Milk Spoilage Over Time
IB Biology Standard Level Internal
Purpose
I was always curious as a kid why there are two main types of cow milk, fresh and long life,
which my parents always bought from grocery stores. This came up in my mind once again
recently and pushed me to find out more about the mechanic of milk spoilage when compared
between long life and fresh milk.
Research Question
How does time duration (0(control),3,5,7 and 10 days) of refrigeration affect the rate of spoilage
of fresh and long-life cow (Bos taurus) milk differently, measured by pH using the pH meter?
Background Information
Cow milk is a liquid produced through the mammalian gland of cows that is nutrient-rich and
provides many needed substances such as vitamin B12, calcium and proteins for the growth of
young mammals 1. This made cow milk abundant in many households thanks to the number of
benefits it provides for bone growth, development of muscles and even in controlling blood
pressure.2 However, over time cow milk tends to spoil and become harmful for human
consumption and there is normally a difference in the lengths of time for spoilage between long-
life and fresh milk, where fresh milk tends to spoil roughly 3-4 days in a refrigerator and sealed
environment compared to long-life milk which usually takes up to 7 days to spoil. 34
Spoilage in cow milk is a very regular occurrence amongst many households and tends to vary in
terms of length and extremity of spoilage when compared between long life and fresh cow milk.
Spoilage in cow milk occurs due to the overgrowth of bacterial communities which inevitably
causes the milk to lose quality, flavour and change its general texture making it unpleasant and
possibly risky to consume. To combat this, most milk produced nowadays, whether long life or
fresh, goes through a pasteurisation process which kills most harmful bacteria that causes major
food illnesses such as E. coli; however, pasteurisation is not enough to kill all the bacteria that
spoils milk which multiplies with the bacteria that enters the milk from the environment
surrounding it when opened to air and long-life milk normally has a much more extreme
pasteurisation process than that of fresh milk where it is heated to 140C° for two seconds to kill
harmful organism, resulting in differing expiration dates for both types of milk. 56
Introduction
1
https://www.sciencedirect.com/topics/food-science/cow-milk
2
https://www.bbc.com/future/article/20191123-what-milk-is-best-cows-dairy-free-
alternative#:~:text=Cow's%20milk%20is%20a%20good,role%20in%20lowering%20blood%20pressure.
3
https://www.sfa.gov.sg/food-for-thought/article/detail/how-long-can-you-keep-fresh-milk-once-it-s-
opened#:~:text=While%20some%20believe%20that%20milk,products%20that%20you%20can%20consume.
4
https://www.dairy.com.au/dairy-matters/you-ask-we-answer/how-long-does-uht-long-life-milk-last-once-
opened#:~:text=UHT%20milk%20or%20long%2Dlife,and%20used%20within%20seven%20days.
5
https://www.healthline.com/nutrition/spoiled-milk#what-it-is
6
https://www.dairy.com.au/dairy-matters/you-ask-we-answer/how-long-does-uht-long-life-milk-last-once-
opened#:~:text=UHT%20milk%20or%20long%2Dlife,and%20used%20within%20seven%20days.
, In this experiment, the goal is to compare the degree of spoilage of fresh and long-life milk
which will be indicated with the pH of each of the samples. Both samples of milk will be placed
in equal-sized open-air beakers in a refrigerator set at 8C° for 10 days, taking a measurement of
both milk’s pH in different durations to contrast the acidity of the fresh and long-life milk in
order to determine which one experienced the most spoilage indicated with a lower pH. They
will both also be observed in terms of texture, odour, colour and formation of bubbles to gain an
understanding of the effects of spoilage on the two types of milk.
A pH meter will be used to take the measure of each sample’s pH over the 10 day period. It is of
utmost importance to make sure both samples of milk are stored at the same temperature and
measured in the same way while also rinsing the pH indicator between measurements to make
sure cross-contamination does not occur and inadequacies are eliminated.
Prediction
Mainly the prediction is fresh milk will display a lower pH indicating more acidity and higher
spoilage when measured over its increments compared to its long-life counterpart. Day 10 for
each sample is expected to show the lowest pH with pH decreasing starting from day 0 which
will show the highest pH all the way to day 10. The more intensive pasteurisation process of
long-life milk should theoretically kill more bacteria in the milk, causing less overall spoilage
and a higher general pH when compared to fresh milk. This is due to the increased prolonged
exposure of milk to the air allowing for more bacteria to form leading to higher overall acidity
and a lower pH.
Variables
Figure 1.1: Variables of experiment
Variable Identify variable Why and how it was
controlled
Dependent The pH of each type of milk measured using pH meter
(representing spoilage). These values will then be used to
measure: 1. Mean pH of all trials 2. Change in mean pH 3.
Percentage change in mean pH 4. Mean rate of spoilage
Independent The type of milk, whether long life or fresh.
Duration of exposure to open air (0,3,5,7,10 days)
*Durations were chosen as such because milk is believed to
spoil in 4-7 days after being opened in the fridge. 7 Day 0
represents the control (original pH of milk), as a reference to
compare other milk samples to.
Controlled Temperature of environment Temperature affects the rate of
spoilage; as such beakers were
kept in the same refrigerator
set at 8C°set using the fridge’s
7
https://www.tasteofhome.com/article/milk-safety/#:~:text=milk%20really%20expire%3F-
,Some%20sources%20say%20it's%20good%20past%20its%20expiration%20date%20and,its%20printed%20date%2
C%20if%20refrigerated.
2|P ag e
IB Biology Standard Level Internal
Purpose
I was always curious as a kid why there are two main types of cow milk, fresh and long life,
which my parents always bought from grocery stores. This came up in my mind once again
recently and pushed me to find out more about the mechanic of milk spoilage when compared
between long life and fresh milk.
Research Question
How does time duration (0(control),3,5,7 and 10 days) of refrigeration affect the rate of spoilage
of fresh and long-life cow (Bos taurus) milk differently, measured by pH using the pH meter?
Background Information
Cow milk is a liquid produced through the mammalian gland of cows that is nutrient-rich and
provides many needed substances such as vitamin B12, calcium and proteins for the growth of
young mammals 1. This made cow milk abundant in many households thanks to the number of
benefits it provides for bone growth, development of muscles and even in controlling blood
pressure.2 However, over time cow milk tends to spoil and become harmful for human
consumption and there is normally a difference in the lengths of time for spoilage between long-
life and fresh milk, where fresh milk tends to spoil roughly 3-4 days in a refrigerator and sealed
environment compared to long-life milk which usually takes up to 7 days to spoil. 34
Spoilage in cow milk is a very regular occurrence amongst many households and tends to vary in
terms of length and extremity of spoilage when compared between long life and fresh cow milk.
Spoilage in cow milk occurs due to the overgrowth of bacterial communities which inevitably
causes the milk to lose quality, flavour and change its general texture making it unpleasant and
possibly risky to consume. To combat this, most milk produced nowadays, whether long life or
fresh, goes through a pasteurisation process which kills most harmful bacteria that causes major
food illnesses such as E. coli; however, pasteurisation is not enough to kill all the bacteria that
spoils milk which multiplies with the bacteria that enters the milk from the environment
surrounding it when opened to air and long-life milk normally has a much more extreme
pasteurisation process than that of fresh milk where it is heated to 140C° for two seconds to kill
harmful organism, resulting in differing expiration dates for both types of milk. 56
Introduction
1
https://www.sciencedirect.com/topics/food-science/cow-milk
2
https://www.bbc.com/future/article/20191123-what-milk-is-best-cows-dairy-free-
alternative#:~:text=Cow's%20milk%20is%20a%20good,role%20in%20lowering%20blood%20pressure.
3
https://www.sfa.gov.sg/food-for-thought/article/detail/how-long-can-you-keep-fresh-milk-once-it-s-
opened#:~:text=While%20some%20believe%20that%20milk,products%20that%20you%20can%20consume.
4
https://www.dairy.com.au/dairy-matters/you-ask-we-answer/how-long-does-uht-long-life-milk-last-once-
opened#:~:text=UHT%20milk%20or%20long%2Dlife,and%20used%20within%20seven%20days.
5
https://www.healthline.com/nutrition/spoiled-milk#what-it-is
6
https://www.dairy.com.au/dairy-matters/you-ask-we-answer/how-long-does-uht-long-life-milk-last-once-
opened#:~:text=UHT%20milk%20or%20long%2Dlife,and%20used%20within%20seven%20days.
, In this experiment, the goal is to compare the degree of spoilage of fresh and long-life milk
which will be indicated with the pH of each of the samples. Both samples of milk will be placed
in equal-sized open-air beakers in a refrigerator set at 8C° for 10 days, taking a measurement of
both milk’s pH in different durations to contrast the acidity of the fresh and long-life milk in
order to determine which one experienced the most spoilage indicated with a lower pH. They
will both also be observed in terms of texture, odour, colour and formation of bubbles to gain an
understanding of the effects of spoilage on the two types of milk.
A pH meter will be used to take the measure of each sample’s pH over the 10 day period. It is of
utmost importance to make sure both samples of milk are stored at the same temperature and
measured in the same way while also rinsing the pH indicator between measurements to make
sure cross-contamination does not occur and inadequacies are eliminated.
Prediction
Mainly the prediction is fresh milk will display a lower pH indicating more acidity and higher
spoilage when measured over its increments compared to its long-life counterpart. Day 10 for
each sample is expected to show the lowest pH with pH decreasing starting from day 0 which
will show the highest pH all the way to day 10. The more intensive pasteurisation process of
long-life milk should theoretically kill more bacteria in the milk, causing less overall spoilage
and a higher general pH when compared to fresh milk. This is due to the increased prolonged
exposure of milk to the air allowing for more bacteria to form leading to higher overall acidity
and a lower pH.
Variables
Figure 1.1: Variables of experiment
Variable Identify variable Why and how it was
controlled
Dependent The pH of each type of milk measured using pH meter
(representing spoilage). These values will then be used to
measure: 1. Mean pH of all trials 2. Change in mean pH 3.
Percentage change in mean pH 4. Mean rate of spoilage
Independent The type of milk, whether long life or fresh.
Duration of exposure to open air (0,3,5,7,10 days)
*Durations were chosen as such because milk is believed to
spoil in 4-7 days after being opened in the fridge. 7 Day 0
represents the control (original pH of milk), as a reference to
compare other milk samples to.
Controlled Temperature of environment Temperature affects the rate of
spoilage; as such beakers were
kept in the same refrigerator
set at 8C°set using the fridge’s
7
https://www.tasteofhome.com/article/milk-safety/#:~:text=milk%20really%20expire%3F-
,Some%20sources%20say%20it's%20good%20past%20its%20expiration%20date%20and,its%20printed%20date%2
C%20if%20refrigerated.
2|P ag e
