Comparing and validating methods for measuring protein content in wheat

Comparison of protein methods

Protein content continues to be an important quality factor for producers and exporters, affecting both the functionality and marketability of grain. We calculate protein content in grain by measuring the amount of nitrogen in a sample and multiplying it with a nitrogen-to-protein conversion factor. The reference method for determining protein content in wheat at the Canadian Grain Commission was Kjeldahl analysis until combustion nitrogen analysis (CNA) was implemented in the 1990s. Compared to the Kjeldahl method, CNA is more precise, takes less time, does not use corrosive chemicals and is more commonly used in labs worldwide. Kjeldahl analysis and CNA both require samples that have been ground. In 1992, near-infrared transmittance (NIT) instruments were introduced at the Canadian Grain Commission for the protein analysis of whole grains. These instruments are calibrated against CNA results, allowing us to predict protein content from NIT data using the mathematical relationship between NIT and CNA results.

Before CNA was implemented as the new protein reference method for wheat at the Canadian Grain Commission, a study was conducted to compare the results of Kjeldahl analysis and CNA. The study found that CNA was more efficient at recovering nitrogen than Kjeldahl analysis and that protein values obtained using CNA were 0.15% to 0.25% higher. The Canadian Grain Commission used these results to calculate wheat protein in the years following the study. In 2022, we conducted a new study to verify the accuracy of our protein calculations for each method of analysis.

Validating protein calculations

We analyzed 19 samples of CWRS wheat that had a protein content ranging from 11% to 15.5% using CNA (AACC International Method 46-30.0), Kjeldahl analysis (AOAC Method 2001.11), and NIT spectroscopy (Foss InfratecTM 2019). Whole grain samples of wheat were divided into equal portions using a Boerner divider. The samples were first analyzed using NIT and then ground using a Perten Labmill 3100. Ground samples were further divided into test portions using a rotary divider and duplicate samples were evaluated using CNA and Kjeldahl analysis. We repeated our analysis on 3 different days.

We found that there was a Kjeldahl and CNA results gave similar results across the range of protein values (Figure 1). Based on our statistical analysis of the data, we can calculate Kjeldahl protein results from CNA results using the following equation:

• Kjeldahl protein = (0.9891 × CNA protein result) - 0.2162

Our statistical analysis also showed that there was good agreement between NIT and CNA results (Figure 1).

The results of our study confirm that our calculations of wheat protein based on CNA and Kjeldahl analysis were accurate. In addition, our NIT results indicate that the monitoring and maintaining of our NIT calibrations were valid and resulted in accurate protein calculations. Every crop year, we will again verify that the calculations used to determine protein are accurate for each method of analysis used at the Canadian Grain Commission.