Microbial Assessments of Bulk Milk Before and After Pasteurization in Two Different Dairy Farms in Zaria, Nigeria
Milk has high nutritional values and an important source of protein, minerals,
vitamins and fat in human diet. It provides excellent medium for growth of both
pathogenic and spoilage microorganism. This study was carried out with the aim
of assessing the microbial quality of bulk milk before and after pasteurization,
in two different dairy farms in Zaria. Bulk milk samples were collected on daily
basis for a period of 30 days during which 30 samples each, of pasteurized and
raw milk were collected from each of the farms making a total of 120 milk samples.
The both samples of raw milk before pasteurization and post-pasteurization of
farm A and B were analyze for total aerobic, coliform plates counts and E.
coli isolation rates. Mean results of aerobic plate counts of raw milk for
farm A and B before pasteurization were; 5.70 and 6.04 log10 CFU
mL-1, respectively. These counts decrease to 3.76 and 4.20 log10
CFU mL-1 after pasteurization of the milk. Similarly, the E. coli
isolation rate for farm A and B were; 20 and 53.3%, respectively. These also
decrease to 6.7 and 13.3% after pasteurization. In addition, coliform counts
also follow similarly trend. The coliform count in farm A and B were; 5.32 and
6.49 log10 CFU mL-1, respectively. The counts decrease
to 3.16 and 3.74 log10 CFU mL-1 after pasteurization of
the milk. Total coliform and aerobic plates counts before bulk pasteurization
in both farms were significantly different (p<0.05), with post-pasteurization
values when subjected to paired t-test. The coliform and aerobic plates demonstrated
poor hygiene practices and inefficient pasteurization methods in both farms.
High isolation rate of E. coli in post pasteurization is an indicative
of fecal contamination of the bulk milk indicative of serious public health
concern. In conclusion, the present study has demonstrated poor method of pasteurization
couple with lack of good hygienic practices such as: proper solid waste management,
potable water, cleanliness of milking area and absent of milking installation.
These are all factors that contributed to production of poor quality milk in
both farms with high coliform, aerobic plates and high isolation rate of Escherichia
coli post pasteurization.
Received: August 09, 2012;
Accepted: October 13, 2012;
Published: November 27, 2012
Milk is defined as a fresh, clean, whole undigested and normal mammary secretion
obtained by draining of the udder of healthy cows that are properly fed, kept
and contains no appreciable colostrums (Frank and Mahony,
1988; Ajogi et al., 2005). Milk is an important
source of protein, minerals, vitamins and fat in human diet (Pirestani
and Eghbalsaeed, 2011) which approximately comprises of 87% water, 3.7%
protein, 4.9% lactose and 0.7% ash 3.6% fat (Ramesh et
al., 2008). With these constituents, milk is described as the most nearly
perfect food (Barrett, 1986). This complex biochemical
composition, nutritional values and high water content render milk an excellent
growth medium for both pathogenic and spoilage microorganism (Bryne,
2004; Parekh and Subhash, 2008; Okonkwo,
Dairy products are consumed by millions on daily basis worldwide and as such
the potential for food-borne illness is a major concern to producers, regulators
and consumers (Bryne, 2004). Fresh milk may be contaminated
with different microorganisms depending on methods used in cleaning and handling
of milk during processing and may originate from udder, the exterior of the
udder, milking equipment used and milkers hand (Bramley
and McKinnon, 1990; Douglas et al., 2002; Oliver
et al., 2005; Bashir and Usman, 2008; Shojaei
and Yadollahi, 2008). Bacteria in raw milk can affect the quality, safety
and consumer acceptability of dairy products (Elmoslemany
et al., 2009). Such microorganisms include Bacillus cereus,
Listeria monocytogenes, Yersinia enterocolitica, Salmonella
spp., Escherichia coli, Staphylococcus aureus and Campylobacter
jejuni (Navratilova et al., 2004; Bashir
and Usman, 2008; Elmoslemany et al., 2009).
Most of the food-borne illnesses associated with milk consumption are linked
to post-pasteurization contamination (Olsen et al.,
2004) as proper pasteurization supposed to destroy most of the pathogenic
bacteria in milk. Post-pasteurization contamination of milk is mostly by contaminated
hands of dairy workers, unsanitary utensils and polluted water supply (Pantoja
et al., 2009). Detection of specific pathogens (bacterial, coliform,
yeast and mould) and their toxins are used as index of contamination of milk
and its products with possibility of presence of pathogens which may constitute
health hazards to consumers (Parekh and Subhash, 2008).
In Nigeria, most dairy farms that produce fresh milk for human consumption
are not subjected to quality control to ascertain the safety of the milk for
public consumption (Bertu et al., 2010). However,
during this study it was commonly observed that the pasteurization methods include
heating of milk in large pots using kerosene, gas or in some instances fire
woods. In all of these methods, automated temperature regulator is absent. Rather,
visual observation of the milk being heated is often carried out to assess pasteurization
parameters. The visual method is ineffective in ascertaining whether the milk
pasteurization temperature is up to 63-76°C. It is also practically impossible
to apply such pasteurization techniques as High Temperature Short Time (HTST)
at 72-76°C for 15 sec which use the function of time temperature designed
to kill pathogenic microorganism (ICMSF, 1998). In addition,
there is high risk of post pasteurization contamination of milk with food-borne
pathogens due to hygiene problems during preparation and handling (Obi
and Ikenebomeh, 2007). The present study was therefore, designed to assess
microbial quality of bulk milk before and after pasteurization and to isolate
Escherichia coli as an index of milk contamination, from two major dairy
farms in Zaria, Nigeria.
MATERIALS AND METHODS
Study area: The study was carried out between November and December,
2010. In two major dairy farms (designated A and B) with different management
systems located in Zaria, North-western Nigeria were used for the study. Farm
A belongs to Friesian-white Fulani (Indigenous cattle breed X Northern Nigeria)
maintained in semi-intensive manner, where animals were allowed to graze in
a pasture field within the farm during the daytime and supplemented with concentrate
in the evenings. The farm also had a parlour, where milking was done. Milk pasteurization
was carried out by heating milk in large pot using cooking gas, once the milk
boils it allowed to cool before packaging into containers or further processes
into yoghurt. Farm B consisted of White Fulani breed of cattle extensively managed,
in which animals move for long distance in search of green fields or crop residues
in farms after harvest. Farmers in both farms used bare hands for milking their
cows. Milking pasteurization in farm B was done by heating of milk in large
pot using fire wood and once the milk boils it allowed to cool and packaged
into plastics container in both farms no thermometer used in measuring temperature.
Milk sampling: About 5 mL of milk was collected into clean sterile sample
bottle from bulk milk in both farms before and after milk pasteurization and
transported immediately to the bacteriology laboratory in ice packs. Sample
collection was carried out daily over a period of 30 days during which 30 samples
each of pasteurized and raw milk were collected from each of the farms making
a total of 120 milk samples.
Laboratory procedures: The samples were diluted using a 10 fold serial
dilution. By pour plate method, 0.1 mL of 103 dilutions were inoculated
in both MacConkey and nutrient agar (Oxoid, UK) and incubated for 24 h at 37°C
for enumeration of total coliform and aerobic plate counts, respectively. Another
0.1 mL was inoculated into Eosin methylene blue agar for isolation of Escherichia
coli. Escherichia coli identification was carried out based on conventional
methods of morphological features, Gram staining and biochemical characterization
as described by Singh and Prakash (2008). Interview
and observations were conducted in the two farms to know how the farmers carry
out milking practice and the method of milk pasteurization.
Data analyses: Results expressed as CFU mL-1 were converted
to log10 values using the methods of Lawan et
al. (2011). Data generated were subjected to paired t-tests to determine
significant differences in total coliform and aerobic plate counts between raw
and pasteurized milk. Percentage isolation rate of E. coli was also calculated.
The aerobic plates count of raw milk for farm A and B before pasteurization
were; 5.70 and 6.04 log10 CFU mL-1, respectively. These
counts decrease to 3.76 and 4.20 log10 CFU mL-1 after
pasteurization of the milk. Coliform counts also follow similar trend. The counts
in farm A and B were; 5.32 and 6.49 log10 CFU mL-1, respectively.
The counts decrease to 3.16 and 3.74 log10 CFU mL-1 after
pasteurization of the milk. The total coliform and aerobic plates counts before
bulk milk pasteurization in both farms were significantly different with post
pasteurization values (p<0.05) when subjected to paired t-test (Table
||Mean values of coliform and aerobic plate counts of pre and
post pasteurized milk sample collected from two dairy farms in Zaria, Nigeria
|Values are Mean±SD, p<0.05 was considered significant,
p-value = 0.000812 for coliform count, p-value = 0.000326 for aerobic count
|| Isolation rates of Escherichia coli from pre and post
pasteurized milk from two dairy farms in Zaria, Nigeria
The isolation rates of E. coli were 20 and 6.7% in farm A, 53.3 and
13.3% in farm B before and after milk pasteurization, respectively (Table
2). Result of the interview and personal observations showed that both farms
carried out hand milking methods. However, milking in farm B was done in open
field and hygiene practices were poor, water was sourced from well which was
not wholesome for milking procedures. In farm A, milking was done in milking
parlour, udder were washed with mild disinfectants before milking, but the hygiene
and sanitation of milking parlour was poor so also was the lightening of the
milking parlour inadequate.
Results of the study revealed that there was a high level of contamination
in the raw milk before pasteurization with mean count of 5.70 and 6.48 log10
CFU mL-1 in farm A and B, respectively. This may be related
to the unhygienic milking practices, unclean environment and absence of potable
water for cleaning procedures in both dairy farms studied. This finding concurred
with the report of Bramley and McKinnon (1990) in which
counts exceeding 105 mL for raw milk were indicative of poor and
unhygienic milking practices.
By comparing the mean counts of both aerobic and coliform counts post pasteurization,
Farm B was observe to have higher values than Farm A (Table 1).
This might be due to the fact that milking in farm B was done in an open space
in which the bulk milk was in direct contact with dust from road and fecal materials
from manure generated in the farm. This may be suggestive of the possible roles
played by environmental and management factors in milk contamination during
milking and processing. Such features were reported to include farm construction
and design, cleanliness of milking area and surrounding buildings and installations,
solid waste management, practice and pollution from within or outside the premises
(Abid et al., 2009). The environmental factors
such as building and milking installations were absent from Farm B and the solid
waste management disposable was also observed to be very poor. This might have
contributed to the level of contamination in milk samples from the farm.
The mean coliform counts of pre and post pasteurized bulk milk from both farms
were high, exceeding the recommended acceptable level 100 cell mL-1
of milk (Shojaei and Yadollahi, 2008). This is suggestive
of unsanitary conditions and poor hygiene practice in the dairy farms. However,
these high coliform counts might also have been contributed by fecal contamination
in the bulk milk and could have been indicative possible presence of other enteric
pathogens like Salmonella, Listeria and E. coli O157:H7
which are of serious public health concern in consumption of such milk.
When both aerobic and coliform counts before and after pasteurization were
subjected to t-test, there were significant differences in counts before and
after pasteurization and even though the counts after pasteurization were slightly
lower than before pasteurization of the bulk milk, the counts still exceeded
the minimum acceptable level specified by World Health Organization of 3x102
CFU mL-1 (Ajogi et al., 2005).
The high counts post pasteurization obtained could be due to ineffective pasteurization,
post pasteurization contamination from the containers, equipment, utensils and
or hands of the handlers as reported by Harding (1999)
that high bacteria counts an indicator of poor production hygiene or ineffective
pasteurization of milk. Although 6 out of the 30 samples in farm A and 3 out
of 30 samples in Farm B were positive for aerobic and coliform before pasteurization
but there were no growths for both aerobic and coliform organism post pasteurization.
The isolation rates of E. coli in both farms also decreased after pasteurization.
However, the isolation rates post pasteurization were still high, indicative
of possible post-pasteurization contaminations or fecal contamination of the
In conclusion, the present study has demonstrated poor method of milk pasteurization
in both farms A and B. Lack of good hygienic practices such as: proper solid
waste management, potable water, cleanliness of milking area and absent of milking
installation. These are all factors that contributed to production of poor quality
milk in both farms with high coliform, aerobic plates and high isolation rate
of Escherichia coli post pasteurization.
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