Fundamental Food Microbiology, Third Edition - Fuad Fathir

314 FUNDAMENTAL FOOD MICROBIOLOGY
processed foods by microbial spoilage, two points are important. One is to predict
how long a food, following production, will stay acceptable under the conditions of
storage normally used for that food, i.e., what will be the expected shelf life under
normal conditions of handling and storage. The other is to determine the current
status, with respect to spoilage, of a food that has been stored for some time. This
information needs to be available well before a food develops obvious detectable
spoilage and therefore becomes unacceptable.
Many criteria have been evaluated for their efficiency as indicators to predict
expected shelf life, as well as to estimate stages of microbial food spoilage. These
criteria or indicators can be grouped as sensory, microbiological, and chemical
(specific microbial metabolites). Sensory tests (e.g., changes in color, odor, flavor,
texture, and general appearance), although easy and quick to perform, have several
drawbacks as indicators, especially if used alone. Changes in texture and flavor
generally appear at the advanced stages of spoilage. Odor changes can be masked
by the spices 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 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:
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.