468 ◾ Fundamental Food Microbiology
suffer reversible injury and death in foods with low A W although not in a predictable manner as
in heat treatment. Finally, reduced A W is also used to retain viability of starter-culture bacteria for
use in food bioprocessing, which is discussed in Chapter 14. In this chapter, preservation of food
by controlling microbial growth at low A W is described.
Mechanism of Action
Microorganisms need water for transport of nutrients, nutrient metabolism, and removal of cellular wastes. In a food, the total water (moisture) is present as free water and bound water; the
latter remains bound to hydrophilic colloids and solutes (it can also remain as capillary water or in
a frozen state as ice crystals) and is not available for biological functions. Thus, only the free water
(which is related to A W ) is important for microbial growth. Microorganisms also retain a slightly
lower A W inside the cells than in the external environment to maintain turgor pressure, and this
is important for cell growth. If the free water in the environment is reduced either by removing
water or by adding solutes and hydrophilic colloids, which cannot readily enter the cells, the free
water from the cells flows outside in an effort to establish equilibrium. The loss of water causes an
osmotic shock and plasmolysis, during which the cells do not grow. The water loss can be quite considerable even with a slight reduction in A W . A 0.005 reduction in A W from 0.955 to 0.950 in the
environment reduces the intracellular water content by 50% in Staphylococcus aureus and reduces
the cell volume by 44% in Salmonella enterica ser. Typhimurium. Hence, even a slight reduction
in A W , necessary for minimal growth of a microbial species or strain, prevents its growth. Unless
a microbial cell regains its intracellular turgor by reducing internal A W , it will either remain dormant or die. This is often the case with microorganisms sensitive to slight A W reduction. However,
some other microorganisms have developed very effective mechanisms such as transporting solutes
inside or metabolizing solutes to overcome plasmolysis and regain turgor. Microorganisms that are
relatively resistant to a great reduction in A W and grow at relatively lower A W have this capability. 2,3
influencing Factors
Nature of Process
Water activity and the total amount of water (% moisture) a food contains are different. The A W
of a food indicates the amount or fraction of the total amount of water available for some chemical
or biochemical reactions. In pure water, both values are the same, but in food, A W is always less
than the total amount of water. Under a set of conditions, the relationship between the moisture
content and the A W of a food can be determined from the sorption isotherm. However, instead
of a single line, the sorption isotherm forms a loop (hysteresis loop), depending on whether it is
determined during removal of water from (desorption) or during addition of water to (adsorption)
a food (see Figure 6.1). At the same moisture level, the A W value obtained by desorption is lower
than that obtained by adsorption. In controlling microbial growth by reducing A W , this is quite
important.
Solutes differ in their ability to reduce A W . The amounts (% w/w) of NaCl, sucrose, glucose,
and inverted sugar required to reduce A W at 25°C of pure water to 0.99 are 1.74, 15.45, 8.9,
and 4.11 g; and to 0.92, they are 11.9, 54.34, 43.72, and 32.87 g, respectively. These solutes do
not freely enter the microbial cells and thus have a greater inhibitory effect on microbial cells
suffer reversible injury and death in foods with low A W although not in a predictable manner as
in heat treatment. Finally, reduced A W is also used to retain viability of starter-culture bacteria for
use in food bioprocessing, which is discussed in Chapter 14. In this chapter, preservation of food
by controlling microbial growth at low A W is described.
Mechanism of Action
Microorganisms need water for transport of nutrients, nutrient metabolism, and removal of cellular wastes. In a food, the total water (moisture) is present as free water and bound water; the
latter remains bound to hydrophilic colloids and solutes (it can also remain as capillary water or in
a frozen state as ice crystals) and is not available for biological functions. Thus, only the free water
(which is related to A W ) is important for microbial growth. Microorganisms also retain a slightly
lower A W inside the cells than in the external environment to maintain turgor pressure, and this
is important for cell growth. If the free water in the environment is reduced either by removing
water or by adding solutes and hydrophilic colloids, which cannot readily enter the cells, the free
water from the cells flows outside in an effort to establish equilibrium. The loss of water causes an
osmotic shock and plasmolysis, during which the cells do not grow. The water loss can be quite considerable even with a slight reduction in A W . A 0.005 reduction in A W from 0.955 to 0.950 in the
environment reduces the intracellular water content by 50% in Staphylococcus aureus and reduces
the cell volume by 44% in Salmonella enterica ser. Typhimurium. Hence, even a slight reduction
in A W , necessary for minimal growth of a microbial species or strain, prevents its growth. Unless
a microbial cell regains its intracellular turgor by reducing internal A W , it will either remain dormant or die. This is often the case with microorganisms sensitive to slight A W reduction. However,
some other microorganisms have developed very effective mechanisms such as transporting solutes
inside or metabolizing solutes to overcome plasmolysis and regain turgor. Microorganisms that are
relatively resistant to a great reduction in A W and grow at relatively lower A W have this capability. 2,3
influencing Factors
Nature of Process
Water activity and the total amount of water (% moisture) a food contains are different. The A W
of a food indicates the amount or fraction of the total amount of water available for some chemical
or biochemical reactions. In pure water, both values are the same, but in food, A W is always less
than the total amount of water. Under a set of conditions, the relationship between the moisture
content and the A W of a food can be determined from the sorption isotherm. However, instead
of a single line, the sorption isotherm forms a loop (hysteresis loop), depending on whether it is
determined during removal of water from (desorption) or during addition of water to (adsorption)
a food (see Figure 6.1). At the same moisture level, the A W value obtained by desorption is lower
than that obtained by adsorption. In controlling microbial growth by reducing A W , this is quite
important.
Solutes differ in their ability to reduce A W . The amounts (% w/w) of NaCl, sucrose, glucose,
and inverted sugar required to reduce A W at 25°C of pure water to 0.99 are 1.74, 15.45, 8.9,
and 4.11 g; and to 0.92, they are 11.9, 54.34, 43.72, and 32.87 g, respectively. These solutes do
not freely enter the microbial cells and thus have a greater inhibitory effect on microbial cells