Conventional, Molecular, and Biosensor-Based Detection Methods ◾ 537
milk products, one needs to use the methods that have been designated as standard by the regulatory agencies. For other foods, however, specific microbiological methods have been recommended
or approved by the regulatory agencies. Although it is advisable to use such a method, there is no
mandatory requirement. Some of the publications in the United States that are approved by the
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 experts. Many other countries have similar
publications for 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 (FAO) of the United Nations, and the International Standards
Organization (ISO) publishes standard methods for foodborne pathogens in food and foodstuffs.
A food microbiology course invariably contains a laboratory component. The methods described
in one or more of these books for the microbiological examination of food, food ingredients, and
environments can be used in the laboratory. This will help the students or interested individuals
become familiar with the standard and recommended methods approved by the regulatory agencies in the United States. Some of the methods are briefly discussed here. Details of these methods
are available in the books listed above and references listed at the end of this chapter.
Sampling for Microbial Analysis
Sample and Sampling Plan 4–6
The microbiological quality of a batch or a lot of a food, which could be liquid, solid, or semisolid, is determined by testing a small portion of the total amount. This tested portion is called a
sample, and it must be the true representative of the total mass. It is very important to develop and
implement an effective sampling plan; otherwise, even the most sensitive testing method will not
provide information about the microbiological quality of a food. This is particularly important for
the highly virulent pathogens, such as Salmonella or Esc. coli O157:H7 or a potent toxin, such as
botulinum. In addition, different laboratories testing the same product but with samples that are
collected differently could give different results, thus making it difficult for regulatory agencies to
decide whether to accept or reject a lot to ensure safety to the consumers.
Several statistical sampling plans have been described in standard or recommended methodology books. Four of these are briefly discussed here. 3,4
1. Single attribute plan: In this plan, a lot is sampled once and tested for the bacterium or
bacteria of interest. The results are used to determine if a lot should be rejected or accepted
(on the basis of the presence or absence of Esc. coli O157:H7). The sample size or number of
samples (n) to be tested is dependent on the batch or lot size (N). Defective unit(s) (C) of n
is dependent on the microbial type.
2. Multiple attributes plan: Initially based on the lot size (N) the number of samples (n 1 ) are
tested. If the defective unit(s) (C 1 ) is above the acceptance level, a second sampling is taken
(n 2 ), tested, and analyzed for defective units (C 2 ) to decide to accept or reject the lot.
milk products, one needs to use the methods that have been designated as standard by the regulatory agencies. For other foods, however, specific microbiological methods have been recommended
or approved by the regulatory agencies. Although it is advisable to use such a method, there is no
mandatory requirement. Some of the publications in the United States that are approved by the
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 experts. Many other countries have similar
publications for 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 (FAO) of the United Nations, and the International Standards
Organization (ISO) publishes standard methods for foodborne pathogens in food and foodstuffs.
A food microbiology course invariably contains a laboratory component. The methods described
in one or more of these books for the microbiological examination of food, food ingredients, and
environments can be used in the laboratory. This will help the students or interested individuals
become familiar with the standard and recommended methods approved by the regulatory agencies in the United States. Some of the methods are briefly discussed here. Details of these methods
are available in the books listed above and references listed at the end of this chapter.
Sampling for Microbial Analysis
Sample and Sampling Plan 4–6
The microbiological quality of a batch or a lot of a food, which could be liquid, solid, or semisolid, is determined by testing a small portion of the total amount. This tested portion is called a
sample, and it must be the true representative of the total mass. It is very important to develop and
implement an effective sampling plan; otherwise, even the most sensitive testing method will not
provide information about the microbiological quality of a food. This is particularly important for
the highly virulent pathogens, such as Salmonella or Esc. coli O157:H7 or a potent toxin, such as
botulinum. In addition, different laboratories testing the same product but with samples that are
collected differently could give different results, thus making it difficult for regulatory agencies to
decide whether to accept or reject a lot to ensure safety to the consumers.
Several statistical sampling plans have been described in standard or recommended methodology books. Four of these are briefly discussed here. 3,4
1. Single attribute plan: In this plan, a lot is sampled once and tested for the bacterium or
bacteria of interest. The results are used to determine if a lot should be rejected or accepted
(on the basis of the presence or absence of Esc. coli O157:H7). The sample size or number of
samples (n) to be tested is dependent on the batch or lot size (N). Defective unit(s) (C) of n
is dependent on the microbial type.
2. Multiple attributes plan: Initially based on the lot size (N) the number of samples (n 1 ) are
tested. If the defective unit(s) (C 1 ) is above the acceptance level, a second sampling is taken
(n 2 ), tested, and analyzed for defective units (C 2 ) to decide to accept or reject the lot.