Fundamental Food Microbiology, Third Edition - Fuad Fathir

316 FUNDAMENTAL FOOD MICROBIOLOGY
on incubation temperature, data can be available in 2 to 7 d. 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 proper standard curve), the level of Gram-negative bacteria in a food
can be estimated. However, this method is not applicable for spoilage by Grampositive
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 (higher the population, faster the reduction). For each method, appropriate
standard curves are used to determine bacterial levels. However, each method
has specific advantages and disadvantages. Also, all methods are not applicable in
all food systems (see Appendix E).
B. Phase-Contrast Microscopy
Small and easy-to-use phase-contrast microscopes are available to rapidly identify
microbial types (morphology, motility, spore, cell arrangement) present in a food
(see cover page). However, the population has to reach to 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
20). 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 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). For a nonliquid
food, a known amount of food can be suspended in sterile water in a 1:1 dilution,
mixed well, and 1 to 2ml of supernatant fluid used on a microscopic slide or a counter
for viewing or counting.