Factors Influencing Microbial Growth in Food ◾ 67
fruit juices, fermented foods (from fruits, vegetables, meat, and milk), and salad dressings are
high-acid (low-pH) foods, whereas most vegetables, meat, fish, milk, and soups are low-acid (highpH) foods. Tomato, however, is a high-acid vegetable (pH 4.1–4.4). The higher pH limit of most
low-acid foods remains below 7.0; only in a few foods, such as clams (pH 7.1) and egg albumen
(pH 8.5), does the pH exceed 7.0. Similarly, the low-pH limit of most high-acid foods remains
above 3.0 except in some citrus fruits (lemon, lime, grapefruit) and cranberry juice in which the
pH can be as low as 2.2. The acid in the foods can either be present naturally (as in fruits), produced during fermentation (as in fermented foods), or added during processing (as in salad dressings). Foods can also have compounds that have a buffering capacity. A food such as milk or meat,
because of good buffering capacity, does not show pH reduction when compared with a vegetable
product in the presence of the same amount of acid.
pH and Microbial Growth
The pH of a food has a profound effect on the growth and viability of microbial cells. Each species
has an optimum and a range of pH for growth. In general, molds and yeasts are able to grow at
lower pH levels than do bacteria, and Gram-negative bacteria are more sensitive to low pH than
are Gram-positive bacteria. The pH range of growth for molds is 1.5–9.0; for yeasts, 2.0–8.5;
for Gram-positive bacteria, 4.0–8.5; and for Gram-negative bacteria, 4.5–9.0. Individual species
differ greatly in lower pH limit for growth; for example, Pediococcus acidilactici can grow at pH
3.8, and Sta. aureus and Salmonella can grow at pH 4.5, but normally, Campylobacter cannot.
The lower pH limit of growth of a species can be a little higher if the pH is adjusted with a strong
acid instead of a weak acid (because of its undissociated molecules). Acid-resistant or acid-tolerant
strains can acquire resistance to lower pH compared with the other strains of a species (e.g., acidresistant Salmonella; Escherichia coli O157:H7).
When the pH in a food is reduced to below the lower limit for growth of a microbial species,
the cells not only stop growing but also lose viability, the rate of which depends on the extent of
pH reduction. This is more apparent with weak acids, especially with those that have higher dissociation constants (pK), such as acetic acid versus lactic acid (with pK values 4.8 and 3.8, respectively). This is because at the same pH, acetic acid has more undissociated molecules than lactic
acid does. The undissociated molecules, being lipophilic, enter into the cell and dissociate to generate H + in the cytoplasm. This causes a reduction in internal pH, which ultimately destroys the
proton gradient between the inside and the outside of the cells and dissipates proton motive force
as well as the ability of the cells to generate energy. The information on the influence of pH on
the growth and viability of microbial cells is important to develop methods to prevent the growth
of undesirable microorganisms in food (e.g., in acidified foods; see Chapter 36), to produce some
fermented foods (e.g., sequential growth of lactic acid bacteria in sauerkraut fermentation), and to
selectively isolate aciduric microorganisms from food (e.g., yeasts and molds in a medium with pH
3.5). 8,9 Acquired acid tolerance by pathogens and spoilage bacteria can impose problems in their
control in low-pH foods; this aspect is discussed in Chapter 10.
Redox Potential, Oxygen, and Growth
Principle
The redox or oxidation–reduction (O-R) potential measures the potential difference in a system
generated by a coupled reaction in which one substance is oxidized and a second substance is
fruit juices, fermented foods (from fruits, vegetables, meat, and milk), and salad dressings are
high-acid (low-pH) foods, whereas most vegetables, meat, fish, milk, and soups are low-acid (highpH) foods. Tomato, however, is a high-acid vegetable (pH 4.1–4.4). The higher pH limit of most
low-acid foods remains below 7.0; only in a few foods, such as clams (pH 7.1) and egg albumen
(pH 8.5), does the pH exceed 7.0. Similarly, the low-pH limit of most high-acid foods remains
above 3.0 except in some citrus fruits (lemon, lime, grapefruit) and cranberry juice in which the
pH can be as low as 2.2. The acid in the foods can either be present naturally (as in fruits), produced during fermentation (as in fermented foods), or added during processing (as in salad dressings). Foods can also have compounds that have a buffering capacity. A food such as milk or meat,
because of good buffering capacity, does not show pH reduction when compared with a vegetable
product in the presence of the same amount of acid.
pH and Microbial Growth
The pH of a food has a profound effect on the growth and viability of microbial cells. Each species
has an optimum and a range of pH for growth. In general, molds and yeasts are able to grow at
lower pH levels than do bacteria, and Gram-negative bacteria are more sensitive to low pH than
are Gram-positive bacteria. The pH range of growth for molds is 1.5–9.0; for yeasts, 2.0–8.5;
for Gram-positive bacteria, 4.0–8.5; and for Gram-negative bacteria, 4.5–9.0. Individual species
differ greatly in lower pH limit for growth; for example, Pediococcus acidilactici can grow at pH
3.8, and Sta. aureus and Salmonella can grow at pH 4.5, but normally, Campylobacter cannot.
The lower pH limit of growth of a species can be a little higher if the pH is adjusted with a strong
acid instead of a weak acid (because of its undissociated molecules). Acid-resistant or acid-tolerant
strains can acquire resistance to lower pH compared with the other strains of a species (e.g., acidresistant Salmonella; Escherichia coli O157:H7).
When the pH in a food is reduced to below the lower limit for growth of a microbial species,
the cells not only stop growing but also lose viability, the rate of which depends on the extent of
pH reduction. This is more apparent with weak acids, especially with those that have higher dissociation constants (pK), such as acetic acid versus lactic acid (with pK values 4.8 and 3.8, respectively). This is because at the same pH, acetic acid has more undissociated molecules than lactic
acid does. The undissociated molecules, being lipophilic, enter into the cell and dissociate to generate H + in the cytoplasm. This causes a reduction in internal pH, which ultimately destroys the
proton gradient between the inside and the outside of the cells and dissipates proton motive force
as well as the ability of the cells to generate energy. The information on the influence of pH on
the growth and viability of microbial cells is important to develop methods to prevent the growth
of undesirable microorganisms in food (e.g., in acidified foods; see Chapter 36), to produce some
fermented foods (e.g., sequential growth of lactic acid bacteria in sauerkraut fermentation), and to
selectively isolate aciduric microorganisms from food (e.g., yeasts and molds in a medium with pH
3.5). 8,9 Acquired acid tolerance by pathogens and spoilage bacteria can impose problems in their
control in low-pH foods; this aspect is discussed in Chapter 10.
Redox Potential, Oxygen, and Growth
Principle
The redox or oxidation–reduction (O-R) potential measures the potential difference in a system
generated by a coupled reaction in which one substance is oxidized and a second substance is