The meaning of „water activity“
T h e A r t o f P r e c is io n M e a s u r e m e n t
What is the meaning of the
measurement parameter
“water activity”?
1) The total water / moisture :
The moisture content is the total water in a
product, including the molecularly bound water, and
is expressed in % of the total weight.
2) The relative humidity:
It means the relative product equilibrium humidity, the freely available
water for micro organisms to grow; its measuring value is called “water
activity”, aw-value.
Water is a basic ingredient in foods, pharmaceuticals and cosmetics.
Its control can be critical to guarantee microbial safety during shelf-life
and consumption, or for optimising a production process.
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Importance of aw
Why is the measurement of aw – water activity
in foods and pharmaceuticals so important?
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Microbial Safety
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The importance of „aw“ measurement :
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microbial safety
to predict shelf life
HACCP requirements
as part of QC
fulfil government
regulations
EU, USA, Japan ….
control chemical
reactions,
physical properties
to guarantee product quality,
good taste, colour, crispness
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Why to measure „aw“?
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Why to measure the “water activity”?
• Water activity, pH, temperature, and other parameters,
have a direct impact on the growth of micro organisms,
thus aw and pH are two of the most important parameters.
• Free water that is available to molds, yeasts, and bacteria
is responsible for their growth and even toxin production.
Or it may participate in chemical/biochemical reactions (eg.
Maillard reactions), which might deteriorate :
•
•
•
•
texture, flavour
colour
taste
nutritional value
of a product, and its stability
shelf-life time.
These are some of the main reasons why a correct
measurement of the “water activity” aw is so important.
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Bacteria, Molds
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Other reasons
• For many food spoilage organisms a minimum aw-value is
known, below which its growth and toxin production is inhibited.
Bacteria, for example:
Staphylococcus aureus :
requires a minimum aw of for growth/toxin production
0.86aw
Clostridium botulinum A and Escherichia coli
0.95aw
Salmonella may start at
0.92aw.
A mold for example:
Aspergillus flavus :
produces toxin above 0.83aw, but does not grow below
Fungi, other molds, mildew, yeasts go lower,
but not beyond
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0.78aw.
0.60aw.
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Examples
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Examples
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Water migration
Water migration
• Two ingredients may have the same
moisture content, but totally different
aw-values.
• The water migrates from regions of high aw
to regions of low aw, and not between
areas of unequal moisture content!
• Water migration between different layers of
a multi-component food causes undesirable textural changes.
• This can be reduced by knowing and
influencing the various levels of aw.
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Regulations
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Some Laws
The recognition of the importance of water activity started in Europe over 30
years ago and was then also incorporated into FDA and USDA regulations.
Today it is one of the critical parameters of the HACCP requirements.
Europe:
Europe:
The EU required already in 1977 in the Official Journal of the
European Communities, No. L 26/97, for imported meat products:
aw < 0.95 and pH < 5.2, or aw only < 0.91
Draft for regulation 77/99/CEE, A, V, p.26/27, aw-measurement
for meat products in general, accuracy:
The instrument shall have a measuring range (for meat products) of
0.74 ... 0.99aw, an accuracy of ±0.01aw, a repeatability better than
0.005aw a temperature control (preferably 25.0°C) of ±0.2°C.
USA :
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The FDA (21 CFR part 113) proposes maximum levels of 0.85aw
and 4.6pH for low-acid canned foods, and the USDA maximum
levels of 0.70aw for peanuts and tree-nuts.
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