Water is the most abundant substance in most foods. Classification of food is divided into three groups according to their water content (high, medium and low humidity). Fruits, vegetables, juices, raw meat, fish and dairy fall into the high humidity category. Breads, hard cheeses, and sausages are examples of medium-humidity foods, while the low-moisture groups are dehydrated vegetables, whole grains, powdered milk and dry soup mixes.
The importance of the function of water in food goes far beyond the quantity in the composition of the food. Water is essential for the good texture and appearance of fruits and vegetables. In such products, loss of water usually results in lower quality. In other hand, water, becomes an important requirement for the occurrence and support of chemical reactions and microbial growth, and prevents chemical spoilage of food.
It is now well known that the effect of water on food stability cannot be related solely to quantitative water content. For example, honey containing 23% water is very stable in storage while dehydrated potatoes will undergo rapid decay at half-high water content. To explain the effect of water, parameters that reflect the quantity and ‘effectiveness’ of water are needed. This parameter is water activity.
Water activity aw is defined as the ratio of the vapor pressure of the food to the vapor pressure of pure water at the same temperature.
aw = P/P0
P = partial pressure of water vapor of food at temperature T
P0 = equilibrium vapor pressure of pure water at temperature T. The same type of ratio also determines the relative humidity of air, RH (usually expressed as a percentage):
RH = (P’/P0)*100%
P’ = Partial pressure of water vapor in the air
If food is in equilibrium with air, then p = p’ . Therefore the water activity of the food is equal to the relative humidity of the atmosphere in equilibrium with the food. For this reason, water activity is sometimes expressed as the equilibrium relative humidity, ERH.
aw = ERH/100
|above 0.95||Fresh fruit, vegetables, milk, meat, fish|
|0.90-0.95||semi-hard cheese, anchovies, bread|
|0.85-0.90||hard cheese, sausage, butter|
|0.80-0.85||fruit juice, jelly, wet pet food|
|0.70-0.80||jam, dry cheese, legumes, plums|
|0.50-0.70||raisins, honey, seeds|
|0.20-0.40||Non-fat milk powder|
|<0.2||biscuits, roasted ground coffee, sugar|
Typical water activity in food
The main mechanisms that contribute to the decrease in water vapor pressure in food are solvent-solute interactions, binding of water molecules to the poles of polymeric substances (e.g. polysaccharides and proteins), adsorption of water on the solid surface matrix and capillary forces. In high-moisture foods, such as fruit juices, depression can be attributed entirely to water-solute interactions. If the food is considered an ‘ideal solution’, this water vapor pressure applies Raoult’s law:
P = XwP0
Where Xw is the water content (in mole fraction) of the food. Therefore the water activity of an ideal aqueous solution is equal to the molar concentration of water Xw. The water activity of foods with high humidity (with an aw of 0.9 or higher) can be calculated quite accurately by this method.
Contributor: Daris Arsyada
Berk, Zeki. 2008. Food Process Engineering and Technology. United States of America: Elsevier