Rapid Methods and Automation in Food Microbiology
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for growth of target microorganisms on selective and non-selective culture media using an agar-less solidifying system (Redigel), have greatly helped to reduce labor time in performing viable cell counts, which is so important in the food industry. Chain and Fung (1991) made a comparative study of aerobic plate counts by comparing the effectiveness of the Petrifilm, Spiral plate, Redigel, and Isogrid systems on the enumeration of seven different foods. They prepared 20 samples each and found that the alternative systems and the conventional method (CFU/ml; or g) were highly comparable at an agreement of r ¼ 0.95. In the same study they also found that the alternative systems cost less than the conventional system for making viable cell counts. According to the authors (Fung et al., 1980), the following Fung Scale is for viable cell counts in common food particles, edible food surfaces, and liquid foods: 2
0–2 log CFU/ml, g, and cm
Low count
Acceptable
3–4 log CFU/ml, g, and cm
Intermediate count
Caution
5–6 log CFU/ml, g, and cm2
High count
Corrective actions
2
7 log CFU/ml, g, and cm
2
8 log CFU/ml, g, and cm2 9 log CFU/ml, g, and cm
2
10 log CFU/ml, g, and cm2
Index of spoilage Odoriferous Slime formation Discard immediately
The scale of viable cell count is useful for ascertaining spoilage potential of most foods. This scale is not for pathogenic bacteria in food. For cooked foods, no pathogens are allowed. For raw ground beef no Escherichia coli O157:H7 is allowed. Of course, for fermented solid and liquid foods high levels of desirable organisms are needed to make excellent products and this scale does not apply.
II.B. Air samples and surface samples The food industry also needs to ascertain the air quality as well as environmental surfaces for product manufacturing, storage, and transportation. An active air sampling instrument such as the SAS system of pbi (Milan, Italy) can ‘suck’ a known volume of air and deposit microorganisms on an agar surface (impaction). After incubation of the agar the number of organisms per cubic meter of air can be obtained. Another method is to suck a known volume of air and pass it through a tube of liquid (impingement) to capture the microorganisms in the liquid and later obtain CFU/meter3 or ml. A variety of