Factors Influencing Microbial Growth in Food ◾ 65
cloves. The inhibitors, depending on their mode of action, can prevent or reduce growth and kill
microorganisms. This aspect is discussed in Chapter 38.
Water Activity and Growth
Principle
Water activity (A W ) is a measure of the availability of water for biological functions and relates to water
present in a food in free form. In a food system, total water or moisture is present in free and bound
forms. Bound water is the fraction used to hydrate hydrophilic molecules and to dissolve solutes, and
is not available for biological functions; thus, it does not contribute to A W . The A W of a food can be
expressed by the ratio of water vapor pressure of the food (P, which is <1) to that of pure water (Po,
which is 1), that is, P/Po. It ranges between 0 and 1 or, more accurately, >0 to <1 because no food can
have a water activity of either 0 or 1. The A W of a food can be determined from its equilibrium relative
humidity (ERH) by dividing ERH by 100 (because ERH is expressed as a percentage). 4–7
A W of Food
The A W of food ranges from approximately 0.1 to 0.99. The A W values of some food groups are
as follows: cereals, crackers, sugar, salt, dry milk, 0.10–0.20; noodles, honey, chocolate, dried
egg, <0.60; jam, jelly, dried fruits, parmesan cheese, nuts, 0.60–0.85; fermented sausage, dry
cured meat, sweetened condensed milk, maple syrup, 0.85–0.93; evaporated milk, tomato paste,
bread, fruit juices, salted fish, sausage, processed cheese, 0.93–0.98; and fresh meat, fish, fruits,
vegetables, milk, eggs, 0.98–0.99. The A W of foods can be reduced by removing water (desorption) and increased by the adsorption of water, and these two parameters can be used to draw a
sorption isotherm graph for a food (Figure 6.1). The desorption process gives relatively lower A W
Water activity (A w )
Desorption
Adsorption
% Water
30
25
20
15
10
5
0
0
0.2
0.4
0.6
0.8
1
0.12
Figure 6.1 Water sorption isotherm showing hysterisis. At the same percentage of water, A W is
lower by desorption than by adsorption.
cloves. The inhibitors, depending on their mode of action, can prevent or reduce growth and kill
microorganisms. This aspect is discussed in Chapter 38.
Water Activity and Growth
Principle
Water activity (A W ) is a measure of the availability of water for biological functions and relates to water
present in a food in free form. In a food system, total water or moisture is present in free and bound
forms. Bound water is the fraction used to hydrate hydrophilic molecules and to dissolve solutes, and
is not available for biological functions; thus, it does not contribute to A W . The A W of a food can be
expressed by the ratio of water vapor pressure of the food (P, which is <1) to that of pure water (Po,
which is 1), that is, P/Po. It ranges between 0 and 1 or, more accurately, >0 to <1 because no food can
have a water activity of either 0 or 1. The A W of a food can be determined from its equilibrium relative
humidity (ERH) by dividing ERH by 100 (because ERH is expressed as a percentage). 4–7
A W of Food
The A W of food ranges from approximately 0.1 to 0.99. The A W values of some food groups are
as follows: cereals, crackers, sugar, salt, dry milk, 0.10–0.20; noodles, honey, chocolate, dried
egg, <0.60; jam, jelly, dried fruits, parmesan cheese, nuts, 0.60–0.85; fermented sausage, dry
cured meat, sweetened condensed milk, maple syrup, 0.85–0.93; evaporated milk, tomato paste,
bread, fruit juices, salted fish, sausage, processed cheese, 0.93–0.98; and fresh meat, fish, fruits,
vegetables, milk, eggs, 0.98–0.99. The A W of foods can be reduced by removing water (desorption) and increased by the adsorption of water, and these two parameters can be used to draw a
sorption isotherm graph for a food (Figure 6.1). The desorption process gives relatively lower A W
Water activity (A w )
Desorption
Adsorption
% Water
30
25
20
15
10
5
0
0
0.2
0.4
0.6
0.8
1
0.12
Figure 6.1 Water sorption isotherm showing hysterisis. At the same percentage of water, A W is
lower by desorption than by adsorption.