Protein (Dumas) Test

Method description

The sample is completely combusted at high temperatures in the presence of pure O2 (99.9%). Combustion products are H2O, CO2, NOx, and N2. Water vapor, oxygen, and CO2 are removed from the resulting gas stream by a series of thermoelectric coolers and chemical sorbents. NOx is reduced to N2 when passed through a column packed with hot copper. The resulting N2 is passed to a thermal conductivity detector where it is quantified (helium is the carrier gas). Protein is then calculated from the nitrogen content using a published conversion factor (e.g. 5.70 for wheat).

Acceptable matrices

This method is universal for the determination of nitrogen content and can be used for nearly all food products.

Unacceptable matrices

Pure oils and shortenings cause fires in the instrument and must be assayed by the Kjeldahl method.

Limit of quantitation

0.125% Nitrogen
0.648% – 0.798% Protein (factor dependent)

Equipment

Protein/Nitrogen by combustion analyzer.

Method reference

AOAC 992.15; AACC 46-30

Reportable unit

% Protein

Sample size requirements

20 grams

Additional information

Available protein factors:

• 6.25 – general-purpose food products/ingredients (default) except as noted below
• 5.70 – wheat, wheat flour, and wheat flour bread
• 6.38 – dairy
• 5.46 – peanuts and brazil nuts
• 5.18 – almonds
• 5.30 – other tree nuts (coconuts, butternuts, cashews, chestnuts, hazelnuts, hickory nuts, pistachios, walnuts, and pecans)