Enumeration of total bacteria in raw and pasteurized milk by reflectance colorimetry: collaborative study.

Seven out of 9 laboratories completed a collaborative study comparing a reflectance colorimetric (RC) bioactivity monitor (Omnispec 4000) method to the standard plate count (SPC) method for estimation of total bacteria in raw and homogenized pasteurized milk. Each laboratory analyzed 12 different samples by the SPC method and 24 samples (12 blind duplicates) by the RC method. For the RC method RSDr was 1.7%, and RSDR was 4.5%. RSDR for the SPC method was 20.8%. The method was adopted first action by AOAC INTERNATIONAL.

Detection of Penicillin and Streptomycin in Milk by Impedance Microbiology 1, 2.

A Bactometer 123 impedance instrument detected .001 IU penicillin or streptomycin/ml of sterile milk inoculated with 5% active lactic culture. Results were available in 5 to 10 h. Inactive lactic culture produced results in less than 24 h. Impedance instruments can monitor incoming milk supplies simultaneously for bacterial load, abnormal milk, and antibiotics.

Comparative Study of the Stomacher and the Waring Blender for Homogenization of High-Fat Dairy Foods 1.

There were no differences between conventional aerobic plate counts on high-fat dairy food products after homogenization 2 min in either a Stomacher 400 or a Semi-Micro Waring blender.

A Flooded Plate Loop Count Procedure 1.

A modification of the plate loop count procedure for milk samples is described which incorporates use of a 0.5-ml diluent loop-rinse onto pre-dried agar medium in a 100-mm petri plate. The sample is distributed using a glass spreader, the plate is dried and incubated using standard procedures. Colony formation was more uniform than with the conventional method. The method did not appear to significantly change the statistical characteristics of the plate loop count on raw milk samples. A correlation coefficient of .91 was obtained between the flooded and the conventional plate loop count.

Optical Somatic Cell Counting and Total Protein Analysis in a Dairy Herd Improvement Program 1.

Milk sample preparation for Optical Somatic Cell Counter II operation was simplified by using a diluter to add fixative, mix, and dilute samples. Potassium dichromate preservative tablets produced a mean increase of 7,000 in somatic cell counts in fresh milk. Samples held at 20-23 C beyond 2 days or at 4-7 C beyond 4 days showed a reduction in somatic cell count. The mean somatic cells in 3 Holstein herds tested over a 6-month period was 3.8 × 105/ml. A 22-month survey of 52.6 thousand Utah Dairy Herd Improvement samples which were shipped under ambient conditions and then held at 5 C until tested, indicated 75% below 400,000 and 2.7% above 1.6 million somatic cells/ml. Casein, noncasein protein, total protein, fat and milk weight data were also obtained on the three herds. Multiple correlations were obtained. The best correlations suggested that testing for total protein and somatic cells in a central laboratory would estimate casein and noncasein protein. Such tests are most valuable for the cheese industry.

Detection of Abnormal Milk with Impedance Microbiology Instrumentation.

Mastitic milk was detected by obtaining conductance measurements using an impedance microbiology Bactomatic 120 SC instrument. Conductance readings separated normal and abnormal milks after 30 min at 25°C when readings differed by more than 2 to 3% and exceeded the variance among instrument module wells. Samples blended from four quarters of a cow indicated milk from one quarter was abnormal if the salt level in the abnormal quarter raised the blend conductivity above that of normal samples and variance among the wells. Either solid or liquid substrates that contained stimulants could be used to accelerate bacterial acid production or reduce impedance detection times and did not affect the ability to detect abnormal milk. However, measurements varied with the volume of sample in the well, suggesting that fixed 1-ml liquid volumes of milk be used. Such volumes would allow detection of abnormal milk and bacterial load on the same sample.

Capillary tube immunological assay for staphylococcal enterotoxins.

A simple assay is reported in which 1 mug of staphylococcal enterotoxins A, B, and D per ml was detected in less than 1 hr. Interfacial reaction of antisera and enterotoxin solutions in a 1-ml internal diameter capillary tube allowed rapid detection of sera type.

A Reflectance Colorimeter Instrument for Measurement of Microbial and Enzymatic Activities in Milk and Dairy Products.

A reflectance color meter has been combined with a Zymate II robot and incubator to measure microbial and enzymatic activity in dairy and food products. Microwells are automatically filled with samples, dyes, and media, and the plates are intermittently removed during incubation to measure color changes of the dye(s). Traditional pH, metabolic, or O/R dyes can be used. The instrument can be programmed and media/dye selected for more rapid estimation of antibiotics, microbial numbers, abnormal milks, coliform counts, product shelf life stabilities, yeast counts, staphylococcal counts, enzymes and culture activity tests, etc. Antibiotic test data are similar to that obtained with impedance instrumentation. Where fewer samples per day are processed, models requiring manual sample preparation are described.

A Survey of Coliforms and Staphylococcus aureus in Cheese Using Impedimetric and Plate Count Methods 1.

A total of 256 cheese samples were analyzed for coliform plate count using violet red bile agar and for an impedance count using BactometerR Coliform Medium with a correlation coefficient between methods of R=-.91. Fifty-four percent of the samples contained 102 to 107 colony forming units/gram (CFU/g). The highest counts were in cream and fresh cheese products. When 27 Cheddar cheese samples were inoculated with from 102 to 107 CFU of Escherichia coli /g a correlation of R=-.97 was found between methods. Two hundred of the cheese samples were analyzed for Staphylococcus aureus using Baird-Parker medium and impedance count using BactometerR S.aureus Medium. Five samples (2%) contained over 103 CFU/g. The strains isolated were coagulase-positive. When 34 samples of cheese were inoculated with 102 to 107 CFU of staphylococci/g, the correlation between the plate and impedance method was R=0.98.

Quantitation of added water in milk by using vapor pressure osmometry.

A vapor pressure osmometer (VPO) was successfully studied in 1975. An improved osmometer with greater range and temperature stability was evaluated. Repeatability on 290 mOs/kg standard solution was 289.5 +/- 1.08. The mean and standard deviation on 36 water-free milk samples was 280.5 +/- 4 compared to 280.1 +/- 3 in the previous study. Instrument data from 2 osmometers and 2 cryoscopes produced acceptable chi-square values, and the correlation coefficients between methods were better than 0.99 for samples containing 1--21% added water. In a survey involving 760 samples, more than 37% of producer samples in one cheese plant contained greater than 3% added water. Additional evidence of added water in market milk samples suggests the immediate application of the VPO to help prevent milk adulteration. The vapor pressure osomometer method for quantitating added water in milk has been adopted as official first action.

A Rapid Psychrometric Procedure for Water Activity Measurement of Foods in the Intermediate Moisture Range 1.

Commercial vapor pressure thermocouple psychrometers (hygrometers) are now generally accepted for measuring water activity, aw (water potential) in plants and soils, and commercial instruments are available. We have adapted them for aw measurements in the 0.99 to 0.60 range using a two-step procedure. Water is first condensed on the thermocouple; then the sample is inserted in the thermocouple chamber and the psychrometric cooling of the wet thermocouple measured. The procedure is calibrated with a series of saturated salt slurries of known aw values. Typical aw values (with standard deviations) for a variety of foods were: Cheddar cheese, 0.95 ± 0.03; Parmesan cheese, 0.76 ± 0.03; milk powder, 0.75 ± 0.02; milk chocolate, 0.60 ± 0.04; luncheon meat, 0.96 ± 0.03; bread, 0.95 ± 0.03; dried raisins, 0.82 ± 0.02; corn syrup, 0.60 ± 0.02; and orange juice concentrate, 0.80 ± 0.03. The coefficients of variation ranged from 1.9 to 5.8%. When compared with published values obtained by other methods, these figures were within the standard errors of measurement. The thermocouple detector did not foul since it had only vapor contact with the sample. An economical sample chamber and instrument is described. Analysis time is 4 to 8 min. The procedure is accurate, convenient and rapid.

Effects of pH, calcium chloride, and chymosin concentration on coagulation properties of abnormal and normal milk.

Individual Holstein cow milk samples were selected for good and poor chymosin-coagulation characteristics. The effect of pH adjustment, addition of .02% calcium chloride, and variation in chymosin concentration on coagulation properties of good and poor coagulating samples was evaluated. Pooling 50% good and 50% poor coagulating samples did not improve the average coagulation properties of the poor samples. Reducing milk pH to 6.3 caused a significant decrease in coagulation time but a less marked increase in curd firmness. The greatest increase in curd firmness was obtained by a combination of reducing milk pH, addition of .02% calcium chloride, and reducing chymosin concentration. High-chymosin concentration at reduced pH decreased coagulation time without substantially increasing curd firmness. Curd disintegration was more apparent at high-chymosin concentration in the poor coagulating samples.

Collaborative evaluation of rolling ball viscometer for measuring somatic cells in abnormal milk.

The Ruakura rolling ball viscometer was evaluated in 3 laboratories along with currently approved instrumental methods for measuring somatic cells in milk and the Wisconsin mastitis test. Replacement of the Teepol reagent with Wisconsin mastitis test reagent in the rolling ball viscometer was also evaluated. Both repeatability and reproducibility were satisfactory for all methods evaluated. The instrumental methods each gave higher readings than the other 3 methods. Use of Wisconsin mastitis test reagent in the rolling ball viscometer improved both repeatability and reproducibility. Additional work on standardization is suggested to match rolling ball viscometer readings with those of the instrumental methods.

Group Comparative Study of VRB-2 Agar in the Recovery of Coliforms from Raw Milk, Ice Cream and Cottage Cheese.

The method of Hartman et al. for recovering stressed coliform bacteria was evaluated in a group study. The experimental medium, VRB-2, was more productive than VRB medium by 31% for raw milk, 70% for ice cream and 61% for cottage cheese. Differences in colony counts of coliforms caused by incubating for 48 vs. 24 h and by boiling vs. autoclaving were not statistically significant. Among 40 samples tested in two laboratories, counts obtained at 30 vs. 32 C were not significantly different.