Control by Low pH and Organic Acids ◾ 481
Parabens (Esters of p-Hydroxybenzoic Acid)
Parabens are used as methyl, ethyl, butyl, or propyl parabens. They are broad-spectrum antimicrobial agents and used in food and cosmetic and pharmaceutical products. Because of high pK
values, they are effective at high pH and against bacteria, yeasts, and molds. They are used at
100–1000 ppm (0.01%–0.1%) in nonalcoholic and alcoholic beverages, fruit fillings, jams and jellies, pickles, confectioneries, salad dressings, spreads, and mustards. The undissociated inhibitory
concentrations are 0.05%–0.1% against yeasts, molds, and bacteria. Propyl and butyl parabens are
more inhibitory than the others.
Parabens produce antimicrobial action by acting on several targets of microbial cells. They may
inhibit the functions of several enzymes. They dissolve membrane lipids and interfere with membrane functions, including transport of nutrients. They also interfere with the synthesis of proteins, RNA, and DNA. In addition, they destroy the membrane potential as other weak organic
acids do. However, toxic side effects of parabens, such as reproductive toxicity resulting from an
estrogenic effect, asthma, allergic response, and dermatitis have raised serious concerns in recent
years. 10,11
Conclusion
A pH lower than the minimum growth pH prevents microbial growth by affecting energy production, enzymatic activity, transportation of nutrients, and others. In addition to controlling growth,
microbial cells are sublethally injured or killed by low pH. Many food-grade organic acids along
with other methods are used to control microbial growth in food. Low pH is also used to prevent
germination of bacterial spores in food. Although many Gram-negative pathogenic bacteria are
very sensitive to low pH, it cannot be used to eliminate these pathogens during storage of food.
The fact that some strains can become acid resistant has created concern about such a practice in
the processing and preservation of some low-pH foods. In addition to low pH and the methods
discussed before, low O-R potential is used to control microbial growth in food and is discussed
in Chapter 37.
QUESTIONS
1. Discuss the mechanisms of antimicrobial action of weak organic acids and low pH. Explain
the differences in sensitivity to low pH among gram-negative and gram-positive bacteria,
bacterial spores, yeasts, and molds.
2. Define the pK of an organic acid and, using acetic and lactic acids as examples, describe their
differences in antibacterial effect at pH 5 and 6.
3. Briefly discuss the influence of the following factors on antimicrobial effectiveness of low
pH: acids, foods, and microorganisms.
4. List the specific acids to be used to inhibit growth of gram-positive bacteria, gram-negative
bacteria, yeasts, and molds.
5. Discuss the antimicrobial properties of acetic acid, propionic acid, lactic acid, benzoic acid,
and parabens.
6. Some low-pH foods, such as apple juice, orange juice, and fermented sausage, have recently
been implicated in foodborne disease outbreaks by strains of Esc. coli O157:H7, Salmonella,
and Listeria monocytogenes. Explain the possible reasons and suggest methods that could be
used to reduce this problem.
Parabens (Esters of p-Hydroxybenzoic Acid)
Parabens are used as methyl, ethyl, butyl, or propyl parabens. They are broad-spectrum antimicrobial agents and used in food and cosmetic and pharmaceutical products. Because of high pK
values, they are effective at high pH and against bacteria, yeasts, and molds. They are used at
100–1000 ppm (0.01%–0.1%) in nonalcoholic and alcoholic beverages, fruit fillings, jams and jellies, pickles, confectioneries, salad dressings, spreads, and mustards. The undissociated inhibitory
concentrations are 0.05%–0.1% against yeasts, molds, and bacteria. Propyl and butyl parabens are
more inhibitory than the others.
Parabens produce antimicrobial action by acting on several targets of microbial cells. They may
inhibit the functions of several enzymes. They dissolve membrane lipids and interfere with membrane functions, including transport of nutrients. They also interfere with the synthesis of proteins, RNA, and DNA. In addition, they destroy the membrane potential as other weak organic
acids do. However, toxic side effects of parabens, such as reproductive toxicity resulting from an
estrogenic effect, asthma, allergic response, and dermatitis have raised serious concerns in recent
years. 10,11
Conclusion
A pH lower than the minimum growth pH prevents microbial growth by affecting energy production, enzymatic activity, transportation of nutrients, and others. In addition to controlling growth,
microbial cells are sublethally injured or killed by low pH. Many food-grade organic acids along
with other methods are used to control microbial growth in food. Low pH is also used to prevent
germination of bacterial spores in food. Although many Gram-negative pathogenic bacteria are
very sensitive to low pH, it cannot be used to eliminate these pathogens during storage of food.
The fact that some strains can become acid resistant has created concern about such a practice in
the processing and preservation of some low-pH foods. In addition to low pH and the methods
discussed before, low O-R potential is used to control microbial growth in food and is discussed
in Chapter 37.
QUESTIONS
1. Discuss the mechanisms of antimicrobial action of weak organic acids and low pH. Explain
the differences in sensitivity to low pH among gram-negative and gram-positive bacteria,
bacterial spores, yeasts, and molds.
2. Define the pK of an organic acid and, using acetic and lactic acids as examples, describe their
differences in antibacterial effect at pH 5 and 6.
3. Briefly discuss the influence of the following factors on antimicrobial effectiveness of low
pH: acids, foods, and microorganisms.
4. List the specific acids to be used to inhibit growth of gram-positive bacteria, gram-negative
bacteria, yeasts, and molds.
5. Discuss the antimicrobial properties of acetic acid, propionic acid, lactic acid, benzoic acid,
and parabens.
6. Some low-pH foods, such as apple juice, orange juice, and fermented sausage, have recently
been implicated in foodborne disease outbreaks by strains of Esc. coli O157:H7, Salmonella,
and Listeria monocytogenes. Explain the possible reasons and suggest methods that could be
used to reduce this problem.