Testing for Lipids
Name: ____________________
Date: ___________ Period: ___
Introduction
Lipids include fats, oils, waxes and sterols. Simple lipids are composed of a glycerol backbone with
three hydrocarbon chains (a chain made of hydrogen and carbon) extending from the glycerol molecule. The
structure of the hydrocarbon chains determines whether the lipids will be solid (a fat) or liquid (an oil) at
room temperature. More complex lipids form interlocking rings waxes and sterols. Examples of sterols
include cholesterol and estrogen.
It is more difficult to test for lipids than carbohydrates. A simple test is the “brown paper test”.
When lipids soak into brown paper it becomes translucent (think of a greasy paper bag). Lipids can also be
stained by a chemical called Sudan IV. Lipids molecules absorb the Sudan IV stain. The best way to
observe the staining is under a microscope on low power.
In this experiment, we will chemically extract the fat from Cheetos Cheese Puffs and compare the
amount of fat extracted from the amount indicated on the nutrition label. We will confirm that the substance
obtained from the extraction is a lipid by using the brown paper test and Sudan IV stain.
Methods:
1. Obtain a sample of Cheetos or Potato Chips (0.45-0.50g) and crush it using a mortar and pestle.
Mass the crushed food and transfer it to a test tube.
2. Add 3.0 ml petroleum ether (Caution: petroleum ether is extremely flammable!!!) to the test tube.
3. Stopper the tube and shake for 30 seconds.
4. Remove the stopper and pipet the liquid into a second test tube. Do not allow any solid to be
transferred to the second test tube.
5. Add another 3.0 ml to the first test tube. Stopper the tube and shake again for 30 seconds. Pipet the
liquid into the second test tube, transferring as much liquid as possible into the second tube (without
any solid).
6. Let the second test tube sit undisturbed for 3-4 minutes. While waiting, thoroughly clean the first
test tube and mass a watch glass.
7. Decant the supernatant liquid in the second tube into the watch glass. Set the watch glass in a safe
place so the petroleum ether can evaporate. (With teacher permission, you may set the watch glass
over a beaker of hot water to speed the evaporation process.)
8. Make sure the watch glass is dry and mass the watch glass and the extracted fat.
Data:
Table 1: Calculating the Mass of Fat Extracted From Snack Foods
Measurement
Mass (g)
Mass of crushed food
Mass of watch glass
Mass of watch glass and extracted fat
Mass of extracted fat
Experimental percent of fat in the snack food
(mass of extracted fat/mass of crushed food) x 100%
Theoretical percent of fat in snack food
(mass of fat per serving/mass of serving size) x 100%
Percent of fat extracted from snack food
(Experimental percent/theoretical percent) x 100%
C. Gay 3/02 Adapted from Where’s The Fat Carolina Science and Math
http://www.carolina.com/apscience/biology_fat.asp.
Table 2: Class Data: the Mass of Fat Extracted From Snack Foods
Initials
Snack Food
% Fat Extracted
Figure 1: Class Data: the Mass of Fat Extracted From Snack Foods
9
8
7
6
Frequency 5
4
3
2
1
50
55
60
65
70
75
80
Percent Fat Extracted
85
90
95
100
Discussion Answer on a separate piece of paper
1. Was all the fat in the snack food extracted? If not, why? How could the experiment be altered to
recover more fat?
2. Why was only the fat extracted from the snack food?
3. Why were the petroleum extracts (the liquids) combined in the second test tube?
4. Would the percentage of fat extracted change if the snack food was crushed into a fine powder
instead of large pieces? Why?
5. What is the role of fats in living organisms?
6. Why does an organism (such as humans) require body fat in order to survive?
Bonus: What are the physical and chemical differences between saturated fats and unsaturated (or
polyunsaturated) fats? Why are some fats hydrogenated (and what does that mean)?
C. Gay 3/02