Fish Microbiology and Quality Assurance

Unit 2 - Growth and survival of microorganisms in food

2.1.3. Moisture content (a_w)
Moisture content of the food affects microorganisms in foods, and the microbial types present in foods depends on the amount of water available. Water requirement for microorganisms is described in terms of water activity
(a_w) in the environment and is defined as the ratio of the water vapor pressure of food substrate to the vapor pressure of pure water at same temperature.

a_w = P/P_o
P = vapour pressure of water in substrate
P_o = vapour pressure of solvent (pure water)
aw is related to relative humidity (RH)
RH = 100 x a_w

Water activity of solutes and requirements of certain microorganisms

• a_w of pure water: 1.0
• NaCl solution (22 %): 0.86
• Saturated NaCl solution: 0.75
• Bacteria generally require higher value of aw than fungi
• G ^{– ve} bacteria require higher aw than G ^+ve bacteria
• Most spoilage bacteria do not grow at a_w below 0.91
• Spoilage molds grow at a_w of 0.80
• Halophilic bacteria grow at a_w of 0.75
• Xerophilic and osmophilic yeasts grow at of 0.61

Microorganisms like halophiles, osmophiles and xerophiles grow better at reduced aw. Microorganisms can not grow below aw 0.60, and in such situations spoilage of food is not microbiological but due to chemical reactions (Ex: oxidation).

Relationship between aw, temperature and nutrition

• Growth of microorganisms decreases with lowering of a_w
• The range of a_w at which the growth is greatest occurs at optimum temperature for growth
• The presence of nutrients increases the range of a_w over which the organisms can survive