526-8 Factors That Influence Microbial Activity

Factors that influence microbial activity

 A knowledge of the factors that favor or inhibit the growth of the microorganisms is important in understanding the principles of food spoilage and preservation.  Six major factors: 1. Moisture 2. Oxygen 3. Temperature 4. Acidity, pH 5. Nutrients 6. Growth inhibitors

Effect of moisture on microbial growth  Water is essential for the growth of all living organisms. Has many important functions in the growth of microoranisms and in enzyme activity.  Preserving techniques such as dehydration, concentration, freezing are based on making water unavailable 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)

Osmotic Effect  Water is transported into and out of the microbial cell through the semi-permeable cell membrane (wall) by osmosis.  Water passes from the less dense medium to the more dense.  If solute concentration of the food (growth medium of microorganisms) is increased water flows from the microbial cell to its surrounding. Microbial cell shrivels (shrinks) when osmoregulatory capacity of the cell is exceeded (plasmolysis). Shrinking of cells retards the metabolic activity.

(0.85% salt)

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 of microorganisms.  Amount of water available for microorganims is expressed in terms of water activity. aw = Po/Pow (ERH) at a given T, 0.0 < aw < 1.0

Effect of oxygen on 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 their oxygen requirements: aerobic : grow in the presence of atm. oxygen anaerobic : grow in the absence of atm. oxygen facultatively anaerobic : grow either in absence or presence of oxygen microaerophilic : grow in the presence of limited oxygen

 In the presence of atmospheric oxygen microbial activity leads to a slow oxidation of foods: Carbohydrates → CO2 and H2O Amino acids → NH3 , CO2 and H2O Fats → free fatty acids, CO2 and H2O  In the presence of limited oxygen oxidation results in intermediary products; alcohol, lactic acid, acetic acid, formic acid. C6H12O6 + 6O2 → 6CO2 + 6H2O + 674 calories C6H12O6 → 2C2H5OH + 22 calories

Effect of temperature on microbial growth  Optimum, minimum and maximum temperatures for microbial growth. min. T. : refrigeration, freezing max. T. : heat processing  Microorganisms are classified w.r.t. their temperature requirements.

 Dependence of growth of microorganisms on temperature is largely related to activation and inactivation of enzyme systems in microorganisms.  Three different effects of temperature contribute to the death of microbial cells: 1. Denaturation of proteins (enzymes) by heat 2. Intoxication due to accelerated metabolic reactions 3. Changes in essential lipids. Melting points of the fats found in the organisms and temperature ranges of death are related.

Effect of acidity on microbial growth Microorgisms affected by pH of foods, since they do not have any mechanism to adjust their internal pH.  Microorganisms have different minimal, maximal and optimal pH-levels for growth: optimum

maximum 9.0

minimum

Bacteria

6.5-7.5

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

 Acid foods : pH < 4.5 . Pathogenic micro-organisms cannot survive.  Pasteurization (heat treatment below boiling temperature) is sufficient for acid foods.  For pH > 4.5, sterilization (severe heat treatment above boiling temperatures) is needed to destroy pathogenic microorganisms and their toxins.

pH ranges for major foods Fruits : 3.0 – 4.5 Vegetables : 4.6 – 6.6 Meat : 5.5 – 6.8 Milk : 6.0 – 6.8  Pickling : Production of acids in foods by fermentation.  Acetic-, citric-, lactic-, propionic-, benzoic-, and sorbic acids added to foods for preservation (acidulants)  pH also affects resistance of microorganisms to destruction during heating, drying and other processes

Effect of nutrients on microbial growth  Microorganisms depend on nutrients for both energy and growth.  Different microorganisms possess different enzyme systems which are specific in breakdown of certain nutrient compounds.  Microbial growth can be enhanced by enriching the growth medium with specific nutrients and it can be retarded using growth inhibitors.  Creating specific nutrient media is a very useful tool both in laboratory work and in industry for isolation and growth of certain microorganisms.

Growth inhibitors  Many chemical compounds selectively inhibit microbial activity  These compounds act either by inactivating a necessary enzyme needed for growth, by denaturing the protein portion of a cell or by causing physical damage to certain parts of the cell like breaking the cell wall.  A microrganism growing in food may produce substances inhibitory to other organisms or to destroy compounds inhibitory to its growth. Basis for production of antibiotics. Example: Pencillin is produced by moulds (aspergillus) and retards growth of bacteria.

Antibiotics used to control microbial growth 1. 2. 3.

Cell wall inhibitors Protein synthesis inhibitors DNA replication inhibitors