Extrinsic factors affecting the growth of microbes food microbiology

Extrinsic factors affecting the growth of microorganisms
1)TEMPERATURE
2)RELATIVE HUMIDITY
3)PRESSURE

1. EXTRINSIC FACTORS AFFECTING THE GROWTH OF MICROBES LAHARI BASAVARAJU 1 ST MSc MICROBIOLOGY

2. EXTRINIC PARAMETERS (ENVIRONMENTAL LIMITATIONS/ STORAGE CONDITIONS ) The characteristics of the environment in which the food is maintained, and can affect the properties of the food as well as the potential for the growth of microorganisms . These are : Temperature of storage. Relative humidity of the environment. Presence and concentration of gases.

3. TEMPERATURE As temperature influences enzymic reactions it has an important role in promoting or preventing microbial growth. Micro-organisms can be categorized into one of four groups depending on their optimum growth temperature and the temperature range at which they will grow.

4. TEMPERATURES FOR MICROBIAL GROWTH GROUP OPTIMUM TEMPERATURE MAXIMIUM GROWTH RANGE THERMOPHILES 55 ⁰C 30-70 ⁰C MESOPHILES 37 ⁰C 10-45 ⁰C PSYCHROTROPHS 20-30 ⁰C 0-40 ⁰C PSYCHROPHILES 15 ⁰C -5 to -20 ⁰C

5. Lower temperatures still result in cell death but at a slower rate. Temperature can therefore be used to eliminate or control the growth of micro-organisms . Heat treatments (pasteurization or sterilization) eliminate contaminating micro-organisms via the application of heat for a specific time period (time and temperatures used being dependent upon the target organism). Refrigeration of a food can prevent spoilage by controlling the growth of thermophiles or mesophilic organisms. Most pathogens are capable of growth at refrigeration temperatures and therefore cannot be controlled via refrigeration alone. Some, for example Listeria , can grow at very low temperatures.

6. RELATIVE HUMIDITY The relative humidity in which a food is stored can have an influence on the water activity of that product and an influence on the growth of microorganisms on the surface of a product. If the growth of micro-organisms in a food is controlled by the water activity of a product then it is very important that the food be stored under relative humidity conditions which will not allow the uptake of moisture from the air, and therefore an increase in water activity. Packaging can be used to limit the migration of moisture into the product.

7. GASEOUS ATMOSPHERE As all micro-organisms have specific requirements for oxygen and carbon dioxide, by altering the atmosphere within a food package the growth of micro-organisms can be controlled . Gram negative bacteria are more sensitive to CO2 than gram positive bacteria.

8. Vacuum packing food removes available oxygen and thereby prevents the growth of aerobic organisms; but still it allows the growth of anaerobes such as C. botulinum . Modified atmosphere packaging (MAP) allows the food producer to select the atmosphere within the package using varying combinations of oxygen, carbon dioxide and nitrogen, depending upon the product type and target micro-organisms

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