298 ◾ Fundamental Food Microbiology
However, each method has specific advantages and disadvantages. Also, all methods are not applicable in all food systems (see Chapter 42).
Phase-Contrast Microscopy
Small and easy-to-use phase-contrast microscopes are available to rapidly identify microbial types
(morphology, motility, spore, and cell arrangement) present in a food. However, the population
has to reach a fairly high level (~10 5–6 /mL) before it can be viewed under a phase-contrast microscope. Also, food particles can interfere with the identification. With practice, it can be a very
quick and easy method to get initial ideas about the predominant microbial types (Chapter 21).
It is also possible to do quick and direct enumeration of cells by a suitable counting device (e.g.,
Petroff Hauser counter). The results can be interpreted in several ways. If a desired level is set
(specification level), such as a spoilage-detection level, one can interpret the result as less than
the level (desirable), very close to the level (should be used immediately), or above the level (and
disposed of). For a nonliquid food, a known amount of food can be suspended in sterile water in
a 1:1 dilution, mixed well, and one to two drops of supernatant fluid can be used on a microscopic
slide or a counter for viewing or counting.
Chemical Criteria
As microorganisms (particularly bacteria) grow in foods, they produce many types of metabolic
byproducts associated with the spoilage characteristics. If a method is developed that is sensitive enough to measure a specific metabolite in very low concentrations and long before spoilage
becomes obvious, then the results can be used to determine the spoilage status of a food. Methods
studied thus far to measure microbial metabolites include H 2 S production, NH 3 production by
colorimetric or titration methods, production of volatile reducing substances, CO 2 production,
diacetyl and acetoin production, and indole production. However, different metabolites are produced by different species and strains of bacteria, and the results are not consistent; they cannot
be used for different types of products.
Change in food pH, especially in meat and meat products, resulting from microbial growth
has also been used to determine the spoilage status of a food. In normal meats, with a pH of
approximately 5.5, metabolism of amino acids by some spoilage bacteria generates NH 3 , amines,
such as histamine, cadaverine, putrescine, and other basic compounds. 2 This shifts the pH to the
basic side (as high as pH 8.0). In contrast, metabolism of carbohydrates (present or added) by
some bacteria produces acids and reduces the pH further to the acidic side. Thus, measurement of
pH of a stored meat product can also give some indication of its type of spoilage bacteria and the
spoilage status (see Table 21.3). As the pH increases, the proteins become more hydrated, that is,
the water-holding capacity (WHC) increases, and, when pressed, this meat has less extract-release
volume (ERV); in contrast, when the pH shifts toward the acidic side, the WHC is lower and ERV
is higher. However, many low-fat processed meat products are formulated with high phosphate
and generally have a pH close to 7.0 (to increase WHC). The buffering action of phosphate may
not allow pH to shift to the basic or acidic side from the microbial metabolism of amino acids and
carbohydrates, respectively. In these products, pH measurement (or WHC or ERV measurements)
may not be good indicators of spoilage status.
None of the microbiological and chemical criteria studied fulfills all the factors necessary for a
good indicator that will indicate the expected shelf life of a fresh product as well as its spoilage status
However, each method has specific advantages and disadvantages. Also, all methods are not applicable in all food systems (see Chapter 42).
Phase-Contrast Microscopy
Small and easy-to-use phase-contrast microscopes are available to rapidly identify microbial types
(morphology, motility, spore, and cell arrangement) present in a food. However, the population
has to reach a fairly high level (~10 5–6 /mL) before it can be viewed under a phase-contrast microscope. Also, food particles can interfere with the identification. With practice, it can be a very
quick and easy method to get initial ideas about the predominant microbial types (Chapter 21).
It is also possible to do quick and direct enumeration of cells by a suitable counting device (e.g.,
Petroff Hauser counter). The results can be interpreted in several ways. If a desired level is set
(specification level), such as a spoilage-detection level, one can interpret the result as less than
the level (desirable), very close to the level (should be used immediately), or above the level (and
disposed of). For a nonliquid food, a known amount of food can be suspended in sterile water in
a 1:1 dilution, mixed well, and one to two drops of supernatant fluid can be used on a microscopic
slide or a counter for viewing or counting.
Chemical Criteria
As microorganisms (particularly bacteria) grow in foods, they produce many types of metabolic
byproducts associated with the spoilage characteristics. If a method is developed that is sensitive enough to measure a specific metabolite in very low concentrations and long before spoilage
becomes obvious, then the results can be used to determine the spoilage status of a food. Methods
studied thus far to measure microbial metabolites include H 2 S production, NH 3 production by
colorimetric or titration methods, production of volatile reducing substances, CO 2 production,
diacetyl and acetoin production, and indole production. However, different metabolites are produced by different species and strains of bacteria, and the results are not consistent; they cannot
be used for different types of products.
Change in food pH, especially in meat and meat products, resulting from microbial growth
has also been used to determine the spoilage status of a food. In normal meats, with a pH of
approximately 5.5, metabolism of amino acids by some spoilage bacteria generates NH 3 , amines,
such as histamine, cadaverine, putrescine, and other basic compounds. 2 This shifts the pH to the
basic side (as high as pH 8.0). In contrast, metabolism of carbohydrates (present or added) by
some bacteria produces acids and reduces the pH further to the acidic side. Thus, measurement of
pH of a stored meat product can also give some indication of its type of spoilage bacteria and the
spoilage status (see Table 21.3). As the pH increases, the proteins become more hydrated, that is,
the water-holding capacity (WHC) increases, and, when pressed, this meat has less extract-release
volume (ERV); in contrast, when the pH shifts toward the acidic side, the WHC is lower and ERV
is higher. However, many low-fat processed meat products are formulated with high phosphate
and generally have a pH close to 7.0 (to increase WHC). The buffering action of phosphate may
not allow pH to shift to the basic or acidic side from the microbial metabolism of amino acids and
carbohydrates, respectively. In these products, pH measurement (or WHC or ERV measurements)
may not be good indicators of spoilage status.
None of the microbiological and chemical criteria studied fulfills all the factors necessary for a
good indicator that will indicate the expected shelf life of a fresh product as well as its spoilage status