Читать книгу Analytical Food Microbiology - Ahmed E. Yousef - Страница 53

Portions of the prepared dilutions are used to inoculate agar media in Petri plates and the process is called “plating” (Figure 3.1). The questions that should be considered by the analyst are: How many dilutions should be prepared, and which dilutions should be selected for plating? In other words, what dilution scheme should the analyst prepare and follow? Preparing all possible dilutions and plating these dilutions is a waste of resources and effort. On the contrary, preparing a limited number of dilutions may lead to the failure of the analyst to accurately determine the population of the organism in the food.

To answer the questions just presented, the dilution scheme should be based on the projected concentration of the targeted microorganism in the product. A product expected to contain a high load of the microorganism should be diluted further than that expected to contain a small load. The microbial load is sometimes easy to predict if the analyst has prior experience in analyzing the same product. In most cases, however, the analyst should check published literature for microbial populations expected in the product. In other situations, the analyst may seek information about the product from the producer, manufacturer, processor, or retailer. Considering that determining a dilution scheme is often based on the analyst’s best guess, the scheme should accommodate a reasonable margin for error. For example, if the food is presumed to contain 10⁷ CFU/g of the microorganism subject to analysis, the dilution scheme should allow determination of populations in the range of 10⁶ to 10⁸ CFU/g. This can be achieved by the scheme shown in Figure 3.1, assuming that an ideal countable plate contains 100 colonies. This scheme can be verified using the population count equations described later in this chapter.

Figure 3.1 Example of a dilution scheme, showing the dilutions (prepared from a homogenized food) and the dilutions selected for plating, presuming the targeted population in the food is 1.0×10⁷ CFU/g.