Fundamental Food Microbiology, Third Edition - Fuad Fathir

558 FUNDAMENTAL FOOD MICROBIOLOGY
B. Sampling Procedure
The individual collecting the samples should know why the samples are being
collected, i.e., what types of testing will be conducted, such as for APC, coliforms,
or a specific pathogen. 1 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 by using proper sanitary measures to prevent any contamination. 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 to 4�C. Each sample should be
labeled to identify date, time, nature of sample, and types of analysis to be conducted,
and the persons who collected the sample. The samples should be transported to the
laboratory under conditions that 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 to prepare the sample
and the procedures of testing for a specific microorganism or microbial group. 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.
V. QUANTITATIVE METHODS FOR MICROBIAL ENUMERATION
IN FOODS
A. Direct Enumeration
1. 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 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. 1–3 Foods that have particles cannot be effectively tested for enumeration
microscopically.
2. Colony-Forming Units (CFUs) 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
by using nonselective media such as plate count agar (PCA), tryptic soy agar, or
nutrient agar. Different media can give different results. However, PCA is recom-