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A total of 100 pasteurized milk samples consisting of 20 each from five
different brands namely A, B, C, D and E were subjected to different bacterial counts
to evaluate the bacterial quality of milk samples available in and around Trichur. An
attempt was also made to isolate and identify E. coli and to test the sensitivity of
these isolates to six antibacterial agents.
Brand D had the lowest total viable count (4.47 ± 4.23 10glO cfu/ml) and the
highest was in C (7.65 ± 7.22 10glO cfu/ml)., Analysis of the data revealed a highly
significant (p<0.01) difference in the count of the samples between brands except
between A and E and B and C.
The psychrotrophic count was lowest in samples belonging to brand D
(3.57±3.23 10glO cfulml and highest in C (6.95± 6.73 10glO cfulml). Highly significant
(p<0.01) difference in the count was noticed between samples belonging to all
brands, except Band C.
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The lowest coliform count was observed in brand D (1.24 ± 1.0410glO cfulml)
and highest in C (5.38 ± 5.13 10glO cfulml). Highly significant (p<0.01) difference
was observed between the counts of samples belonging to different brands except
between A and E and B and C.
Samples of brand D were found to be free from E. coli. The highest E. coli
count was found in brand B (3.39 ± 2.74 10glO cfu/ml) and lowest in C (0.87 ± 0.87
10glO cfulml). Highly significant (p
observed between brands except between D and C, e and E and E and A. Of the
hundred pasteurized milk samples examined, 82% did not meet the total viable count
criteria prescribed by BIS (1992). Only 75% samples from brand D and 15% from E
met the criteria.
Eighty five per cent samples from brand D met the coliform count limit
prescribed for pasteurized milk by BIS (1992). The per cent of samples belonging to
brands E and A that met the above criteria was 35 and 25, respectively. Only five per
cent samples from brands Band e met this en teria.
Sixty four isolates were identified as E. coli . Antibiogram of these isolates
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showed that 96.9% were sensitive to gentamicin and norfloxacin. The per cent of
isolates that showed sensitivity to tetracycline, cotrimazine, cephotaxime and
nitrofurantoin were 92.2, 67.2, 56.3 and 54.7 respectively.
To study the effect of pasteurization on the bacterial quality of raw milk and
to find out the shelf life under refrigerated conditions, one raw milk sample and 12
sachets of pasteurized milk were collected from a batch on each day from the dairy
plant. This was repeated at varied interval for ten days. The total viable,
psychrotrophic, coliform and E. coli counts of raw milk samples were 6.74 ± 6.17,
6.67 ± 6.13, 3.41 ± 2.93 and 2.42 ± 2.27 10glO cfu/ml. No E. coli was found in freshly
pasteurized milk samples. Total viable count of these samples was 5.05 ± 4.38 10glO
cfu/rnl. The psychrotrophic and coliform counts were 4.85 ± 4.26 10glO cfu/ml and
2.36 ± 2.01 10glO cfu/ml respectively.


Pasteurization reduced total viable and psychrotrophic count by 2 log and the
coliform count by one.
In the refrigerated sample the initial bacterial growth rate was very slow, and
there was no significant increase in total viable and psychrotrophic counts for up to
seven days of storage. Thereafter the increase in these counts were significantly
different from that of initial count. An increase in coliform count was observed but it
did not differ significantly from that of freshly pasteurized milk.
Off odour was detected for the first time on the io" day and it was noticed in
30% of the samples and by 11 th day 100% samples showed off odour. Only on 1 ih
(
days clot on boiling was noticed for the first time, that too only in one sample from a
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batch.
As far as pasteurized milk is concerned, clot on boiling is not a suitable test
for assessing the quality of pasteurized milk. The storage life was found to be at least
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nine days for pasteurized milk kept in a refrigerator. Inspite of the fact that bacterial
counts were well above the suggested norms by BIS, the milk had a keeping quality
of nine days. Hence there is an indication for modification of the existing norms.