66 ◾ Fundamental Food Microbiology
values than the adsorption process does at the same moisture content of a food. This has important
implications in the control of a microorganism by reducing the A W of a food. The A W of a food
can be reduced by several means, such as adding solutes, ions, hydrophilic colloids, and freezing
and drying. 4–7
A W and Microbial Growth
The free water in a food is necessary for microbial growth. It is necessary to transport nutrients and
remove waste materials, carry out enzymatic reactions, synthesize cellular materials, and take part
in other biochemical reactions, such as hydrolysis of a polymer to monomers (proteins to amino
acids). Each microbial species (or group) has an optimum, maximum, and minimum A W level for
growth. In general, the minimum A W values for growth of microbial groups are as follows: most
molds, 0.8, with xerophilic molds as low as 0.6; most yeasts, 0.85, with osmophilic yeasts, 0.6–0.7;
most Gram-positive bacteria, 0.90; and Gram-negative bacteria, 0.93. Some exceptions are growth
of Staphylococcus aureus at 0.85 and halophilic bacteria at 0.75. The A W needed for spore-forming
bacteria to sporulate, for the spores to germinate, and for the toxin-producing microorganisms
to produce toxins is generally higher than the minimum A W needed for their growth. Also, the
minimum A W for growth in an ideal condition is lower than that in a nonideal condition. As an
example, if the minimum A W for growth of a bacterial strain at pH 6.8 is 0.91, then at pH 5.5, it
can be 0.95 or more. When the A W is reduced below the minimum level required for growth of a
microorganism, the cells remain viable for a while. But if the A W is reduced drastically, microbial
cells in a population lose viability, generally rapidly at first and then more slowly. This information
is used to control spoilage and pathogenic microorganisms in food as well as enhance the growth
of desirable types in food bioprocessing (such as adding salt in the processing of cured ham, see
Chapter 35) and in laboratory detection of microorganisms (adding salt to media to create a selective environment for enumeration of Sta. aureus, Listeria monocytogenes, etc.).
pH and Growth
Principle
pH indicates the hydrogen ion concentrations in a system and is expressed as –log [H
+
], the negative logarithm of the hydrogen ion or proton concentration. It ranges from 0 to 14, with 7.0 being
neutral pH. [H
+
] concentrations can differ in a system, depending on what acid is present. Some
strong acids used in foods, such as HCl and phosphoric acid, dissociate completely. Weak acids,
such as acetic or lactic acids, remain in equilibrium with the dissociated and undissociated forms:
[
] [ ] [
]
.
HCl
H
CI , pH of
NHCl is
CH COOH
3
→
+
.
→
+
−
0 1
1 1
[ [ ] [
]
H
CH COO , pH of
NCH COOH is
+
−
+
.
.
3
3
0 1
2 9
Acidity is inversely related to pH: A system with high acidity has a low pH and vice versa. 8,9
pH of Food
Depending on the type, the pH of a food can vary greatly. On the basis of pH, foods can be
grouped as high-acid foods (pH below 4.6) and low-acid foods (pH 4.6 and above). Most fruits,
values than the adsorption process does at the same moisture content of a food. This has important
implications in the control of a microorganism by reducing the A W of a food. The A W of a food
can be reduced by several means, such as adding solutes, ions, hydrophilic colloids, and freezing
and drying. 4–7
A W and Microbial Growth
The free water in a food is necessary for microbial growth. It is necessary to transport nutrients and
remove waste materials, carry out enzymatic reactions, synthesize cellular materials, and take part
in other biochemical reactions, such as hydrolysis of a polymer to monomers (proteins to amino
acids). Each microbial species (or group) has an optimum, maximum, and minimum A W level for
growth. In general, the minimum A W values for growth of microbial groups are as follows: most
molds, 0.8, with xerophilic molds as low as 0.6; most yeasts, 0.85, with osmophilic yeasts, 0.6–0.7;
most Gram-positive bacteria, 0.90; and Gram-negative bacteria, 0.93. Some exceptions are growth
of Staphylococcus aureus at 0.85 and halophilic bacteria at 0.75. The A W needed for spore-forming
bacteria to sporulate, for the spores to germinate, and for the toxin-producing microorganisms
to produce toxins is generally higher than the minimum A W needed for their growth. Also, the
minimum A W for growth in an ideal condition is lower than that in a nonideal condition. As an
example, if the minimum A W for growth of a bacterial strain at pH 6.8 is 0.91, then at pH 5.5, it
can be 0.95 or more. When the A W is reduced below the minimum level required for growth of a
microorganism, the cells remain viable for a while. But if the A W is reduced drastically, microbial
cells in a population lose viability, generally rapidly at first and then more slowly. This information
is used to control spoilage and pathogenic microorganisms in food as well as enhance the growth
of desirable types in food bioprocessing (such as adding salt in the processing of cured ham, see
Chapter 35) and in laboratory detection of microorganisms (adding salt to media to create a selective environment for enumeration of Sta. aureus, Listeria monocytogenes, etc.).
pH and Growth
Principle
pH indicates the hydrogen ion concentrations in a system and is expressed as –log [H
+
], the negative logarithm of the hydrogen ion or proton concentration. It ranges from 0 to 14, with 7.0 being
neutral pH. [H
+
] concentrations can differ in a system, depending on what acid is present. Some
strong acids used in foods, such as HCl and phosphoric acid, dissociate completely. Weak acids,
such as acetic or lactic acids, remain in equilibrium with the dissociated and undissociated forms:
[
] [ ] [
]
.
HCl
H
CI , pH of
NHCl is
CH COOH
3
→
+
.
→
+
−
0 1
1 1
[ [ ] [
]
H
CH COO , pH of
NCH COOH is
+
−
+
.
.
3
3
0 1
2 9
Acidity is inversely related to pH: A system with high acidity has a low pH and vice versa. 8,9
pH of Food
Depending on the type, the pH of a food can vary greatly. On the basis of pH, foods can be
grouped as high-acid foods (pH below 4.6) and low-acid foods (pH 4.6 and above). Most fruits,