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Factors that influencemicrobial activity
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A knowledge of thefactors that favor or inhibitthe growth of the microorganisms is important inunderstanding the principles of food spoilage andpreservation.
Six major factors:1. Moisture2. Oxygen3. Temperature4. Acidity, pH
5. Nutrients6. Growth inhibitors
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Effect of moisture
on microbial growth Water is essential for the growth of all living
organisms. Has many important functions in thegrowth of microoranisms and in enzyme activity.
Preserving techniques such as dehydration,concentration, freezing are based on making waterunavailable for the microorganisms.
Mainly 3 methods for making water unavailable:1. Increasing the solute concentration;
removing water, adding of solutes (salt, sugar)
2. Addition of hydrophilic (water-binding)colloids (gels, pectins, gums)
3. Bringing water to a solid phase (freezing)
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Osmotic Effect
Water is transported into and out of themicrobial cell through the semi-permeable cellmembrane (wall) by osmosis.
Water passes from the less dense medium to the
more dense.
If solute concentration of the food (growthmedium of microorganisms) is increased water flowsfrom the microbial cell to its surrounding. Microbial
cell shrivels (shrinks) when osmoregulatory capacityof the cell is exceeded (plasmolysis). Shrinking ofcells retards the metabolic activity.
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(0.85% salt)
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Existance of water in foods
Bound water : An integral part of the living tissue,vital to all processes associated with the cell.
Free water : Exists in and around the tissue or cell.Important for metabolism and survival ofmicroorganisms.
Amount of water available for microorganims isexpressed in terms of water activity.
aw = Po/Pow (ERH) at a given T, 0.0 < aw < 1.0
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Effect of oxygenon microbial growth
Essential element for the metabolic activity
Free atmospheric oxygen in air. Some organisms can
utilize oxygen in C-H-O
Microorganisms are classified according to theiroxygen requirements:
aerobic : grow in the presence of atm. oxygen
anaerobic:
grow in the absence of atm. oxygenfacultatively anaerobic : grow either in absenceor presence of oxygen
microaerophilic : grow in the presence of limitedoxygen
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In the presence of atmospheric oxygenmicrobial activity leads to a slow oxidation of foods:Carbohydrates CO2 and H2OAmino acids NH3 , CO2 and H2OFats free fatty acids, CO2 and H2O
In the presence of limited oxygen oxidation resultsin intermediary products; alcohol, lactic acid, aceticacid, formic acid.
C6H12O6 + 6O2 6CO2 + 6H2O + 674 calories
C6H12O6 2C2H5OH + 22 calories
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Effect of temperature
on microbial growth
Optimum, minimum and maximumtemperatures for microbial growth.min. T. : refrigeration, freezing
max. T. : heat processing
Microorganisms are classified w.r.t.their temperature requirements.
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Dependence of growth of microorganisms ontemperature is largely related to activation andinactivation of enzyme systems in microorganisms. Three different effects of temperature contributeto the death of microbial cells:
1. Denaturation of proteins (enzymes) by heat2. Intoxication due to accelerated metabolic
reactions3. Changes in essential lipids. Melting points of the
fats found in the organisms and temperatureranges of death are related.
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Effect of acidity on microbial growth
Microorgisms affected by pH of foods, since theydo not have any mechanism to adjust their internalpH.
Microorganisms have different minimal, maximal
and optimal pH-levels for growth:
optimum maximum minimum
Bacteria 6.5-7.5 9.0 4.5
Molds 4.0-6.8 8.0-11.0 1.5-3.5
Yeasts 4.5-6.5 8.0-8.5 1.5-3.5
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Acid foods : pH < 4.5 . Pathogenic micro-organismscannot survive.
Pasteurization (heat treatment below boilingtemperature) is sufficient for acid foods.
For pH > 4.5, sterilization (severe heat treatmentabove boiling temperatures) is needed to destroypathogenic microorganisms and their toxins.
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pH ranges for major foods
Fruits : 3.0 4.5Vegetables : 4.6 6.6Meat : 5.5 6.8Milk : 6.0 6.8
Pickling : Production of acids in foods byfermentation. Acetic-, citric-, lactic-, propionic-,
benzoic-, and sorbic acids added to foodsfor preservation (acidulants)
pH also affects resistance of
microorganisms to destruction duringheating, drying and other processes
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Effect of nutrients on microbial growth
Microorganisms depend on nutrients for bothenergy and growth.
Different microorganisms possess differentenzyme systems which are specific in breakdownof certain nutrient compounds.
Microbial growth can be enhanced by enrichingthe growth medium with specific nutrients and itcan be retarded using growth inhibitors.
Creating specific nutrient media is a veryuseful tool both in laboratory work and inindustry for isolation and growth of certainmicroorganisms.
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Growth inhibitors
Many chemical compounds selectively inhibitmicrobial activity
These compounds act either by inactivating anecessary enzyme needed for growth, bydenaturing the protein portion of a cell or by
causing physical damage to certain parts of the celllike breaking the cell wall. A microrganism growing in food may producesubstances inhibitory to other organisms or to
destroy compounds inhibitory to its growth. Basisfor production of antibiotics.Example: Pencillin is produced by moulds(aspergillus) and retards growth of bacteria.
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Antibiotics used to controlmicrobial growth
1. Cell wall inhibitors
2. Protein synthesis inhibitors3. DNA replication inhibitors
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