Fundamental Food Microbiology, Third Edition - Fuad Fathir

490 FUNDAMENTAL FOOD MICROBIOLOGY
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 on 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 35.
REFERENCES
1. Ray, B. and Sandine, W.E., Acetic, propionic and lactic acid of starter culture bacteria
as biopreservatives, in Food Biopreservatives of Microbial Origin, Ray, B. and Daeschel,
M.A., Eds., CRC Press, Boca Raton, FL, 1992, p 103.
2. Corlett, D.A. and Brown, M.H., pH and acidity, in Microbial Ecology, Vol. I, Silliker,
J. H., Ed., Academic Press, New York, 1980, p. 92.
3. Baird-Parker, A.C., Organic acids, in Microbial Ecology, Vol. I, Silliker, J.H., Ed.,
Academic Press, New York, 1980, p. 126.
4. Booth, I.R. and Kroll, R.G., The preservation of foods by low pH, in Mechanisms of
Action of Food Preservation Procedures, Gould, G.W. Ed, Elsevier Applied Science,
New York, 1989, p. 119.
5. Eklund, T., Organic acids and esters, in Mechanisms of Action of Food Preservation
Procedures, Gould, G.W. Ed., Elsevier Applied Science, New York, 1989, p. 181.
6. Brown, M.H. and Booth, I.R., Acidulants and low pH, in Food Preservatives, Russell,
N. J. and Gould G.W. Ed., Van Nostrand Reinhold, New York, 1990, p. 22.
7. Doors, S., Organic acid, in Antimicrobials in Foods, Davidson, P.M. and Branen,
A.L., Eds., Marcel Dekker, New York, 1993, p. 95.
8. Sofos, J.N. and Busta, F.F., Sorbic acid and sorbates, in Antimicrobials in Foods,
Davidson, P.M. and Branen, A.L., Eds., Marcel Dekker, New York, 1993, p. 49.
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.