Chapter 10
Culture, Microscopic, and
Sampling Methods
The examination of foods for the presence, types, and numbers of microorganisms and/or their
products is basic to food microbiology. In spite of the importance of this, none of the methods
in common use permits the determination of exact numbers of microorganisms in a food product.
Although some methods of analysis are better than others, every method has certain inherent limitations
associated with its use.
The four basic methods employed for “total” numbers are as follows:
1. Standard plate counts (SPC) or aerobic plate counts (APC) for viable cells or colony forming
units (cfu).
2. The most probable numbers (MPN) method as a statistical determination of viable cells.
3. Dye reduction techniques to estimate numbers of viable cells that possess reducing capacities.
4. Direct microscopic counts (DMC) for both viable and nonviable cells.
All of these are discussed in this chapter, along with their uses in determining microorganisms from
various sources. Detailed procedures for their use can be obtained from references in Table 10–1. In
addition, variations of these basic methods for examining the microbiology of surfaces are presented
along with a summary of methods and attempts to improve their overall efficiency.
CONVENTIONAL STANDARD PLATE COUNT
By the conventional SPC method, portions of food samples are blended or homogenized, serially
diluted in an appropriate diluent, plated in or onto a suitable agar medium, and incubated at an
appropriate temperature for a given time, after which all visible colonies are counted by use of a
Quebec or electronic counter. The SPC is by far the most widely used method for determining the
numbers of viable cells or colony-forming units (cfu) in a food product. When total viable counts
are reported for a product, the counts/numbers should be viewed as a function of at least some of the
following factors:
1. Sampling methods employed
2. Distribution of the organisms in the food sample
217
Culture, Microscopic, and
Sampling Methods
The examination of foods for the presence, types, and numbers of microorganisms and/or their
products is basic to food microbiology. In spite of the importance of this, none of the methods
in common use permits the determination of exact numbers of microorganisms in a food product.
Although some methods of analysis are better than others, every method has certain inherent limitations
associated with its use.
The four basic methods employed for “total” numbers are as follows:
1. Standard plate counts (SPC) or aerobic plate counts (APC) for viable cells or colony forming
units (cfu).
2. The most probable numbers (MPN) method as a statistical determination of viable cells.
3. Dye reduction techniques to estimate numbers of viable cells that possess reducing capacities.
4. Direct microscopic counts (DMC) for both viable and nonviable cells.
All of these are discussed in this chapter, along with their uses in determining microorganisms from
various sources. Detailed procedures for their use can be obtained from references in Table 10–1. In
addition, variations of these basic methods for examining the microbiology of surfaces are presented
along with a summary of methods and attempts to improve their overall efficiency.
CONVENTIONAL STANDARD PLATE COUNT
By the conventional SPC method, portions of food samples are blended or homogenized, serially
diluted in an appropriate diluent, plated in or onto a suitable agar medium, and incubated at an
appropriate temperature for a given time, after which all visible colonies are counted by use of a
Quebec or electronic counter. The SPC is by far the most widely used method for determining the
numbers of viable cells or colony-forming units (cfu) in a food product. When total viable counts
are reported for a product, the counts/numbers should be viewed as a function of at least some of the
following factors:
1. Sampling methods employed
2. Distribution of the organisms in the food sample
217