One approach to verifying that undeclared GMOs in raw materials are compliant with legal thresholds is to test for GMOs in the manufactured product and then extrapolate the GMO content in the ingredient using the pro-rata recipe proportions. This is an attractive approach for enforcement testing which does not always have access to the raw materials.
The authors of this paper (purchase required) investigated the inherent bias in this approach. They prepared in-house model processed foods (heat-treated soybeans) containing GM events and then tested them using a GMO quantification method incorporating taxon-specific real-time PCR with longer amplicons They observed that the extrapolated GMO content increased with the length of the amplicon used in the taxon-specific PCR assay. When a longer amplicon was deliberately employed, the GMO content calculated for the processed food always exceeded that expected by pro-rata calculation from the raw material.
They conclude that this finding can be used to advantage. The use of longer amplicons in taxon-specific PCR can lead to an overestimation of GMO content at the raw material stage based on the measurements from processed foods. If the overestimated value remains below the labelling threshold, the appropriateness of GMO labelling can still be confirmed. The proposed method offers a simplified and practical screening approach for use in routine inspections.
