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Where's the Fat?

How much fat is in the junk food you eat?

Students learn extraction procedures by separating fat from a snack food. They analyze the effectiveness of their chemical extraction procedure by comparing their actual results with their predicted results based on the snack food's label information.

Materials

Although your students can perform this classroom experiment with existing laboratory supplies, our " Where's the Fat" kit makes things simple for you.  The materials included in this kit are sufficient for 30 students working in pairs. The materials listed are to be used in this experiment only. Carolina Biological Supply Company disclaims all responsibility for any other uses of these materials.

Materials included in this kit are:

  • 15 calibrated 1-mL plastic transfer pipets
  • 15 rubber stoppers
  • Bottle of petroleum ether (125 mL)

Other materials needed, but not provided:

  • 15 watch glasses
  • 30 test tubes (13 X 100 mm)
  • Several beakers
  • Mortar and pestle
  • Snack food

Laboratory safety

Petroleum ether is volatile, so be sure to keep it away from fire or sparks. For detailed safety information, read the MSDS included in this kit.

Discussion

The science of separating substances is essential in the production of materials. Although impurities may not be a problem in some cases, they lower the quality of the final product and can make foods, beverages, and medicines dangerous to consume. Separation science also increases production efficiency by enabling manufacturers to separate and reuse materials important to producing finished goods. Separation and reuse of component materials can also reduce waste and help conserve landfill space. In this laboratory exercise, students examine a process used to separate substances—extraction.

Fat is soluble in nonpolar solvents, such as petroleum ether, and may be extracted from snack food by crushing the food and mixing it with the solvent. The solvent, which may contain suspended solid particles, is transferred by pipet to another container. The solid is allowed to settle to the bottom, and the supernatant liquid (liquid above a precipitate) is removed by decanting (pouring). The solvent is evaporated, the extracted fat weighed, and the efficiency of the separation determined. Typical results are tabulated on the following chart.

Hint:

Thin foods that allow complete penetration of the solvent, such as potato chips and corn puffs, work best.

Snack food % of fat extracted
Corn chips 65
Popcorn 85
Potato chips 90
Corn puffs 95

Procedure

Name of snack food: ________________________

  1. Mass a sample of snack food (0.45—0.50 g) and crush it using a mortar and pestle. Mass the crushed food. Transfer the crushed food to a test tube.
  2. Add petroleum ether (3 mL) to the test tube.
  3. Stopper the test tube and shake for several seconds.
  4. Remove the stopper and pipet the liquid into the second test tube, using the plastic micropipet. Do not allow any solid to be transferred to the second test tube.
  5. Add another portion of petroleum ether (3 mL) to the first tube. Stopper the test tube and shake. Pipet the liquid into the second test tube, transferring as much liquid as possible.
  6. Allow the liquid in the second test tube to sit for 2 to 3 min.
    While you are waiting, mass a watch glass.
  7. Decant the supernatant liquid in the second test tube onto the watch glass. Fill a beaker with hot tap water and place the watch glass on the beaker in the hood to evaporate the solvent (5 to 10 min).
  8. Wipe the condensation off the bottom of the watch glass and mass the watch glass and the extracted fat.

Experimental data

Name of snack food: ________________________

  1. Mass of crushed snack food
  2. Mass of watch glass
  3. Mass of watch glass and extracted fat
  4. Mass of extracted fat
  5. Observed percent of fat in snack food
    (mass of extracted fat/mass of crushed snack food) X 100%
  6. Theoretical percent of fat in snack food derived from
    label information (mass of fat per serving/mass of serving size) X 100%
  7. Percent of snack food fat extracted
    (Answer from 5/ Answer from 6) X 100%

Lab questions

  1. Was all the fat in the snack food extracted? If not, why? How could the experiment be changed to recover more fat?
  2. Why was only the fat extracted from the snack food?
  3. Why were the petroleum extracts combined in the second test tube?
  4. How would the separation efficiency change if you changed the volume of solvent from 3 mL to 0.3 mL? Why?
  5. How would the separation efficiency change if you crushed the snack food into very fine particles instead of large pieces? Why?

Answers to lab questions

  1. To recover more fat, grind the food into a fine powder and increase the amount of solvent used, or increase the number of extractions, or both.
  2. Only the fat was extracted from the snack food because fat is the only part of the snack food soluble in petroleum ether.
  3. The petroleum extracts were combined in the second test tube in order to allow the solid bits of food to settle. In this way the liquid was made free of solid particles.
  4. If you decreased the volume of the solvent from 3 mL to 0.3 mL, the separation efficiency would decrease because the amount of fat would exceed its solubility in the petroleum ether. The excess fat will not dissolve nor be extracted.
  5. Crushing the snack food into fine particles increases the surface area of the food and exposes the fat to solvent. More fat will dissolve in the petroleum ether, and the separation efficiency will increase.
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