Determination of Nitrogen/Protein in Soybean Meal

Importance of the Topic

Soybean meal is the leading plant‐based protein source in animal and aquaculture feeds. Accurate protein quantification is critical for formulation, nutritional evaluation, and quality control in feed production. Combustion analysis of nitrogen, converted to protein via a calibration factor, ensures reliable assessment of crude protein levels.

Objectives and Study Overview

This study evaluates the performance of the LECO FP828 combustion nitrogen/protein determinator for measuring crude protein in soybean meal, soy flour, and ground soybean samples. Key aims include optimizing sample preparation, selecting carrier gas and aliquot loop size, and demonstrating method precision and accuracy against certified reference materials.

Methodology

Samples were ground to pass through a 1 mm sieve and analyzed as received. Moisture content was determined separately at 80 °C for 2 hours and used to correct results on a dry basis. Nitrogen was measured by combusting approximately 0.15 g of sample in a vertical quartz furnace under pure oxygen. Combustion gases were dried, mixed in a ballast, and a defined gas aliquot (3 cm3 or 10 cm3) was carried by helium or argon to a thermal conductivity detector. Calibration and drift correction employed linear, force‐through‐origin calibration with certified reference materials following AOAC 990.03 and AOCS BA4F‐00 procedures.

Instrumentation

The LECO FP828 Series features pure oxygen combustion, a thermoelectric moisture trap, and selectable aliquot loops. Key parameters:

• Furnace temperature 950 °C and afterburner at 850 °C
• Carrier gas: helium for highest sensitivity or argon for cost reduction
• Aliquot loop sizes: 10 cm3 for low‐range precision, 3 cm3 to extend reagent life
• Detector: thermal conductivity cell
• Accessories: tin foil cups or quick‐cap capsules and spatula
• Reference materials: LECO certified reference materials and NIST standards

Main Results and Discussion

Analysis of soy flour, ground soybeans, and soybean meal under four conditions (helium or argon, 10 cm3 or 3 cm3 loops) yielded consistent nitrogen determinations (RSD typically below 0.5 %) and protein contents matching expected values (39 % to 49 %). Helium provided slightly better sensitivity and lower detection limits, while argon offered cost advantages with marginal loss of precision. The 10 cm3 loop delivered optimal precision at low nitrogen levels. Correcting for moisture on the day of analysis minimized bias from hygroscopic sample behavior.

Advantages and Practical Applications

• Fast, robust combustion analysis with minimal sample handling
• High throughput with automated sample introduction and data processing
• Wide dynamic range enabled by selectable aliquot loops
• Compliance with official methods for regulatory and commercial testing
• Applicability in feed mills, QA/QC laboratories, and research facilities

Future Trends and Applications

Advances may include integration with automated sample preparation, development of alternative low‐cost carrier gas strategies, multi‐element detection in a single combustion run, and enhanced data connectivity with laboratory information management systems. Miniaturization and greener reagents could further reduce operating costs and environmental impact.

Conclusion

The LECO FP828 combustion nitrogen/protein determinator delivers accurate, precise, and efficient protein analysis for soybean meal and related feed materials. Its flexibility in carrier gas choice and aliquot loop configuration meets diverse laboratory needs and supports stringent quality assurance requirements.

References

• AOAC Official Method 990.03 Protein in Animal Feed by Combustion
• AOCS Method BA4F-00 Crude Protein in Soybean Meal by Combustion
• USDA Circular No. 183 Protein Conversion Factors