538 ◾ Fundamental Food Microbiology
3. Threeclass attribute plan: Microbial levels are divided into three groups: ≤ m is acceptable;
> m but ≤ M is marginal; and > M is unacceptable. Number of units (C 1 ) allowed between
> m and ≤ M among a number of samples tested (n) is defined for a microbial type.
4. Plan for low level of contamination: This is important for some highly potent pathogens.
Units, depending upon the lot size, are tested to determine the presence or absence of a
pathogen in the unit to make a decision for acceptance or rejection of a lot.
Sampling Procedure
The individual collecting the samples should know why the samples are being collected, that is,
what type(s) of testing will be conducted, such as for APC, coliforms, or a specific pathogen. The
person should also have information about the product if it has been implicated in a foodborne
outbreak and the nature of the product (liquid, solid, or semisolid; frozen; bulk or single units).
The sample should be collected aseptically using proper sanitary measures to prevent any contamination and in large enough volumes to allow repeat analysis, if needed. Following collection and
until tested, the samples should not be handled to avoid growth or death of microorganisms. If the
product is frozen, samples should be kept frozen until analyzed. Otherwise they can be stored at
0°C–4°C. Each sample should be labeled to identify date, time, nature of sample, types of analysis
to be conducted, and the person(s) who collected the sample. The samples should be transported
to the laboratory under proper conditions to avoid microbial contamination, growth, or death.
The laboratory that will test the samples should examine the conditions, such as temperature,
appearance, and the sampling information, and note the time of receiving the samples. Once
received in good condition, a sample should be tested as soon as possible. Standard or recommended methods should be used for the preparation of the sample and the procedures of testing
for a specific microorganism or microbial group(s). The unused portion of the sample should be
stored under proper conditions until the results are available. In some specific situations, a sample
might need to be preserved for a considerable period of time (e.g., in an outbreak involving a lawsuit). The results should be recorded immediately and properly in permanent form.
Quantitative Methods for Microbial enumeration in Foods
Direct Enumeration
Microscopic Counts
Either stained cells under a bright field or live cells under a phase contrast microscope can be counted
and, using an appropriate microscopic factor, these counts can be expressed as microscopic counts
per milliliter or gram of the food sample. However, viable and dead cells cannot be differentiated
by this method. In addition, a sample must have large numbers (≈10 6 /mL, g) of microorganisms
for effective use of this method. 4 Foods that have particles cannot be effectively tested for enumeration microscopically. A commercially available LIVE/DEAD BacLight stain kit allows one to count
fluorescent-labeled live gram-positive or gram-negative bacteria under a fluorescence microscope.
Colony-Forming Units (CFU) in Nonselective Agar Media
Aliquots from selected dilutions of a serially diluted sample (generally based on the number
expected in a food; see Chapter 4) are either pour-plated or surface-plated using nonselective
3. Threeclass attribute plan: Microbial levels are divided into three groups: ≤ m is acceptable;
> m but ≤ M is marginal; and > M is unacceptable. Number of units (C 1 ) allowed between
> m and ≤ M among a number of samples tested (n) is defined for a microbial type.
4. Plan for low level of contamination: This is important for some highly potent pathogens.
Units, depending upon the lot size, are tested to determine the presence or absence of a
pathogen in the unit to make a decision for acceptance or rejection of a lot.
Sampling Procedure
The individual collecting the samples should know why the samples are being collected, that is,
what type(s) of testing will be conducted, such as for APC, coliforms, or a specific pathogen. The
person should also have information about the product if it has been implicated in a foodborne
outbreak and the nature of the product (liquid, solid, or semisolid; frozen; bulk or single units).
The sample should be collected aseptically using proper sanitary measures to prevent any contamination and in large enough volumes to allow repeat analysis, if needed. Following collection and
until tested, the samples should not be handled to avoid growth or death of microorganisms. If the
product is frozen, samples should be kept frozen until analyzed. Otherwise they can be stored at
0°C–4°C. Each sample should be labeled to identify date, time, nature of sample, types of analysis
to be conducted, and the person(s) who collected the sample. The samples should be transported
to the laboratory under proper conditions to avoid microbial contamination, growth, or death.
The laboratory that will test the samples should examine the conditions, such as temperature,
appearance, and the sampling information, and note the time of receiving the samples. Once
received in good condition, a sample should be tested as soon as possible. Standard or recommended methods should be used for the preparation of the sample and the procedures of testing
for a specific microorganism or microbial group(s). The unused portion of the sample should be
stored under proper conditions until the results are available. In some specific situations, a sample
might need to be preserved for a considerable period of time (e.g., in an outbreak involving a lawsuit). The results should be recorded immediately and properly in permanent form.
Quantitative Methods for Microbial enumeration in Foods
Direct Enumeration
Microscopic Counts
Either stained cells under a bright field or live cells under a phase contrast microscope can be counted
and, using an appropriate microscopic factor, these counts can be expressed as microscopic counts
per milliliter or gram of the food sample. However, viable and dead cells cannot be differentiated
by this method. In addition, a sample must have large numbers (≈10 6 /mL, g) of microorganisms
for effective use of this method. 4 Foods that have particles cannot be effectively tested for enumeration microscopically. A commercially available LIVE/DEAD BacLight stain kit allows one to count
fluorescent-labeled live gram-positive or gram-negative bacteria under a fluorescence microscope.
Colony-Forming Units (CFU) in Nonselective Agar Media
Aliquots from selected dilutions of a serially diluted sample (generally based on the number
expected in a food; see Chapter 4) are either pour-plated or surface-plated using nonselective