Acidity AACC Method 02-01APage 1 of 2

Fat Acidity—General MethodFinal approval October 3, 1984; Reapproval November 3, 1999

ObjectiveNatural fats are mixtures of esters of fatty acids with glycerol. They are

essentially neutral. Unfavorable storage conditions may cause partial hydrolysisof the glycerides. The resultant free fatty acids increase the acidity, which is anindication of deterioration in quality. This method determines total fatty acids ingrain and grain products by a general titrimetric procedure.

Apparatus1. Laboratory mill, suitable for grinding small samples of grain so that 90% or

more will pass through no. 40 U.S. Standard sieve.2. Fat-extraction device; Soxhlet or similar type equipped with double-

thickness paper thimbles (porosity 5 µm).

Reagents1. Petroleum ether; boiling range 35–60°.2. Toluene-alcohol-phenolphthalein (TAP) solution. Mix equal parts by vol-

ume of chemically pure toluene and 95% ethyl alcohol. Add 0.2 g phenolphthal-ein per liter to form 0.02% solution.

3. KOH. Prepare CO2-free standard solution, 0.0178N (1 ml = 1 mg KOH).

Color standardsThe intensity of yellow color in grain varies, depending on type of grain;

therefore, a color standard is helpful in making titration end points uniform.Prepare as follows:

To 50 ml water in flask of type used for titrating, add dropwise 0.05% potas-sium dichromate until water solution matches in color the grain extract solutionto be titrated. Add 2.5 ml freshly prepared 0.01% potassium permanganatesolution and mix. Color of titration end point should match this standard. Preparecolor standard for titration blank by adding 2.5 ml 0.01% potassium permanga-nate to 50 ml water.

ProcedureFor most accurate results, moisture content of grain should not exceed 11.0%.

Higher percentages of moisture at time of extraction have been found to raise fatacidity values significantly.

1. Grind at least 40 g of representative sample of small grains such as wheat,or 200 g of larger grains such as corn. Preferably grind sample so that 90% ormore will pass through no. 40 U.S. Standard sieve. Once ground, sample must becarried to extraction step within 1 hr to forestall changes caused by lipolyticenzymes.

Acidity AACC Method 02-01APage 2 of 2

Fat Acidity—General Method (continued)

2. Extract 10 g ground sample with petroleum ether in extractor at rate of onesiphoning about every 3 min. Extract approximately 16 hr. If thimble is elevated1 in. above bottom of extracting chamber by a glass rod, extraction time can beshortened to 4 hr. Plug of cotton or glass wool in top of thimble will prevent lossof sample.

3. Evaporate petroleum ether from extract and redissolve extract in extractionflask with 50 ml TAP solution.

4. Titrate extract solution with 0.0178N KOH to end point matching color ofstandard described under Reagents.

5. Determine blank by titrating 50 ml TAP solution to end point matchingcolor of standard for titration blank described under Reagents.

NoteIn the case of grain having high fat-acidity values, emulsions are sometimes

formed during titration, partially masking the end point. When emulsion appears,50 ml additional TAP solution may be added to ensure a clear solution for titra-tion. The blank titration value in this case must be double that determined on thesingle 50-ml portion of solvent.

CalculationReport fat acidity as mg KOH required to neutralize free fatty acids from 100

g grain on dry-matter basis by formula:

Fat acidity value =×

×( – )

–

T B

W

10

100100

where T = ml 0.0178N KOH required to titrate sample extract, B = ml 0.0178NKOH required to titrate blank, W = g water in 100 g of sample.

References1. Baker, D. 1984. The use of toluene in fat acidity methods. Cereal Foods World 29:265.2. Baker, D., Neustadt, M. H., and Zeleny, L. 1957. Application of the fat acidity test as an index

of grain deterioration. Cereal Chem. 34:226.3. Neustadt, M. H. 1942. An improved Soxhlet extractor. Ind. Eng. Chem. 14:431.4. Zeleny, L., and Coleman, D. A. 1938. Acidity in cereals and cereal products, its determination

and significance. Cereal Chem. 15:580.5. Zeleny, L., and Coleman, D. A. 1939. The chemical determination of soundness in corn. U.S.

Dep. Agric., Tech. Bull. 644.