Schaller Messtechnik Humimeter AW3 User Manual page 33

Materials with a moisture content above the fiber saturation point, this means an aw
value above 1, cannot be measured. For such materials only the water content can be
determined.
The water activity must not be confused with the water content – the percentage of
water contained in a product.
The water content is used for billing according to the dry content of food and mate-
rials, it shows the ratio of water to the total mass in percent (kg/kg)x 100.
The water activity influences the following characteristics of a product:
• microbiological stability
• chemical stability
• enzymatic stability
• color, taste and nutritional value
• content of proteins and vitamins
• stability of composition
• shelf life
• storage and packaging
All forms of life are dependent on water. The water activity indicates the amount of
water available for micro-organisms such as bacteria, fungi, moulds, etc. Each type of
microorganism needs a certain minimum water activity value for being able to grow.
Typical minimum water activity values taken from literature:
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Water activity
aw = 0.91 - 0.95
aw = 0.88
aw = 0.80
aw = 0.75
aw = 0.70
aw = 0.65
Organism
Bacteria
Yeast
Mould
Halophilic bacteria
Osmiophilic yeast
Xerophilic mould