Fundamental Food Microbiology, Third Edition - Fuad Fathir

INDICATORS OF MICROBIAL FOOD SPOILAGE 315
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II. MICROBIOLOGICAL CRITERIA
A. Enumeration of Colony-Forming Units (CFUs)
Previous discussions indicated that the initial levels and types of spoilage microorganisms
differ with the products or, more accurately, with the sources and
intrinsic and extrinsic environments of the products. It is rational to select the
microorganisms predominantly involved in spoilage of a food (or a food group)
as the indicators of spoilage for that food. As an example, refrigerated ground
meat during aerobic storage is normally spoiled by Gram-negative psychrotrophic
aerobic rods, most importantly by Pseudomonas spp. Thus, the population level
of psychrotrophic Gram-negative rods should be the most appropriate indicator of
spoilage for this product (or for raw meats stored under the same conditions), both
for predicting shelf life of the product and estimating the status of spoilage during
storage. Aerobic plate count (APC), which measures mesophilic populations, may
not be a good indicator for this product as many mesophiles do not multiply at
psychrotrophic temperature and, conversely, some psychrotrophic bacteria do not
multiply at 35�C in the 2 d used to enumerate APCs in meats. However, APC (also
standard plate count or SPC for dairy products) has special importance in food
microbiology. In fresh products, it indicates the effectiveness of sanitary procedures
used during processing and handling and before storage of the product. A
high APC or SPC in a food product such as chicken salad or pasteurized milk is
viewed with suspicion, both for stability and safety. Thus, it is good to include
APC or SPC along with the method suitable to detect the load of an appropriate
spoilage indicator group for a food, based on its specific type and storage conditions.
Some of the specific microbial groups that can be used as spoilage indicators in
different foods (or food types) are listed here. Details of the procedure can be found
in books on microbiological examination of foods (also see Appendix E).
1. Refrigerated Raw (Fresh) Meats Stored Aerobically. Enumeration of CFUs/g or
/cm 2 of psychrotrophic aerobes, especially Gram-negative aerobes. Data can be
available in 2 to 7 d, depending on the agar media and indicator, plating methods
(pour or surface), and incubation temperature (10 to 25�C) used.
2. Refrigerated Raw (Fresh) Meats Stored Anaerobically (Vacuum Packaged). Enumeration
of CFUs/g or /cm 2 of psychrotrophic lactic acid bacteria (by plating in
a suitable agar medium adjusted to pH 5.0 with lactic acid) as well as psychrotrophic
Enterobacteriaceae (in violet red bile glucose agar medium) and
probably Gram-negative bacteria. Depending on incubation temperature, data can
be available in 2 to 7 d. The plates may be incubated in a CO 2 environment for
lactic acid bacteria. The products can also be tested for psychrotrophic Clostridium
spp., such as Clo. laramie, by specific methods (strict anaerobic conditions).
3. Refrigerated Low-Heat-Processed Vacuum-Packaged Meat Products. Enumeration
of CFUs/g or /cm 2 of psychrotrophic lactic acid bacteria (by plating in an
agar medium adjusted to pH 5.0) as well as psychrotrophic Enterobacteriaceae
(in violet red bile glucose agar medium) and Gram-negative bacteria. Depending