Fundamental Food Microbiology, Third Edition - Fuad Fathir

478 FUNDAMENTAL FOOD MICROBIOLOGY
sandwich or a meal with different items in the same package) will generate a gradient.
This can lead to microbial growth in an item preserved by reduced A w alone and
stored with an item of high A w containing a preservative. Also, condensation of water
during storage with temperature fluctuation, followed by dripping of moisture in
some areas of food, can alter a safe A w level in these areas to an unsafe state.
The minimum A w for growth of microorganisms in a food can be higher than
that in a broth. Thus, Sta. aureus has a minimal A w of growth of 0.86 in a broth, but
it does not grow in shrimp at an A w of 0.89. As the A w is reduced, anaerobic bacteria
will require more of an anaerobic environment for growth. Clostridium perfringens
grew in a broth with an A w of 0.995 at an O–R potential of +194 mV; when the A w
was reduced to 0.975, an O–R potential of +66 mV was required for growth. As the
incubation temperature is moved in either direction from optimum without changing
the A w, the microorganisms require a longer time to grow. In a broth of A w of 0.975,
a Clo. botulinum E strain grew in 6 d at 30ºC, in 19 d at 15ºC, and in 42 d at 7.2ºC.
The minimum A w for growth of Sta. aureus was 0.865 at 30ºC, but changed to 0.878
at 25ºC. Reduced A w and low pH interact favorably in inhibiting microbial growth.
A Clo. botulinum B strain grew at an A w of 0.99 up to pH 5.3 and at an A w of 0.97
up to pH 6.0, but at an A w of 0.95, it failed to grow even at pH 7.0. Similarly, a
spoilage strain of Clo. butyricum grew at an A w of 0.98 up to pH 3.8, but at an A w
of 0.97 failed to grow at pH 4.5 even after 30 d at 30ºC. Many chemical preservatives
enhance the inhibitory effect of lower A w on microbial growth. In the presence of
low concentrations of sorbate, citrate, and phosphate, different microorganisms were
found not to grow at the lowest A w in which they grew in the absence of these
chemicals.
Food composition can influence microbial death rate even at the same A w. At an
A w of 0.33, Escherichia coli counts reduced by log 102.8 in ice cream powder, log 104.8
in dried potatoes, but by more than log 106 in coffee. Under the same conditions, the
death rate of Enterococcus faecalis was much less. Although the survivors remain
dormant in a low-A w food, as soon as it is rehydrated, the microorganisms regain
the ability to metabolize and multiply. Thus, a rehydrated food should be treated as
a perishable food that, unless effective control methods are used, can be unsafe and
spoiled. 1–3
C. Nature of Microorganisms
Microorganisms differ greatly in their minimal A w requirement for growth, sporulation,
and germination (Table 34.1). In general, molds and yeasts can grow at lower
A w values than bacteria; among pathogenic and spoilage bacteria, Gram-negatives
require a slightly higher A w than Gram-positives for growth. Sta. aureus, however,
can grow at an A w of 0.86. Sporulation by sporeforming bacteria occurs at A w values
in which the species and strains grow, whereas germination may occur at slightly
lower A w values. Toxin production may occur at the A w of growth (by Clo. botulinum)
or at a slightly higher than minimum A w of growth (by Sta. aureus at 0.867 at 30ºC
and 0.887 at 25ºC). The minimum A w for microbial growth can vary with the type
of solutes in a food. Clo. botulinum Type E failed to grow below an A w of 0.97 when
NaCl was used as a solute, but grew up to an A w of 0.94 when glycerol was the