526-8 Factors That Influence Microbial Activity

8/14/2019 526-8 Factors That Influence Microbial Activity

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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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526-8 Factors That Influence Microbial Activity