Fundamental Food Microbiology, Third Edition - Fuad Fathir

532 FUNDAMENTAL FOOD MICROBIOLOGY
B. Low Storage Temperature
Clostridium botulinum grows at 35ºC at an A w of 0.95. However, if the storage
temperature is reduced to 20ºC, it does not grow unless the A w is increased to 0.97.
Similarly, Lis. monocytogenes grows at 25ºC in a broth containing 6.5% NaCl in 3
d, but fails to grow under similar conditions at 14ºC.
C. Low pH
Clo. botulinum grows at pH 7.0, 37ºC, and an A w of 0.94; however, when the pH is
dropped to 5.3, no growth is observed, even at an A w of 0.99. A Salmonella strain
grows at pH 5.8 at an A w of 0.97, but when the pH is reduced to 5.0, an A w of 0.99
is required for growth. Clo. botulinum produces a toxin during incubation at 16ºC
in 28 d at pH 5.5, but at pH 5.2 under the same conditions, no toxin is produced.
In using acid to reduce pH, it is important to recognize that organic acids, such
as acetic, propionic, and lactic acids, are more effective than HCl and phosphoric
acid. Also, acetic or propionic acids are more effective than lactic acid. Some acids,
such as citric and phosphoric acids, act by both lowering pH and chelating divalent
cations that are important for microbial functions.
D. Low A w
Some of the examples used show how a reduced A w interacts with other parameters
in adversely affecting microbial growth. Sta. aureus can grow at an A w of 0.86 and
high pH. However, it fails to grow even at an A w of 0.93 (in the presence of NaCl)
at pH 4.6, a pH which favors growth of this bacterium at higher A w. Similarly, Sta.
aureus grows at 12ºC, pH 7.0, and an A w of 0.93. However, if the A w is reduced to
0.90, it will not grow under the same conditions.
It is also important to recognize that at a low A w, some microorganisms may
develop resistance to the killing effect of heat.
E. Modified Atmosphere
In vacuum-packaged (N 2 or CO 2) foods, growth of aerobes is prevented and that of
facultative anaerobes can be reduced. But there is a concern that vacuum packaging
can selectively facilitate growth of anaerobic spoilage and pathogenic bacteria. Thus,
suitable combinations of other factors, such as low pH, A w, or preservatives, must
be used with modified atmosphere to control their growth.
F. Preservatives
Some preservatives such as NaCl and BHA act synergistically to increase the antimicrobial
action of sorbates. Organic acids are also effective at low pH because of
higher concentrations of undissociated molecules. Some preservatives may not be
effective at higher pH and some may lose potency during storage. Bacteriocins may
be destroyed by proteolytic enzymes present in the food (raw food). The bactericidal
effect of bacteriocins can be enhanced when used with acids, SDS, and EDTA.