Poultry Science: Extension & Outreach: Soybeans

Protein Dispersibility Index (PDI)
Measure of Adequately Processed Soybean meal

Procedure for PDI:
American Oil Chemists Society, 1980. Protein Dispersibility Index. Official Method
Ba 10-65. American Oil Chemists Society, Champaign, IL

Definition
This method (A.O.C.S. Official Method Ba 10-65) determines the dispersible protein in soybean products under the conditions of the test. In contrast to the alternate slow stir method for Nitrogen Solubility Index (NSI), the faster stirring technique used in this method will give generally higher results than those obtained by the slow stir method.

Apparatus

1. Sorvall Omni-Mixer or Hamilton Beach Drinkmaster (modified to accommodate Waring Blender blade cup); standardized to the point where the rotating shaft shows 8,500 rpm with adequate amount of water used in the cup.
2. Centrifuge. Capable of 2,700 rpm delivering 1,400 relative centrifugal force at the tip with 50 ml tubes or any equivalent.
3. Standard Kjeldahl equipment for protein analysis.

Reagents

1. Distilled water, neutral.
2. Standard Reagents as used for protein determination.

Standardization of Blender

a. Sorvall Omni-Mixer

1. Measure 150 ml of distilled water into the mixer cup and place in position on the mixer.
2. Remove cap which cover the tope of the drive shaft. Using the proper tip, place tachometer in position on rotating shaft.
3. Gradually increase the built-in rheostat setting until the shaft shows 8,500 rpm on the tachometer in position on rotating shaft.
4. Standardize the blender before each series of tests to eliminate errors due to possible fluctuation in line voltage.

b. Hamilton Beach Drinkmaster, modified

1. Measure 300 ml of distilled water into the blender cup and place in position on the mixer.
2. Remove chrome cap which covers the top of the drive shaft. Using the proper tip, place tachometer in position on the rotating shaft.
3. With the switch in high position, gradually increase the transformer setting until the shaft shows 8,500 rpm on the tachometer. Note voltmeter reading and transformer setting and use for blending of sample.
4. Standardization of the blender should be done before each series of tests.

Procedure

a. Sorvall Omni-Mixer

1. Weigh accurately the required amount of soy product (depends on the dispersibility in water, for raw bean – use 8 g), record weight.
2. Transfer the weighed sample quantitatively to the mixer cup containing a small portion of the total distilled water. Stir with a spatula to form a paste. Add remainder of the distilled water with stirring to form a smooth slurry, using last of the water to rinse the spatula and mixer cup walls. Total amount of distilled water used in 150 ml.
3. Place up in position for blending.
4. Blend at 8,500 rpm for 10 minutes.
5. Remove mixer cup and pour the slurry into a beaker. Allow the slurry to stand for a few minutes. After the slurry has separated, decant a portion (approximately 40 ml) into a 50 ml centrifuge tube and centrifuge for 10 minutes at 2,700 rpm.
6. Pipette an adequate amount of supernatant (2 ml if an 8 g sample is used) into a micro-kjeldahl flack, and then determine protein (dispersible protein). Also using Kjeldahl determine the total protein for the original soy product.

b. Hamilton Beach Drinkmaster

1. Weigh 16 g soy product.
2. Fill a 300 ml volumetric flack with distilled water at 25°. Pour about 50 ml of the water into the Blender cup. Transfer the weighed sample to the blender cup. Stir with a spatula to form a paste. Add remainder of the water to rinse spatula and blender cup walls.
3. Place the cup in position for blending.
4. Blend at 8,500 rpm for 10 minutes.
5. Remove mixer cup and pour slurry into a beaker. After the slurry has separated, decant a portion (approximately 40 ml) into a 50 ml centrifuge tube and centrifuge 10 minutes at 2,700 rpm.
6. Pipette an adequate amount of supernatant (2 ml if 16 g of soy sample is used) into a micro-kjeldahl flask, and determine protein (dispersible protein). Also determine the total protein in the original soy product.

Calculation

% Protein Dispersibility Index (PDI) = (% Dispersible Protein / % Total Protein) x 100

Note: Water dispersible protein is related to temperature, so the water temperature and the blender cup should be maintained at room temperature. Two single determinations performed in one laboratory should not differ by more than 4.375.

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