Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY LOW PH AND ORGANIC ACIDS 487
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 strains, 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 9).
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.
A. Acetic Acid
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Table 35.2 Minimum pH at which Growth Occurs
Microorganism
Gram-Negative Bacteria
Minimum Growth pH
Escherichia coli 4.4
Pseudomonas spp. 5.6
Salmonella 4.5
Vibrio spp. 4.8
Serratia spp.
Gram-Positive Bacteria
4.4
Bacillus cereus 4.9
Bacillus stearothermophilus 5.2
Clostridium botulinum 4.6
Clostridium perfringens 5.0
Enterococcus faecalis 4.4
Lactobacillus spp. 3.8
Staphylococcus aureus 4.0
Listeria monocytogenes
Yeasts
4.6
Candida spp. 1.5 to 2.3
Saccharomyces spp. 2.1 to 2.4
Hansenula spp. 2.1
Rhodotorula spp.
Molds
1.5
Aspergillus spp. 1.6
Penicillium spp. 1.6 to 1.9
Fusarium spp. 1.8
V. ACIDS USED
Acetic acid is used usually as vinegar (5 to 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