326 Modern Food Microbiology
susceptible by rupturing the outer membrane and thus potentiating the effect of fatty acids or fatty
acid esters. An antioxidant such as BHA would exert effects against bacteria and molds and serve as
an antioxidant at the same time. By use of such a system, the development of resistant strains could
be minimized and the pH of a food could become less important relative to the effectiveness of the
inhibitory system.
ACETIC AND LACTIC ACIDS
These two organic acids are among the most widely employed as preservatives. In most instances,
their presence in the subject foods is due to their production within the food by lactic acid bacteria. Products such as pickles, sauerkraut, and fermented milks, among others, are created by the
fermentative activities by various lactic acid bacteria, which produce acetic, lactic, and other acids.
The antimicrobial effects of organic acids such as propionic and lactic acid is due to both the
depression of pH below the growth range and metabolic inhibition by the undissociated acid molecules.
In determining the quantity of organic acids in foods, titratable acidity is of more value than pH
alone, because the latter is a measure of hydrogen-ion concentration and organic acids do not ionize
completely. In measuring titratable acidity, the amount of acid that is capable of reacting with a known
amount of base is determined. The titratable acidity of products such as sauerkraut is a better indicator
of the amount of acidity present than pH. When E. coli 0157:H7 and five other genera of foodborne
pathogens were exposed to 10% acetic acid at 30
◦ C for 4 days, none grew.
54 The same concentration
of acetic acid reduced E. coli 0157:H7 by 6 log cycles in 1 minute. Lactic acid has been shown to
function as a permeabilizer of the outer membrane of Gram-negative bacteria and thus possibly acts
as a potentiator of other antimicrobials.
3
The bactericidal effect of acetic acid can be demonstrated by its action on certain pathogens. When
two species of Salmonella were added to an oil-and-vinegar-based salad dressing, the initial inoculum
of 5 × 10
6 S. Enteritidis could not be detected after 5 minutes nor could S. Typhimurium be detected
after 10 minutes.
129
Organic acids are employed to wash and sanitize animal carcasses after slaughter to reduce their
carriage of pathogens and to increase product shelf life, and this topic is discussed in Chapter 4.
Salts of Acetic and Lactic Acids
The sodium and potassium salts of acetic and lactic acid are widely used in foods, and they have a
long history of use. For example, sodium diacetate (CH 2 COONa · CH 3 COOH · xH 2 O) is used widely
in the baking industry to prevent moldiness of bread and cakes. Interest in these multifunctional
compounds has increased during the past 2–3 decades in large part because of their potential to extend
shelf life of processed meats. The targeted shelf life for refrigerated cooked meat products in the
United States is 75–90 days,
13 and some of these compounds are important in that regard. Another
reason for the increased interest is because of their activity against psychrotrophic pathogens such
as L. monocytogenes. The salts in this section are bacteriostatic rather than cidal. Some of the more
recent findings are summarized below. For a review of lactates, see Shelef
177 and Shelef and Seiter.
178
To control L. monocytogenes in frankfurters, sodium lactate (3 or 6%), sodium acetate (0.25 or
0.5%), and sodium diacetate (0.25 or 0.5%) were tested on the surface of peeled frankfurters inoculated
with log 10 304 cfu/cm
2 of the pathogen and stored at 4
◦ C vacuum packaged, and the pathogen was
inhibited for 20–70 days.
13 The most effective was 3% sodium lactate (0.25% sodium acetate was the
least effective). Growth of the pathogen was completely inhibited for more than 90 days with sodium
lactate at 6% or sodium diacetate at 0.5%. In another study, wiener surfaces were inoculated with 10
5 cfu
susceptible by rupturing the outer membrane and thus potentiating the effect of fatty acids or fatty
acid esters. An antioxidant such as BHA would exert effects against bacteria and molds and serve as
an antioxidant at the same time. By use of such a system, the development of resistant strains could
be minimized and the pH of a food could become less important relative to the effectiveness of the
inhibitory system.
ACETIC AND LACTIC ACIDS
These two organic acids are among the most widely employed as preservatives. In most instances,
their presence in the subject foods is due to their production within the food by lactic acid bacteria. Products such as pickles, sauerkraut, and fermented milks, among others, are created by the
fermentative activities by various lactic acid bacteria, which produce acetic, lactic, and other acids.
The antimicrobial effects of organic acids such as propionic and lactic acid is due to both the
depression of pH below the growth range and metabolic inhibition by the undissociated acid molecules.
In determining the quantity of organic acids in foods, titratable acidity is of more value than pH
alone, because the latter is a measure of hydrogen-ion concentration and organic acids do not ionize
completely. In measuring titratable acidity, the amount of acid that is capable of reacting with a known
amount of base is determined. The titratable acidity of products such as sauerkraut is a better indicator
of the amount of acidity present than pH. When E. coli 0157:H7 and five other genera of foodborne
pathogens were exposed to 10% acetic acid at 30
◦ C for 4 days, none grew.
54 The same concentration
of acetic acid reduced E. coli 0157:H7 by 6 log cycles in 1 minute. Lactic acid has been shown to
function as a permeabilizer of the outer membrane of Gram-negative bacteria and thus possibly acts
as a potentiator of other antimicrobials.
3
The bactericidal effect of acetic acid can be demonstrated by its action on certain pathogens. When
two species of Salmonella were added to an oil-and-vinegar-based salad dressing, the initial inoculum
of 5 × 10
6 S. Enteritidis could not be detected after 5 minutes nor could S. Typhimurium be detected
after 10 minutes.
129
Organic acids are employed to wash and sanitize animal carcasses after slaughter to reduce their
carriage of pathogens and to increase product shelf life, and this topic is discussed in Chapter 4.
Salts of Acetic and Lactic Acids
The sodium and potassium salts of acetic and lactic acid are widely used in foods, and they have a
long history of use. For example, sodium diacetate (CH 2 COONa · CH 3 COOH · xH 2 O) is used widely
in the baking industry to prevent moldiness of bread and cakes. Interest in these multifunctional
compounds has increased during the past 2–3 decades in large part because of their potential to extend
shelf life of processed meats. The targeted shelf life for refrigerated cooked meat products in the
United States is 75–90 days,
13 and some of these compounds are important in that regard. Another
reason for the increased interest is because of their activity against psychrotrophic pathogens such
as L. monocytogenes. The salts in this section are bacteriostatic rather than cidal. Some of the more
recent findings are summarized below. For a review of lactates, see Shelef
177 and Shelef and Seiter.
178
To control L. monocytogenes in frankfurters, sodium lactate (3 or 6%), sodium acetate (0.25 or
0.5%), and sodium diacetate (0.25 or 0.5%) were tested on the surface of peeled frankfurters inoculated
with log 10 304 cfu/cm
2 of the pathogen and stored at 4
◦ C vacuum packaged, and the pathogen was
inhibited for 20–70 days.
13 The most effective was 3% sodium lactate (0.25% sodium acetate was the
least effective). Growth of the pathogen was completely inhibited for more than 90 days with sodium
lactate at 6% or sodium diacetate at 0.5%. In another study, wiener surfaces were inoculated with 10
5 cfu