Free Experiment Aimed At Examining The Microbiology Of Different Types Of Milk Report Example

Type of paper: Report

Topic: Milk, Viruses, Experiment, Microorganisms, Business, Plate, Products, Colony

Pages: 7

Words: 1925

Published: 2021/01/02


Milk and milk products are widely used as food items universally. Whole milk is very rich in nutrients. This fact explains why young ones of lactating animals are able to survive exclusively on milk for some period of time. In addition to being nutritious, milk secreted into the udder of a healthy cow is usually sterile (Jay, Golden, Jay-Loessner-Golden, and Loessner, 2005). Unlike other food substances, milk has no native flora. However, it is important to note that once in the udder, the milk may not be necessarily free from bacteria especially if the cow is infected with a bacteria. Bacteria can gain entrance into the udder through the duct. Nevertheless, milk in the udder has fewer bacteria than the milk that has already been exposed to the environment. Given that milk is a rich source of nutrients, it provides a suitable condition for the growth of bacteria and other microorganisms. Some of the microorganisms of concern in milk are campylobacter jejuni, Coxiella burnetii, Escherichia coli, listeria monocytogenes, and others (International Commission on Microbiological Specifications for Foods., 2011).
Most microorganisms of concern in milk gain access to milk from the environment. Therefore, the environment determines the quality of milk and milk products. Milk products vary in terms of their microorganism content. In this case, the milk products that have been subjected to effective treatment methods are likely to contain less number of microorganisms than the products that have not been treated properly. The nature of the environment determines the number of pathogenic microorganism that is likely to be present in the environment. Less hygienic environment poses higher number of pathogenic microorganisms than more hygienic environment.
Different milk products undergo different treatments; hence, they have different content of microorganisms. Pasteurization is one of the most common methods used to preserve milk. It involves the heating of milk at a temperature of 71.6ºC for approximately 30 minutes in order to kill bacteria. The milk is then cooled very fast to prevent the bacteria from growing (Motarjemi, Moy, and Todd, 2014). On the other hand, ultra heating involves heating of milk at high temperature.
This experiment sought to examine the level of presence of microorganisms in fresh pasteurized milk, sour milk, raw milk, and UHT milk. This helped determine the level of contamination of each milk sample. In this case, the students undertook both the aerobic colony count and coliform plate count on all the samples provided. The aerobic colony count experiment was undertaken in order to help determine the total number of viable bacteria that are capable of growing under the conditions under which the samples were incubated. On the other hand, the coliform plate count was intended to help determine the standard of hygiene of the environment from where the milk was obtained. This experiment is important since it helps to determine the milk that is most vulnerable to spoilage or poisoning. In both experiments, pour plate method was used.


This experiment involved the use of pour plate method in conducting aerobic colony count and coliform plate count experiments. The following materials were used: six 9 ml of ¼ strength ringers (diluent), ten 1 ml disposable sterile pipettes, fourteen sterile Petri dishes, six test tubes, and one 150 ml of molten milk agar Milk sample Marker.

Aerobic colony count

First, serial 10 fold dilutions of milk samples were made. In this case, six test tubes were taken and labelled with respective range of dilution factors (10-1 to 10-2). 1 mL of diluted milk was then transferred to 9 mL of the ringer solution (diluent) contained in the correctly labelled test tube. The pipette was then discarded. Next, 1 mL of the 1:10 dilution was transferred to 9 mL diluent using a fresh pipette. The pipette was then discarded. The task was repeated until a dilution of 1:1,000,000 (10-6) had been obtained. The final pipette was then discarded.
Next, each of the seven petri dishes was labelled with the participating students’ names, date, respective dilution, sample name, and test. 1 mL of the appropriate dilution was then transferred to each plate starting with the most dilute using the same pipette. 20mL of the cooled milk agar was then poured on each plate. The contents were then mixed by gently rotating the dish four times clockwise, four times anticlockwise, four times back & forth and four times side to side. However, care was taken to avoid agitating too vigorously to prevent agar from getting on lids. This step was conducted for all the samples. All the plates were then incubated at 30ºC for 70 hours.

The Coliform Count

This experiment was performed using the dilutions prepared from the aerobic colony count experiment. First, seven petri dishes were obtained and each labelled with the students’ name, date, respective dilution, sample name, and test. 1mL of milk aliquots of each dilution was then pipetted into the appropriate plate. 15mL of the cooled violet red bile agar (VRBA) was then poured onto each of the plates. The plate was then swirled gently on a flat surface and the agar allowed to solidify. 5 ml of VRBA agar to each plate was then added to each plate. The plates were then swirled gently again and then allowed to solidify. All the plates were then incubated at 32ºC for 24 hours.


The results of the experiment conducted on all the samples of milk of different levels of dilutions obtained by all the class groups are shown in table 1 below. The calculated counts provide an estimate of the actual counts of the microbes in each sample.
The results for the aerobic colony count show that highly diluted samples produced the least number of cfu/g. This shows that the most diluted samples contained a smaller population of the microorganisms than the samples with lower levels of dilution. The results show that the greater the level of dilution is, the higher the number of cfu/g becomes. This observation is explained by the fact that highly diluted samples contained smaller amount of the milk sample, hence, smaller population of the microorganisms.
The results obtained from the aerobic colony count experiment further show that both the raw and the sour milk samples produced higher number of the cfu/g than the pasteurized and UHT milk samples. This observation was made in all the dilutions. The results also show that raw milk produced more number of total viable counts than the sour milk. On comparing the results obtained for the pasteurized milk and the UHT milk, the experiment revealed that more total viable colonies were recorded for the pasteurized milk than for the UHT milk. On the other hand, the results for the coliform count experiment show that raw milk has the highest coliform count of all the samples followed by sour milk. Both UHT milk and pasteurized milk showed lower coliform count than sour milk and raw milk.


Aerobic colony count requires a temperature of 30ºC for 70 hours. On the other hand, the coliform count requires temperature of 32ºC for 2 hours. In both experiments, the count limit is 25-250 cfu. Using plates with the number of colony forming units outside this range would bring induce errors in the calculations. Since milk is nutritious, it is prone to contamination by various pathogens. Some of the pathogens commonly found in milk are the following: Brucella spp, Coliforms, campylobacter jejuni, mycobacterium bovis, Listeria monocytogenes, Psychotrophic bacteria, salmonella spp, and mycobacterium tuberculosis (Burlage, 2012).
The results from this experiment support the theory that raw milk contains higher population of microorganisms than the sour milk (Behr and Macguire, 2011.). As a matter of fact, fresh milk can carry numerous microorganisms ranging from a broad spectrum of bacteria to yeast. Most of these bacteria can survive at temperatures above 16ºC. In addition, most bacteria that survive well in milk multiply rapidly at warm temperature. In the aerobic colony count experiment, raw milk recorded higher value of cfu/ml than the raw milk. This shows that the raw milk had a higher population of microorganisms at the beginning of the than the sour milk. The explanation for this observation is that sour milk exhibits lower pH value than raw milk. Consequently, some species of bacteria or other microorganisms that can survive in raw milk can either not survive or multiply in sour milk. Fermentation is helps in preservation of milk since it induces low pH.
Pasteurized milk also recorded a low total viable count. A possible explanation for this observation is that the number of microorganisms was reduced during the treatment process. Pasteurization involves heating of milk at moderately high temperatures for about 30 minutes and then cooling it. This process leads to the killing of many bacteria and other microorganisms present in milk that may be harmful. UHT milk recorded the lowest total viable count. This indicates that the milk is suitable for consumption since it has low bacterial count. UHT treatment involves the heating of milk at very high temperature in order to kill bacteria and other microorganisms that may be harmful. During the treatment process, many microorganisms die resulting into very low population of microorganisms in the milk.
The results from the coliform count experiment show that raw milk has the highest coliform count of all the samples. A possible explanation for this observation is that the milk was kept in an environment that was less hygienic than the other milk samples. Alternatively, the observation may imply that raw milk was more prone to contamination than the other milk samples. The experiment also showed that UHT milk and pasteurized milk had lower coliform count than both the raw and sour milk. The milk standards in the experiments were within the recommended level for consumption. However, the coliform counts and the total viable counts differ from one sample to another.
Even though efforts were to ensure that error is minimised, the experiment was still prone to several sources of error. Some of the possible sources of errors could have been the following: changes in the physical properties of both the apparatus and the chemicals used in the experiment during the experiment. This might have resulted to certain inconsistencies, hence leading to the obtaining of results that deviate from the literature values. Inaccurate calibration of the apparatus used in the experiment. Inaccurate calibration leads to systematic error.


This experiment finds that the raw milk is the least safe for consumption of all the samples used in the experiment. In addition, the experiment finds that UHT milk is the most sterile milk since it has the least coliform and total viable count of all the experiment. The experiment was also successful since the students learnt much.

BibliographyTop of Form

Aub, T., 2013. Rational production and treatment of milk. [S.l.], Book On Demand Ltd.
Behr, E., and Macguire, J., 2011. The art of eating cookbook: essential recipes from the first 25 years. Berkeley, University of California Press.
Burlage, R. S., 2012. Principles of public health microbiology. Sudbury, MA, Jones & Bartlett Learning. Bottom of Form
Bille, Pg, Haradoeb, Br, and Shigwedha, 2010. Evaluation Of Chemical And Bacteriological Quality Of Raw Milk From Neudamm Dairy Farm In Namibia. African Journal of Food, Agriculture, Nutrition and Development (ISSN: 1684-5358) Vol 9 Num 7. Rural Outreach Program.
Ciantar, S., 1991. Ultra heat treatment of milk products. Werribee, Vic, Victorian College of Agriculture & Horticulture, Gilbert Chandler Campus.
Coulston, A. M., Boushey, C., and Ferruzzi, M. G., 2013. Nutrition in the prevention and treatment of disease. Oxford, Elsevier/Academic.
International Commission On Microbiological Specifications For Foods., 2011. Use of data for assessing process control and product acceptance. New York, Springer.
Jay, J. M., Golden, D. A., Jay-Loessner-Golden, and Loessner, M. J., 2005. Modern food microbiology. New York, NY, Springer.
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