Special Properties of Fats, Oils, and Shortenings
AACC Method 58-16 Page 1 of 3
—Acetic ic AcidAcid-Ch Chlor lorof ofor orm m Determination of Peroxide Value —Acet Method First approval October 26, 1977; Reapproval November 3, 1999
Objective To determine the peroxide value of fats and oils, in terms of meq peroxide per 1000 g of sample. The method is applicable to all normal fats and oils including margarine. It is highly empirical, and any variation in procedure may result in variation in results. This method is equivalent to Official Method Cd 8-53 of the American Oil Chemists’ Society (AOCS).
Apparatus 1. Pipet, Mohr, measuring type, 1-ml capacity. 2. Erlenmeyer flasks, glass-stoppered, 250-ml.
Reagents 1. Acetic acid-chloroform solution. Caution. See Note 1. Mix 3 parts by volume glacial acetic acid, reagent grade, with 2 parts by volume chloroform, U.S. Pharmacopeia grade. See Note 2. 2. KI solution, saturated solution of KI, ACS grade, in recently boiled distilled water. Make sure solution remains saturated as indicated by presence of undissolved crystals. Store in dark. Test daily by adding 2 drops starch solution (see Note 3) to 0.5 ml KI solution in 30 ml acetic acid-chloroform solution. If blue color is formed that requires more than 1 drop 0.1 N sodium sodium thiosulfate solution to discharge, discard KI solution and prepare fresh solution. 3. Sodium thiosulfate solution, 0.1 N , accurately standardized. 4. Sodium thiosulfate solution, 0.01 N , accurately standardized. This solution may be prepared by accurately pipetting 100 ml 0.1 N solution into 1000-ml volumetric flask and diluting to volume with recently boiled distilled water. 5. Starch indicator solution, 1.0% of soluble starch in distilled water.
Procedure for Fats and Oils 1. Weigh 5.00 ± 0.05 g sample into 250-ml glass-stoppered Erlenmeyer flask and add 30 ml acetic acid-chloroform solution. Swirl flask until sample is dissolved in solution. Add 0.5 ml saturated KI, preferably using Mohr type measuring pipet. 2. Allow solution to stand with occasional shaking for exactly 1 min and then add 30 ml distilled water. See Note 4. c onstant and vigorous 3. Titrate with 0.1 N Na Na2S2O3, adding gradually and with constant shaking. Continue titration until yellow color has almost disappeared. Add about 0.5 ml starch indicator solution. Continue titration, shaking flask vigorously near end point to liberate all iodine from chloroform layer. Add thiosulfate dropwise until blue color has just disappeared. See Note 5.
Special Properties of Fats, Oils, and Shortenings
AACC Method 58-16 Page 2 of 3
Determination of Peroxide Value —Acetic Acid-Chloroform Method (continued) 4. Conduct blank determination of reagents daily. Blank titration must not exceed 0.1 ml 0.1 N Na2S2O3 solution.
Calculation Peroxide value as milliequivalents peroxide per 1000 g sample
(S =
−
B )( N )( 1000)
weight of sample
where B = titration of blank, S = titration of sample, N = normality of Na2S2O3 solution.
Procedure for Margarine 1. Melt sample by heating with constant stirring on hot plate set at low heat, or by heating in air oven at 60–70°. Avoid excessive heating and particularly prolonged exposure of oil to temperatures above 40°. 2. When completely melted, remove sample from hot plate or oven and allow to settle in warm place until aqueous portion and most of milk solids have settled to bottom. 3. Decant oil into clean beaker and filter through Whatman no. 4 paper (or equivalent) into another clean beaker. Do not reheat unless absolutely necessary for filtration. Sample should be clear and brilliant. 4. Proceed with test on decanted oil as described in steps 1–4 in Procedure for Fats and Oils (above).
Notes 1. Chloroform is a known carcinogen. It is toxic by inhalation and has anesthetic properties. Avoid contact with the skin. Prolonged inhalation or ingestion can lead to liver and kidney damage and may be fatal. It is nonflammable but will burn on prolonged exposure to flame or high temperature. The threshold limit value (TLV) is 10 ppm in air. A fume hood should be used at all times when using chloroform. Acetic acid in the pure state is moderately toxic by ingestion and inhalation. It is a strong irritant to skin and tissue. The TLV in air is 10 ppm. 2. Isooctane has been proposed as a replacement for chloroform in this method. The method using isooctane is now available as AOCS Official Method Cd 8b-90. The acetic acid-chloroform version of the method is intended for deletion by AOCS. 3. “Potato starch for iodometry” is recommended, because this starch produces a deep blue color in the presence of the iodonium ion. “Soluble starch” is not recommended because a consistent deep blue color may not be developed when some soluble starches interact with the iodonium ion.
Special Properties of Fats, Oils, and Shortenings
AACC Method 58-16 Page 3 of 3
Determination of Peroxide Value —Acetic Acid-Chloroform Method (continued) 4. The test should be carried out in diffuse daylight or in artificial light shielded from a direct light source. A report on a coulometric method for the measurement of peroxide value indicates that the iodide-peroxide reaction is complete at the end of 1 min and that the liberation of iodine is affected by light. 5. If the titration is less than 0.5 ml using 0.1 N sodium thiosulfate, repeat the determination using 0.01 N sodium thiosulfate.
References 1. American Oil Chemists’ Society. 1998. Official Methods and Recommended Practices, 4th ed. Method Cc 8-53. The Society, Champaign, IL. 2. AOAC International. 1995. Official Methods of Analysis of AOAC International, 16th ed. Method 965.33. The Association, Gaithersburg, MD.
