Analytical Food Microbiology. Ahmed E. Yousef

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of the analytical sample subjected to analysis, thus improving the detection limit of the technique. The tubes are incubated at the appropriate temperature for the appropriate time, and the tubes are inspected for turbidity or for reactions specific to the microorganisms of interest. The results are used in calculating the MPN of the microorganism in g of food, as described later. An example of an MPN dilution and inoculation scheme is shown in Figure 3.6. To enumerate microbial population in a liquid food, the sample may be decimally diluted directly, an appropriate set of MPN tubes are prepared, and MPN/ml is calculated.

      Scoring the Tubes and Calculating MPN

Following incubation, each tube is scored as positive or negative, depending on the target microorganism’s characteristic of interest to the analyst. The positive tubes per each dilution are tallied and the number of positives from consecutive dilutions are reported. The outcome is reported as a series of numbers separated with commas from least dilute to most dilute. Using the results presented in Figure 3.6, the MPN score would be 2, 2, 0.

TABLE 3.2 Most Probable Number (MPN) estimatesa based on the number of positive tubes resulting from three consecutive dilutions (three tubes each) representing 0.1 g, 0.01 g, and 0.001 g of food.

Number of positive tubes	MPN/g	Number of positive tubes	MPN/g	Number of positive tubes	MPN/g	Number of positive tubes	MPN/g
0.1 g	0.01 g	0.001 g	0.1 g	0.01 g	0.001 g	0.1 g	0.01 g	0.001 g	0.1 g	0.01 g	0.001 g
0	0	0	<3	1	0	0	3.6	2	0	0	9.2	3	0	0	23
0	0	1	3	1	0	1	7.2	2	0	1	14	3	0	1	38
0	0	2	6	1	0	2	11	2	0	2	20	3	0	2	64
0	0	3	9	1	0	3	15	2	0	3	26	3	0	3	95
0	1	0	3	1	1	0	7.4	2	1	0	15	3	1	0	43
0	1	1	6.1	1	1	1	11	2	1	1	20	3	1	1	75
0	1	2	9.2	1	1	2	15	2	1	2	27	3	1	2	120
0	1	3	12	1	1	3	19	2	1	3	34	3	1	3	160
0	2	0	6.2	1	2	0	11 Скачать книгу В начало < 18 19 20 21 22 23 24 25 > В конец e-mail: [email protected]