Evaluation of a Rapid Impedimetric Method for Determining the Keeping Quality of Milk.

Shelf-life of 151 pasteurized milk samples was recorded and correlation coefficients calculated using various microbiological factors: standard plate count (SPC), psychrotrophic plate count (PPC), coliform count (CC), and the impedance response detection time (DT) with incubation at both 21 and 32°C. These data were obtained for milk samples on the day of pasteurization as well as 4 and 8 d thereafter. Various treatments (media, dilution factors, temperature and sample volume) were compared. Of the SPC, PPC, CC and DT taken on the day of pasteurization, only the DT achieved a significant correlation with shelf-life. A correlation coefficient of 0.55 was obtained for one treatment applied to 61 samples and correlation coefficients of 0.28 to 0.32 were obtained for several other treatments applied to the entire 151 samples. Values as large as these could occur by chance in uncorrelated data with p<0.0005. Thus, of the total 61 samples, 80% were correctly classified by the impedance detection time test. It is concluded that for prediction of shelf-life on the day of pasteurization, the impedance method is superior to the SPC and the PPC. In addition, the impedance method is more rapid, i.e., 14 h vs.2 d for the SPC and 10 d for the PPC.

Electrical impedance measurements: rapid method for detecting and monitoring microorganisms.

A conceptually simple and east-to-use technique is described that uses continuous impedance measurements for automated monitoring of microbial growth and metabolism. The method has been applied to a wide range of microorganisms. Optical clarity is not required. The sensitivity and reproducibility of the method are demonstrated. The mechanism whereby microbial growth alters the impedance of the medium is discussed, as well as potential applications of the method to clinical microbiology.

Impedimetric screening for bacteriuria.

A rapid, automated instrumental procedure for distinguishing urine cultures containing greater than 10(5) organism per ml is described. The method is based upon the measurement of changes in impedance that take place as microorganisms alter the chemical composition of the medium. The time required to detect impedance change is inversely related to the initial concentration of microorganisms in the sample. By defining an impedance-positive culture as one that gives detectable impedance change within 2.6 h, 95.8% of 1,133 urine cultures tested were correctly classified as containing more than or fewer than 10(5) organisms per ml. Selection of a longer detection time decreases false negative results at the cost of increased false positive results. Impedance screening is compared with screening data reported in the literature using adenosine-5'-triphosphate detection, microcalorimetry, electrochemical measurements, and optical microscopy.

Automated Impedance Measurements for Rapid Screening of Milk Microbial Content.

The electrical impedance of media is altered with chemical changes brought about by microbial metabolism and growth. Time required to bring about readily detectable change (detection time - DT) is a function of the initial levels of microorganisms in the sample. DTs were compared to Standard Plate Counts for 407 milk samples - homogenized, low fat, skim and raw. Using the criterion that a sample of pasteurized milk with a DT of 7 h or less was indicative of a plate count of 10,000/ml or greater, 323 of 380 samples were correctly classified. For raw milk, the DT was 10 h to resolve samples into greater or less than 10,000 organisms per ml. Results of a preliminary study on estimation of psychrotrophs in pasteurized milk showed that impedance monitoring at 21 C provided a 22-h screen correctly classifying 88% of the samples into categories of more than or less than 1,000 organisms per ml. Better agreement (91%) in a shorter time (13.7 h) was obtained with a screen for 10,000 organisms. Finally, for the first 22 samples analyzed, keeping quality data on pasteurized milk have correlated better with post-pasteurization impedance measurements than with either post-pasteurization total counts or psychrotrophic counts.

Rapid detection of microbial contamination in frozen vegetables by automated impedance measurements.

Automated impedance measurements can be used to rapidly assess whether a sample of frozen vegetables contains greater or less than 10(5) organisms per g. Microorganisms growing pureed food samples cause a change in the impedance of the medium when the organisms reach a threshold concentration of between 10(6) and 10(7) organisms per ml. Estimates of the concentration of microorganisms initially present in the food sample can be made by recording the time required for the organisms in the sample to replicate to threshold levels. In this study, the detection times for 357 samples of frozen vegetables were compared with standard plate counts for each sample. The agreement between the two methods in distinguishing samples containing more than 10(5) organisms per g was 92.6% for 257 assorted frozen vegetables and somewhat higher (93 to 96%) when separate cutoff times were used for each type of vegetable. The time required for analysis was about 5 h, compared to the 48 to 72 h required for standard plate counts.