Farming Magazine - November, 2013


Researching the Taste of Maple

By Tamara Scully

A close-up of an evaporator at the University of Vermont's Proctor Maple Research Center.
Photos courtesy of the Proctor Maple Research Center.

The sap that flows from sugar maple trees isn't a known, immutable product. In fact, it changes not only from season to season, but also alters in composition throughout the season. Sap is a complex compound with a chemical composition that changes with nature's cycles and is possibly influenced by various factors, including those inherent in a specific tree, in the soil, or even in the air. The chemical composition of sap and the factors that can affect it aren't entirely known.

What factors influence the flavor of maple syrup? Does modern processing equipment change the flavor? Is one geographic area going to produce a different syrup flavor than another? These are the age-old questions and beliefs that haunt sugar makers today, and the state of Vermont has a major stake in finding some answers.

Controlling flavor

At the University of Vermont's Proctor Maple Research Center (PMRC), Dr. Abby van den Berg studies elements that might influence the flavor of maple syrup. "There is very minimal knowledge of what is in sap," she said, and there is an "enormous set of complicated reactions" that occur in sap production. Add to that the steps needed to turn sap into syrup. It's difficult to nail down exactly what causes distinctive syrup flavors, but it seems to have the most to do with the inherent characteristics of the sap itself, which appears to override any differences caused by any particular production method.

At the University of Vermont's Proctor Maple Research Center, Dr. Abby van den Berg studies factors that might influence the flavor of maple syrup.

"The problem is, maple syrup flavor is affected by an enormous number of variables, and the flavor of the syrup produced by an individual producer naturally varies throughout the production season, and from season to season. The biggest influence on syrup flavor - both within and between seasons - is sap quality. As the season progresses, sap quality tends to change. Typically, you see a larger proportion of invert sugar due to warmer temperatures and increased microbial action, and this changes the color and flavor profiles of the syrup produced," van den Berg explained. "The effects of different types of equipment, if any, are largely overwhelmed by this."

Warmer temperatures cause increased microbial action, which in turn causes a larger proportion of invert sugar in the sap. This will affect the syrup's flavor and color. Any practice that increases microbial activity could, therefore, also alter the flavor.

What role do production practices play in the ultimate flavor of the syrup produced?

In order to study whether or not production practices can influence the flavor of syrup, controlled experiments, in which the sap used in each trial is identical, are needed. Identical pieces of equipment - like the four identical evaporators at the PMRC - are required to control for any equipment differences.

Through controlled experiments, researchers have tried to isolate various production practices in order to determine how much of an influence any piece of the process might have on maple syrup flavor. "If the quality of the sap is equal, then what is the impact these production practices have?" van den Berg said.

Reverse osmosis

Experiments at PMRC used identical sources of sap but processed them differently - some with reverse osmosis (RO) - and then conducted blind taste tests. They then processed identical sap samples, using RO for all, but concentrating the sugar at various degrees from 8 to 22 percent.

Reverse osmosis - more accurately, membrane separation - takes the sap from its natural 2 percent sugar content and concentrates it further by removing the water before heat processing of the sap occurs. Heat processing, whether using wood or oil as fuel, requires a lot of energy and time, so it is a major expense. By removing a portion of the water prior to heating, RO reduces the time needed to concentrate the sap, and thus the fuel cost. RO equipment runs on electricity, requires a fraction of the energy to concentrate the sugars, and represents a large reduction in the overall cost of production.

Many sugar makers believe the use of membrane separation in the production process may impact the flavor of syrup. However, the experiments van den Berg conducted don't support that theory.

There were no discernible differences in flavor or chemical composition between syrup made from unconcentrated, non-RO-treated sap versus identical sap that was concentrated first with RO. Sap concentrated to 22 percent sugar resulted in syrup with very few differences in composition or flavor compared to sap concentrated at 8 percent, an industry standard. Some producers are beginning to concentrate sap to these higher levels in an effort to increase output while decreasing cost of production.

Using sap with differing sugar concentrations did result in color differences in the syrup, van den Berg noted. Using RO, particularly with the highest concentration, resulted in lighter-colored syrup. But the color difference did not translate into any difference in flavor.

The color differences between RO and non-RO identical sap trials were discounted by conducting blind taste tests of the final product. These tests were good at "eliminating out how much we taste with our eyes," van den Berg said. Study participants noted no or minimal flavor discrepancies between the syrup samples made from sap concentrated to different levels via RO.

The take-home message for sugar makers: It's safe to do RO and concentrate to 8 percent or higher, without the risk of negatively impacting syrup quality. "It's a safe way to save money, increase profits and save time, without affecting the quality of syrup that you produce," van den Berg concluded.

These evaporators are used to conduct research at the University of Vermont Proctor Maple Research Center.
Photo by Mark Isselhardt, Proctor Maple Research Center.

Best practices

Air injection, which was popular a few years back, absolutely produces a lighter-colored syrup. However, the taste difference was determined to be minor in studies. This production practice, much like RO, changed the color but not the flavor, giving more credence to the idea that flavor comes primarily from sap properties, not specific equipment used in production.

Lighter-colored syrups, which naturally arise from early-season sap, have traditionally been valued more highly, but even that is changing as customer demand for darker syrups is increasing. So any gain simply from having lighter-colored syrup, no matter its taste, may no longer be achieved.

While PMRC hasn't conducted a processing experiment in two years due to financial constraints, van den Berg is interested in studying tube versus bucket sap collection. This is another factor that some producers say changes the flavor of syrup, but van den Berg isn't so sure.

To create a scientific study, identical sap would have to be collected at the exact same rate for the tube and the bucket extraction, van den Berg said. It would be processed simultaneously, using identical evaporators and equipment. Her guess: As long as the controls were in place, no taste difference would be demonstrated between collection methods.

However, a difference in taste might result if the tubing was old or dirty and loaded with microorganisms; if the sap was collected at variable rates; or if the sap was left sitting around before processing.

When you alter how much or how long microorganisms get to act in sap, there will be resulting changes in color and flavor, van den Berg said. "Good production practices are really the key to maintaining good quality."

A new study in the works at PMRC is being designed to maximize the yield and quality of syrup. The study will look at various ways of cleaning and sanitizing production systems, as well as the impact on the final product. They will also develop best practice guidelines for producers.

Finding flavor

One thing is certain: Microorganisms play a role in sap quality and characteristics by breaking the bonds of the sap's chemical composition, which is 2 percent sucrose (a nonreducing sugar), and converting it into the reducing sugars of glucose and fructose. Heat can act more readily on the reducing sugars, and this is what causes flavor to develop. Anything altering the amount of reducing sugars in the sap prior to heating would result in flavor differences.

Everyone agrees that there are discernible flavor differences among batches of syrup from the same producer from year to year and from tree to tree, or from producer to producer. Sap does vary tremendously from site to site, tree to tree or from within the same tree, depending on the season. The problem lies in teasing out the effects of so many possible variables, van den Berg said.

As of this time, no conclusive data is available to link specific soil characteristics to certain maple syrup flavors. Designing tests to control for all of the other potential variables would be a daunting task. The weather, genetic characteristics of the tree, the proximity of other trees, and other environmental factors could all play a role in sap quality. Other compounds in sap - phenolic and amino acids and minerals - have not been fully analyzed yet, and how they change over the season or from tree to tree isn't known. Newer technology is making it more likely that this type of research could occur in the near future.

Until then, the question remains: What is it about the sap that makes the flavor of maple syrup vary?

Even without knowing all the answers, maple syrup flavor is arguably important to producers, and branding a flavor profile can help with marketing and sales. To this end, research led by Dr. Amy Trubek at UVM developed into the "Map of Maple," a tool that assists in describing the facets of maple flavor, much as is done in wine tasting.

The map includes characteristics for mouthfeel, intensity, aroma and flavor. Meant as a tool to assist producers and customers with giving Vermont maple syrup a "taste of place," it allows producers to better describe their syrup's characteristics. It also allows customers to better recognize and experience taste subtleties inherent in maple syrup.

Testers - two sugar makers, one maple specialist, three wine experts, one beer specialist, three faculty members of the Department of Nutrition and Food Sciences at UVM, and four other food professionals - were trained in maple syrup evaluation and flavor identification. Some of the flavors and aromas found in maple syrup include raisin, peach, vanilla, butter, baked apple, mushroom, grass and bourbon.

The project, which was a part of the "Taste of Place" initiative of the Vermont Agency of Agriculture, also includes the map to "off flavors" and relates the possible flavor issues back to their most likely origins. For example, a lingering aftertaste and somewhat chocolaty aroma is correlated to sap collected after budding, while a burnt flavor and thick body is linked back to scorching during processing. It is meant to assist sugar makers in making adjustments to their production practices in order to increase syrup quality.

Vermont's sugar makers don't know exactly what gives syrup the characteristics that identify it as their own, but they do know there is something that distinguishes one syrup from another. The subtleties may have to do with sight, smell, atmosphere and other factors, all of which seem to play a role in taste beyond the characteristics inherent in the sap. Mapping the flavors of maple syrup may help to make those elusive differences more describable to customers, while ongoing research will help producers make the most of the sap they have, transforming it into the best-quality syrup it can be.

The author is a freelance contributor based in New Jersey. Comment or question? Visit and join in the discussions.