296 ◾ Fundamental Food Microbiology
used in many products. Odor changes from volatile metabolites may not be detected in a product
that is exposed to air as compared with the same product in an airtight package. Color changes,
such as in meat exposed to air, may not be associated with microbial growth. Finally, individuals
differ greatly in their perception of organoleptic criteria. However, sensory criteria can be used
advantageously along with microbiological or chemical criteria or both.
Studies by many researchers have clearly revealed that a single microbiological or chemical test
is not effective in predicting either the shelf life of a product or its spoilage status. The contributing
factors in microbial spoilage of a food include the type of product, its composition, methods used
during processing, contamination during processing, nature of the packaging, temperature and
time of storage, and possible temperature abuse. Because these factors differ with products, it may
be rational to select indicators on the basis of a product or a group of similar products. Some factors to be considered in selecting a microbial or chemical indicator for a product (or several similar
types of products) are the following:
1. In a good fresh product, it can be present in low numbers (microbial) or absent (chemical).
2. Under normal conditions of storage (temperature, time, packaging), it should increase
(microbial or chemical) in quantity to reach a very high level.
3. When spoilage occurs under normal storage conditions, it should be the predominant causative agent (microbial or chemical).
4. It can be detected rapidly (microbial or chemical).
5. It can be used reliably to predict shelf life and spoilage status (microbial or chemical).
6. It should have a good relationship with the sensory criteria of spoilage of the particular product (microbial or chemical).
Different microbial groups and their metabolites (chemicals) have been evaluated for their suitability as indicators of food spoilage. As bacteria are the most predominant microbial group in
food spoilage, the effectiveness of some bacteria and metabolites as indicators is briefly discussed.
In addition, the effectiveness of testing microbial heat-stable enzymes in predicting shelf life of
products susceptible to spoilage by them is also discussed.
Microbiological Criteria
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. Total aerobic plate count (TAPC), which measures mesophilic populations, may not be a good indicator for this product as many mesophiles
do not multiply at psychrotrophic temperatures and, conversely, some psychrotrophic bacteria do
not multiply at 35°C in the two days used to enumerate APCs in meats. However, TAPC (also
used in many products. Odor changes from volatile metabolites may not be detected in a product
that is exposed to air as compared with the same product in an airtight package. Color changes,
such as in meat exposed to air, may not be associated with microbial growth. Finally, individuals
differ greatly in their perception of organoleptic criteria. However, sensory criteria can be used
advantageously along with microbiological or chemical criteria or both.
Studies by many researchers have clearly revealed that a single microbiological or chemical test
is not effective in predicting either the shelf life of a product or its spoilage status. The contributing
factors in microbial spoilage of a food include the type of product, its composition, methods used
during processing, contamination during processing, nature of the packaging, temperature and
time of storage, and possible temperature abuse. Because these factors differ with products, it may
be rational to select indicators on the basis of a product or a group of similar products. Some factors to be considered in selecting a microbial or chemical indicator for a product (or several similar
types of products) are the following:
1. In a good fresh product, it can be present in low numbers (microbial) or absent (chemical).
2. Under normal conditions of storage (temperature, time, packaging), it should increase
(microbial or chemical) in quantity to reach a very high level.
3. When spoilage occurs under normal storage conditions, it should be the predominant causative agent (microbial or chemical).
4. It can be detected rapidly (microbial or chemical).
5. It can be used reliably to predict shelf life and spoilage status (microbial or chemical).
6. It should have a good relationship with the sensory criteria of spoilage of the particular product (microbial or chemical).
Different microbial groups and their metabolites (chemicals) have been evaluated for their suitability as indicators of food spoilage. As bacteria are the most predominant microbial group in
food spoilage, the effectiveness of some bacteria and metabolites as indicators is briefly discussed.
In addition, the effectiveness of testing microbial heat-stable enzymes in predicting shelf life of
products susceptible to spoilage by them is also discussed.
Microbiological Criteria
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. Total aerobic plate count (TAPC), which measures mesophilic populations, may not be a good indicator for this product as many mesophiles
do not multiply at psychrotrophic temperatures and, conversely, some psychrotrophic bacteria do
not multiply at 35°C in the two days used to enumerate APCs in meats. However, TAPC (also