Fundamental Food Microbiology, Third Edition - Fuad Fathir

556 FUNDAMENTAL FOOD MICROBIOLOGY
used now enumerate or estimate “total microbes,” “total bacteria” or “total viable
population” (terms used by many) present in a food, although it can be obtained for
a pure culture of a microbial strain growing in a sterile liquid or a solid medium.
Rather, each method enumerates or estimates a specific group among the total
microbial population normally present in a food and that grows or multiplies preferentially
under the conditions or methods of testing. These include composition of
an enumeration medium, temperature and time of incubation and oxygen availability
during incubation, pH, and treatments given to a sample before enumeration and
estimation. Examples of some of the quantitative methods used are aerobic plate
counts (APCs, or standard plate counts, SPCs, for dairy products), anaerobic counts,
psychrotrophic counts, thermoduric counts, coliform counts, Staphylococcus aureus
counts, and yeasts and molds counts.
In contrast, qualitative methods are designed to determine whether a representative
amount (a sample) of a food or a certain number of samples in a batch of a
food contain a specific microbial species among the total microbial population. These
methods are used to detect the possible presence of certain foodborne pathogens,
especially those capable of inflicting high fatality rates among consumers. Salmonella,
Clostridium botulinum, Escherichia coli O157:H7, and probably Listeria
monocytogenes in ready-to-eat food, are some that fall into this group.
III. STANDARD AND RECOMMENDED METHODS
Many methods and variations of different methods that can be used for quantitative
and qualitative detection of microorganisms in foods are reported in the literature.
However, it is desirable to use methods that have been approved by regulatory
agencies. They can be either standard or recommended methods. In the U.S., for
the microbiological examinations of milk and milk products, one needs to use the
methods that have been designated as standard by regulatory agencies. For other
foods, however, specific microbiological methods have been recommended or
approved by regulatory agencies. Although it is advisable to use such a method,
there is no mandatory requirement. Some of the books in the U.S. approved by
regulatory agencies are Standard Methods for the Examination of Dairy Products;
Standard Methods for the Examination of Water and Waste Water; Standard Methods
for the Examination of Seawater and Shellfish; Compendium of Methods for the
Microbiological Examination of Food (all four are published by the American Public
Health Association, Washington, D.C.), and Bacteriological Analytical Manual of
Food and Drug Administration (prepared by the FDA and published by the Association
of Official Analytical Chemists, Arlington, VA). The last two publications
include recommended methods. All these books are revised when necessary to
incorporate the new methods and to update old methods by different groups of
expertise. Many other countries have similar publications for the standard and
recommended methods. Similar books are published by international organizations,
such as specific branches of the World Health Organization and the Food and
Agricultural Organization of the United Nations.