Investigative project unit 6
Testing the spoilage of milk in different fat contents:
How does the rate of bacterial growth become affected by different fat
contents in milk?
The purpose of my project is to understand the growth of bacteria changes in
different fat contents of milk. This finds out where milk would spoil the
quickest leading to whether you would want to buy semi skimmed, skimmed
or full fat milk.
Research:
https://newprairiepress.org/cgi/viewcontent.cgi?
referer=&httpsredir=1&article=3226&context=kaesrr#:~:text=in%20raw
%20milk.-,Milk%20fat%20content%20did%20not%20affect%20bacteria
%20counts.,protein%20degradation%20became%20more%20pronounced.
This was an investigation completed by S. Zimmerman, I.J. Jeon, L. McVay who
studied Bacterial degradation of milk components is affected by storage
temperature and time. This relates to my investigation as they also tested if
the different fat contents had any effect on bacterial growth. The main
difference in their investigation to mine was that the milk they used was raw
which mean it has not been pasteurised whereas the milk I used in
investigation was pasteurised. This can have an effect to the results they
achieved in their investigation compared to mine as they could have a different
outcome to mine. In their investigation it was concluded that the fat contents
in the milk did not have any effect on how much bacterial growth there was
during the bacterial count. Another difference in their investigation was that
they stored the milk at a different temperature to what I did. They had the
temperature to 3.9 degrees Celsius for 4 days. They then completed the
bacterial count at the 8th day. In my investigation I left the petri dishes in a 25
degrees Celsius incubator for 5 days. The reason for them to use raw milk was
due to its great ability to produce more bacteria than pasteurised milk. From
this study they suggest that the bacterial growth rate was influenced by the
time and temperature. The longer they left the milk to spoil the more bacteria
was produced. When the temperature of the milk being stored in was
increased more bacteria was produced. Bacterial counts were done
throughout the experiment but in my investigation, I only did one bacterial
count at the end of my investigation which was the fifth day. In their report
they stated that the milk after day 2 when it was kept at 3.9 C degrees it could
still be pasteurised and drank safely as any bacteria produced was not harmful
, at that time. The milk that was kept at a higher temperature on the second day
would not be accepted as there would be bacteria that can be considered
dangerous to humans. The different fat contents used in the experiment where
different to the fat contents I chose for my experiment. This can have an
impact on the results to whether fat content has an impact on the bacterial
growth rate. From this we would not be able to conclude if the fat content
does impact it and this is comparable to my experiment as I can only go as far
to say that in my experiment, I found that it did or did not impact the bacterial
growth rate. This means I could not ultimately conclude the different fat
contents in milk increases or decrease the bacterial growth of the milk.
Bacterial growth in milk is caused by lactobacilli reproduction. Lactobacillus
bulgaricus reproduce asexually by binary fission. This happens by the DNA in
the parent cell replicates it then attaches to multiple sides of the cell
membranes. The bacterium in milk needs the water in it to help growth. Any
lower than 98 aw the more difficult it becomes for them to grow. The bacteria
grow as the milk is a liquid with no barriers. Found in spoiled milk lactic acid is
classed as healthy bacteria for people to consume. It is beneficial by creating a
balance in the microbial content in the gastrointestinal tract. Another benefit is
that they fight pathogens which can cause illnesses. This research relates to my
investigation as it explains what the bacteria in milk grow and how the bacteria
is able to reproduce. It also identifies how lactic acid even though it is found in
milk that has spoiled it can be healthy to humans.
http://bioweb.uwlax.edu/bio203/s2007/kahl_ambe/
life_style.htm#:~:text=Lactobacillus%20bulgaricus%2C%20like%20most
%20bacteria,into%20two%20identical%20daughter%20cells.
Three questions for my project need to be answered.
Does the different fat content of milk change the rate of growth of the
bacteria?
What bacteria has grown in each milk sample?
Is the pH of each different milk types different?
Bacteria in dairy products grow in a temperature between 4 ͦC and 50 C ͦ
however the best temperature for them to grow is between 25 C ͦ and 35 ͦC.
The pH and acidity of the milk changes as it spoils, as it decreases it becomes
more acidic. Lactobacilli reproduction and growth is the cause of the milk
spoilage. The species of bacteria found in spoiled milk are bacillus polymyxa,
lactic acid and B cereus. This explains that the pH and acidity of the milk
changes as it spoils this is important to my research connected to the list of
what bacteria is produced in spoiled milk which would grow in my petri dishes
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