An acidic solution of monosulfonic azo dye, Acid Orange 12, reacts with basic groups on proteins to form an insoluble complex. The protein dye-binding capacity is genetically controlled and is essentially constant for any given protein system. The estimate of protein content is based on a colorimetric measurement of unbound dye.


Applicable to beef, pork, lamp and other meat, fish or poultry products. The dye does not bind with non-protein nitrogen.

Apparatus and Reagents

UDY Model MT-DP Protein Analyzer including an UDY Colorimeter, Reagent Dye Dispensor, Electronic Balance, Blender, and a Syringe-Pipet along with standard Reagent and Reference Dye Solutions.


1. Prepare a 10-fold dilution by blending about 20 g of product with fixative diluent solution(one part methanol and two parts 2.0% citric acid monohydrate). The amount of diluent to use is nine times the actual weight of sample. Blend until homogeneous (2 to 5 min). If desired, this can be preserved with UDY-Pol for a shelf-life of several weeks at 20C.

2. Set 5 ml Syringe-Pipet to deliver 2.25 or 3.00 g of homogenate as appropriate for protein level expected. Fill by pumping with rapid short strokes. This will mix the homogenate and simultaneously remove air bubbles from the Syringe-Pipet.

3. Dispense homogenate aliquot(s) into a tared 60 ml Sample Bottle. Use 3.00, 4.50, or 6.50 g of homogenate depending on the expected protein level, and set colorimeter accordingly. Contact UDY Corporation for settings. Actual weight can vary by 5% of specified weight. (See Note 1).

4. Add 40.00 ml (40.37 "0.08 g) of Reagent Dye Solution. Cap and shake vigorously for 15 to 20 seconds. Verify that longer times do not make a difference in reading. (See Note 2).

5. Adjust temperature to 25C or make correction to reading. (See Table III).

6. Place a 19 mm fiberglass filter disk in a dropper cap; then filter 25 to 30 drops into the cuvet of preset UDY Colorimeter. Take reading, and make weight and temperature corrections as needed.


g Sample %Protein Range ml Sample A CI Po
3.00 13.5 - 23.8 2.89 34.9 1.231 19.9
4.50 8.4 - 15.4 4.33 24.0 1.191 12.7
6.00 5.8 - 11.3 5.77 18.6 1.154 9.2


NOTE 1: When actual weight of aliquot(s) varies more than 0.02 g from that specified, multiply % Protein reading by specified weight and divide by actual weight.

NOTE 2: Longer shaking times may be needed for cooked products. To verify adequate reaction time, retest using a longer shaking time.