Impedance Detection Time Research Articles

Enumeration of Brettanomyces in Wine Using Impedance

Brettanomyces bruxellensis is a wine spoilage concern in wineries around the world. In order to maintain wine quality during storage and ageing, it is imperative to control and monitor this yeast. Being a fastidious slow growing yeast, which requires 5 to 14 days of incubation for visible growth in agar plates, it is difficult to detect growth (colonies) by conventional agar plate count method. Yeast enumeration by impedance was investigated because previous research using other microorganisms has shown that it is potentially faster than plate counting. The relationship between plate counting and impedance detection times was investigated for Brettanomyces inoculated in red wine samples. A linear relationship between log plate count concentrations and impedance detection times was found. Incubation time was reduced from 120 h down to 0.9 and 57.7 h for samples with 6.7 × 107 and 1.8 × 102 cfu/mL, respectively, using the ‘indirect’ impedance method. The ‘direct’ method also reduced the incubation times to 9.5 and 81.9 h, for the same concentrations. The ‘indirect’ impedance method has the potential to be used by the wine industry to control and monitor the Brettanomyces numbers in wines.

Impediometric Technique as a Rapid Estimation to Detect Pathogenic Bacteria in Quality Control

It is of interest for drug- and food-producing industries to acquire information about the microbiological quality of their products as early as possible. Conventional mi- crobiological methods for the detection and identification of pathogens include microbiological culturing and isola- tion of the pathogens, followed by approbation with bio- chemical and serological tests. These methods are time- consuming and labor-intensive, not suitable for a rapid re- sponse for high-risk pathogens. As a result, several instru- ments have been developed for use much earlier, including nucleic acid-based tests, serological methods, and some automated instrumental diagnostic assays (1). The impedance measurement is used as a rapid method to detect microbial growth. It is based on a measurement of the changes in electrical conductance of the culture media that are produced by microbial metabolism. Impedance is the resistance to flow of an alternating current through a conducting material. The examined microorganisms are cultivated in impedance glass tubes (cells) in fluid growth medium, with highly uncharged molecules that are fitted with measuring electrodes. The culture medium is metabolized by micro- bial activity, and the impedance changes and decreases because of the production of small charged molecules by the microorganism's metabolites (2). The impedance measurement is a complex entity composed of a combi- nation of a conductive element and a capacitive element. Therefore, in monitoring microbial growth, conductance measurements can indicate changes taking place in the bulk solution or medium (3). The specific growth medium used in the impediometric technique supports the target bacteria, and must be formulated to provide optimal impedance signals to provide specificity to the impedance microbiological method (2). Impedance measurements are based on the bacte- rial growth and metabolic processes that produce electri- cally measurable changes in the growth medium. These changes are due to the metabolism of high-molecular- weight nutrients into smaller charged ionic molecules, which increase the electricity conductivity of the spe- cific medium. Therefore, the impedance technique dis- tinguishes viable and dead pathogens rapidly, within 24 hours (4). Variation in electrical conductivity is related to the change in bacterial number, and thus bacterial growth can be measured. Accordingly, impedance measurements are considered for quality control in the food, drug, and hygiene-product industries, especially for the identifica- tion and enumeration of indicator microorganisms (2, 5). In glass tubes, the electrical change is recorded using a pair of electrodes immersed in the growth medium. The impedance measurement can be done by two methods: di- rect or indirect. In the direct method, a pair of metal elec- trodes is submerged in the medium that is inoculated with the testing sample. The electrical change caused by the re- lease of ionic metabolites from live cells in the medium is monitored over time (6). In the indirect method, the electrodes are immersed in a separate solution (usually a potassium hydroxide solution) instead of the inoculated growth medium. The gases (mainly CO2) released from the metabolism of bacteria are absorbed by the potassium hy- droxide solution, which leads to decreased conductance of the alkaline solution. The indirect method is applicable to all microorganisms, including yeasts and molds (2, 6). In the impedance technique, by means of an elec- trode system in the glass tubes, impedance values can be recorded over time for a pre-selected incubation period (7). The concentration of ions generated by the bacteria reach a magnitude at which a measurable increase in conductiv- ity can be detected. The bacterial level associated with this change in conductance is called the bacterial threshold level, and the impedance detection time (IDT) is defined as the point in hours. Usually, the IDT is inversely propor-

Bactometer system versus traditional methods for monitoring bacteria populations in salchichón during its ripening process.

The performance of the Bactometer system (an impedimetric microbial monitoring system) compared with traditional methods (microbial colony counts) for monitoring bacterial populations (lactic acid bacteria [LAB], Enterobacteriaceae, and coliforms) was studied in 90 samples of an experimental salchichón (a type of Spanish ripened dry sausage) during its ripening process. The population quantitations were carried out with fresh sausage, semiripened sausage (14 days of ripening), and finished product (28 days of ripening). The results showed a high correlation between the traditional microbial colony count (in CFU per gram) and the impedance detection time: -0.98, -0.97, and -0.94 for coliforms, Enterobacteriaceae, and LAB, respectively (P < 0.01). Considering the results obtained with regard to the enumeration of populations of Enterobacteriaceae, coliforms, and LAB in salchichón during its ripening process, the advantages of impedance with respect to plate counts for monitoring the microbial dynamics of ripening processes are notable, especially in its time-saving aspects: between 19 and 21 h in the case of Enterobacteriaceae, between 7 and 20 h for coliforms, and between 32 and 46 h for LAB.

IMPEDANCE MICROBIOLOGY TO SCREEN VARIOUS ANTIMICROBIALS ON WHOLE AND FILLET CHANNEL CATFISH

Whole (deheaded and eviscerated) channel catfish were dipped in 1 or 2% lactic acid or sodium percarbonate (Pergenox ®). Whole and fillets were also washed with high‐pressure water or 8% sodium tripolyphosphate (STPP). Impedance detection time (IDT) was recorded for rinsates of the products. Fish dipped in 2% lactic acid had higher initial IDT than those dipped in 1 % lactic acid. Regardless of lactic acid concentration, IDT were the same in products held more than one day at 4C. Similar results were achieved for products dipped in Pergenox®. Fillets spray‐washed in STPP had higher IDT than those dipped or spray‐washed with water; the difference being pronounced after six days at 4C. For whole fish, STPP had a very significant initial reduction in microbial counts (higher IDT). Exposing the product to high‐pressure spray wash for a short time did not have a significant effect on IDT, but the addition of STPP in the water did increase IDT, thus shelf‐life.

Rapid Impedance Method for Predicting the Potential Shelf Life of Packaged Pasteurized Milk

The potential shelf life of packaged pasteurized milk was studied using a rapid impedance method. The results showed a high correlation between the shelf life of Fengxing and Xiang Man-lou pasteurized milks and between the detection time and the logarithm of colony-forming units per milliliter. The impedance detection time was measured after preliminary incubation at 30 degrees C for 6 h for 100 and 200 microL milk samples, or at 37 degrees C for 6 h for 100-400 microL milk samples for Fengxing pasteurized milk, and after 6 h preliminary incubation at 30 degrees C for 300 and 400 microL milk samples or 6 h at 37 degrees C for 100-400 microL milk samples for Xiang Man-lou pasteurized milk. Regressive equations were then constructed to predict the potential shelf life. Compared with the traditional method, the impedance method plus pre-incubation of milk at elevated temperatures (30 and 37 degrees C) was rapid, accurate, and convenient. The entire estimation process was completed within 11-14 and 14-20 h for Fengxing and Xiang Man-lou pasteurized milks, respectively.

Impedimetric method for estimating the residual activity of freeze-dried Lactobacillus delbrueckii ssp. bulgaricus

The residual activity of Lactobacillus delbrueckii ssp. bulgaricus cultures was analysed using pH and various impedimetric methods (impedance detection time (IDT), conductance and capacitance) to quantify the loss in activity following freeze-drying. The large variation recorded in IDT values for similar levels of activity suggests that IDT is not an adequate parameter for estimating the culture's fermentative activity. Comparison of the impedance signals generated revealed that capacitance yields values that are more reproducible than those of conductance, and also gives a better correlation with pH. Statistical analysis (p<0.05) indicated that there are no significant differences between the capacitance and the pH method when attempting to estimate residual activity.

Bacteriocidal Activity of Sanitizers against Enterococcus faecium Attached to Stainless Steel as Determined by Plate Count and Impedance Methods

Enterococcus faecium attached to stainless steel chips (100 mm2) was treated with the following sanitizers: sodium hypochlorite, peracetic acid (PA), peracetic acid plus an organic acid (PAS), quaternary ammonium, organic acid, and anionic acid. The effectiveness of sanitizer solutions on planktonic cells (not attached) was evaluated by the Association of Official Analytical Chemists1 (AOAC) suspension test. The number of attached cells was determined by impedance measurement and plate count method after vortexing. The decimal reduction (DR) in numbers of the E. faecium population was determined for the three methods and was analyzed by analysis of variance (P < 0.05) using Statview software. The adhered cells were more resistant (P < 0.05) than nonadherent cells. The DR averages for all of the sanitizers for 30 s of exposure were 6.4, 2.2, and 2.5 for the AOAC suspension test, plate count method after vortexing, and impedance measurement, respectively. Plate count and impedance methods showed a difference (P < 0.05) after 30 s of sanitizer exposure but not after 2 min. The impedance measurement was the best method to measure adherent cells. Impedance measurement required the development of a quadratic regression. The equation developed from 82 samples is as follows: log CFU/chip = 0.2385T2 − 0.96T + 9.35, r2 = 0.92, P < 0.05, T = impedance detection time in hours. This method showed that the sanitizers PAS and PA were more effective against E. faecium than the other sanitizers. At 30 s, the impedance method recovered about 25 times more cells than the plate count method after vortexing. These data suggest that impedance measurement is the method of choice when evaluating the number of bacterial cells adhered to a surface.

Modelluntersuchungen zur Impedanzwirksamkeit Lebensmittelhygienisch Relevanter Bakterienatten

The impedance technique mostly meets today's requirements of microbiological rapid methods. At relatively high prime cost for the equipment the advantages are marked by low personnel and material costs as well as swiftness combined with highly flexible usage. The method is applicable for both quantitative and qualitative examinations but can fail occasionally in total count determination, especially if the sample material contains heterogeneous microbes. In model investigations with 53 strains of 17 different genera Enterobacteriaceae strains, Aeromonads and Enterococcus strains proved to be highly impedance effective. Lactobacillus strains and Pseudomonads as well as Staphylococcus aureus strains showed a low impedance effectiveness. Several strains, for example of the genera Micrococcus, Acinetobacter and Brochothrix, did not show any changes of the medium impedance under the chosen conditions. Criterion for characterization of impedance effectiveness was the impedance detection time starting with identical initial counts (10(3) cfu/ml). Impedance effectiveness of microbes was determined at highly varying degree by the parameters of generation time, lag-phase duration and relative activity. This can lead either to wrong negative (underestimations) or wrong positive (overestimations) results of bacterial count.

The effect of evisceration on visible contamination and the microbiological profile of fresh broiler chicken carcasses using the Nu-Tech Evisceration System or the conventional Streamlined Inspection System

Experiments were conducted to determine the effect of evisceration on visible contamination and the microbiological profile of fresh broiler chicken carcasses using the Nu-Tech System or the Streamlined Inspection System (SIS). Visible contamination was evaluated on the inside cavity and outside surface of 1,000 and 7,825 carcasses from Plants A and B, respectively, during 5 d of processing. For the microbiological study, in five separate trials, 80 broiler carcasses were collected each day from two processing plants (A and B). Two groups of 20 carcasses each were collected immediately prior to entering the Nu-Tech eviscerator, bagged separately, and 20 were labeled as NT1 and 20 as SIS1. The third group of 20 carcasses was collected immediately after the cropper on the Nu-Tech line and labeled NT2. For the fourth group, 20 carcasses were aseptically transferred from the Nu-Tech line just prior to evisceration, placed on the SIS line prior to evisceration, allowed to be eviscerated by the SIS eviscerator, collected immediately after the cropper, and labeled SIS2. Whole carcass rinses were conducted and aerobic plate counts (APC), total coliform counts (TC), total Escherichia coli counts (TEC), aerobic mesophilic impedance detection times (DT), and coliform impedance detection times (CDT) were determined within 30 h of collection. The Nu-Tech System was superior to SIS with regard to visible carcass contamination. For Plant A, evisceration using the Nu-Tech System or SIS did not significantly affect APC, TC, or TEC; however, evisceration using the Nu-Tech System resulted in an increase in DT and CDT (indicating a reduction in bacteria); whereas evisceration using SIS resulted in no significant decrease in bacterial levels. For Plant B, evisceration using the Nu-Tech resulted in lower APC, TC, TEC, and higher DT and CDT (indicating a reduction in bacteria); whereas evisceration using SIS resulted in no significant decrease in bacterial levels. Evisceration using the Nu-Tech System was at least equal to or in some cases better than SIS with regard to APC, TC, TEC, DT, and CDT.

The Effect of Temperature Mishandling at Various Times During Storage on Detection of Temperature Abuse of Fresh Broiler Chicken Carcasses

Experiments were conducted to determine the effect that temperature mishandling at various times during storage has on detection of temperature abuse of fresh broiler chicken carcasses using populations of mesophilic and coliform bacteria as indicators of abuse. Forty-five broiler carcasses were used in each of three independent trials. Five carcasses were analyzed at day of collection (Treatment 0 control). Five carcasses, on each day of the next 7 d (Treatments 1 to 7) were temperature-abused for 12 h at 25 C, stored at 3 C until Day 9, and then sampled. The five remaining carcasses were stored at 3 C and sampled on Day 9 (Treatment 8 control). Carcasses were sampled using a whole carcass rinse procedure and assayed for aerobic plate counts (APC), total coliform counts (TCC), impedance detection times (DT), and impedance coliform detection times (DTCM). Although significant differences were noted using all four microbiological methods, temperature abuse at different times during storage had no consistent effect on populations of mesophilic or coliform bacteria; therefore, temperature abuse determinations may be conducted without regard to the day on which the carcasses were subjected to elevated temperatures.

The Effect of Refrigerated and Frozen Storage on Populations of Mesophilic and Coliform Bacteria on Fresh Broiler Chicken Carcasses

Experiments were conducted to determine whether lots of broiler chicken carcasses analyzed at Day 0 or subjected to various refrigeration or freezing temperatures could be microbiologically differentiated using impedance microbiological methods. Sixty ready-to-cook broiler chicken carcasses were collected from a processing facility and used in each of three replicate trials. For each trial, 10 carcasses each were: sampled immediately (Day 0 control), sampled after holding at 3 C for 13 d (Refrigerated control), sampled after holding at 0 C for 12 d and tempering at 3 C for 24 h (Refrigerated 0), sampled after holding at −3.9 C for 12 d and tempering at 3 C for 24 h (Frozen −3.9), sampled after holding at −7.8 C for 12 d and tempering at 3 C for 24 h (Frozen −7.8), and sampled after holding at −27.0 C for 12 d and tempering at 3 C for 24 h (Frozen −3.9), sampled after holding at −7.8 C for 12 d and tempering at 3 C for 24 h (Frozen −7.8), and sampled after holding at −27.0 C for 12 d and tempering at 3 C for 24 h (Frozen −27). Carcasses were sampled using a whole carcass rinse procedure. Mesophilic impedance detection times (MDT) and coliform impedance detection times (CDT) at 42 C were conducted in duplicate on each carcass rinse. Lots of carcasses held at 0 C or below had significantly fewer mesophiles than those held at 3 C. The MDT would be suitable for distinguishing carcasses held at 0 C or below from those held at 3 C. Using CDT, lots of carcasses frozen at −7.8 or −27.0 C could be distinguished from lots analyzed at Day 0 or held at 0 or 3 C. Using CDT, lots of carcasses frozen at −3.9 could not be separated from those held at 3 C, but could be distinguished from those held at 0 C. Monitoring coliforms using impedance would be useful for identifying lots of carcasses subjected to temperatures of −7.8 C or below but would not be suitable for determining freezing temperature.

A rapid bactometer method for screening of biocides against sulfate-reducing bacteria

A rapid and efficient bactometer method was developed for screening biocides against sulfate-reducing bacteria. The method is based on impedance microbiology principles and uses double-layer API (American Petroleum Institute) agar medium supplemented with 0.1% sodium thioglycolate as a reducing agent. Compared to the conventional API procedure, which requires 28 days, the present technique takes only 1 day to obtain test results. Excellent linear correlation (r=−0.98) was found between the impedance detection time and log initial cell concentration. The results of the bactometer test were comparable to that of the API bottle test.

Antibacterial effect of protamine assayed by impedimetry

Impedimetric measurements were used to assay the antibacterial effect of protamine. A good linear correlation between the impedance detection time and the initial cell counts was obtained (r = 0.99, n = 2). As basic peptides may cause clumping of cells, this correlation curve was used when estimating the cell number after protamine treatment, rather than colony counts. Protamine from salmon killed growing Gram-positive bacteria and significantly inhibited growth of Gram-negative bacteria in Tryptone Soy Broth (TSB) at 25 degrees C. In general Gram-positive bacteria were more sensitive to protamine than Gram-negative bacteria; the minimum inhibitory concentrations (MIC) determined for Gram-positive strains varied from 20 to 1000 micrograms ml-1 and for Gram-negative strains from 500 micrograms ml-1 to more than 4000 micrograms ml-1. The effect of protamine on non-growing Listeria monocytogenes Scott A suspended in buffer was not lethal as was the effect on growing cells; however, protamine (50-500 micrograms ml-1) killed the Gram-negative fish spoilage bacteria Shewanella putrefaciens when the live cells were suspended in buffer.

Effect of Freezing on the Recovery of Mesophilic Bacteria from Temperature-Abused Broiler Chicken Carcasses

The effects of freezing on the recovery of mesophilic bacteria from broiler chicken carcasses were determined. Fifty fresh broiler carcasses were used in each of three independent trials. Ten carcasses were sampled at time of collection (Treatment 1: Day 0 control). The other 40 carcasses were sampled at Day 7 following one of four additional treatments with 10 carcasses each: held at 3 C (Treatment 2: 7-d control), frozen and then held at 3 C (Treatment 3: frozen control), temperature abused at 25 C for 12 h and held at 3 C (Treatment 4: temperature abused), and temperature abused at 25 C for 12 h, frozen, and held at 3 C (Treatment 5: temperature abused and frozen). Carcasses were sampled by rinsing, and total plate counts (TPC) expressed as log10 colony-forming units per milliliter and impedance detection times (DT) in hours were determined using incubation temperatures of 42 and 43 C, respectively. Results for TPC for Treatments 1 to 5 were 3.89, 3.52, 2.86, 6.53, and 5.57 log10 cfu/mL, respectively. Results for DT for Treatments 1 to 5 were 5.41, 6.50, 7.10, 2.55, and 3.42 log10 cfu/mL, respectively. Freezing reduced the ability to detect temperature abuse because of a significant reduction in the number of mesophiles recovered using either microbiological technique. Freezing appears to decrease, but may not completely eliminate, the ability of selective incubation to detect temperature abuse.

The use of impedance for preservative efficacy testing of pharmaceuticals and cosmetic products

Impedance was investigated for its applicability to preservative efficacy testing of pharmaceuticals and cosmetics. A good correlation between impedance detection time (Td) and total colony counts (colony-forming units (cfu)) was obtained for untreated suspensions of Staphylococcus aureus, Candida albicans, Aspergillus niger and Pseudomonas aeruginosa in phosphate-buffered saline (PBS). A good correlation between Td and the number of cfu was also obtained for suspensions of test organisms treated for varying contact periods with selected concentrations of chlorhexidine, methyl paraben and phenoxyethanol in PBS, and methyl paraben in cetomacrogol cream, but these correlations were significantly different from those for untreated suspensions. It was found that for any given number of cfu the Td for preservative treated cells was extended. It is concluded that impedance represents a valid method for preservative efficacy testing of pharmaceuticals and cosmetics which could be used to achieve more comprehensive but economic screening of formulations against a wider range of preservative systems and concentrations than is the current approach where only a limited range of systems are tested because of the workload involved.

An Evaluation of a Rinse Procedure Using Sodium Bicarbonate and Hydrogen Peroxide on the Recovery of Bacteria from Broiler Carcasses

A patent entitled “Reduction of Bacteria Count on Poultry Being Processed into Food at a Poultry Processing Plant” (U.S. Patent No. 4,683,618) claimed that a three-step rinse process using sodium bicarbonate and hydrogen peroxide solutions would remove bacteria from the surface of broiler carcasses. In three replicate trials, 40 broilers were obtained postchill from a commercial processing plant. Broilers (n = 20) were treated according to the patent by spraying the inside and outside surfaces of each carcass with a 2% NaHCO3 solution for 5 s and rinsing with water, repeating, spraying with a 3% H2 O2 solution for 5 s, and rinsing a final time with water. Controls (n = 20) were treated identically except that in each of the rinse steps tap water was used in place of the test solutions. Whole carcass rinses were conducted and total aerobic plate counts (TPC) and impedance detection times (DT) were determined after 1 h and 7 days at 4 C. The NaHCO3 + H2 O2 treatment resulted in no significant difference in TPC at 1 h post-treatment but did result in lower TPC after 7 days and greater DT at both 1 h and 7 days. The procedure was effective in reducing the recovery of bacteria at 7 days post-treatment by .3 log10 but was not effective in removing the bacteria to the extent implied in the patent. Based on previous studies using H2 O2, these results are not unexpected, but commercial applicability remains questionable based on actual reduction levels.

A Rapid Method for the Determination of Temperature Abuse of Fresh Broiler Chicken

Ready-to-cook broiler chicken carcasses were obtained immediately postchill from a commercial processing plant. A control group was held at 4 C for 10 days. Test groups were held at 4 C for 2 days, temperature abused at 10, 15, or 20 C for 0, 1, 2, 4, 6, 8, 12, or 24 h, and then held at 4 C for an additional 4 days. Both control and test birds were sampled using whole carcass rinse techniques and bacteria enumerated using impedance detection times (DT) and total plate count (TPC) techniques at incubation temperatures of 18 and 42 C. The DT and TPC showed a significant increase (P < .05) in the number of organisms recovered at 18 C but did not show a significant increase (P < .05) in organisms recovered at 42 C when carcasses were held at 4 C continuously for up to 10 days. Carcasses that were temperature abused at 10 C for up to 24 h were not significantly different (P < .05) from the unabused controls, using either DT or TPC, following incubation at 18 and 42 C. At an incubation temperature of 18 C, carcasses abused at 15 or 20 C had significantly more growth of bacteria than controls at 1, 6, 12, and 24 h and 4, 8, and 12 h as determined by DT and at 6, 12, and 24 h and 1, 2, 4, 8, and 12 h, respectively, as determined by TPC. Carcasses abused at 15 or 20 C were different from controls at 12 and 24 h and 4, 8, and 12 h, respectively, using DT, and at 12 and 24 h and after 2 h, respectively, using TPC, upon incubation at 42 C. These data suggest that carcasses abused at 15 C for 12 h or 20 C for as little as 4 h can be differentiated from carcasses not subjected to temperature abuse.

Heterotrophic plate counts of surface water samples by using impedance methods

Membrane filtration, spread plating, and pour plating are conventional methods used to determine the heterotrophic plate counts of water samples. Impedance methods were investigated as an alternative to conventional methods, since sample dilution is not required and the bacterial count can be estimated within 24 h. Comparisons of impedance signals obtained with different water samples revealed that capacitance produced faster detection times than conductance. Moreover, the correlation between heterotrophic plate count and detection time was highest (r = 0.966) when capacitance was used. Linear and quadratic regressions of heterotrophic plate count and impedance detection time were affected by incubation temperatures. Regressions between heterotrophic plate counts based on conventional methods and detection times of water samples incubated at less than or equal to 25 degrees C had R2 values of 0.878 to 0.933. However, regressions using detection times of water samples incubated at greater than or equal to 30 degrees C had lower R2 values, even though water samples produced faster detection times. Comparisons between broth-based versions of R2A medium and plate count agar revealed that the latter correlated highly with heterotrophic plate count, provided that water samples were incubated at 25 degrees C and impedance measurements were conducted with the capacitance signal (r = 0.966). When the linear regression of this relationship was tested with 100 water samples, the correlation between predicted and actual log10 CFU milliliter-1 was 0.869. These results indicate that impedance methods provide a suitable alternative to conventional methods.

Estimation of Potential Shelf-life of Pasteurized Fluid Milk Utilizing a Selective Preliminary Incubation

Selective preliminary incubation, followed by bacterial enumerations or detection techniques, was used to indicate potential shelf-life of pasteurized fluid milk. Commercial whole milk samples, stored at 7°C, were analyzed for bacterial and biochemical parameters and potential shelf-life using daily organoleptic evaluation. Prior to analysis, each sample was subjected to the following preliminary incubations: milk alone, milk with benzalkonium chloride, milk and broth, milk and broth with benzalkonium chloride, and milk and a dairy gram-negative broth. The following bacterial enumerations were conducted: Psychrotrophic Bacteria Count, modified Psychrotrophic Bacteria Count (Petrifilm and agar methods), and Moseley Keeping Quality test (Petrifilm and agar methods). Catalase detection (headspace pressure and flotation time) and impedance detection times were also determined. Initial Standard Plate and Coliform counts (Petrifilm and agar methods) were conducted on each fresh sample but were not used for shelf-life prediction. Many of the preliminary incubations, in conjunction with enumeration or detection combinations, (especially modified Psychrotrophic Bacteria Count and impedance microbiology) gave good correlations to shelf-life (−0.89 and 0.91, respectively). Thus, these methods could be used to indicate the potential shelf-life of pasteurized fluid milk.

Impedance Measurement of Changes in Activity of Lactic Cheese Starter Culture After Storage at 4°C

Impedance, titratable acidity, and change in pH were used to measure activity of lactic cheese starter culture held up to 7 d at 4°C. The impedance microbiology, as quantified by the impedance detection time method, provided an activity estimate within 37 to 53min, and the results were similar to the activity measurements using titratable acidity (3.5h) and change in pH (2.5h). The impedance detection time, titratable acidity, and pH activity tests showed that the culture activity decreased most rapidly between 2 and 5 d. The slope of the impedance curve, 15min after the impedance detection time, decreased as storage time increased with the sharpest decrease occurring during the first 3 d of storage.