Introduction: (Initial Observation)
Milk can not be kept for a long time. It spoils and smells very bad. Refrigeration delays spoiling milk. Milk that normally spoils in a few days, can last a few weeks with proper refrigeration. Storing milk in a clean environment will also slow down the spoiling process. It appears that all these considerations can only delay spoiling milk, but they can not stop it completely. Milk is a valuable food and preserving it is important. We are hoping that this research project will help us to know what causes the milk to spoil. When we know what causes the milk to spoil, we can focus on proper ways of preserving milk.
Information Gathering:
Find out about milk properties and benefits. Read books, magazines or ask professionals who might know in order to learn about the factors that may cause the milk to spoil. Keep track of where you got your information from.
Following are some of our gatherings about milk:
Milk is a very good food for both people and bacteria.
From the time the milk leaves the cow till you drink it, it can become contaminated with many different kinds of bacteria–some good, but some bad. At the dairy, the milk is heated (pasteurized) to kill bacteria, and sealed. This kills most of the bacteria, but not all.
When you open the milk at home, new bacteria from the air, from your hands, and from the glass you use can get into the milk. Normally this is OK, because you drink the milk right away. However, if you keep the milk, you are encouraging all kinds of bacteria to grow. You must be careful not to touch, drink or handle the spoiled milk. Depending on how you treat the spoiled milk, they may contain harmful bacteria. A good rule of food safety is that if it smells bad, do not eat or touch it. Please be careful; wash your hands and be careful with the spoiled milk, when you throw it out.
Now about what happens to the milk. As the bacteria grow in the milk, they eat some of the protein and some of the milk sugar. As they eat the sugar, the bacteria produce acid, and the acid causes the milk to clot.
A similar thing happens when you cut your finger; the blood clots. After you keep the milk clot for a while, the clot shrinks and a yellow fluid (whey) is released. You can make this happen more quickly by squeezing a little lemon juice (acid) into a small amount of milk. The curds are the white caseins, or milk proteins, and they are sticky (people once used them as glue).
However, when milk clots we do not know if good or harmful bacteria caused the milk to clot. The bacteria are also partially eating the fat and protein. This releases the bad smell. And since we don’t know which kind of bacteria are at work, PLEASE be careful.
All milk should be promptly refrigerated, otherwise it will turn sour within a matter of hours. Dairy products last longer and taste better when kept cold–at 45°F or below. If the temperature of milk is allowed to reach 50°F, the shelf life is halved.
Milk containers should be sealed, closed, or covered, as milk readily picks up flavors and aromas from other foods. It’s best to leave fresh milk in its original container, where it should keep for three to five days after purchase.
Canned evaporated or condensed milk can be stored at room temperature for six months. Once opened, it should be transferred to a clean, opaque container, covered tightly and refrigerated; the milk should keep for three to five days.
How milk is made?
Milk is produced by the mammary glands of female mammals. Mammary glands are highly specialized sweat glands. The female ability to produce milk is one of the defining characteristics of mammals. It provides the primary source of nutrition for newborns before they are able to digest other types of food.
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.
What causes the milk to spoil? The purpose of this project is to find out what factors cause the milk to spoil, so we can prevent or slow down spoiling milk.
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.
There are three different factors that may accelerate or slow down the spoilage of the milk. These factors are:
- Storage temperature
- Exposure to contamination
- Pre-storage preparation method (None, Pasteurization, Boiling, Chemical preservatives)
Research on each of these factors may be subject of a different science project. You must select one of these factors for your project.
1. Storage Temperature: This is how you define variables if you want to study on the effect of storage temperature.
Independent variables (also known as manipulated variables) is the milk storage temperature.
Dependent variable is the time it takes for the milk to spoil.
Constant is the milk.
2. Exposure to Contamination: This is how you define variables if you want to study on the exposure to contamination.
Independent variables (also known as manipulated variables) is the exposure to contamination. Possible options to try are storage in an open container and storage in a closed container.
Dependent variable is the time it takes for the milk to spoil.
Constant is the milk.
3. Preparation Method: This is how you define variables if you want to study on the effect of preparation method.
Independent variables (also known as manipulated variables) is the preparation method. Possible values are boiling prior to storage and not boil prior to storage.
Dependent variable is the time it takes for the milk to spoil.
Constant is the milk.
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the spoiling process of the milk. Identifying variables is necessary before you can make a hypothesis.
It is important to remember that a true scientific hypothesis must be testable. For example, ask yourself how you would test the following hypotheses.
These are sample hypotheses for the three conditions you may want to test:
- Milk will spoil faster in warmer environment (Tested in experiment 1)
- Milk will spoil more quickly in an uncovered container. (Tested in experiment 2)
- Milk will spoil slower if boiled prior to storage. (Tested in experiment 3)
Remember hypothesis are just what we think now and they may prove to be wrong after our experiments.
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.”
We design three experiments, one for each hypothesis. While performing these experiments, wear latex gloves, cover your hair and wear a filter mask to cover your nose and mouth. These precautions will reduce the chance of contamination to the milk and to your body.
Experiment 1:
Introduction:
This experiment will show if the milk will spoil faster in warmer environment. For this experiment you will need a thermometer, 4 or 5 small clean cups and some fresh milk.
Procedure:
Add the same amount of milk to all cups, put a lid for them and place them in different temperatures. For example place one cup in the refrigerator, another one in a cool place at your home, third one in normal room temperature and the last one in a warm place. Behind the refrigerator and inside the oven when it is off, you may find warm spaces. Make sure you select location and place your samples with the help of an adult and put proper sign so it will not be accidentally knocked off.
Inspect your samples 3 times a day and record the results. Inspect the samples by first smelling them and then tasting them. (if it smells very bad, don’t taste them). Use a small tea spoon, take out a small amount of milk and taste it. Use a different clean spoon for each sample taste and wash the spoons immediately, so milk will not spoil on the spoon and it will be ready for next taste. Make sure you put back the lid immediately after you smell or take out the milk for taste from each sample.
The table that you use to record the results can look like this:
| Sample condition after: | 40º Fahrenheit |
60º Fahrenheit |
72º Fahrenheit |
85º Fahrenheit |
| 4 hours | No bad smell/ Tastes good | No bad smell/ Tastes good | No bad smell/ Tastes good | No bad smell/ Tastes good |
| 8 hours | No bad smell/ Tastes good | No bad smell/ Tastes good | No bad smell/ Tastes good | No bad smell/ Tastes sour |
| 12 hours | No bad smell/ Tastes good | No bad smell/ Tastes good | No bad smell/ Tastes good | No bad smell/ Tastes sour |
| 24 hours | No bad smell/ Tastes good | No bad smell/ Tastes good | No bad smell/ Tastes good | Bad smell |
| 36 hours | No bad smell/ Tastes good | No bad smell/ Tastes good | No bad smell/ Tastes good | Bad smell |
| 48 hours | No bad smell/ Tastes good | No bad smell/ Tastes good | Bad smell | Bad smell |
| 60 hours | No bad smell/ Tastes good | Bad smell | Bad smell | Bad smell |
Data in the above table are fake and offered just as an example, your temperatures and results will vary from this table. Your table will be longer than this because you will continue until the refrigerated milk also becomes sour and smells bad.
Warning: If you have trouble with smelling, be careful and use someone help. Not being able to smell, taste or feel hazardous conditions can put you in a serious risk when you deal with chemicals, bacteria or hot objects.
Need a control?
The sample placed at room temperature is your control. You will compare the speed of spoiling with this sample so you can show that the storage temperature affects the speed of spoiling.
Make a graph
You can make a bar graph to visually present your results. Use one vertical bar for each of the temperatures you test. The height of each bar will show the number of hours it takes for milk to spoil in that temperature.
You may consider the sour taste as the start for spoilage of the milk.
Experiment 2:
Introduction:
This experiment will show if the Milk will spoil more quickly in an uncovered container. For this experiment you will need 2 small clean cups and some fresh milk.
Procedure:
Add the same amount of milk to both cups, put a lid for one of them and leave the other open. Place them next to each other in the normal room temperature. Make sure you select location and place your samples with the help of an adult and put proper sign so it will not be accidentally knocked off.
Inspect your samples 3 times a day and record the results. Inspect the samples by first smelling them and then tasting them. (if it smells very bad, don’t taste them). Use a small tea spoon, take out a small amount of milk and taste it. Use a different clean spoon for each sample taste and wash the spoons immediately, so milk will not spoil on the spoon and it will be ready for next taste. Make sure you put back the lid immediately after you smell or take out the milk for taste from the covered sample.
The table that you use to record the results can look like this:
| Sample condition after: | ||
| 2 hours | No bad smell/ Tastes good | No bad smell/ Tastes good |
| 4 hours | No bad smell/ Tastes good | No bad smell/ Tastes good |
| 8 hours | No bad smell/ Tastes good | No bad smell/ Tastes good |
| 12 hours | No bad smell/ Tastes good | No bad smell/ Tastes good |
| 16 hours | No bad smell/ Tastes good | No bad smell/ Tastes good |
| 20 hours | No bad smell/ Tastes good | No bad smell/ Tastes good |
| 24 hours | No bad smell/ Tastes good | Bad smell |
Data in the above table are fake and offered just as an example, your results will vary from this table. Your table may be longer than this because you will continue until both samples becomes sour and smells bad.
Make a graph
You can make a bar graph to visually present your results. Use one vertical bar for the covered cup and another bar for the open cup. The height of each bar will show the number of hours it takes for milk to spoil in that cup.
You may consider the sour taste as the start for spoilage of the milk.
Experiment 3:
Introduction:
This experiment will show if the Milk will spoil slower if boiled prior to storage. For this experiment you will need 2 small clean cups and some fresh milk.
Procedure:
Boil some of your milk for five minutes. Let it cold while covered. Add some boiled milk to one cup and the same amount un-boiled milk to the other cup. cover both cups with lids. Place them next to each other in the normal room temperature. Make sure you select location and place your samples with the help of an adult and put proper sign so it will not be accidentally knocked off.
Inspect your samples 3 times a day and record the results. Inspect the samples by first smelling them and then tasting them. (if it smells very bad, don’t taste them). Use a small tea spoon, take out a small amount of milk and taste it. Use a different clean spoon for each sample taste and wash the spoons immediately, so milk will not spoil on the spoon and it will be ready for next taste. Make sure you put back the lid immediately after you smell or take out the milk for taste from each sample.
The table that you use to record the results can look like this:
| Sample condition after: | ||
| 2 hours | ||
| 4 hours | ||
| 8 hours | ||
| 12 hours | ||
| 16 hours | ||
| 20 hours | ||
| 24 hours |
You may repeat this experiment one more time, but this time don’t put the lids and leave the samples open. We did not write any sample results in the table because the results are quite surprising. So do it yourself and send a copy of results to your project advisor. Your table may be longer than this because you will continue until both samples becomes sour and smells bad.
Make a graph
You can make a bar graph to visually present your results. Use one vertical bar for the boiled milk and another bar for the un-boiled milk. The height of each bar will show the number of hours it takes for milk to spoil.
You may consider the sour taste as the start for spoilage of the milk.
Materials and Equipment:
Material used for this experiment may vary based on your final experiment design. Following is a sample list of material:
- Cups (Glass, ceramic or plastic)
- Spoons
- Milk
- Thermometer (at least 1 room thermometer)
Note: This project is about natural milk such as cow milk. Juices from vegetables do not provide the same results even if their commercial name includes the word MILK.
Results of Experiment (Observation):
The above tables after completion of your experiments are your results.
Calculations:
No calculations are required for this project.
Summary of Results:
The results of experiments 1 showed that milk spoils faster in warmer temperature.
The results of experiment 2 showed that milk exposed to contamination and not covered properly will spoil faster.
Experiment 3 however showed that a boiled milk may spoil much faster and boiling does not prevent or slow down the spoiling process.
Conclusion:
Since our experiments proved that milk spoils faster in warmer temperature, we have a clear solution to keep milk for a longer time and that is keeping it cold.
Experiments also proved that milk exposed to contamination and not covered properly will spoil faster. So we can store milk on sealed containers to prevent any contact with contamination.
The fact that boiled milk spoiled faster was quite surprising. Boiling kills all bacteria, so we expected the boiled milk to stay fresh for ever or at least for a very long time. Spoiling faster lead us to this conclusion that when natural milk bacteria are all dead, natural environmental bacteria in the air or in our cup will become dominant bacteria and get chance to grow and reproduce faster. That is why a boiled milk may spoil much faster when exposed to any environmental contamination.
Related Questions & Answers:
If you want to continue your research, you can test on the effect of vinegar on the milk. You can also test to see if adding some salt can slow down the spoiling process of the milk. You may even compare non-fat and low fat milk with high fat milk to see which one spoils faster.
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.
