RANCID RESEARCH
PROBLEM:

Tyson Foods has large vats of oil that have been used to fry chicken. It is their concern that the oil may have oxidized or gone bad. They have hired you as a food chemist to determine if the oil is still good.

BACKGROUND INFORMATION:

The extent of oxidation that has taken place in a sample of lipid or lipid fat can be expressed in terms of peroxide value. Peroxides produced by oxidation of the oil are measured using the technique based on their ability to liberate iodine from potassium iodide. Peroxide value is determined by measuring iodine released from potassium iodide. Sodium thiosulfate titration is used for this purpose.

The amount of iodine liberated from KI by oxidative action of peroxides present in the oil is determined by titration in a biphasic system with Na 2 S 2 O 3 solution.

ROOH + 2KI --> I 2 + 2KOH + RO -
Hydroperoxide + potassium iodide --> Iodine + potassium hydroxide + oxidized peroxide

I 2 + Na 2 S 2 O 3 --> S 2 O 3 + 2NaI

Iodine + Sodium thiosulfate --> Sulfur Trioxide + Sodium iodide

Oil samples are dissolved in an acetic acid-chloroform mixture and saturated KI is added and the liberated I 2 is titrated with Na 2 S 2 O 3 using a starch solution as an indicator.

OBJECTIVE:

The student will determine whether or not an oil has become rancid (spoiled) by determining the peroxide value of the oil using titration techniques.

MATERIALS:

  • Magnetic stirrer
  • 250 ml Erlenmeyer flasks
  • Pipettes
  • 50 ml graduated cylinders
  • Acetic acid-chloroform solution (3:2 v/v)
  • Sodium thiosulfate solution (Na 2 S 2 O 3 ) 0.1N
  • Sodium thiosulfate solution (Na 2 S 2 O 3 ) 0.01N
  • Starch indicator (5%, heat to disperse)
  • Oil (Soybean)*
  • Tenox (Butylated hydroxyanisole (BHA) with Butylated hydroxytoluene BHT)*
  • Tertiary butylated hydroxyquinone (TBHQ)*

    *If these compounds are not available to you, part A can be replaced by the extension. The extension can be performed using precooked oils from places of business or home.

    SAFETY AND DISPOSAL:

    Keep acetic acid-chloroform solution under the fume hood at all times. Dispose of the acetic acid-chloroform solution in waste containers in the hood before cleaning. Do not wash these down the drain.

    TIME:

    Preparation time: 11 hours if oxidizing your own oils. If the extension is chosen, there is little preparation time. Students should be asked to bring in precooked oils. If you oxidize your own oils, you do have a basis of comparison regarding time vs. rancidity.

    Activity: 1 class period

    PROCEDURE:

    1. To be completed by the teacher:
      1. Preparation of oil samples (To be completed by the instructor prior to lab):
        • Label Erlenmeyer flasks from 1 - 10 and prepare oil by heating at 110C for 4 hours, 8 hours and 10 hours.
          • _ Sample 1 - Oil control heated for 4 hours
          • _ Sample 2 - Tenox and oil (200 ppm = 200 mg/kg oil) heated for 4 hours
          • _ Sample 3 - TBHQ and oil (200 ppm) heated for 4 hours
          • _ Sample 4 - Oil control heated for 8 hours
          • _ Sample 5 - Tenox and oil (200 ppm)heated for 8 hours
          • _ Sample 6 - TBHQ and oil (200 ppm) heated for 8 hours
          • _ Sample 7 - Oil control heated for 10 hours
          • _ Sample 8 - Tenox and oil (200 ppm) heated for 10 hours
          • _ Sample 9 - TBHQ and oil (200 ppm) heated for 10 hours
          • _ Sample 10 - Oil control not heated
    2. To be completed by the student:
      1. Weigh accurately 2.0 grams of oil sample 1 into a 250 ml Erlenmeyer flask. Repeat this for all other samples (2-10).
      2. Add 30 ml of acetic acid-chloroform solution (3:2 v/v) and swirl the flask until the sample is dissolved in the solution. DO THIS UNDER THE FUME HOOD!!
      3. Add 0.5 ml of saturated potassium iodide solution (KI).
      4. Allow to stand for 1 minute with occasional shaking.
      5. Add 30 ml of deionized water and shake.
      6. Titrate with sodium thiosulfate (Na 2 S 2 O 3 ) solution (use 0.01N solution for oil controls that were not oxidized, and 0.1N solution for oxidized oils) with constant and vigorous shaking. Continue the titration until the yellow color has almost disappeared.
      7. Add 0.5 ml of the starch indicator solution (5%). The solution will turn blue (if oxidized). Continue the titration; add sodium thiosulfate solution drop-by-drop until the blue color has just disappeared (end point). Record the ml of Na 2 S 2 O 3 used (S).
      8. Repeat steps 3-8 with the rest of the samples (2-10).
      9. Run a blank comprising all the solutions but without the oil sample and record the ml of sodium thiosulfate solution (B) used to reach the end point.
      10. Calculate the peroxide value of each oil sample using the following formula as milliequivalents of peroxide per kilogram (1000 g) of sample:

        PV (meq/kg oil) = (S-B) x 1000 x N
        PV (meq/kg oil) = (S-B) x W
        B= Titration of blank
        W= Weight of sample (g)
        S= Titration of sample
        N= Normality of Na 2 S 2 O 3

    Report PV as meq/kg oil

    EXTENSIONS:

  • Students could bring in oil that has been used to fry a food from home or at their place of work (MacDonald's, Wendy's, etc.) to test for rancidity.

    QUESTIONS:

    1. How do antioxidants minimize/prevent rancidity?
    2. Name the essential fatty acid(s).
    3. Oxidative rancidity can affect vitamins A, C, D, E, and K. Why?

    FOR ADDITIONAL INFORMATION CONTACT:

    Dr. Navam Hettiarachchy
    Marlene Janes, Microbiology, Food Science, U of A-Fayetteville,
    mejanes@comp.uark.edu

    REFERENCES:

    A.O.C.S. 1990. Official Method Cd 8-53, American Oil Chemists Society, Champaign, Il.

    Aurand, L.W. and A.E. Woods. 1977. Laboratory Manual in Food Chemistry. AVI Pub. Co. Inc., Westport, CT.

    Pomeranz, Y. and C.E. Meloan. 1978. Food Analysis: Theory and Practice. Revised ed. AVI

    Pub. Co. Inc., Westport, CT.

    SAMPLE NORMALITY OF Na 2 S 2 O 3 VOLUME OF

    Na 2 S 2 O 3

    		PV in meq/kg of oil
    SAMPLE 1

    Oil control

    4 hours

    		     
    SAMPLE 2

    Tenox + oil

    4 hours

    		     
    SAMPLE 3

    TBHQ + oil

    4 hours

    		     
    SAMPLE 4

    Oil control

    8 hours

    		     
    SAMPLE 5

    Tenox + oil

    8 hours

    		     
    SAMPLE 6

    TBHQ + oil

    8 hours

    		     
    SAMPLE 7

    Oil control

    10 hours

    		     
    SAMPLE 8

    Tenox + oil

    10 hours

    		     
    SAMPLE 9

    TBHQ + oil

    10 hours

    		     
    SAMPLE 10

    Oil Control

    Not heated

    		     
       

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