Just How Greasy Are Those French Fries?
If you are observant, you have probably noticed that the paper bags in which oily or greasy fast foods are stored have transparent grease spots. You might notice the same effect if you drop French fries on your homework. The presence of those spots is usually caused by the presence of fats in the food. In this experiment you will extract fat from a food sample by dissolving the fat in a fat-soluble solvent, namely acetone. You may be surprised at the amount of fat in just a few French fries!
WHAT YOU NEED
1. 2 Sheets of Plain White Paper, one for the experiment and one to write down your data
2. Acetone
3. Glass Jar with Pipette Lid
4. Paper Towels
5. A Watch
6. A Scale
PROCEDURE
1. Put on your lab safety glasses and an apron. If there are rubber gloves, you should also use them.
2. Select 4-5 French fries and place them on one sheet of paper. Allow them to sit there for 30 seconds.
3. Observe the appearance of the paper as a result of the fat. Write down what you see.
4. Use the scale to record the mass of the group of French fries, making sure to include the decimal place!
5. Tear the French fries into 1 inch long pieces, and place them into the glass jar.
6. Using a beaker, transfer acetone from the large plastic bottles into your jar with the French fries. Use enough acetone so that the fries are completely covered. What color is the solvent?
7. Using the dropper in the jar lid, place a drop of the clean solvent on a piece of white paper. After the acetone evaporates, examine the area where the drop was. What do you see?
8. Place the lid onto the jar and swirl the acetone around in the jar, opening the lid to vent frequently! Taking turns in your group and venting often, gently swirl the mixture for no less than 5 minutes. What do you see happening to the solvent? What is the final color of the solvent? Write this information down.
9. After 5 minutes, use the dropper in the lid to extract a small amount of the fat-saturated solvent and place it on the white paper. Wait till the acetone evaporates, and examine the area where the drop was. What do you see?
10. Carefully pour out the remaining solvent from the French fry jar into the large waste beaker, making sure not to loose any French fries. When the jar is empty, add a little bit more acetone, swirl for a few seconds, and dump the remaining solvent into the waste to ensure that all of the fat-saturated solvent is removed.
11. Carefully remove the French fries from the jar, and place them on a paper towel. You may need to try a few techniques to remove the fries, but be careful because if you break them your data will not be as accurate. Allowing the French fries to sit on the towel for a few minutes will allow the acetone inside of them to evaporate off, giving you more accurate data. You can make this go faster by turning the fries every now and then, and moving them to dry parts of the paper towel. This should not take more than 4 minutes.
12. After the French fries are dry, use the scale to record the new mass of the group of French fries. What has happened to the mass? Why?
13. Assuming that all of the mass lost was due to the fat being extracted from the French fries and solubilized by the acetone, calculate the percentage of fat that WAS in the fries.
HOMEWORK QUESTIONS
In addition to all of the questions above, please answer the following questions to the best of your ability. This assignment will be due on Friday, and will be graded!
Assuming that Sonic French fries are about 12% fat, by what percentage did you value differ from the value reported by Sonic?
What are some of the reasons why the value you calculated were less than that of Sonic’s (12%)? Give at least 3 reasons.
Why do you think I recommend ripping the French fries into small pieces for testing?
We used acetone as our solvent. What are 3 other common uses for acetone?
BONUS QUESTION
This one is very tricky, but whoever gets it right will get a surprise! Why did the acetone appear cloudy after mixing with the French fries for 5 minutes? *HINT* It has to do with the size of the fat molecules!