Indicators of Microbial Food Spoilage ◾ 297
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 TAPC 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 TAPC 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 Chapter 42).
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 two to seven
days, depending on the agar media and indicator, plating methods (pour or surface), and
incubation temperature (10°C–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 two to seven days. 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 lowheatprocessed vacuumpackaged 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 on incubation temperature, data can be available in two to seven days. The
plates for lactic acid bacteria can be incubated in a CO 2 environment. The products can be tested
for psychrotrophic Clostridium spp., such as Clostridium laramie, by specific methods.
4. Raw milk. SPC, psychrotrophic gram-negative bacteria, thermoduric bacteria.
5. Pasteurized milk. SPC, psychrotrophic bacteria (gram-negative and gram-positive).
6. Butter. Lipolytic microorganisms.
7. Cottage cheese. Psychrotrophic, especially gram-negative bacteria.
8. Fishery products (raw). Psychrotrophic gram-negative bacteria.
9. Beverages. Aciduric bacteria, yeasts, and molds.
10. Salad dressing and mayonnaise. Lactobacillus spp. (especially Lactobacillus fructivorans) and
yeasts.
The major disadvantage of microbiological enumeration methods is that it takes several days for
population levels of the indicator microorganisms from enumeration of CFUs to become available. To overcome this problem, several indirect methods that indicate the probable population of
microorganisms in foods have been devised. One such method is to determine lipopolysaccharides
(LPS) present in a food. LPS is specifically found in gram-negative bacteria. Thus, by measuring
LPS concentration (with a proper standard curve), the level of gram-negative bacteria in a food
can be estimated. However, this method is not applicable for spoilage by gram-positive bacteria.
Several other indirect methods studied are measurement of ATP (ATP concentrations increase
with high numbers of viable cells), impedance or conductivity (electric conductivity decreases
with increase in cell numbers), and dye reduction time (the higher the population, the faster the
reduction). For each method, appropriate standard curves are used to determine bacterial levels.
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 TAPC 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 TAPC 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 Chapter 42).
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 two to seven
days, depending on the agar media and indicator, plating methods (pour or surface), and
incubation temperature (10°C–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 two to seven days. 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 lowheatprocessed vacuumpackaged 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 on incubation temperature, data can be available in two to seven days. The
plates for lactic acid bacteria can be incubated in a CO 2 environment. The products can be tested
for psychrotrophic Clostridium spp., such as Clostridium laramie, by specific methods.
4. Raw milk. SPC, psychrotrophic gram-negative bacteria, thermoduric bacteria.
5. Pasteurized milk. SPC, psychrotrophic bacteria (gram-negative and gram-positive).
6. Butter. Lipolytic microorganisms.
7. Cottage cheese. Psychrotrophic, especially gram-negative bacteria.
8. Fishery products (raw). Psychrotrophic gram-negative bacteria.
9. Beverages. Aciduric bacteria, yeasts, and molds.
10. Salad dressing and mayonnaise. Lactobacillus spp. (especially Lactobacillus fructivorans) and
yeasts.
The major disadvantage of microbiological enumeration methods is that it takes several days for
population levels of the indicator microorganisms from enumeration of CFUs to become available. To overcome this problem, several indirect methods that indicate the probable population of
microorganisms in foods have been devised. One such method is to determine lipopolysaccharides
(LPS) present in a food. LPS is specifically found in gram-negative bacteria. Thus, by measuring
LPS concentration (with a proper standard curve), the level of gram-negative bacteria in a food
can be estimated. However, this method is not applicable for spoilage by gram-positive bacteria.
Several other indirect methods studied are measurement of ATP (ATP concentrations increase
with high numbers of viable cells), impedance or conductivity (electric conductivity decreases
with increase in cell numbers), and dye reduction time (the higher the population, the faster the
reduction). For each method, appropriate standard curves are used to determine bacterial levels.