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Food Microbiology

  Water activity or aw is a measurement of the energy status of the water in a system. It is defined as the vapor pressure of water divided by that of pure water at the same temperature; therefore, pure distilled water has a water activity of exactly one.

      aw = p/p0

Where p is the vapor pressure of water in the substance, and p0 is the vapor pressure of pure water at the same temperature.

    Relativehumidity(RH) = aw*100

          Uses for Water Activity

  1. Water activity is an important considerationfor food product design and food safety.
  2. Food Product Design
  3. Food designers use water activity toformulate products that are shelf stable. If a product is kept below a certain water activity, then mold growth is inhibited. This results in a longer shelf-life.
  4. Water activity helps limit or slowcertain undesirable reactions, such as non-enzymatic browning, fat oxidation, vitamin degradation, enzymatic reactions, protein denaturation, starch gelatinization and starch retrogradation. This too maintains product quality and extends shelf life.

  Water activity is used in many cases as a Critical Control Point for Hazard Analysis and Critical Control Points (HACCP) programs.Samples of the food product are periodically taken from the production area and tested to ensure that water activity values are within a specified range for food quality and safety.

    Water Activity Measurement

  Water activity values are obtained by either a capacitance or a dew point hygrometer.

    aw Values of Microorganism Inhibition

  1. Clostridium botulinum E 0.97
  2. Pseudomonas fluorescens 0.97
  3. Escherichia coli 0.95
  4. Clostridium perfringens 0.95
  5. Salmonella 0.95
  6. Vibrio cholerae 0.95
  7. Clostridium botulinumA, B 0.97
  8. Bacillus cereus 0.93
  9. Listeria monocytogenes 0.92
  10. bacillus subtilis 0.91
  11. Staphylococcus aureus0.87
  12. Most Fungi 0.70

Foodborne pathogens   
Staphylococcus aureus
Escherichia coli
Clostridiumbotulinum and
Clostridium perfringens

Bacillus cereus
Water activity
Microbiology of soft drinks and fruit juices
   Incidence of Foodborne              Disease
Epidemiology of food borne diseases
Microbial food safety risk assessment
  Foodborne Infections
Salmonella infections  
Campylobacter jejuni andrelated pathogens
Yersinia infections
Listeria monocytogenes infections
Clostridium perfringens

Vibrio infections
Escherichia coli infections
Infections with other bacteria
Infections with viruses and prions
Foodborne parasites
   Foodborne Intoxications
Clostridium botulinum
Staphylococcal intoxications
Bacillus cereus gastroenteritis
Prevention of Foodborne Disease Effects of food processing on disease agents
Food safety
Principles of Toxicology
Determination of Toxicants in Foods
Natural Toxins in Animal Foodstuffs
Natural Toxins in Plant Foodstuffs
Fungal Toxins Occurring in Foods
Toxic Food Contaminants from Industrial Wastes
Pesticide Residues in Foods
Food Additives
Toxicants Formed during Food Processing