How does bacteria turns milk into Yogurt?

The Project will answer "How bacteria can convert milk into yogurt?" this includes further researches on the bacterias along with the yogurt making experiment. We will also work with different kinds of liquids, how they work and react with bacteria.
Elaine Lo Farah Maja Farah
Grade 9


If the lactose bacteria are added into cow milk, then the milk would turn into yogurt because the bacterias would break down the proteins, and convert the lactose into lactic acid, which forms yogurt. 

The oat milk would not convert into yogurt, nor solidify as good as the cow milk did because the lack of lactose that the oat milk contain,  the kinds of protein that the oat milk contains does not meet the same amount of protein that cow milk contain, therefore, the oat milk would not solidify as well.

The mixture made out of egg and vinegar would not turn into yogurt because it does not contain lactose and casein, but it might solidify better than the oat milk since it contains more protein than oat milk does. The vinegar can also prevent the egg from getting perfectly cooked during the boiling process. 



How the Bacteria is turning milk into Yogurt;

It is known that the lactic bacterial break down the protein inside the milk into amino acid, and convert the lactose into lactic acid(C3H6O3). The thermophilus provide the condition that allow the balgarius to develop, while the balgarius break down the proteins into acid. 

The amino acid is a compound that is essential for human life to functioned, with key elements of carbon, hydrogen, oxygen, and nitrogen. Normally human body can only create 10 of the 20 amino acids, while the other rely on consume nutrition from food. Amino acid can be found in every food that contains protein, such as eggs and milk or even grains like buckweat and quinoa. 

The main job for amino acid is to build up proteins. Human body would also require amino acid to help break down foods, repair body tissues, and perform other body functions. Essential amino acids cannot be produced in human body, while the non-essential ones could. We can only consume the essential amino acid through foods. There are also conditional amino acid who are nonessential most of the time, but essential when a person is having serious illness or stress. 

Since the bacteria break down the lactose sugar into lactic acid, people with lactose intolerance can also consume yogurt, which provide them a good source to intake the nutrition that they are unable to consume due to lactose intolerance. 


LAB - Lactic Acid Bacteria:

The starter [we are using Lactobacillus Bulgaricus,] ferments lactose, producing lactic acid. Lactic acid decreases PH [Power of Hydrogen], causing the milk to form the soft yogurt texture. Lactic acid is mostly used as a curing agent, flavouring agent, and food preservation. 

Types of Yogurt Bacterias;

Yogurt contains lots of different types of good bacterias; Streptococcus thermophilus is one of the most common bacteria used as a yogurt starter pack. The bacteria naturally lives inside a human's body without harming or causing any type of diseases. 

Another friendly bacteria called Bifidobacterium animalis are also naturally found inside the human and animal's body, mainly inside the stomach and large intestines and are used to improve digestion. 

Lastly Lactobacillus bulgaricus; another common bacteria used for dairy products such as yogurt. This bacteria plays a huge role in maintaining an individual's health, it keeps human’s intestinal walls strong, according to the National Institute of Health, Lactobacillus bulgaricus helps to prevent diarrhea, common cold and reduces a big chance of getting diseases. 

Both Streptococcus thermophilus and Lactobacillus bulgaricus can help prevent harmful microrganisms from harming human intestine system.


Casein is a type of protein that are found inside mammal's milk, and are made up of fat, lactose sugar, or other kinds of dairy products. It is a slow digesting dairy protein that feeds our cells with amino acid at a low level over a long period of time, casein also helped our cell's synthesis protein and reduce muscle break down.  In some occassion, the casein allergy would occur when human body mistakenly consider casein as a harmful substance. Therefore the body would release antibodies for self defense/protection, which trigger the release of body chemicals that can cause symptoms  such as; swelling on facial part, and skin reactions. 


Manipulative variable: The type of liquid that has been used during each experiment.

Responding variable: The solidification of the liquid.

Controlled variable:

- The temperature of the Yogurts fermentation.

- The amount of liquid and Yogurt Bacteria that has been used in the experiment.

- The same amount of time for fermentation.

- The same kind of pot that were used to boil the liquid. 

- The same kind of container(s) used to incubate the yogurt. 


1. Prepare all the materials that is needed in the experiment;

- Liquids [Milk, Egg+Vinegar mixture.]

- Yogurt starter/Yogurt Bacteria.

- Cooking thermometer.

- Pots.

- Measuring spoons.

- Container(s). 

2. Drain 500 ml of liquid into the pot.

3. Heat up the liquid until it is boiled [100C]

4. Drain the liquid into the container.

5. Wait for the liquid to cool down to 40 celcius, make sure to use cooking thermometer to measure the temperatures. [If cooking thermometer is not avaliable, dipping your pinky inside the mixture and count to 5 comfortably.]

6. Once the liquid cool down to 40 celcius, add 1 Teaspoon of "Yogurt starter pack/Yogurt bacteria" (1 quater out of the yogurt pack) into the liquid.

7. Continue to stir the liquid until the powder dissolve completely [Around 4-5 minutes.] 

8. Put the lid on the container, make sure to close the lid losely so the air can get in.

9. Let the liquid ferment in room temperature overnight or for around 16 hours.

10. Repeat step 2 till' step 9 for other kinds of liquids. 


We used 3 different kinds of liquid for this experiment, which are; cow milk, oat milk, and a mixture formed by egg and apple vinegar. 

As there are more plant-based food products in recent years, we wanted to know if the lactose bacteria could also ferment with the plant based oat milk.

Then, we also decided to make another type of experiment with the yogurt bacteria but mixed with a mixture made out of egg and vinegar, since the vinegar provides acid and enzyme, while the egg contains a certain amount of protein and amino acid. 

Liquids Observation Taste
Cow milk Did solidify and turned into yogurt. But the texture is not as thick comparing to the store sell yogurt.   Taste as sour as the sugar free yogurt in the groccery store, but taste relatively astringent than the store sell yogurt. 
Oat milk The liquid did not solidify, the texture nor the colour changed too.  Does not seem like there had any changes during the culturing process.  It taste just like normal oat milk. Does not have any sourness or creamy taste. 
Egg + vinegar Did solidified thicker than cow milk when heating up, however the consistency did not change after the incubating process. Which means the bacteria did not ferment in the liquid.  The solidified egg taste like sour version of pudding, but it is because the vinegar itself is sour



The fact that both samples without dairy milk cannot solidified shows a result of lack on lactose and casein in both liquids. Since these two compounds can only be found in dairy product, the oat milk and the egg vinegar mixture, as a non-dairy product, obviously cannot convert into yogurt via lactose bacteria successfully.


Difference between home made and store sell yogurt

Food factories would add in ingredients to make the solidification more obvious, and add in huge amount of sugar to make the smooth taste of yogurt. Although the sugar free yogurt taste sour too, there are still xylitol or sodium cyclamate(MSG) being added in the yogurt, which makes it have less astringent taste than the home-made sugar free yogurt. 

There are vegan yogurt starter that can convert the oat milk or other kinds of plant-based milk into yogurt, through the help from probiotic and beneficial gut bacteria.Even though some vegan starter contain the lactose bacteria that our yogurt starter also have, but because of the  different ingredients for other probiotic, the oat milk in our experiement still cannot turn into yogurt. 


Difference between the making process of types of yogurt: 

If we add more water to mix with the milk, the yogurt would dilute and become a liquid yogurt drink. On the other hand, if there is more milk being added, the yogurt would concentrate and form a thicker texture. Greek yogurt is an example of yogurt that is made out of higher concentration, which is why it contain more protein and calcium than other kind of yogurt. 



Yogurt can be heavily influenced by the type of milk that is used, since milk comes from various different animals that also contains different nutritions inside them, this can affect the yogurt in many ways such as; the texture and thickness of the yogurt.

Cow milk is one of the most popular choices for making home-made yogurt, cow milk usually creates the best homemade yogurt [texture, taste, thickness] comparing to other types of milk. The cow milk did solidify as we expected,  product had the yogurt texture, thick and creamy, along with the yogurt scent.  

However the oat milk and egg and vinegar mixture did not actually solidified, both samples remained the same before and after adding in the yogurt bacteria.  Because both samples do not contain lactose, which do not satisfied the condition to convert into yogurt. 




"Why is our experiment important?"

We decided to do an experiment/research about yogurt because we believe a lot of people underestimate the power of yogurt. Yogurt is a good source for protein, calcium, and vitamins, it contains good bacterias that can prevent lots of sickness, such as; diarrhea, keeps human's body stays healthy and strong, improves digestions, reducing the chances of getting diseases, and the lists goes on. 

This project shows that there are thousands of different types of bacterias out there and that not all bacterias are bad. Yogurt bacterias can be very beneficial to lots of individuals, and we are teaching others on how to make homemade yogurt or maybe try a fun mini science experiment at home.


Sources Of Error

There is no changes for the oat milk even after the Yogurt bacteria was added. We were expecting the oat milk to at least have a sour taste, but the taste remain the same. Additionally, we also expected the oat milk to have a yogurt-like texture, thicker than regular milk, however, the texture also remains the same.   


Thank you to the William D. Pratt science teacher Mrs. Friesen, for helping and giving us support on our project. It was a pleasure and definetely a new experience joining the Science Fair Club.