272 Modern Food Microbiology
Impedance
Although the concept of electrical impedance measurement of microbial growth was advanced by
G.N. Stewart in 1899, it was not until the 1970s that the method was employed for this purpose.
Impedance is the apparent resistance in an electric circuit to the flow of alternating current, corresponding to the actual electrical resistance to a direct current. When microorganisms grow in cultured
media, they metabolize substrates of low conductivity into products of higher conductivity and thereby
decrease the impedance of the media. When the impedance of broth cultures is measured, the curves
are reproducible for species and strains, and mixed cultures can be identified by use of specific growth
inhibitors. The technique has been shown capable of detecting as few as 10–100 cells (Table 11–1).
Cell populations of 10
5 –10
6 /ml can be detected in 3–5 hours and 10
4 –10
5 /ml in 5–7 hours.
226 The times
noted are required for the organisms in question to attain a threshold of 10
6 –10
7 cells per milliliter.
Some applications of impedance to foods are summarized below.
1. In one study, 200 samples of pur´ eed vegetables were assessed, and a 90–95% agreement was
found between impedance measurements and plate count results relative to unacceptable levels
of bacteria.
76 Impedance analyses required 5 hours, and the method was found to be applicable
to cream pies, ground meat, and other foods.
2. The microbiological quality of pasteurized milk was assessed by using the impedance detection
time (IDT) of 7 hours or less, which was equivalent to an aerobic plate count (APC) of 10
4 /ml or
more bacteria.
25 Of 380 samples evaluated, 323 (85%) were correctly assessed by impedance.
Using the same criterion for 27 samples of raw milk, 10 hours were required for assessment.
In a collaborative study of raw milk involving six laboratories, impedance results varied less
than standard plate count (SPC) results among laboratories.
59 In yet another study with raw
milk, impedance was found useful when a 7-hour cutoff time (10
5 cfu/ml) was used to screen
samples.
68 A scattergram relating IDT to APC on 132 raw milk samples is presented in Figure
11–6.
3. The brewing industry test for detecting spoilage organisms in beer was shortened from 3 weeks
or more to only 2–4 days by use of impedance.
52 Yeasts growing in wort caused an increase in
impedance, whereas bacteria caused a decrease.
4. For raw beef, IDTs for 48 samples were plotted against log bacterial numbers and a regression
coefficient of 0.97 was found.
59 The IDT for meats was found to be about 9 hours. In another
study, the relative level of contamination of meat surfaces by impedance was assessed.
22 With
10
7 cells/cm
2 and above,
22 detection could be made accurately within 2 hours.
5. For frozen orange juice concentrate, an impedance method was used to classify acceptable (<10
4
cfu/ml) or unacceptable (>10
4 cfu/ml).
220 By using cutoff times of 10.2 hours for bacteria, and
15.8 hours for yeasts, 96% of 468 retail samples could be correctly classified.
6. For coliforms in ground beef, a new selective medium was used, and impedance was assessed on
70 samples.
133 In this study, 79% of impedimetric results fell within the 95% confidence limits
of the three-tube most probable number (MPN) procedure for coliforms, and fewer than 100–
21,000 cells per gram could be detected with results obtained within 24 hours. In another study,
a new selective medium was developed that yielded impedance signals that were consistent with
cfu results.
60 From an inoculum of 10 coliforms into the new medium, the average IDT was 3.8
hours and of 96 meat samples, a correlation coefficient of 0.90 was found between impedance
and corrected coliform counts on violet red bile agar (VRBA). Further, an IDT of 6.5 hours was
required for meat samples with 10
3 coliforms, and it was suggested that an impedance signal in
5.5 hours or less denoted meat with coliforms >10
3 /g, whereas the inability to detect in 7.6 hours
Impedance
Although the concept of electrical impedance measurement of microbial growth was advanced by
G.N. Stewart in 1899, it was not until the 1970s that the method was employed for this purpose.
Impedance is the apparent resistance in an electric circuit to the flow of alternating current, corresponding to the actual electrical resistance to a direct current. When microorganisms grow in cultured
media, they metabolize substrates of low conductivity into products of higher conductivity and thereby
decrease the impedance of the media. When the impedance of broth cultures is measured, the curves
are reproducible for species and strains, and mixed cultures can be identified by use of specific growth
inhibitors. The technique has been shown capable of detecting as few as 10–100 cells (Table 11–1).
Cell populations of 10
5 –10
6 /ml can be detected in 3–5 hours and 10
4 –10
5 /ml in 5–7 hours.
226 The times
noted are required for the organisms in question to attain a threshold of 10
6 –10
7 cells per milliliter.
Some applications of impedance to foods are summarized below.
1. In one study, 200 samples of pur´ eed vegetables were assessed, and a 90–95% agreement was
found between impedance measurements and plate count results relative to unacceptable levels
of bacteria.
76 Impedance analyses required 5 hours, and the method was found to be applicable
to cream pies, ground meat, and other foods.
2. The microbiological quality of pasteurized milk was assessed by using the impedance detection
time (IDT) of 7 hours or less, which was equivalent to an aerobic plate count (APC) of 10
4 /ml or
more bacteria.
25 Of 380 samples evaluated, 323 (85%) were correctly assessed by impedance.
Using the same criterion for 27 samples of raw milk, 10 hours were required for assessment.
In a collaborative study of raw milk involving six laboratories, impedance results varied less
than standard plate count (SPC) results among laboratories.
59 In yet another study with raw
milk, impedance was found useful when a 7-hour cutoff time (10
5 cfu/ml) was used to screen
samples.
68 A scattergram relating IDT to APC on 132 raw milk samples is presented in Figure
11–6.
3. The brewing industry test for detecting spoilage organisms in beer was shortened from 3 weeks
or more to only 2–4 days by use of impedance.
52 Yeasts growing in wort caused an increase in
impedance, whereas bacteria caused a decrease.
4. For raw beef, IDTs for 48 samples were plotted against log bacterial numbers and a regression
coefficient of 0.97 was found.
59 The IDT for meats was found to be about 9 hours. In another
study, the relative level of contamination of meat surfaces by impedance was assessed.
22 With
10
7 cells/cm
2 and above,
22 detection could be made accurately within 2 hours.
5. For frozen orange juice concentrate, an impedance method was used to classify acceptable (<10
4
cfu/ml) or unacceptable (>10
4 cfu/ml).
220 By using cutoff times of 10.2 hours for bacteria, and
15.8 hours for yeasts, 96% of 468 retail samples could be correctly classified.
6. For coliforms in ground beef, a new selective medium was used, and impedance was assessed on
70 samples.
133 In this study, 79% of impedimetric results fell within the 95% confidence limits
of the three-tube most probable number (MPN) procedure for coliforms, and fewer than 100–
21,000 cells per gram could be detected with results obtained within 24 hours. In another study,
a new selective medium was developed that yielded impedance signals that were consistent with
cfu results.
60 From an inoculum of 10 coliforms into the new medium, the average IDT was 3.8
hours and of 96 meat samples, a correlation coefficient of 0.90 was found between impedance
and corrected coliform counts on violet red bile agar (VRBA). Further, an IDT of 6.5 hours was
required for meat samples with 10
3 coliforms, and it was suggested that an impedance signal in
5.5 hours or less denoted meat with coliforms >10
3 /g, whereas the inability to detect in 7.6 hours