478 ◾ Fundamental Food Microbiology
Nature of Foods
The normal pH of foods varies greatly from a very acid range (pH 3.0; citrus juice) to an alkaline
range (pH 9.0; egg albumen). The initial pH can strongly influence the antimicrobial effect of an
acid. An acid is more inhibitory in a food at a lower pH than in one at a higher pH. The buffering
action of the food components also reduces the effectiveness of low pH. Nutrients can also facilitate repair of sublethal acid injury of microorganisms.
Nature of Microorganisms
Microorganisms important in food vary greatly in the lower limit of pH that allows growth
(Table 36.2). In general, gram-negative bacteria are more sensitive to low pH than are grampositive bacteria, and yeasts and molds are the least sensitive. Fermentative bacteria are more
resistant to lower pH than are respiring bacteria, probably because they are able to resist changes
in external pH as well as withstand slightly lower internal pH. The ability of yeasts and molds to
withstand low pH is also a result of these factors. 5 The antimicrobial property of an organic acid is
enhanced by heat, low A W , the presence of some other preservatives, and low storage temperatures.
The inhibitory effect of weak acids is reduced at a higher microbial load. Also, in a mixed
microbial population, the metabolism of an acid (such as lactate) by one resistant species can
reduce its effective concentration against another sensitive species, allowing the latter to grow.
Some microorganisms important in food, such as some Salmonella serovars and Esc. coli O157:H7,
seem to have genetic determinants that enable them to grow at higher acid concentrations (or
lower pH) than other strains of the same species. The acid tolerance seems to be related to overproduction of a group of proteins (stress proteins) by these strains (see Chapter 10).
Finally, microorganisms differ in their sensitivity to different organic acids. Yeasts and molds
are particularly sensitive to propionic and sorbic acids, and bacteria are more sensitive to acetic
acid. Bacterial spores at lower pH become susceptible to heat treatment and do not germinate and
outgrow at minimum A W of growth. Also, the inhibitory effect of NO 2 against spores is more
pronounced at the lower pH range of growth.
Acids Used
Acetic Acid
Acetic acid is used usually as vinegar (5%–10% acetic acid) or as salts of sodium and calcium at
25% or higher levels in pickles, salad dressings, and sauces. It is more effective against bacteria
than yeasts and molds. Bacteria that grow better above pH 6.0 are more inhibited. The inhibitory
concentrations of undissociated acid are 0.02% against Salmonella, 0.01% against Staphylococcus
aureus, 0.02% against Bacillus cereus, 0.1% against Aspergillus spp., and 0.5% against Saccharomyces
spp. The inhibitory action of acetic acid is produced through neutralizing the electrochemical gradient of the cell membrane as well as denaturing proteins inside the cells. Besides its use in food,
acetic acid has been recommended for use (1%–2%) in carcass wash to reduce bacterial loads. 1,2
Propionic Acid
Propionic acid is used as salts of calcium and sodium at 1000–2000 ppm (0.1%–0.2%) in bread,
bakery products, cheeses, jams and jellies, and tomato puree. It is effective against molds and
Nature of Foods
The normal pH of foods varies greatly from a very acid range (pH 3.0; citrus juice) to an alkaline
range (pH 9.0; egg albumen). The initial pH can strongly influence the antimicrobial effect of an
acid. An acid is more inhibitory in a food at a lower pH than in one at a higher pH. The buffering
action of the food components also reduces the effectiveness of low pH. Nutrients can also facilitate repair of sublethal acid injury of microorganisms.
Nature of Microorganisms
Microorganisms important in food vary greatly in the lower limit of pH that allows growth
(Table 36.2). In general, gram-negative bacteria are more sensitive to low pH than are grampositive bacteria, and yeasts and molds are the least sensitive. Fermentative bacteria are more
resistant to lower pH than are respiring bacteria, probably because they are able to resist changes
in external pH as well as withstand slightly lower internal pH. The ability of yeasts and molds to
withstand low pH is also a result of these factors. 5 The antimicrobial property of an organic acid is
enhanced by heat, low A W , the presence of some other preservatives, and low storage temperatures.
The inhibitory effect of weak acids is reduced at a higher microbial load. Also, in a mixed
microbial population, the metabolism of an acid (such as lactate) by one resistant species can
reduce its effective concentration against another sensitive species, allowing the latter to grow.
Some microorganisms important in food, such as some Salmonella serovars and Esc. coli O157:H7,
seem to have genetic determinants that enable them to grow at higher acid concentrations (or
lower pH) than other strains of the same species. The acid tolerance seems to be related to overproduction of a group of proteins (stress proteins) by these strains (see Chapter 10).
Finally, microorganisms differ in their sensitivity to different organic acids. Yeasts and molds
are particularly sensitive to propionic and sorbic acids, and bacteria are more sensitive to acetic
acid. Bacterial spores at lower pH become susceptible to heat treatment and do not germinate and
outgrow at minimum A W of growth. Also, the inhibitory effect of NO 2 against spores is more
pronounced at the lower pH range of growth.
Acids Used
Acetic Acid
Acetic acid is used usually as vinegar (5%–10% acetic acid) or as salts of sodium and calcium at
25% or higher levels in pickles, salad dressings, and sauces. It is more effective against bacteria
than yeasts and molds. Bacteria that grow better above pH 6.0 are more inhibited. The inhibitory
concentrations of undissociated acid are 0.02% against Salmonella, 0.01% against Staphylococcus
aureus, 0.02% against Bacillus cereus, 0.1% against Aspergillus spp., and 0.5% against Saccharomyces
spp. The inhibitory action of acetic acid is produced through neutralizing the electrochemical gradient of the cell membrane as well as denaturing proteins inside the cells. Besides its use in food,
acetic acid has been recommended for use (1%–2%) in carcass wash to reduce bacterial loads. 1,2
Propionic Acid
Propionic acid is used as salts of calcium and sodium at 1000–2000 ppm (0.1%–0.2%) in bread,
bakery products, cheeses, jams and jellies, and tomato puree. It is effective against molds and