Water activity (aw) is defined as the ratio of the vapor pressure of water in a food to the vapor pressure of pure water at the same temperature. It indicates how available the water is in a food for microbial growth and chemical reactions. Most bacteria do not grow below an aw of 0.91 and molds below 0.80, while S. aureus can grow down to 0.86. Measuring aw can predict spoilage and determine enzyme and vitamin activity in foods. Stability depends on both aw and pH. Common methods to control aw include drying, adding salt or sugar, and freezing.

1. Water Activity (aw)
P. Karthika
Assistant Professor
Department of Food Science and Nutrition
Periyar University, Salem -11

2. Introduction
• All the food products contain varying amount of
water in their tissues. Various qualities and
processing attributes like deterioration, dehydration,
spoilage etc., in raw as well as processed foods, are
influenced by the water content. Water exists in
three forms:
• As solid (ice) below 0 º C
• In liquid form as water between 0º C and 100º C and
• As gas above 100º C at the atmospheric pressure.

3. • The water present in foods interacts
with major hydrophilic nutrients such
as carbohydrates, proteins, water-
soluble vitamins, and minerals etc.
(polar hydrogen-bonding
interactions).
• The alkyl chains of fats and proteins
are hydrophobic in nature, and
therefore do not tend to interact with
water.

4. Definition
Water activity is defined as the ratio of the vapour pressure of
water in a food item (p) to the vapour pressure of pure water
(po) at the same temperature.

5. Role of Water Activity in food spoilage
On the basis of moisture content, the food products can be broadly put into
three categories as:
• Low moisture foods e.g., dried or freeze dried foods having moisture content of
5-15 per cent.
• Intermediate moisture foods e.g., cakes and dates with a moisture content of
20-40 per cent.
• High moisture foods e.g. fresh fruits and vegetables of greater than 40 per cent
of moisture.
Though moisture content is important, it is the water activity (aw) that is
critical factor for the Shelf life of a food item. It may be used to predict stability
with respect to physical properties like texture and caking, rates of
deteriorative reactions and microbial growth.

6. Water Activity and Quality
• Most bacteria, for example, do not grow at water activities below
0.91, including pathogens such as Clostridium botulinum.
• Below 0.80 most molds cannot be grown and below 0.60 no
microbiological growth is possible. The risk of food poisoning
must be considered in low acid foods (pH > 4.5) with a water
activity greater than aw 0.86.
• Staphylococcus aureus, a common food poisoning organism, can
grow down to this relatively low water activity level.
• Foods that may support the growth of this bacterium include
cheese and fermented sausages stored above correct refrigeration
temperatures.

7. • measuring water activity - to predict which
microorganisms will and will not be potential sources
of spoilage.
• water activity - determining the activity of
enzymes and vitamins in foods and can have a major
impact their color, taste, and aroma.
• Stability and food security depends on water activity
and pH in the food environment.
• water activity is higher with the products are
perishable.

8. Aw Value Effect on Food
0.2–0.3 A monolayer of moisture is formed. It represents the optimal moisture content for the
maximum shelf life of the dehydrated foods.
0.35–0.45 The water is available outside the monolayer and the water phase requiring chemical
reactions begin to occur. This may cause changes in physical state, e.g., loss of crispness,
stickiness, and re-crystallization of amorphous state sugars.
0.4–0.5 The soft materials like raisins etc., become hard due to drying out.
0.6 Considered a critical point where there is high potential for growth of microbes if the
moisture content increases.
0.6–0.8 The rates of chemical reactions that require an aqueous phase increase and cause
deterioration of foods, reaching a maximum. The rates however fall at higher moisture
content.
0.85 Another critical point, above which bacterial spoilage occurs and pathogens begin to grow.
If we want to prevent the food spoilage or the action of microbes and maintain food quality,
the water activity of the food product needs to be lowered. Conventionally, the water
activity in foods has been controlled by drying, addition of sugar or salt, and by freezing.
The added salt or sugar dissolves in the free water and makes it bound or unavailable. In
other words, it decreases the water activity and makes the food less prone to spoilage.
Lowering the temperature checks the activity of the enzymes in the food and also makes
conditions unfavorable for the growth of microorganisms. Drying of the product acts as a
means of preserving the food by lowering the water activity.