Water Activity

Food Spoilage

Types of food spoilage




Microbial growth

Primary cause


Moisture content

Single most important factor in food spoilage

Moisture content is usually expressed as % moisture

Can be expressed as

Wet basis

Used for food composition table

Dry basis

Used for processing calculations

Determination of % moisture

Wet basis



Dry basis



Moisture content (%MC)

Knowledge of moisture content is not sufficient to predict stability of food

Need to know the availability of water

Microbial activity

Enzymatic activity

Chemical Activity


Types of Water

Tightly bound

First water molecules to interact with functional groups of solutes

Loosely bound

Interacts with functional groups but not as strongly


Not bound and acts as bulk water

Available for microbial growth, enzymatic and chemical reactions

Water Activity (aw)

Water activity

The measure of the amount of water available for microbial growth, enzymatic activity and chemical reactions

aw is defined as a ratio:

Moisture content and Water Activity

Relationship of aw to %MC

BET monolayer (Brunauer-Emmett-Teller)

Water that is tightly bound to specific sites

BET monolayer value

%MC where food is most stable

%MC below increase lipid oxidation

%MC above increase of maillard browning, enzymatic and microbial activity

BET equation






Determination of BET monolayer value

Plot aw/M(1-aw) vs aw

Known as a BET Plot

Should yield a straight line

Calculate M1


Effect of Water Activity on microbial growth

aw of 0.9 and below bacterial activity inhibited

aw of 0.8 and below yeast activity inhibited

aw of 0.7 and below fungi activity inhibited

   aw of 0.6 and below all microbial activity inhibited


Effect of Water Activity

Interaction of aw, pH and temperature