Genuine maple syrup is a treat for the taste buds, whether you prefer light golden varieties or robust darker syrups. But sometimes batches can have off-putting flavors. Scientists at the University of Montreal in Quebec, Canada, have developed an artificial "tongue" using gold nanoparticles that can weed out bad batches early on. It's not so much an electronic device as a simple, portable chemistry test that detects a color change when an off flavor is present in a sample, according to a recent paper published in the journal Analytical Methods.
"Especially here in Canada, we take maple syrup for granted," said co-author Jean-François Masson of the University of Montreal. "But it is much more complicated than we had anticipated. It has some of the same complexities as fine wine and whiskey." Quebec is the largest producer of maple syrup, accounting for about 70 percent of the world's supply.
He is not referring to cheap knockoffs whose primary ingredients are high-fructose corn syrup with imitation maple flavoring. To be considered a true maple syrup, at least in Canada, a product must be made entirely from maple sap collected from maple trees, usually sugar maple, red maple, or black maple varieties. Maple syrup is mostly sugar, water, and a small amount of organic molecules that are responsible for the final product's flavor profile. Those compounds account for just 1 percent of the content, but it is a crucial 1 percent, determining whether a given syrup is caramelized, smoked, salty, or woody, among the 60 or so possible categories.
The maple trees store starch in their roots and trunks before winter, which converts into sugar. Come late winter or early spring, as the sap rises, it can be collected by drilling holes in the trunks and inserting a spout to funnel the sap into containers. The sap is then boiled down into pure syrup in a "sugar shack"—typically a shack louvered at the top to let off steam. ("It's a cross between Daniel Boone's shack and a sweet factory," said Masson.) After boiling, the syrup is filtered to remove any crystalline sugar that has formed (aka "sugar sand").
Since 2014-2015, the United States and Canada have adopted roughly the same grading standards. Testing is typically done after filtering, while the syrup is still hot. Grade A syrups are those sold for consumer use—the stuff you pour on your pancakes, waffles, and so forth. If a batch falls short of the criteria, it is deemed to be Processing Grade and can't be sold in containers smaller than five gallons. Failing even that, the batch will be deemed substandard.
The Grade A category includes golden-colored syrups, amber, dark, and very dark syrups. Color classification is determined by measuring a syrup's internal transmittance by passing a 560nm wavelength of light through a 10mm sample. For a golden syrup, there must be 75 percent or higher transmittance. A very dark syrup (typically used for cooking or baking) will have less than 25 percent transmittance.
When a batch is found to have off flavors, it's usually due to some kind of contamination, microorganisms, or fermentation byproducts, for example. One of the most common is a "buddy" off flavor. A late harvest can alter the chemistry of the sap, resulting in higher levels of amino acid and small organic thiols. This, in turn, can affect the flavor profile. It's a situation that has been exacerbated by climate change, according to Masson, as Quebec now regularly experiences large temperature swings from day to day. "For example, one day there were snowflakes, which is unusual for May," he said. "The day before that, we were in the upper 60s. That's not good for producing maple syrup."
Masson's expertise is in developing field-deployable instruments, which is why the Quebec Maple Syrup Producers approached him about the possibility of a quick and easy test to weed out off-flavored batches on site. The current method for grading and classifying syrups involves trained human taste testers who are very rarely augmented in high-tech industrial settings with fluorescent spectroscopy. But there are only four such facilities in the world, according to Masson, and spectroscopy isn't available at all to smaller producers. Fluorescent spectroscopy also can't be easily deployed in the field.
Masson thought a colorimetric test would be well-suited to this application, and he settled on using a plasmonic sensor, which can be configured for naked-eye detection and would be suitable for the high-throughput 96-well plates used at maple sugar shacks. Plasmonic-based "noses" and "tongues" have been used to distinguish between normal cells and cancer cells, for instance, or between different types of proteins. A 2019 paper even reported on the use of a plasmonic-based array to detect different signatures of Scottish whisky.
Masson and his team designed their artificial "tongue" to react specifically to "buddy" flavor profiles. They used spherical gold nanoparticles stirred into ultra-pure water to create a reagent solution. Several molecules believed to be associated with off flavors in maple syrup also bind to gold surfaces. So if they are present in sufficient concentrations in a maple syrup sample, they will cause the gold nanoparticles to clump together, leading to a stark change in resonance wavelength and a corresponding visible change in color.