Scientists working within the Government Chemist programme have developed a pioneering new DNA test that, along with mass spectrometry, is able to distinguish between almond and mahaleb - a spice made from the seeds inside a species of cherry stones - to resolve a scientific dispute.
A sample of cumin was referred to the Government Chemist by the Food Standards Agency after it appeared that almond, a species known to be an allergy risk for some individuals, was present in the sample.
Almond is a member of the genus ‘Prunus’ - trees and shrubs, which includes the plums, cherries, peaches, nectarines, apricots and mahaleb. Prunus mahaleb was previously little known in the UK but was said also to have been handled in the cumin supply chain. The Government Chemist was asked if it is possible to tell whether almond or mahaleb or both is present in the sample.
The analytical chemistry and molecular biology of Prunus species in spices are not well represented in the scientific literature. ELISA, the most commonly applied technique for the detection of food allergens, reacts to several common Prunus species, as do commercially available PCR DNA assays. There is no available DNA assay that appears specific to almond only.
In order to investigate the issue, a multidisciplinary team of scientists looked at routine ELISA tests used previously and applied advanced DNA and mass spectrometry techniques to determine the differences in DNA and protein structures that distinguish almond and mahaleb. Evidence from all three techniques was used to resolve the problem.
Prunus species protein was confirmed present in the sample above the limit of quantification of three ELISA platforms with statistical significance. A specially developed real-time PCR method generated a response consistent with mahaleb DNA being present in the sample. The significant sequence homology across Prunus species prevented the simultaneous development of a set of DNA assays specific to almond and other common Prunus species. However chromatographic and mass spectrometric signals related to almond and / or mahaleb kernels suggest that mahaleb protein is present in the laboratory sample. No peptide occurring solely in almond kernel was detected.
Although limitations still remain in the state of the science that prevent the presence of almond being completely ruled out, the results of the investigation indicate that the queried sample contains Prunus protein and DNA, the origin of which is consistent with mahaleb rather than almond.
Michael Walker, consultant referee analyst, said:
Almond and other Prunus species in spices had received little attention. We now know that ELISA detection is useful but only as a screening test.
There are unusually high similarities in the DNA and protein of these related species that make it very difficult to tell them apart in spices.
But thanks to the expertise of the molecular biologists and protein chemists in LGC we have developed what is, to the best of our knowledge, the world’s first DNA test for mahaleb and discovered subtle mass spectrometry differences to distinguish almond and mahaleb proteins.