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The Influence of the pH of Agar Media Upon the Bacterial Counts of Raw and Pasteurized Milk
THE
INFLUENCE OF THE pH OF AGAR MEDIA THE BACTERIAL COUNTS OF RAW AND PASTEURIZED MILK*
UPON
JACK FABER
Departmenf of Bacferiolooy, Universify of Maryland, College Park, Maryland
A study of this subject was suggested by a statement in the American Public Health Association's publication entitled, "Standard Methods of Milk Analysis" (fourth edition, 1923, page 5) which is as follows: A medium consisting of the above ingredients, including a suitable peptone, ordinarily has a reaction between pH = 6.2 and 7.0. If within these limits, the reaction requires no adjustment for milk analysis. The most desirable reaction is about pH = 6.5 to 6.6, but any reaction between pH = 6.2 and 7.0 is allowable. No change in reaction should be made without carefully determining the H-ion concentration of the finished medium by the method described below. The specific problems, therefore, are, first, to find whether or not there is a material variation in bacterial counts when beef extract agar is employed with the following p H values 6.2, 6.4, 6.6, 6.8, and 7.0--for plating raw milk. Second, to find the most satisfactory p H value when beef extract agar is employed with the following p H values--6.2, 6.4, 6.6, 6.8, and 7.0--for plating pasteurized milk. The raw milk samples were taken from milk shipped to the University of Maryland Dairy. The individual shippers were selected because of their sanitary methods. Samples were taken both from the milk of individual shippers and from the pooled milk of several shippers. A portion of the pasteurized milk samples were collected just after the milk had been cooled, others were taken from milk which had been held in the ice box for five or six hours at t e m peratures varying from 34 ° to 40°F., while still other samples * Received for publication June 13, 1928. 401
402
JACK FABER
were taken from milk which had been carried on trucks over a milk route, and returned to the refrigerator. The directions given in the before mentioned edition of Standard Methods of Milk Analysis were followed in the preparation of the media, plating, incubation, and in counting. The experiment was started about the first of October and ran through the month of March. One hundred samples each of raw and pasteurized milk were examined. Five samples of raw and five of pasteurized milk were plated, in dilutions of 1:100, 1:1,000, and 1:10,000 each week. Agar of p H values of 6.2, 6.4, 6.6, 6.8, and 7.0 were prepared every two weeks for plating this milk. The reaction was deterTABLE 1 Influence of pH of a¢ar media upon bacterial counts of milk RAW ,~IT.~
PaSTItU~ZaD M ~ r
pH Average count, 99 samples
6.2 6.4 6.6 6.8 7.0
51,453 52,797 53,103 53,210 50,739
Mean variation
4-6,824 ~7,911 =1:7,621 ~8,491 ±6,608
Average count, 100 samples
30,342 31,293 30,634 30,396 29,857
Mean variation
4-3,575 ±3,585 ±3,413 ±3,501 ±3,521
mined by the colorimetric method, using the LaMotte 3 B. set and following the procedure recommended by its manufacturers. The indicator, brom thymol blue, with a range of 6.0 to 7.6 was used. After the pH was determined, the agar was adjusted to the desired point. It is well known that media does not always show an increase in titrable acidity after autoclaving. To correct for such a change, a s~mple was autoclaved to determine the drop and enough alkali was added to make up for any deficiency. Most of the media made showed all initial titration of from 6.0 to 6.2. The media was then tubed and autoclaved at 15 pounds pressure for twenty minutes, ooolad and placed in the ice box until used. Table 1 shows the data.
pH
OF AGAR MEDIA
403
The results obtained by plating 99 samples of raw milk showed but little variation in counts. The medium having a hydrogen ion concentration of 6.8 gave the highest average bacterial count. The other media ranged as follows: 6.6, 6.4, 6.2, and 7.0. The results obtained by comparing the results of one hundred samples of pasteurized milk plated on beef extract agar, titrated to the before mentioned pH's, like raw milk, showed but little variation. The medium having a hydrogen ion concentration of 6.4 gave the highest average bacterial count, the others ranged in the following order: 6.6, 6.8, 7.0, and 6.2. CONCLUSION
After a study of the available literature, together with the foregoing data, the following conclusion would seem warranted: If the bacterial counts of raw and pasteurized milk plated on beef extract agar are signit~cantly affected by the hydrogen ion concentration of the media wheI; ranging between 6.2 and 7.0, the 199 samples are not sufficient to show this significance.
INFLUENCE OF THE pH OF AGAR MEDIA THE BACTERIAL COUNTS OF RAW AND PASTEURIZED MILK*
UPON
JACK FABER
Departmenf of Bacferiolooy, Universify of Maryland, College Park, Maryland
A study of this subject was suggested by a statement in the American Public Health Association's publication entitled, "Standard Methods of Milk Analysis" (fourth edition, 1923, page 5) which is as follows: A medium consisting of the above ingredients, including a suitable peptone, ordinarily has a reaction between pH = 6.2 and 7.0. If within these limits, the reaction requires no adjustment for milk analysis. The most desirable reaction is about pH = 6.5 to 6.6, but any reaction between pH = 6.2 and 7.0 is allowable. No change in reaction should be made without carefully determining the H-ion concentration of the finished medium by the method described below. The specific problems, therefore, are, first, to find whether or not there is a material variation in bacterial counts when beef extract agar is employed with the following p H values 6.2, 6.4, 6.6, 6.8, and 7.0--for plating raw milk. Second, to find the most satisfactory p H value when beef extract agar is employed with the following p H values--6.2, 6.4, 6.6, 6.8, and 7.0--for plating pasteurized milk. The raw milk samples were taken from milk shipped to the University of Maryland Dairy. The individual shippers were selected because of their sanitary methods. Samples were taken both from the milk of individual shippers and from the pooled milk of several shippers. A portion of the pasteurized milk samples were collected just after the milk had been cooled, others were taken from milk which had been held in the ice box for five or six hours at t e m peratures varying from 34 ° to 40°F., while still other samples * Received for publication June 13, 1928. 401
402
JACK FABER
were taken from milk which had been carried on trucks over a milk route, and returned to the refrigerator. The directions given in the before mentioned edition of Standard Methods of Milk Analysis were followed in the preparation of the media, plating, incubation, and in counting. The experiment was started about the first of October and ran through the month of March. One hundred samples each of raw and pasteurized milk were examined. Five samples of raw and five of pasteurized milk were plated, in dilutions of 1:100, 1:1,000, and 1:10,000 each week. Agar of p H values of 6.2, 6.4, 6.6, 6.8, and 7.0 were prepared every two weeks for plating this milk. The reaction was deterTABLE 1 Influence of pH of a¢ar media upon bacterial counts of milk RAW ,~IT.~
PaSTItU~ZaD M ~ r
pH Average count, 99 samples
6.2 6.4 6.6 6.8 7.0
51,453 52,797 53,103 53,210 50,739
Mean variation
4-6,824 ~7,911 =1:7,621 ~8,491 ±6,608
Average count, 100 samples
30,342 31,293 30,634 30,396 29,857
Mean variation
4-3,575 ±3,585 ±3,413 ±3,501 ±3,521
mined by the colorimetric method, using the LaMotte 3 B. set and following the procedure recommended by its manufacturers. The indicator, brom thymol blue, with a range of 6.0 to 7.6 was used. After the pH was determined, the agar was adjusted to the desired point. It is well known that media does not always show an increase in titrable acidity after autoclaving. To correct for such a change, a s~mple was autoclaved to determine the drop and enough alkali was added to make up for any deficiency. Most of the media made showed all initial titration of from 6.0 to 6.2. The media was then tubed and autoclaved at 15 pounds pressure for twenty minutes, ooolad and placed in the ice box until used. Table 1 shows the data.
pH
OF AGAR MEDIA
403
The results obtained by plating 99 samples of raw milk showed but little variation in counts. The medium having a hydrogen ion concentration of 6.8 gave the highest average bacterial count. The other media ranged as follows: 6.6, 6.4, 6.2, and 7.0. The results obtained by comparing the results of one hundred samples of pasteurized milk plated on beef extract agar, titrated to the before mentioned pH's, like raw milk, showed but little variation. The medium having a hydrogen ion concentration of 6.4 gave the highest average bacterial count, the others ranged in the following order: 6.6, 6.8, 7.0, and 6.2. CONCLUSION
After a study of the available literature, together with the foregoing data, the following conclusion would seem warranted: If the bacterial counts of raw and pasteurized milk plated on beef extract agar are signit~cantly affected by the hydrogen ion concentration of the media wheI; ranging between 6.2 and 7.0, the 199 samples are not sufficient to show this significance.