1. MEASURING THE PH OF MILK AS IT TURNS SOUR
1.0 ENGAGE
Introduction
Milk contains a large number of a weak acid or a weak base in its salt. So, it behaves as a
buffer solution. Fresh cow milk has a pH of between 6.7 and 6.5. Therefore, it is slightly
acidic. Values higher than 6.7 denote of mastitic milk while values lower than 6.5 denote the
presence of bacterial deterioration. As milk is a buffer solution, considerable acid
development may occur before the pH changes. pH value lower than 6.5 therefore indicates
that considerable acid development has taken place. This is normally due to the bacterial
activity.
pH measurements are often used as acceptance tests for milk. pH is the abbreviation
of the weight of hydrogen. This term was introduced in 1909 by the Danish biochemist S. P.
L. SØrensen (1868-1939). The pH scale covers the active concentration of the H+ ions and
OH- ions and therefore the pH value is defined as the negative common logarithm of the
active hydrogen ion concentration in an aqueous solution. The value of pH for neutral, acidity
and alkalinity condition are 7, less than 7 and more than 7, respectively.
Before this, litmus papers are widely used to test the acidity of a solution while pH
papers or pH meters are used to know the pH value of a solution. But nowadays, data loggers
are more often used in experiments because the data collected by them are more accurate and
the experimental procedure become easier. However, the litmus papers and pH papers or pH
meters are still being used in certain experiments.
Data loggers offer more flexibility and are available with a greater variety of input
types. Most data loggers collect data which may be directed transferred to a computer. Data is
normally collected in non volatile memory for later download to a computer. Data logger can
collect data independently of a computer. So, the computer does not need to be present during
the data collection process. This makes them ideally suited for applications requiring
portability. Besides, mistakes are not made in reading the results. Compared to human, data

2. loggers will not misread the data. Graphs and tables of results also can be produced
automatically by the data logging software. This advantage can save the time to draw the
graphs manually.
However, data loggers also have disadvantages. The main disadvantage is the cost of
purchasing the equipment. The price of the equipment is not too economical. If the data
logging equipment breaks down or malfunctions, some data would be lost or not recorded.
The cost needed to repair the equipment is higher. So, the tendency of not being repaired is
also higher. Besides, by producing the graphs and tables automatically, the drawing skills of
the user will become weaker.
For this experiment, it looks at the changes in pH of milk as it goes sour. Milk is
contaminated by bacteria even before it leaves the udder, although at this point they are
harmless and few in numbers. Processing and handling activities can introduce further
microorganisms. The two main groups of bacteria in milk are lactic acid bacteria and
coliforms. Lactic acid bacteria are the normal bacteria present in milk and dairy products.
They are anaerobic and preferably live in a low pH environment. Coliforms are the main
reason for milk spoilage.
One of the main ingredients of milk is lactose. This is the substance that the bacteria
use to create lactic acid. The lactic acid bacteria live on the lactose, which they turn in
glucose and then turn them into lactic acid:
Lactose  Glucose  Lactic acid
Measuring the milk acidity is an important test to determine the milk quality. Acidity
measurements are also used to monitor processes such as cheese-making and yogurt-making.
This experiment demonstrates the changes in the pH of milk over a several day period,
depending on room temperature. This experiment is suitable for ages 13 and above. The
equipment used in this experiment to monitor the pH of the milk under the exposed condition
is data logger.
Problem Statement

3. Ahmad bought milk from the nearby shop. He drank in a little amount and then left it on the
table. He forgot to store the milk into the refrigerator. The next day, drank the leftover milk.
But, he found out that the taste was totally different than yesterday. The milk tasted sour. He
wonders what happened to the milk when it is left and exposed.
1. Why does the taste of the milk changes as it is exposed?
2. Is the sour milk acidic or basic?
3. If the milk is acidic or basic, what is the pH value?
2.0 EMPOWERING
To conduct the experiment, the following are the equipment and methodologies.
Equipment required
1. Data logger connected to a PC
This data logger by DrDAQ contain all these features:
 4 digital inputs and outputs
 3 sockets for external sensors
 High impedance input for pH or Redox measurements
 100 kHz, 8–bit oscilloscope with a ±10 V range
 Signal generator with AWG for creating your own waveforms
 Captures fast signals
 USB connected and powered
 Use up to 20 USB DrDAQs on a single PC
Data logger is an electronic device that records data over time and stored them. It is
based on a digital processor or computer. Data logger is used as specific devices that can
measure a range of measurement applications to a very specific device for measuring in one
application type only. Data logger has the ability to automatically collect data on a wide
range of time. Therefore, an accurate data of the environment conditions being monitored can
be obtained.
To start using data logger, first the data logger is connected to a PC via USB interface.
Accompanying data logger software is used to select logging parameters and activate the

4. logger. The logger is then connected to a sensor. After the desired monitoring period, the data
connected to the computer and will read out the data and display the measurement in a graph.
Diagram 1: Data logger Diagram 2: data logger compartments
2. pH sensor
Diagram 3: pH sensor
The pH sensor is used to measure the pH of milk in this experiment. The advantages of
using pH sensor compared to a traditional pH meter are this sensor provides automatic
data collection, graphing, and data analysis when connected to the data logger. This
sensor can measured pH from range 0 to 14.
3. Bottle of milk

5. Methodologies
1. The milk from bottle is pour into a glass.
2. The milk is then allow to reach the room temperature without cooking and re-
pasteurising it.
3. The pH probe is plug into the data logger socket.
4. The pH probe is inserted into the milk.
5. Run the software.
6. All data are recorded and displayed in graph form.
Diagram 4: data logger connected to PC
Diagram 5: pH probe inserted into milk

6. Result
Graph 1
Discussion
The experiment is to investigate and measure the pH of milk as it goes sour. The pH
probe will measured the pH of the milk and send the data to data logger to be processed by
the software. The data obtained is then plotted in graph 1. The graph 1 shows that the pH of
the milk will decrease as days passed by. The decreasing in the pH of the milk indicating that
the milk becomes acidic. As the milk turns sour, the pH decreasing. We can also see from the
graph that as the time of milk exposed to the environment increasing, its pH value decreased.
From the graph 1, the reduction in pH can be seen shortly after starting the
experiment. However, the changes are little. This depends on the freshness of the milk that
we used. If the milk is no longer fresh, we may see rapid decreasing in the pH value because
there is bacteria content inside the milk. Therefore, when exposed to air, these bacteria will
start to grow rapidly contributing to the sourness of the milk. After few days, we can see that
the pH starting to decrease to a lower value.
Bacteria and other organism is the main contribution to the changes in pH of the milk.
Milk contains lactose, but at the same time milk also contained bacteria. Some bacteria work

7. efficiently in warm conditions and causes milk become sour. Eventhough pasteurized milk
undergo series of steps, bacteria are not completely killed in this process. Therefore, the left
bacteria will gradually reproduce and turns the milk into sour. The main reason why milk
turns sour is because the lactose in the milk is converted to lactic acid. As the acid level
increases, the pH levels drops. If milk is exposed, the bacteria in the milk will start to produce
and forms gases and chemicals which change its smell, taste, texture.
Over the entire experiment, the reduction in pH is not constant. From the graph, we
can see that the gradient significantly increases half way through for fresh pasteurized milk.
As the time increases or from first day to fourth day, we can see that the pH is reduced.
Reduction in pH means that the milk has increases in its acidity.
As milk begins to sour, its pH levels fall sharply. This makes milk even more acidic. The
sugars in milk are converted into acids by certain bacteria named Lactobacilli, thereby
reducing its pH. Completely sour milk has a pH of about 4.4. Knowing the pH of milk may
also help to cut down intake of high acidic foods and to keep acid-base related disorders.
The extent of change in pH can be calculated by subtracts the pH value of milk in first
day with the value pH value of milk in fourth day. From the graph, the pH of milk in first day
is 6.75. In contrast, the pH of milk in fourth day is 4.8. So, the change in pH is
𝑇𝑕𝑒 𝑒𝑥𝑡𝑒𝑛𝑡 𝑜𝑓 𝑐𝑕𝑎𝑛𝑔𝑒 𝑖𝑛 𝑝𝐻 = 6.75 − 4.8 = 1.95
UHT stands for “Ultra High Temperature” processing, which is just another way of
heating milk in order to kill the bacterial spores. In the UHT process, the milk is heated at the
temperature of 135°C for one to two seconds. This extreme heat is said to kill off any spores
in the milk. UHT milk tastes different than its pasteurized milk. It tastes like it comes from a
box.
Since a single microbial cell of an organism capable of growth in UHT milk may
cause spoilage in a container when the product may be stores for six months or more at
ambient temperatures, the methods used for detecting contamination must be very sensitive.
This sensitivity is usually achieved by doing pre incubation. So, we can say that if we
incubate UHT milk in a closed arrangement over the same time period, this will provides
very high bactericide effect and decreased the contamination risk down to zero. The obtained

8. result of this incubation can be considered as very stable because there are no bacteria and the
taste of the milk will remain the same.
Microorganisms are everywhere. They can be found in the air, in water, in soil, on
animals and even on humans. Some are beneficial and others can cause spoilage. A small
number are pathogenic and can cause disease, which is food borne disease. Food borne illness
or food borne disease is any illness result from the consumption of contaminated food. Three
types of microorganisms can contaminate food and cause foods borne illness are bacteria,
viruses and parasites. The most common symptoms of food borne illness are diarrhea,
vomiting, fever, sore throat with fever and jaundice.
Our stomach is an acidic place. Once we start eating, acid is secreted into the stomach
to start the digestion process. The secretion then mix with the food in the stomach. The
resulting mixture can have a pH of very acidic condition. The acid content in the food that we
have eaten have the ability to impact the digestive process. When too many foods that are
high in acid content are consumed, they may trigger miserable symptoms which include sour
tummy, acid reflux and an uncomfortable bloated state.
1.3.1 ENHANCE
1.3.1 Cheese
Figure 1 (a): Curd Cheese Figure 1 (b): Curd Cheese

9. Figure 1 (c): Curd Cheese
Many of the cheese are produced in food industry. Cheese can be produced by using milk and
soy milk. How curd or cheese can be produced by own at home or in kitchen?
A very simple cheese can make by anyone with nothing but milk, vinegar or lemon
juice, a piece of cloth, and a few utensils that everyone has around the house. A wide variety
of milks commercially purchased pasteurized homogenized cow’s milk, raw’s cow milk,
goat’s milk, sheep or soy milk can be used to make cheese, each with unique flavour. The
best cheese is from unpasteurized milk but should be cured for 2-4 months if there is any
doubt about pathogens in the milk. If used pasteurized milk, need to add little calcium
chloride to firm up the curd because the heat makes the calcium unavailable.
Basically, when acid (either produced by bacteria or added to the mix) and/ or rennet
act on milk to cause the milk fat and protein to separate from the liquid, leaving a substance
that we call cheese. Most milk simply curdled by a combination of heat and added acid, such
as vinegar or lemon juice. Once the cheese has curdled the following is being made. A soft,
fresh cheese might be drained at this point and can be eaten immediately. If want hard cheese,
might be heated further to remove moisture, pressed into mold, and then aged to develop a
characteristic intense flavour.
Besides the milk, acidifiers are needed to convert milk sugar to lactic acid and “ripen”
milk. An acidifier can be something as simple as lemon juice or bacterial special starter
cultures purchased from cheese-making suppliers. Bacteria are added to acidify the milk, so

10. that the rennet will work, and to aid in the curing. Some recipes add chemicals acid to acidify
the milk.
Make cheese is one way to keep milk and its nutritional value around for a relatively
long time. Milk would sour quickly without refrigeration, so cheese from the milk made it
possible to make use the milk’s nutritional value over a longer period.
1.3.2 Yogurt
Yogurt is one of the Malaysian’s choices. Yogurts come from milk that may help
prevent from Osteoporosis. How yogurt can be made from milk?
Milk is fermented with the help of lactose-loving bacteria to make yogurt. The
lactose-loving bacteria consume lactose (the sugar in the milk) and produce lactic acid as
waste product. This lactic acid is responsible for both the tart flavor and the thickening by
acting on the casein protein in the milk.
The yogurt-like products each varies by thickness and acidity by used different
compositions of lactic-acid bacteria. For example, yogurt in the Unites States is made with
lactobacillus bulgaricus and streptococcus thermophilus. The milk needs to be fairly warm
between 105 and 115 degrees in order to the organisms in this culture to function.
1.3.3 Further Studies
For enhance phase, there are four further studies that can be done. The four further studies are
1. Repeating the experiment using unpasteurized and Ultra Heat treated milk,
2. Keeping the container of milk at different temperature whether in fridge or an
incubator,
3. Using different milk source such as goat’s milk, cow’s milk and soy milk in
experiment, and
4. Using different milk product such as Dutch Lady, Marigold, Everyday, Nestle and
Farmhouse Milk (F&N).
The experiment can be done by using the unpasteurized milk and Ultra Heat Treated
(UHT) milk to study the pH of milk when leaved for a few days. The different between

11. unpasteurized milk and UHT milk results in difference pH after leave for a few days. The
difference is due to the presence of bacteria in unpasteurized milk and UHT milk.
The unpasteurized milk is raw milk that can harbor dangerous microorganisms that
can affect serious health risks. The unpasteurized milk can have dangerous bacteria such as
Salmonella, E.coli and Listeria that responsible for causing numerous food borne illnesses.
These bacteria can cause vomiting, diarrhea, abdominal pain, fever, headache and body ache.
Usually, people are more likely to consume or drink Ultra Heat Treated Milk (UHT
Milk) or ultra-pasteurization. This is because UHT milk undergoes sterilization by heating
milk for an extremely short period, around 1-2 seconds at temperature exceeding 135 degree
Celsius (temperature required to kill spores in the milk). UHT milk have typical shelf of 6
month to 9 month, until opened.
Second, the pH of milk can be measured also when milk is placed whether in fridge or
in incubator. In this experiment, we are study whether the temperature affect the pH value of
milk. We can compare the pH value between milk in fridge that has low temperature and
incubator that has high temperature.
The next further studies are using different milk source from goat, cow and soy.
From this experiment, we can identify which milk that has higher acidity after leave for a few
days. Milk that has higher acidity means that have more bacteria presence that produces
toxicity within a few days. In addition, we can also replaced by using different milk brand
like Dutch Lady, Marigold, Nestle, Everyday and Farmhouse Milk (F&N). Then, we can
know which milk brand has higher acidity.

12. Conclusions
The pH of the milk from 6.75 in the first day reduced to 4.8 in the fourth day. This
means that the acidity of the milk increases from the first day to the fourth day. The milk has
a high acidity in the fourth day because of the presence of the bacteria that convert the sugars
in the milk into acid.
The benefits use of data logger are the low-cost and used as educational data logger
throughout the world. The most important of data logger use in educations is it motivate
students in learning process. In future,data logger can be used in school in Malaysia and also
educational institutions. The properties of data logger that can increase the student’s interest
and motivate student as well help student in learning process. The usage of data logger can
also make students more familiar with technology instrument or computer.
Not only that, the data obtain both during and after experiment are viewable in both
spreadsheet and graphical format. The data that viewable make students more easily to
analyse the data. The pH measuring kit, allows to accurately measure and record the acidity
and alkalinity of solution. So, the data logger can be used for schools and educational
institutions in Malaysia.
Furthermore, many applications of data logger ranging from industrial operations to
biological processes requires accurate measurement and analysis of pH is needed. Some of
the applications include biochemistry, agronomy, food science, chemical research and
engineering, and environmental research and pollution control. These applications demands a
low cost portable devices and also a low power.

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