Ensuring the quality of durum wheat products with new DNA-based methods

Canada exports durum wheat to many different countries, where it is often coarsely milled into semolina used to manufacture pasta with a bright yellow colour and firm texture. A team of researchers from our Microbiology and Grain Genomics, Grain Biotechnology, and Bread Wheat and Durum Research programs has developed new DNA-based methods to identify and quantify trace amounts of other grain contaminants in semolina and pasta made from durum wheat.

A crop of durum wheat can sometimes become contaminated with trace amounts of other grains during handling, such as bread wheat and soybeans. Grain inspectors visually identify contaminant grains by their size, shape or colour. Canada’s grain grading system sets limits on the amount of contaminant grain allowed in bulk loads, which helps ensure that Canadian grain meets the expectations of buyers and exporters.

Once grain is ground into flour or made into other food products, it becomes difficult to detect the presence of contaminant grains because they occur at very low levels and are no longer visually distinct. Although the limit for bread wheat and soybeans in the highest grade of Canadian durum wheat is 2% and 0.2%, respectively, their presence at even lower levels in semolina might still affect its quality and safety. Some consumers may have strict requirements for the colour or texture of pasta and the purity of durum wheat can be a concern to consumers with allergies. In these circumstances it can be beneficial to test for contaminants at a molecular level.

Based on previous work on wheat genotypes by the Canadian Grain Commission’s Grain Research Laboratory, the research team identified DNA markers in durum wheat that distinguish it from bread wheat and soybeans. They then prepared samples of durum wheat semolina and ground pasta with known levels of contamination ranging from 0.5% to 55% for bread wheat and 0.01% to 5% for soybeans, based on mass. Droplet digital polymerase chain reaction (ddPCR) techniques were used to calculate the amount of bread wheat and soybean DNA in the samples.

Since durum wheat, bread wheat and soybeans differ in their genome size, grain size and grain composition, the amount of their DNA measured using ddPCR may not accurately reflect their relative abundance by mass in a sample. To account for this, the research team developed correction constants and formulas. When the correction constants were applied, the ddPCR results were consistent with the amount of bread wheat and soybeans known to be present in the durum wheat samples, confirming that these methods can accurately identify and quantify soybean and bread wheat contamination in semolina and pasta made from durum wheat.

The development of these DNA-based methods gives the grain sector, regulators and policy makers new tools and knowledge for the detection of grain contaminants in processed durum wheat products.

Quote

“The methods developed in this study better equip the Canadian Grain Commission and its partners in the grain sector with tools to detect contaminants in milled or processed durum wheat products, such as semolina and pasta.”

Full research article

Lee, S-J, Bamforth, J., Eckhardt, M., Demeke, T. Perry, D., Taylor, D., Wang, K., Fu, B.X. and S. Walkowiak. 2025. Detection of adventitious bread wheat and soybean presence in durum wheat semolina and pasta by droplet digital PCR. Can. J. Plant Sci. 105: 1-8.
https://doi.org/10.1139/cjps-2025-0041