) potential ---(symbol =Eh)
Definition: ease with which the substrate loses or gains
electrons.
When electrons are transferred from one compound to another,
a potential difference is created between the two compounds
and expressed as millivolts (mV).
When an element or compound loses electrons, the substrate is
oxidized,
When a substrate that gains electrons becomes reduced.
A substance that readily gives up electrons is a good reducing
agent.
A substance that readily takes up electrons is a good oxidizing
agent.
The more highly oxidized a substance, the more positive will
be its electrical potential.
The more highly reduced a substance, the more negative will
be its electrical potential.
When the concentration of oxidant and reductant is equal, a
zero electrical potential exists.
Aerobic microorganisms: require positive Eh values
(oxidized) for growth.
Anaerobes: require negative Eh values (reduced) .
Microaerophiles: Aerobic bacteria actually grow better
under slightly reduced conditions such as lactobacilli and
campylobacters.
According to Frazier The *O/R potential of a food is
determined by the following:
The characteristic O/R potential of the original food.
The poising capacity (resistance to change in potential of
the food)
The oxygen tension of the atmosphere about the food.
The access that the atmosphere has to the food.
pH: (Eh tends to be more negative under progressively
alkaline conditions)
Microbial activity: Microorganisms decrease the Eh of
their environments during growth:
As aerobes grow, O2 in the medium is depleted,
resulting in the lowering of Eh.
By production of metabolic such as H2S (lower Eh to
−300 mV)
Reducing substances in some foods:
–SH groups in meats
Ascorbic acid and reducing sugars in fruits and vegetables
Eh of some food:
Plant foods, especially plant juices, tend to have Eh values
of from +300 to 400 mV.
Cheeses of various types have been reported to have Eh
values on the negative side, from −20 to around −200 mV.
Solid meats have Eh values of around −200 mV; in
minced meats, the Eh is generally around +200 mV.
Cheese : -200mv
Fruit Juices: 200-300mv
Change in pH of meat pre and post slaughter:
Eh of muscle after death is +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.
Thus, anaerobic bacteria do not multiply until the onset
of rigor mortis because of the high Eh in pre-rigor
meat.
4. Nutrient content
Microorganisms to grow and function normally require
the following:
1. Water
2. Energy
3. Nitrogen
4. Vitamins
5. Minerals
The importance of water to the growth and welfare of
microorganisms is obvious.
With respect to the other four groups of substances
requirement:
Molds < Gram-negative bacteria < Yeasts < Gram-positive bacteria.
As sources of energy, microorganisms may utilize:
Sugars
Alcohols
Amino acids (Also as nitrogen source)
Complex carbohydrates such as starches and cellulose are also
used by some microorganisms as sources of energy by first
degrading these compounds to simple sugars
Fats are also used by few microorganisms as sources of energy.
In general, simple compounds such as amino acids will be
utilized by almost all organisms before any attack is made
on the more complex compounds such as high-molecular-
weight proteins.
The same is true of polysaccharides and fats.
Microorganisms may require B vitamins in low quantities,
and almost all natural foods have an abundant quantity for
those organisms that are unable to synthesize their
essential requirements.
In general, Gram-positive bacteria are the least synthetic
and must therefore be supplied with one or more of these
compounds before they will grow.
The Gram-negative bacteria and molds are able to
synthesize most or all of their requirements.
Thus, Gram-negative bacteria and molds may be found
growing on foods low in B vitamins.
Whys fruits spoilage by molds more than bacteria?
1) Fruits have lower B vitamins (not suitable for most bacteria)
2) Fruits have lower pH (not suitable for most bacteria)
3) Fruits have positive Eh (not suitable for most bacteria)
5. Antimicrobial constituents
Some plant species are known to contain essential oils that
possess antimicrobial activity. Among these are:
Eugenol in cloves
cinnamic aldehyde and eugenol in cinnamon,
Allicin in garlic
Allyl isothiocyanate in mustard
Antilisterial compounds in carrot (not exactly determined
yet but probably: phytoalexin 6-methoxy-mullein)
eugenol and thymol in sage, and carvacrol isothymol and thymol
in oregano.
phytoalexins and the lectins. Lectins are proteins that can
specifically bind to a variety of polysaccharides, including the
glycoproteins of cell surfaces. Through this binding, lectins can
exert a slight antimicrobial effect.
Cow’s milk contains several antimicrobial substances,
including:
Lactoferrin (an iron-binding glycoprotein)
Conglutinin
A rotavirus inhibitor in raw milk (susceptible to pasteurization)
Casein
Free fatty acids
Lysozyme
Lactoperoxidase system.
Lactoperoxidase system in milk
This is an inhibitory system that occurs naturally in bovine
milk, and it consists of three components:
1) Lactoperoxidase
2) Thiocyanate (SCN−)
3) H2O2
All three components are required for antimicrobial
effects.
Activation of Lactoperoxidase system:
The quantity of lactoperoxidase in bovine milk is enough.
But the quantity of H2O2 and thiocyanate maybe lower
than required.
So, lactoperoxidase system in raw milk is activated by
adding thiocyanate and H2O2 and finally hypothiocyanate
be generated.
Then, shelf life was extended to 5 days compared to 48
hours for controls.
Gram-negative psychrotrophs such as the pseudomonads
are sensitive.
The system was more effective at 30◦C than at 4◦C.
The lactoperoxidase system can be used to preserve raw
milk in countries where refrigeration is uncommon.
Lacto peroxidase system
Add about 10 parts per million (PPM) of thiocyanate
increases the overall level to 15 PPM (5 PPM naturally
present).
The solution is thoroughly mixed for 30 seconds and then an
equimolar amount (8.5 PPM) of hydrogen peroxide is added
(generally in the form of a granulated sodium carbonate
peroxyhydrate).
This treatment extends the shelf life of raw milk under tropical
conditions for a further 7 to 8 hrs.
Eggs:
Lysozyme
Conalbumin (appears to be the inhibitory substance in
raw egg white that inhibits Salmonella enteritidis)
Fruits, vegetables, tea, molasses:
The hydroxy-cinnamic acid derivatives (p-coumaric,
ferulic, caffeic, and chlorogenic acids)
6. Biological structures
The natural covering of some foods provides excellent
protection against the entry and subsequent damage by
spoilage organisms such as:
Outer covering of fruits
Shell of nuts
Hide of animals
Shells of eggs
Skin covering of fish
