Water activity, not moisture content, determines the lower limit of “available” water for microbial growth. Since bacteria, yeast, and molds require a certain amount of “available” water to support growth, designing a product below a critical water activity level provides an effective means to control growth.

1. WATER ACTIVITY
By,
Shefi Varghese
1st M.Sc FTQA
To,
Prof.Anil
Mount Royal College

2. INDEX
 What do you meant by water activity…?
 Factors influencing water activity
 Role of water activity
 Significance of water activity
 Reference

3. What do you mean by Water Activity…?
 Water activity (aw) is defined as the ratio of vapour pressure (P) in food to the vapour pressure of pure
water (p*)at same conditions .
 The definition of aw is aw=p/po

4.  Water activity is a measure for the amount of available water.
 Water activity of pure water is 1
 Foods with water activity greater than 0.95 ;it supports the growth of micro
organisms.
Food Product Typical water activity
Fresh fruits & vegetables 0.99
Fresh meat 0.99
Milk 0.99
Rice 0.5 - 0.6
Peanut Butter 0.7
Jams 0.8
Honey 0.5 – 0.6
Roasted Coffee Beans 0.1 – 0.3

5. Factors Influencing water Activity(aw)
 Drying: Water activity is decreased by physically removing water (Ex: beef jerky).
 Solutes: Water activity is decreased by adding solutes such as salt or sugar (Ex: jams, cured meat)
 Freezing: Water activity is decreased by freezing (Ex: water is removed in the form of ice)
 Surface Tension:Surface tension decreases water activity increases

6. Role of Water Activity(aw)
 Predicting safety and stability
 Limiting microbial growth
 Moisture Migration
 Physical properties
 Caking, clumping, collapse and stickiness
 Shelf life/packaging
 Chemical or biochemical activity

7. Significance of Water Activity
 Water activity is one of the most critical factors in determining quality and
safety of foods .Water activity affects the shelf life ,safety
,texture,flavour,and smell of foods.
 While temperature , ph and several other factors can influence if and how
fast oraganisms will grow in a product,water activity may be the most
important factor in controlling spoilage.
 Most bacteria for example,do not grow at water activities below 0.91, and
most molds cease to grow at water activities below 0.80

8.  By measuring water activity, it is possible to predict which microorganisms will and
will not be potential sources of spoilage.
 Water activity determines the lower limit of available water for microbial growth.
 In addition to influence microbial spoilage,water activity can play a significant role
in determining the activity of enzymes and vitamins in foods and can have a major
impact in their color,taste and aroma.

9. Significance of water Activity…
 Food industry
 Pharmaceutical industry, Cosmetic industry
 Seed storage
 Tobacco industry
 Industrial manufacturing

10. Reference
 John A. Troller. Statistical Analysis of a w Measurements. Journal of Food Science,
Vol. 42, 1977.
 Norman N. Potter. Food Science. The AVI Publishing Co., Inc. 1968.