Food Microbiology

• Water_activity

  Water activity or a_w 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.

a_w 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