Intrinsic and Extrinsic Parameters of Foods That Affect
Microbial Growth
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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.
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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. Oxidationreduction potential (Eh) 4. Nutrient
content 5. Antimicrobial constituents 6. Biological structures
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1. pH It has been well established that most microorganisms
grow best at pH values around 7.0 (6.67.5), whereas few grow below
4.0. Sensitivity to acidity: Bacteria > yeasts> molds
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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.
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With respect to temperature, the pH of the substrate becomes
more acid as the temperature increases. salt 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.
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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
Enterococcus faecalis 0 C to >40 C Some organisms such as
Enterococcus faecalis can grow over a range from 0 C to >40 C.
Most thermophilic bacteria of importance in foods belong to the
genera: Bacillus Paenibacillus Clostridium Geobacillus
Alicyclobacillus Thermoanaerobacter
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Molds have wider ranges of temperature Molds have wider ranges
of temperature than bacteria (like pH, osmotic pressure and
nutrient content). moldsgrow at refrigerator
AspergillusCladosporium Thamnidium Many molds are able to grow at
refrigerator temperatures, notably some strains of Aspergillus,
Cladosporium, and Thamnidium. Yeasts not within the thermophilic
range. Yeasts grow over the psychrotrophic and mesophilic
temperature ranges but generally not within the thermophilic
range.
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refrigerator temperatures this is not always best Although it
would seem desirable to store all foods at refrigerator
temperatures or below, this is not always best for the maintenance
of desirable quality in some foods. bananas keep better if stored
at 1317C than at 57C. For example, bananas keep better if stored at
1317C than at 57C. vegetables such potato as are favored by
temperatures of about 10C. A large number of vegetables such potato
as are favored by temperatures of about 10C.
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Temperature he most important parameter that affects the
spoilage Temperature of storage is the most important parameter
that affects the spoilage of highly perishable Foods. The rate of
spoilage of fresh poultry : at 10C is about twice that at 5C a15C
is about three times that at 5C
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2. Relative humidity of environment is important both The RH of
the storage environment is important both from: aw The standpoint
of aw within foods growthsurfaces The growth of microorganisms at
the surfaces When foods with low aw values are placed in
environments of high RH, the foods pick up moisture until
equilibrium has been established. Likewise, foods with a high aw
lose moisture when placed in an environment of low RH. Relationship
between RH and temperature: The higher the temperature, the lower
the RH, and vice versa.
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, surface spoilage deep spoilage Surface spoilage: In the
refrigerator, surface spoilage occurred before deep spoilage occurs
in food such as improperly wrapped beef cuts. Why? High RH of the
refrigerator Meat-spoilage biota is generally aerobic (deep
sections of meat have often low O 2 ).
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3. Presence and concentration of gases CO 2 Carbon dioxide (CO
2 ) is the single most important atmospheric gas that is used to
control microorganisms in foods. O 2 and O 3 (Ozone) are important
in modified atmosphere packaged (MAP) foods. O 3 : has
antimicrobial properties. strong oxidizing agent. should not be
used on high-lipid-content foods since it would cause an increase
in rancidity. is recognized GRAS (generally recognized as safe) in
Australia, France, and Japan and USA.
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4. Presence and activities of other microorganisms organisms
produce substances inhibitory or lethal Some foodborne organisms
produce substances that are either inhibitory or lethal to others;
These include: Antibiotics Bacteriocins Hydrogen peroxide Organic
acids
