Introduction: (Initial Observation)
Yogurt is a tangy, nutritionally excellent dairy product that can be made at home.
My question is how liquid milk becomes a solid yogurt? Where does the water go? The other thing that I would like to know is whether making yogurt is a chemical process or biological process?
Knowing this will help us to control the process for a better product in less time. For example, heat, stirring, and catalysts can accelerate chemical reactions. So maybe we don’t have to wait one day for milk to become yogurt and we can do it in a few minutes.
On the other hand if making yogurt is a biological process such as growing a plant or growing mold, then environmental parameters such as temperature and light may affect the result and need to be controlled for the best results.
The subject of my project is “Making Yogurt” and I am hoping to find answers to my entire question with experiments that I will perform.
Information Gathering:
Find out about what you want to investigate. Read books, magazines or ask professionals who might know in order to learn about the effect or area of study. Keep track of where you got your information from.
I collected some information from the Internet and learned that the activity of certain bacteria will convert milk to yogurt. I also learned that the small amount of yogurt that we add to the milk to start the process is called culture and must contain live bacteria. In other words, if you boil some yogurt, you will kill the live bacteria and you cannot use that yogurt as culture any more. Finally this material suggested that the best temperature for making yogurt is about 110 degrees Fahrenheit.
Question/ Purpose:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to find out:
– How liquid milk becomes solid yogurt?
– Is making yogurt a chemical or biological process?
– How can we control the process for faster and better product?
Although each of these questions can be the foundation for a different Science Project, you may prefer to work on a more specific question. Following are some questions that you can choose from.
- How does the amount of yogurt starter culture affect the time it takes for yogurt to be done, its firmness and quality or taste?
- How does the initial boiling/heating of milk before adding starter culture affect the time it takes for yogurt to become firm and the quality of yogurt?
- How does the culturing temperature (temperature of milk when you add yogurt starter culture) affect the time it takes for yogurt to become firm and yogurt quality?
You may also have another question for storage temperature. Usually we keep the milk with added yogurt culture in a warm temperature until yogurt is done. What will happen if you keep it in cold or hot temperatures?
Identify Variables:
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Depending on the specific question that you choose to study, following is how you define variables.
For question 1:
Independent variable (also known as manipulated variable) is the amount of yogurt starter culture (or the ratio of yogurt culture to milk).
Dependent variables (also known as responding variable) are the time it takes for yogurt to be done, its firmness and taste.
Constants are the amount and type of milk, container, culturing temperature, storage temperature, method and procedures of preparation and adding starter culture.
Controlled variables are air flow, wind and other environmental conditions that may deliver unexpected bacteria to the milk.
For question 2:
Independent variable (also known as manipulated variable) is the initial heating/boiling temperature.
Dependent variables (also known as responding variable) are the time it takes for yogurt to be done, its firmness and taste.
Constants are the amount and type of milk, amount and type of starter culture, container, culturing temperature, storage temperature, method and procedures of preparation and adding starter culture.
Controlled variables are air flow, wind and other environmental conditions that may deliver unexpected bacteria to the milk.
For question 3:
Independent variable (also known as manipulated variable) is the culturing temperature.
Dependent variables (also known as responding variable) are the time it takes for yogurt to be done, its firmness and taste.
Constants are the amount and type of milk, amount and type of starter culture, container, storage temperature, method and procedures of preparation and adding starter culture.
Controlled variables are air flow, wind and other environmental conditions that may deliver unexpected bacteria to the milk.
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Following are some examples:
For question 1:
As the amount of yogurt culture increases, It will take less time for yogurt bacteria to reproduce and saturate the milk with yogurt bacteria. So the yogurt will be done faster. Also the taste and quality of yogurt will be better because our yogurt culture will be the dominant bacteria in the milk. This reduces the chance for other bacteria to grow and produce a bad taste in the yogurt. My hypothesis is based on information that I have gathered about bacteria and how they grow.
For question 2:
Milk must be boiled for a few minutes before using it to make yogurt. Boiling will kill all existing bacteria in the milk, so when we add yogurt starter culture, it will be the dominant bacteria. If the milk is not boiled properly, that may leave some bacteria that later will grow and give a bad taste to the yogurt or may prevent production of yogurt.
For question 3:
I think the culturing temperature must be warm. If the milk is very hot and we add culture, many of live bacteria in our starter culture may die and we may not get yogurt.
Experiment Design:
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Read the yogurt recipe at the bottom of this page and try it once before working on your experiments.
Experiment 1:
Introduction:
For this experiment you must try making yogurt with different amounts of culture. The ratios of culture to milk that you may try are:
0.002 or 0.2% (2 grams of yogurt culture in 1000 grams of milk)
0.005 or 0.5% (5 grams of yogurt culture in 1000 grams of milk)
0.01 or 1% (10 grams of yogurt culture in 1000 grams of milk)
0.02 or 2% (20 grams of yogurt culture in 1000 grams of milk)
0.05 or 5% (50 grams of yogurt culture in 1000 grams of milk)
0.10 or 10% (100 grams of yogurt culture in 1000 grams of milk)
0.20 or 20% (200 grams of yogurt culture in 1000 grams of milk)
You may skip some of the above ratios.
Procedure:
- Prepare 3 small Styrofoam or ceramic cups for each of the culture ratios that you want to test. Write the culture ratio for each group on a sticker and attach the stickers to the cups.
- Boil enough milk for all the cups. Let the milk boil for 10 minutes. Control the heat to prevent overflow of foamed milk.
- Stop the heat and let it cool to warm temperature (about 45º Celsius). Keep it covered to prevent air bacteria from entering the milk.
- Transfer the milk to the cups. Fill each cup to about 1/2.
- Stir your yogurt culture to make it smooth. Use a pipette or any other fine measuring cup to enter different amounts of yogurt culture into the cups as planned. For example if you have 100 mL milk in each cup, you may add 1 mL yogurt culture to make it 1%; or you may add 0.2 mL yogurt culture to make it 0.2%.
- After you add proper amounts of yogurt culture to all the cups, cover them properly and keep them in a warm place.
- Inspect the cups every 2 hours. Tilt the cup slightly to see if it is still liquid or it has solidified. It is best if you make at least 5 observations in the first 10 hours after adding the culture. Record the results in a table like this:
Condition of samples at different time intervals after adding yogurt culture.
| Culture Ratio | 2 hours | 4 hours | 6 hours | 8 hours | 10 hours |
| 0.2% | |||||
| 0.5% | |||||
| 1% | |||||
| 2% | |||||
| 5% | |||||
| 10% | |||||
| 20% |
Experiment 2:
Introduction:
For this experiment you must try making yogurt with different pre heat conditions. In this way you will know if heating or boiling the milk before adding culture has any affect on the production of the yogurt. In either case add the culture at warm room temperature (about 40ºC).
Procedure:
- Prepare 3 small Styrofoam or ceramic cups for each of the pre heat conditions that you want to test. Write the preheat condition for each group on a sticker and attach the stickers to the cups.
- Keep some milk at warm room for a few hours so it gets to the room temperature. Use this milk for your first group.
- Warm up some milk with heat to about 65ºC. Use this milk for your second test group.
- Heat up some milk until it gets very hot, but stop the heat before boiling. Use this milk for your third test group.
- Boil some milk for 1 minute. Use it for the fourth test group.
- Boil some milk for 5 minute. Use it for the fifth test group.
- Boil some milk for 10 minute. Use it for the sixth test group. (optional)
- Wait until all milks cool off to the room temperature.
- Transfer the prepared milk of each group to the cups reserved and marked for the same group. Fill each cup to about 1/2.
- Stir your yogurt culture to make it smooth. If your cups contain 100 mL milk, add 10 grams yogurt culture to each cup. If your cups are smaller or larger, adjust the amount of culture accordingly so you will use 10% culture in all test groups.
- After you add proper amounts of yogurt culture to all the cups, cover them properly and keep them in a warm place.
- Inspect the cups every 2 hours. Tilt the cup slightly to see if it is still liquid or it has solidified. It is best if you make at least 5 observations in the first 10 hours after adding the culture. Record the results in a table like this:
Condition of samples with different preheat conditions.
| Preheat condition | 2 hours | 4 hours | 6 hours | 8 hours | 10 hours |
| Cold Milk (no preheat) | |||||
| Warm Milk | |||||
| Hot Milk | |||||
| Boiled 1 minute | |||||
| Boiled 5 minutes |
Make a graph:
You can use a bar graph to visually present your results. Now that you know how many hours does it take for yogurt to form with each pre-heat condition, make one vertical bar for each condition and adjust the height of each bar based on the number of hours it took for yogurt to form with that specific preheat condition.
(For example it may take 6 hours for the sample “Boiled for One Minute” to form yogurt. You may make a bar that is 6 inches tall for that condition)
You may try multiple experiments at the same time and try to make multiple samples of yogurt in different conditions. In this way you will be able to test the first and second hypothesis at the same time.
To do this, heat up some milk and get samples in different stages, wait until it cools off, add different amount of yogurt culture, keep it at room temperature and wait for the result.
Record the results in the following table.
| 0.002 culture | 0.005 Culture | 1% culture | 2% culture | |
| Cold Milk | ||||
| Warmed Milk | ||||
| Hot Milk | ||||
| Boiled 1 minute | ||||
| Boiled 5 minute |
Additional experiments
Test the Incubation temperature: Repeat this experiment a second time, but this time keep the samples in a warm cabinet (about 110 degrees Fahrenheit).
Test culture temperature: And finally repeat the experiment and add the culture while the milk is still very hut.
Experiment 3:
In this experiment I want to add some yogurt to the boiling milk, stir it for a few seconds and let it cool off. At hot boiling temperature, all bacteria from the milk and from the yogurt that I add will be killed. Now if this gives me a good product, I can conclude the making yogurt can be a chemical process because we killed all live microorganisms at boiling temperature.
Materials and Equipment:
List of material may vary based on what you have available for yourself.
You will need about 2 gallons of milk to complete all your experiments.
Results of Experiment (Observation):
Experiment 1,
Culture added at room temperature, Results after 24 hours kept at room temperature.
| 0.002 culture | 0.005 Culture | 1% culture | 2% culture | |
| Cold Milk | ||||
| Warmed Milk | ||||
| Hot Milk | ||||
| Boiled 1 minute | ||||
| Boiled 5 minute |
As you see the results are very unexpected and disappointing.
Experiment 2,
Culture added at warm temperature, Results after 24 hours at 110 degrees Fahrenheit
| 0.002 culture | 0.005 Culture | 1% culture | 2% culture | |
| Cold Milk | ||||
| Warmed Milk | ||||
| Hot Milk | ||||
| Boiled 1 minute | ||||
| Boiled 5 minute |
Experiment 3,
Culture added at hot temperature, Results after 24 hours at 110 degrees Fahrenheit
| 0.002 culture | 0.005 Culture | 1% culture | 2% culture | |
| Hot Milk | ||||
| Boiled 1 minute | ||||
| Boiled 5 minute |
Those who are marked bad, they smelled bad. Even though they looked like soft yogurt, they were not yogurt and I did not take the chance to taste them.
Side Experiments:
Now that I had everything ready, I also did some side experiments. One side experiment was comparing identical samples with only one difference. In one I added the culture without stirring and in the other with stirring about 15 seconds. Results were identical or very close.
The other side experiment was using microwave heat to heat up the milk. I did not notice any obvious difference.
Calculations:
Description
Summary of Results:
As the above tables show, only in certain conditions we were able to produce a good yogurt. Higher amount of culture and proper temperature control normally resulted a better product.
Conclusion:
The fact that conversion will happen only at certain warm temperature range and it takes a long time is an indication that a biological activity is happening. In other words certain bacteria at that temperature feed the nutrition that exists in milk and reproduce.
Bad smell in some samples in the last experiment can be explained in this way that since we added a small amount of culture to the hot milk, we possibly killed or highly reduced the live bacteria in our culture, as a result other types of bacteria from the environment or from the milk got chance to reproduce and became the dominant bacteria in that sample.
Not getting result in the first experiment shows that yogurt bacteria cannot grow or reproduce efficiently at room temperature.
Yogurt tastes slightly sour and it becomes sourer as time passes. This indicates that yogurt bacteria are producing some acid. Acids coagulate milk proteins, but in yogurt this process happens very slowly, so water gets trapped in a structure of coagulated proteins. And that’s what makes yogurt to be like solid. Otherwise the water contents of our product do not change, as milk becomes yogurt.
The milk used to make yogurt contains a higher concentration of solids than normal milk. By increasing the solids content of the milk, a firm, rather than soft, end product results. Addition of nonfat dry milk or boiling milk for a longer time is the easiest at-home method for doing this.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
References:
Visit your local library and find some food related books that also discuss yogurt and other dairy products.
Yogurt Making Recipe
This is my yogurt making recipe after some research and some experiments.
Yogurt is made by inoculating certain bacteria (starter culture), usually Streptococcus thermophilus and Lactobacillus bulgaricus, into milk. After inoculation, the milk is incubated at approximately 110°F ± 5°F until firm; the milk is coagulated by bacteria-produced lactic acid. Since this process happens gradually, water traps inside a structure of coagulated proteins.
Making yogurt at home is fun and less expensive than buying it. It can be made with ordinary kitchen utensils. Following is the materials and directions necessary for making yogurt.
Starter Culture
Any non-pasteurized plain yogurt contains live bacteria for making yogurt and can be used as starter culture.
Temperature
For rapid coagulation and a good taste, temperature must be maintained in about 110°F ± 5°F.
Ingredients
1 gal skim milk
10 oz dry milk
Method for making yogurt
- Mix one gallon milk with 10 oz dry milk
- Heat this milk to boiling and cool immediately to 110°F, discard any skin that may have formed on the milk. Heating the milk to boiling kills any undesirable bacteria that might be present and also changes the properties of the milk protein so that it gives the yogurt a firmer body and texture.
- To one gallon of milk, add one cup of warm 110°F starter culture, Mix well but gently. Do not incorporate too much air. If too much air is mixed in, the starter culture will grow slowly.
- Pour milk into clean containers, and cover with lid. Incubate filled containers at 110°F. Do not stir the yogurt during this period. A Styrofoam box with light bulb may be used as an incubator.
- Maintain 110°temperature until the milk coagulates with a firm custard-like consistency (3-6 hours). Then refrigerate. You may keep yogurt in the refrigerator for two to three weeks.
