Fundamental Food Microbiology, Third Edition - Fuad Fathir

INDICATORS OF MICROBIAL FOOD SPOILAGE 317
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III. CHEMICAL CRITERIA
As microorganisms (particularly bacteria) grow in foods, they produce many types
of metabolic by-products 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 2S 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, due to microbial
growth has also been used to determine the spoilage status of a food. In normal
meats, with a pH of ca. 5.5, metabolism of amino acids by some spoilage bacteria
generates NH 3, amines, and other basic compounds. 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 20.3). As the pH increases,
the proteins become more hydrated, i.e., 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 during storage. More emphasis needs to be
given to develop suitable indicators to reduce the loss of food by microbial spoilage.
In the future, biosensors may be developed that could be effective for indicating
changes in specific metabolites by a group of bacteria with similar characteristics,
that are considered important spoilage bacteria in a food group.
IV. ASSAY OF HEAT-STABLE ENZYMES
A. Heat-Stable Proteinases in Milk
Proteinases of some psychrotrophic bacteria, such as Pseudomonas fluorescens strain
B52, even when present as low as 1 ng/ml raw milk, can reduce the acceptance
quality of UHT-treated milk during normal storage. 5 Because of this, it is very
important that sensitive assay methods be used in their estimation to predict the