Microbial Ecology of Spoilage 387
12.3.2.2.2 pH and Acidity
Increasing the acidity of foods, either through fermentation or the addition of weak
acids, has been used as a preservation method since ancient times. In their natural
state, vegetables are slightly acidic, while most fruits are moderately acidic. Table
12.4 lists the pH ranges of some common fruits and vegetables.
TABLE 12.3
Approximate Minimum aw Values for Growth of
Selected Microorganism at Optimal Temperature
Microorganisms awBacteria
Bacillus cereus 0.95
B. subtilis 0.90
Campylobacter spp. 0.98
Clostridium botulinum type E 0.97
Clostridium botulinum types A and B 0.94
Clostridium perfringens 0.94
Enterohemorrhagic Escherichia coli 0.95
Enterobacter aerogens 0.94
Listeria monocytogenes 0.92
Microbacterium spp. 0.94
Salmonella spp. 0.94
Shigella spp. 0.97
Staphylococcus aureus growth 0.83
Staphylococcus aureus toxin 0.88
Vibrio vulnificus 0.96
Vibrio parahaemolyticus 0.94
Yersinia enterocolitica 0.97
Mold
Alternaria citri 0.84
Aspergillus candidus 0.75
A. niger 0.77
Botrytis cinerea 0.93
Chrysosporium fastidium 0.69
Pencillium brevicompactum 0.81
Rhizopus nigricans 0.93
Yeast
Debaryomyces hansenii 0.83
Saccharomyces bailii 0.80
S. cerevisiae 0.90
S. rouxii 0.62
Sources: Troller, J.A. and Christian, J.H.B., Water Activity and
Food, Academic Press, New York, 1978 and ICMSF, Microbial
Ecology of Foods, Vol. 1, Factors Affecting Life and Death of
Microorganisms, Academic Press, Orlando, FL, 1980.