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Poultry Science: Extension & Outreach: Soybeans

KOH Protein Solubility

Described by: Araba M. and N. M. Dale. 1990. Evaluation of protein solubility as an indicator of overprocessing of soybean meal. Poultry Science 69:76-83.

Whittle, E. and M. Araba, 1992. Sources of variability in the protein solubility assay for soybean meal. J. Appl. Poultry Res. 1:221-225.

Procedure:
Place approximately 1.5 g of sample in a 250 mL beaker. Important: sample should be finely ground - # 60 mesh
Add 75 mL of 0.2% KOH solution.

Stir mixture for 20 minutes at 22o C at 75% of maximum velocity using Corning Model PC-351 stir plate with a magnetic stir bar 3.6 cm in length.
At end of 20 minutes, approximately 50 mL of liquid is collected into centrifuge tubes.

Centrifuge (Beckman Model GPR) at 1250 x g for 10 minutes.
Collect a 15 mL aliquot of the supernatant.

Perform macro Kjeldahl procedure on 15 mL aliquot (mg N/mL setting; input 15 as the sample size) and original sample (percent protein setting) to determine crude protein level.

Calculate protein solubility as percentage of total in the original sample.

Solution preparation:
KOH Solution Recipe (weight/volume)

0.2% KOH = 0.2 g of KOH in 100 mL deionized distilled water. Mix in hood.
0.5% KOH = 0.5 g of KOH in 100 mL deionized distilled water. Mix in hood.

Calculation Example:

Sample weight - 1.5072 g
Liquid Kjeldahl value - 1.125 mg N/mL
Dry Kjeldahl value - 47.47% protein

1.125 mg N x 75 (how much KOH added) = 84.4 mg N = 55.98 mg N = 5.598 g N
mL 1.5072 g mL 100 g sample

5.598 g N x 6.25 = 34.99% CP

(34.99/47.47) x 100 = 73.68% solubility

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