Physical methods to microbial control | Part 1

 

 Physical methods to microbial control | Part 1

Physical Methods of Microbial Control

1. ·         Heat
2. ·         Moist heat
3. ·         Dry heat
4. ·         Filtration
5. ·         Low temperature
6. ·         Desiccation
7. ·         Osmotic pressure radiation

1: Heat

It destroys microorganisms by denaturing their enzymes and other proteins.  Heat resistance among microbes varies widely.

Thermal Death Point (TDP):

 It is lowest temperature at which all microbes will be destroyed in ten minutes present in a liquid suspension.

Thermal Death Time (TDT):

It is minimal length of time in which all bacteria will be destroyed at a given temperature.

Decimal Reduction Time (DRT):

Time in minutes at which ninety percent of bacteria will be destroyed at a given temperature. It is used in canning industry.

2: Moist heat

It kills microorganisms by coagulating their proteins. Moist heat is significantly more effective than dry heat in general.

Boiling: at sea level, heat to 100oC or more. Kills, within 10 minutes or less, the vegetative forms of bacterial pathogens, almost all viruses, and fungi and their spores. Endospores and some viruses are not killed by this quickly. However brief boiling will destroy most pathogens.

Hepatitis virus: it can remain alive up to 30 minutes of boiling.

Endospores: it can remain alive up to 20 hours or more of boiling.

Autoclave: it is a chamber filled with hot steam under pressure.

·         It is a preferred method of sterilization, unless medium is damaged by heat, moisture, or high pressure.

·         Steam temperature reaches 121oC at twice atmospheric pressure.

·         Most effective when organisms touch steam directly or are contained in a small amount of liquid.

·         All organisms and endospores are destroyed within 15 minutes.

·         Need more time to reach center of solid or large volumes of liquid.

Pasteurization: it is developed by Louis Pasteur to prevent the spoilage of beverages. It is used to reduce microorganisms responsible for spoilage of beer, milk, wine, juices, etc.

Classic Method of Pasteurization: Milk was treated at 65oC for 30 minutes.

High Temperature Short Time Pasteurization (HTST): This is used today. At 72 degrees Milk is treated for 15 seconds.

Ultra-High Temperature Pasteurization (UHT):  At 140 degrees, milk is exposed for 3 seconds and then it is cooled very quickly in a vacuum chamber.

Advantage: Milk can be stored for several months at room temperature.