The Science of Maple Syrup


“We get a little twitch in our shoulder when March comes around,” professes Dr. David Higgs. It’s maple syrup season! This past year, Biological Sciences Department’s botanists, Dr. David Higgs and Dr. David Rogers along with Vince Shaff and other faculty, staff and students launched a small project to try and harvest sap on Parkside’s campus in order to make maple syrup. On Wednesday, March 11, Higgs and Rogers shared their interesting work during Parkside’s Science Night with the surprising science behind maple syrup. With Rogers specializing in plant and forest ecology and Higgs in plant molecular biology and physiology, it made for a fascinating night.

The sap harvesting season usually starts in March. Higgs and Rogers have already distributed their sap siphons and buckets throughout the Greenquist trees. You may spot a few of their blue buckets hanging on the trees. They use a more traditional way of collecting the sap, using a siphon and hanging a small container on the tree and then transporting it to a cooker to make the syrup. While this traditional way still exists in some places, today it is more common to use piping systems, instead of the buckets, that move the sap by gravity flow or vacuum systems into tanks where reverse osmosis starts the conversion into syrup instead of cooking. To make syrup, you need to remove 98 percent of the water in the sap, whether it be through reverse osmosis or cooking. What is left after that is your syrup. So if 98 percent of the water is removed, then what is syrup really made up of?

Maple syrup is mainly made up of carbohydrates, and the makeup of a majority of those carbohydrates is sucrose. Essentially, sucrose is sugar and is what gives maple syrup that sweet taste that we all crave. Usually the syrup is about 88 to 99 percent sucrose. The concentration all depends on the time of year that it was harvested. In the late season there is less of a concentration of sucrose and more fructose and glucose. Fructose and glucose are also sugars, and actually derive from sucrose. Fructose and glucose are monosaccharides, simple sugars, and are the building blocks of sucrose, a disaccharide. When sucrose breaks down it is broken into those simple sugars. This breaking down process in the sap actually happens when the sap is sitting in the buckets, hanging on the trees. When it’s later in the season the weather becomes warmer and becomes an incubator for yeasts and bacteria in the buckets, the culprit of splitting sucrose into fructose and glucose. This process is called inverting and is fueled by the enzyme invertase. This higher concentration of fructose and glucose still makes perfectly good syrup, just a different appearance and taste. When the sap is cooked down, the fructose and glucose actually caramelize which gives the syrup its darker coloring. This darker syrup is usually referred to as Grade B syrup.

The sweetness comes from the sucrose, but what gives maple syrup that maple flavor? Higgs says that it is not completely understood, but is thought to be a combination of a specific mix of amino acids paired with the presence of the compounds maple furanone, strawberry furanone and maltose.

So we have looked at sap at a chemical level, but what about the physiological side of it? Why are we able to draw out sap from a tree to make syrup? Higgs explains that the harvesting season only provides this small window because it is the time when the tree is transporting the sucrose from the roots to the branches. The syrup is essentially the product of last seasons photosynthesis. In the fall, before the tree looses it’s leaves, the tree transports its energy that it stored from photosynthesis in the leaves down to the roots to store it over winter. This energy reserve in the roots is what the tree uses in the spring to make new leaves. The energy is stored as starch in the roots and leaves, but before being transported to either end of the tree, it is converted into sucrose, the sap we collect.

While almost any maple tree has sucrose in the spring that can be used to make syrup, Dr. David Roger explained how to best identify the sugar maple tree (the best type of tree to tap). The Norway maple is the one maple tree that has a bitter sap that you can’t make syrup from, and unfortunately is most similar looking to the sugar maple. You can tell the difference between these trees by looking at the leaves. The Norway maple has more lobes and long tapered teeth on their leaves while the sugar maple’s leaves are smoother. But at the time of harvesting sap, the trees don’t have any leaves left so the best way to identify the sugar maple is by looking at the buds and the bark of the tree. The sugar maple’s buds are quite distinct. They are long and stiff and are made up of about 11 to 13 scales. They also often have ancillary buds. The bark is another way of identification. The sugar maple’s bark has soft ridges and valleys and is a beautiful, light-grayish color.

Once you have correctly identified the sugar maple trees, harvesting sap and making syrup is a very fun and interesting hobby. But it’s not just a fun hobby. Wisconsin is actually the fourth largest producer of syrup in America, and Vermont makes the most syrup in the U.S. at about 42 percent. But worldwide, Canada is the boss in the maple syrup industry, making 80 percent of the world’s maple syrup.

Though the maple syrup industry booms and it’s tastier than ever, Rogers brings to light studies that show that sugar maple trees are being greatly affected by climate change and global warming. Scientists’ estimates show that the number of sugar maple trees will greatly decrease over the years. Now just imagine how sad your mornings will be with no syrup on your pancakes. It’s time to stop global warming.

Article by Liv Gripko

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