H. LAXMINARAYANA AND E. V. SESHACHARYULU 53
have modified the standard milk agar by decreas-
ing the concentration of peptone and lemco from
0.5 and 0.3 per cent to 0.2 and 0.1 per cent
respectively. These authors claim that their
modified medium is as good as the standard milk
agar, and the mean difference in counts obtained
by the use of these two media is not significant.
Hence they recommend the use of the modified
medium as a suitable substitute for standard milk
agar for the plate counts of milk.
Barkworth and Davis have examined raw (cow)
and pasteurized milks with the modified medium.
As any medium suggested should have a general
applicability to all kinds of milk produced and
handled under different conditions, the present
investigation was undertaken with a view to
testing the suitability of the modified milk agar
when compared with the standard milk agar for the
plate counts of different types of milk, such as
cow, buffalo, mixed (cow and buffalo), pasteurized
and village produced milks. Altogether 308
samples of milk were examined for the purpose.
EXPERIMENTAL
The dilution technique of the Ministry of
Agriculture [1934] adopted by Barkworth and
Davis was used in the present study.
The logarithms of the plate counts obtained in
each case by the use of the two media were taken
into consideration, and the significance of the mean
differences in logarithms between the two was
determined by carrying out the t test as done
by Barkworth and Davis.
The mean differences in logs of the plate counts
obtained by the use of the two media, standard
errors of the mean differences and the significance
of the differences are given in the following Table.
Types of milk
Cow herd milk *(I.D.I. Farm) . . . . .
Buffalo herd milk (I.D.I Farm)
.
.
.
.
Mixed milk (cow and buffalo) (I.D.I. Farm)
Pasteurized milk (I.D.I. Farm)
.
.
.
.
.
Village produced milk
.
.
.
.
.
.
Number
of
samples
79
80
52
45
52
Mean differ-
ences in logs
of the plate
counts
—0.0405
—0.0359
—0.0416
—0.0430
—0.0263
Standard
error of
the mean
difference
0.0482
0.0454
0.0380
0.0604
0.0476
Significance of the
difference
't' = 0.8402 N.S.†
't' = 0.7907 N.S.
't' = 1.0947 N.S.
't' = 0.7119 N.S.
't' = 0.5525 N.S.
*I.D.I.—Imperial Dairy Institute
†N.S.—Not significant
DISCUSSION
Barkworth and Davis have stated that a medium
could not be considered as a suitable substitute if
the mean difference exceeded 0.1 log. The above
results show that the mean differences in logs of
the plate counts of the two media are less than
0.05 log in all the cases, and also they are not
significant as shown by the t test. The results
obtained by this investigation, therefore, support
the findings of the authors of the modified milk
agar. The modified medium can, therefore, be
suitably used in the place of the standard milk
agar for the routine bacteriological examination
of the milk of the five types, referred to above, thus
economizing in the use of material during the war
exigency.
It has also been stated by Barkworth and
Davis that the modified medium compares
favourably with the standard milk agar from the
point of view of mean count and colony size,
but in the course of this investigation it was
observed that the size of the colony was compara-
tively smaller on the former medium than on the
latter.
SUMMARY AND CONCLUSIONS
1. The suitability of the modified milk agar
recommended by Barkworth and Davis as com-
pared with the standard milk agar for determining
the number of bacteria in milk by the plate method
has been studied.
2. The mean colony counts obtained on the
modified milk agar with all the types of milk
examined are not significantly different from those
obtained on the standard milk agar.
3. The colony size on the modified medium is
comparatively smaller than that on the standard
milk agar.
4. The modified medium may be used in the
place of the standard milk agar, even for the
kinds of milk available in India for their routine
bacteriological examination, thus economizing in
the use of costly material especially in war time.
ACKNOWLEDGEMENTS
Thanks are due to Mr Zal R. Kothavalla,
Director of Dairy Research, for his keen interest
in the work.
REFERENCES
Barkworth, H. and Davis, J. G. (1942). J. Hyg. Camb.
42, 218.
Min. Agric. (1934). Bull. 42, 5th Ed.