Overview
This lecture covers three physical methods of microbial control: cold (refrigeration and freezing), pressure (Pascalization), and desiccation (drying and moisture removal).
Cold: Refrigeration and Freezing
- Refrigeration inhibits microbial metabolism, making it bacteriostatic (slows growth but does not kill organisms).
- Freezing below -2°C can kill some microbes but not all; psychrophilic microbes can survive low temperatures.
- Ultra-low temperature freezing (e.g., liquid nitrogen at -196°C) can be used for microbial control and preservation.
Pressure (Pascalization)
- High pressure (100–800 megapascals) denatures proteins and ruptures microbial cells, similar to heat.
- Pascalization is used mainly in food (juices, deli meats) to maintain quality and shelf life.
- High pressure effectively kills vegetative cells but is less effective against spores.
- 800 megapascals is over 116,000 pounds per square inch, far greater than standard autoclave or tire pressure.
Desiccation (Drying)
- Removing water through drying stops metabolism, slowing or preventing microbial growth.
- Examples include sun drying fruits (raisins, jerky); not sterile but reduces spoiling organisms.
- Adding salt or sugar creates a hypertonic environment, removing water from cells by osmosis (used in jams, jellies, canned goods).
- This water loss from cells is called plasmolysis, which inhibits microbial activity but does not kill all spores.
- Lyophilization (freeze-drying under vacuum) rapidly removes moisture for long-term microbial preservation.
Key Terms & Definitions
- Bacteriostatic — inhibits the growth of bacteria without killing them.
- Bactericidal — kills bacteria.
- Pascalization — microbial control using very high pressure.
- Hypertonic environment — condition where the surrounding solution causes water to leave the cell.
- Plasmolysis — process where cells lose water in a hypertonic environment, inhibiting metabolism.
- Lyophilization — rapid freeze-drying under vacuum to preserve biological materials.
Action Items / Next Steps
- Review definitions of key terms: bacteriostatic, Pascalization, plasmolysis, lyophilization.
- Be able to compare and contrast cold, pressure, and desiccation as microbial control methods.