Aoac official method 939 05

32.2.04
AOAC Official Method 939.05
Fat Acidity—Grains
Titrimetric Method
First Action 1939
Final Action
Codex-Adopted–AOAC Method*

A. Reagents

(a) Toluene–alcohol–phenolphthalein solution.—0.02%. To 1 L
toluene add 1 L alcohol and 0.4 g phenolphthalein.
(b) Alcohol–phenolphthalein solution.—0.04%. To 1 L alcohol
add 0.4 g phenolphthalein.
(c) Potassium hydroxide standard solution.—0.0178M, carbonate-free. 1 mL = 1 mg KOH.
B. Apparatus

(a) Grain mill.—Suitable for grinding small test samples.
(b) Fat extraction device.—Soxhlet or other suitable type. (Durable paper thimbles or Alundum RA-360 thimbles are suitable for
extraction.)
C. Method I

Obtain representative test sample of ca 50 g grain (corn, 200 g) by
hand quartering or by use of mechanical sampling device. Preferably
grind test sample so that ≥90% will pass No. 40 sieve (somewhat
coarser grind will not materially affect results). If test sample is too
moist to grind readily, dry at temperature of ca 100°C just long
enough to remove excess moisture.
Extract 10 ± 0.01 g test portion with petroleum ether ca 16 h in extractor. Start extraction as soon as possible after grinding and never
let ground test sample remain overnight. Completely evaporate solvent from extract on steam bath. Dissolve residue in extraction flask
with 50 mL toluene–alcohol–phenolphthalein solution and titrate

with standard KOH solution to distinct pink, or in case of yellow solution to orange-pink. If emulsion forms during titration, dispel by
adding second 50 mL portion of toluene–alcohol–phenolphthalein solution. End point should match color of solution made by adding
2.5 mL 0.01% KMnO4 solution to 50 mL K2Cr2O7 solution of proper
strength to match color of original solution being titrated. (Add 0.5%
K2Cr2O7 solution dropwise to 50 mL H2O until color matches. Then
add 2.5 mL 0.01% KMnO4 solution.)

Make blank titration on 50 mL toluene–alcohol–phenolphthalein
solution and subtract this value from titration value of test sample. If
additional 50 mL portion toluene–alcohol–phenolphthalein solution
was added, double blank titration. Report fat acidity as mg KOH required to neutralize free fatty acids from 100 g grain (dry basis).
Fat acidity = 10 × (titration − blank)
References: JAOAC 22, 526(1939); 23, 493(1940);
24, 587(1941); 50, 198(1967); 71, 84(1988).
D. Method II—Rapid Method for Corn

(Result may be obtained in <1 h.)
Prepare test portion as in C. Weigh 20 ± 0.01 g into 100 mL
glass-stoppered flask or bottle. Add exactly 50 mL benzene, insert
stopper, shake few s to saturated air in flask with benzene vapor, momentarily loosen stopper to release pressure, and replace stopper.
Shake flask 30 min in mechanical shaker, or periodically by hand
45 min. Tilt flask and let meal settle at an angle ≥3 min. Carefully decant as much liquid as possible into 15 cm folded paper inserted in
8 cm glass funnel, and cover funnel with cover glass to reduce evaporation. Collect exactly 25 mL filtrate in 25 mL volumetric flask.
Transfer this filtrate to 250 mL Florence flask. Refill volumetric
flask to 25 mL mark with alcohol–phenolphthalein solution and
transfer to flask containing benzene extract.
Using color standard prepared as in C, titrate extract with standard
KOH solution to distinct pink in case of white corn, and to orange-pink
for yellow corn. If emulsion forms during titration, dispel by adding
25 mL each of benzene and of alcohol–phenolphthalein solution. Determine blank titration on mixture of 25 mL benzene and 25 mL alcohol–phenolphthalein solution. If additional benzene and alcohol were

added, double blank titration. Report fat acidity as mg KOH required to
neutralize free fatty acids from 100 g corn (dry basis).
Fat acidity = 10 × (titration − blank), calculated on dry basis
References: JAOAC 23, 493(1940); 24, 587(1941).
* Adopted as a Codex Defining Method (Type I) for titrimetry of fatty
acidity in wheat flour.

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