The official Flavor of Vermont is Maple, and our State Tree is the Sugar Maple. There’s a friendly rivalry in the Northeast of who has the best Maple Syrup: Vermont? New Hampshire? Maine? Or our neighbors to the North 🇨🇦 in Quebec?
Beyond where the tree was tapped and the sap was boiled, there are many more options: Grade A, Grade B, Fancy, Light, Golden, Dark, Amber, bourbon aged. What you buy is a personal preference, but we can help boil down (pun intended) what these variants mean and why they exist.
Turns out that maple syrup producers rely on optical characterization to grade and quality check their product for commercial sale - much like the tissue phantoms we make for different biomedical applications. And the USDA has standards for how maple syrup is labelled for commercial sale, ensuring consumers get what they expect.
During one of our quarterly Off-road days in the office, we get to use work resources for personal projects. This fall, we opted to measure maple syrup optical properties to see how they differ across variants. Here, with some samples from our friends at Mac’s Maple (Yes, they’re over the river in New Hampshire, but we still ♥️ them), we walk through how we characterized the optical properties of some varieties of local maple syrup.
Syrup grades A through C were originally used to describe the color of syrup for commercial sale, with Grade A being the lightest and C being the darkest. The syrup flavors differ between the variants, ranging from light and bright at the lighter end, to buttery and robust on the darker end. Practically, final syrup grade ends up being primarily influenced by the time of season when sap is harvested - later harvests produce darker grades.
More recently, the grading system homologated commercial maple syrup all to “Grade A”, which are now broken down by color categories: Golden, Amber, Dark, and Very Dark. You can learn more about the history from our friends at Vermont Maple Farmers.
Grading can be roughly estimated by eye, but it is ultimately determined by measuring the light transmittance through the syrup with tightly defined illumination and sample preparation constraints. Relative transmittance of 25, 50, and 75% at 560 nm establish the cut-offs between each of the four commercial variants of maple syrup. Ultimately, maple syrup producers care about the color of their syrup, flavor, and the sugar content in the final product.
Fortunately, the same methods we use to design our tissue phantoms are what’s used to measure maple syrup - refractometry and spectrophotometry. Today, we are going to take you through these steps practically with our locally made maple syrup.
If you’re in the area, like maple syrup (and soft serve), we can’t recommend them enough!
Refractometers measure how much light bends or slows down through a given material. We rely on refractive index measurements for optical property calculations after spectrophotometer measurements.
The refractive index we measured from our maple syrup variants were all above 1.476, which calculates to >66% sugar content by weight (otherwise known as the Brix scale). Since the USDA specification for commercial grade maple syrups is 66-69 Brix, our measurements align with expectations.
Using a UV light, you can see the blue-green fluorescence from the syrup - which is likely from soluble aromatic compounds formed through a couple of possible mechanisms:
These chemical phenomena both give rise to aromatic compounds that influence food color and flavor. The aromatic molecules are what drive UV fluorescence activity in the finished product.
We prepared the syrup in cuvettes to measure their excitation and emission properties in our spectrophotometer. These cuvettes are specially designed for spectrophotometry, letting us make precise measurements of reflectance and transmittance through a sample.
The spectrophotometer tells us what color of light (and how much of it) passes through our sample when illuminated with a fixed color of light. We sweep an illumination source through all of it’s possible colors and measure what colors come out of the cuvette - you can read more about this in our spectrophotometry blog post. Ultimately, we get transmittance (%T) and reflectance (%R) measurements out for each sample out of the spectrophotometer.
When we look at the Excitation-Emission Matrices (EEMs) for our different syrups, we can see that maple syrup fluoresces under UV light and changes distinctly between the different maple syrup categories.
As the syrup grade darkens, we notice a longer shift in the peak fluorescence emission wavelength from ~525nm (Golden), to 565nm (Amber), to 570-579nm (Dark and Bourbon). Similarly, the peak excitation wavelengths shift longer from Golden (~435nm), to Amber/Dark/Bourbon (475nm). Qualitatively, these maple syrup variants have distinct fluorescence spectral properties.
While transmittance (%T) measurements are used commercially to differentiate maple syrup types, that measurement can change based on how much syrup is being measured - namely the length of the light path traveling through the maple syrup. The USDA’s standardized method specifies a 10 mm path length, however our measurements used a 6mm path length.
To get around this discrepancy, we extrapolate these transmittance (and reflectance) measurements to calculate their generalized spatially-invariant optical properties called the absorption coefficient (µa) and the reduced scattering coefficient (µs’). We do this to help us develop models and analyses to design our products around. But here, we can use these parameters to extrapolate what USDA-compliant measurements would be on our instrument.
If you’re curious about how we do this, you can read more about Scott Prahl’s IAD algorithm on his website or through our blog post.
With some reference measurements and anisotropy assumptions, we get the following data:
As expected, the darker syrups had higher absorption coefficients when measured via spectrophotometry. Most of the absorption appears shorter than 700 nm wavelengths and appears to increase approaching the 400nnm and into UV range - suggestive of complex aromatic chromophores. This data tells us a bit about the color of the syrups based on the colors it absorbs and transmits. As syrups darken, they gradually absorb more blue, yellow, and even a little bit of red light. The end result is a progressively dark orange-ish brown appearance in the syrup as the variants darken.
Under a closer look, we can look at the absorption coefficient values at 560nm, which is where the USDA specifies measuring syrup transmittance for commercial grading. Absorption values at 560nm trend with the different “darknesses” maple syrup variants.
The bourbon maple syrup is noticeably darker than the amber and dark syrup variants. This could be from the bourbon barrels, which tend to be charred prior to neutral spirit exposure during bourbon production. This charred wood presents the similar complex aromatic compounds that give maple syrup its color, which when added to maple syrup might make it darker. Since there are no USDA-specifications for bourbon-style maple syrup labelling, this data is simply for curiosity.
We found that maple syrup optical absorption properties track with their commercial variants. But what we found even more interesting was how distinctly fluorescent maple syrup variants are. More data is needed to judge whether fluorescence spectroscopy provides any unique insight to maple syrup quality or production control, or whether scattering measurements could be useful for clarity and adulteration. But we think there are some interesting questions to answer on this front. Who knows? Maybe there’s a market for maple syrup phantoms as reference standards for the industry to calibrate their instruments against?
We can't conclude much about fluorescence spectroscopy’s prospects in the maple syrup industry here, but are relieved to know that our analytical methodologies line up with what’s used in practice to produce our favorite condiment. And we’re even a little hopeful that there’s some interesting support we can provide to the local industry.