A solution of Acid Orange 12 dye is mixed with a weighed portion of sample and the dye reacts with basic amino acid groups in proteins to form an insoluble complex. The protein is determined by colorimetric measurement of unreacted dye remaining in solution. The protein's dye binding capacity is essentially constant and true protein readings may be obtained. Whey and whey products, especially US concentrates, vary in non-protein nitrogen composition. Therefore, for results comparable to Kjeldahl measurements (measurement of true protein plus average NPN), requires settings that vary with the product/process. Once instrument settings have been established for a process, the normally remain constant.


Applicable to all whey products by variation in sample size and use of instrument settings determined from comparable samples.

Apparatus and Reagents

UDY Model MT-DP Protein System including an UDY Colorimeter, Reagent Dye Dispensor, Balance, Reagent Dye Solution and other accessories.

Samples containing over 4% protein can be tested without preliminary dilution if a Balance with 0.001 g accuracy is used for weighing. Samples below 4% protein and dilutions of samples above 4% protein can be tested using a Balance with 0.01 g accuracy.

Dilute samples, if required, by weighing (to 0.01 g) 10 to 12 g of sample in a 120 ml Sample Bottle. Add nine times the sample weight of distilled water (" 0.10 g) and mix thoroughly. While testing the diluted samples when the UDY Colorimeter Range switch is in the HIGH position (read out to 0.1% Protein), the readings obtained are the original sample composition. When the Range switch is in the LOW position (read out to 0.01% Protein), the readings are the percent protein in the diluted sample.

Test using appropriate sample size and UDY Colorimeter settings based on expected protein content and sample source as follows:

A. Dispense aliquot of well mixed sample into tared 60 ml Sample Bottle and weigh. If dispensing the correct sample (within 0.2%) is not convenient, dispense sample to within 5% of the desired weight and make a weight correction to the subsequent reading (Reading times desired weight divided by actual weight).

B. Add 40.0 ml (40.37 " 0.08 g) Reagent Dye Solution and cap bottle with solid cap.

C. Shake sample vigorously long enough to complete reaction. Liquid samples usually require 5 to 15 seconds. Powdered samples which dissolve quickly usually take 10 - 15 seconds after all lumps have broken up. Difficult to dissolve or wet samples may require a vigorous mechanical shaker.

D. Measure sample/dye mixture temperature. Adjust to 25C or correction to ready may be required.

E Place dropper cap with a fiberglass filter inserted in the cap of sample bottle.

F. Invert bottle and gently squeeze 25 to 30 drops of liquid into the cuvet of previously set UDY Colorimeter.

G. Obtain colorimeter reading and make weight and temperature corrections, if needed.

See Instruction Manual for additional information and guidelines for individual steps in the procedure.