Water Activity in Foods


  1. Introduction
  2. The Structure of Water
  3. Water In Foods
    • Water and Food Safety
    • Water Activity (aw)
    • Effects of Reduced aw on Food Safety
    • To Reduce aw
  4. Measure of Water Content and aw
    • Relationship between Water Activity and Water
    • Analysis of Water Content
    • Analysis of Water Activity
    • Water Activity as a CCP

1. Introduction

Water is the most abundant constituent of food and in terms of food safety the most significant. Its presence, quantity and nature determines many chemical and biochemical processes important for the control of product safety and quality. In many HACCP plans, water is frequently referred to as an intrinsic parameter required in product safety and while we most of us understand its importance we often fail to understand the reasons why. A better understanding of water and in particular water activity aw, can assist in developing robust and scientifically supported food safety plans.

2. The Structure of Water

The chemical structure of water is H2O. It does not exist in an absolutely pure form in nature due to its chemical properties. One molecule of water is composed of two hydrogen atoms covalently bonded to a single oxygen atom. It commonly occurs in all three states i.e. liquid, solid and gas.

At standard temperature and pressure water is a liquid. It is also tasteless and odorless. The molecule itself is non-linear, polar with an electrical dipole movement. It is a good polar solvent and is typically referred to as the universal solvent. Hydrophilic (water-loving) substances dissolve readily while hydrophobic (water-fearing) substances are immiscible. At standard temperature and pressure the boiling point of water is 100 °C (212 °F).The density of liquid water is 1,000 kg/m3 (62.43 lb/cu ft) at 4 °C. Ice has a density of 917 kg/m3 (57.25 lb/cu ft).