Gold may be among the most distinctive colours in nature, but the element shows a very different character at smaller scales. At sizes of less than 100 nanometers, they absorb green light from the spectrum and turn a burgundy red. As they bind with other molecules, these nanoparticles can clump together to absorb different wavelengths of light, eventually turning a deep shade of blue.

Université de Montréal chemistry Professor Jean-François Masson has previous applied these properties to chemotherapy, a field in which he and his colleagues developed a visual marking system to help oncologists measure the level of cancer-fighting drug in a patient’s bloodstream. Now they have turned their attention to a much sweeter subject — maple syrup.

“We were approached in 2017 by the Quebec Maple Syrup Producers (QMSP),” he says, referring to the industry group that oversees the production of most of the world’s supply of maple syrup. “They were concerned about ‘off’ flavours that occur in batches collected later in the harvest period.”

Masson points out that maple syrup is 66% sugar and 33% water; the remaining 1% of its content is made up of organic molecules, which are the essence of its unique flavour. A traditional analytical approach would be far too labour intensive to provide maple syrup producers with a convenient means of identifying changes in the chemical profile of these molecules. But plasmonic gold nanoparticles, which can signal these changes by absorbing different parts of the spectrum, make the task much more straightforward.

“A simple colorimetric test detects off-flavour profiles of maple syrup in minutes, which are detectable by the naked eye,” states a paper in the journal Analytical Methods, co-authored by Masson and his research associates, Julien Coutu, Simon Forest, and Trevor Théorêt. Slightly different from the earlier work on methotrexate, this test takes advantage of non-specific interactions between target molecules and nanoparticles, which create clumps depending on their respective proportions in solution, then display various colours ranging from red to blue.

Much of the work consisted of calibrating those colours with different types of flavour profiles, which were originally determined by QMSP inspectors, who routinely taste-test batches as well as submitting them to fluorescence spectroscopy. A hierarchy of established flavour profiles ranks the level of defects according to a code named VR, which runs from VR1 — woody or burnt taste — to VR5 — known as “buddy”, since it resembles tree buds.

“For this study we focused on the extremes of the flavour profile of maple syrup,” says Masson. “We had about 800 ‘normal’ flavoured samples and about the same number of “off flavour”, VR5 samples. We calibrated against that, using these two extreme flavour profiles as a benchmark.”

He adds that QMSP was eager to try out the new test on this year’s maple syrup harvest, which is among the largest on record, but much of this work has been shelved because of shutdowns associated with the coronavirus pandemic and will resume once labs reopens. Masson’s lab, which is now operating at just a fraction of its earlier capacity, is instead focusing on research related to the virus.

Nevertheless, he is looking forward to returning to the nanoparticle work, which could have further applications for testing the flavour of refined products such as wine. Like maple syrup, wine consists almost entirely of two essentially flavourless elements (alcohol and water) and 1% organic molecules.

“That 1% of organic molecules is what gives a distinct flavour to both products.”