Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY REDUCED A W
solute. Pseudomonas fluorescens similarly showed a minimum A w of 0.957 for
growth with NaCl but 0.94 with glycerol. This is because glycerol enters freely
inside the cell and thus does not cause osmotic stress as nonpermeable NaCl, sucrose,
and similar solutes. Germination of spores in glycerol occurs at lower A w because
Clo. botulinum E spores, with glycerol, germinate at an A w of 0.89, but do not grow
below an A w of 0.94. The growth rate of microorganisms also decreases as the A w
value is lowered. The growth rate of Sta. aureus reduces to about the 10% level at
an A w of 0.90 of its optimum growth rate at an A w of 0.99.
Among microbial groups, halophiles, osmophiles, and xerophiles grow better or
grow preferentially at lower A w values. Halophiles, such as some vibrios, need NaCl
in varying amounts for growth. Osmophiles and xerophiles, yeasts, and molds grow
at an A w less than 0.85, as they do not have competition from bacteria.
Optimum growth of most microorganisms in foods occurs at an A w � 0.98. At
A w � 0.98, Gram-negatives, having a faster growth rate, predominate if other needs
for optimum growth are fulfilled. As the A w drops to 0.97, Gram-positive bacteria,
such as bacilli, lactobacilli, micrococci, and clostridia, predominate. Below an A w
of 0.93, Gram-positive bacteria, such as micrococci, staphylococci, enterococci, and
pediococci, as well as yeasts and molds grow preferentially. As the A w drops below
0.86, osmophilic yeasts and xerophilic molds predominate. They can grow in foods
with an A w up to 0.6; as A w drops below 0.6, microbial growth stops. 1–3
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Table 34.1 Minimum A w for Microbial Growth at
Optimum Growth Temperature
Microorganism Aw Bacteria
Bacillus cereus 0.95
Bacillus stearothermophilus 0.93
Clostridium botulinum Type A 0.95
Clostridium botulinum Type B 0.94
Clostridium botulinum Type E 0.97
Clostridium perfringens 0.95
Escherichia coli 0.95
Salmonella spp. 0.95
Vibrio parahaemolyticus 0.94
Staphylococcus aureus 0.86
Pseudomonas fluorescens 0.97
Lactobacillus viridescens
Yeast
0.94
Saccharomyces cerevisiae 9.90
Saccharomyces rouxii 0.62
Debaryomyces hansenii
Molds
0.83
Rhizopus nigricans 0.93
Penicillium chrysogenum 0.79
Penicillium patulum 0.81
Aspergillus flavus 0.78
Aspergillus niger 0.77
Alternaria citri 0.84
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