Science to Live By: Maple Syrup—A Sweet Mystery

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Maple syrup tap. Photo: iStock

As a young lad, our family vacationed several times in New England during the winter holidays. I remember vividly, as we huddled with hot cocoa around the hearth and fire of the 19th century farmhouse at which we stayed, a local newspaper’s headline boldly proclaiming “May Warm to Zero.” That may not sound balmy, but it was certainly a lot better than 35 below!

Whenever the weather did warm above zero, we skied. Sugarbush, then a recently opened ski resort located in the Mad River valley of Vermont, was a favorite venue. Growing up outside Washington, D.C., I had no idea as I have come to understand now that “sugar bush” meant a stand of maple trees from which to gather sap; and “sugarbush” was a colloquial name for a farm that produced maple syrup. As an adult, I continue to learn fascinating things about the science, production, and history of maple syrup.  Here are a few examples.

In our backyard a few winters ago, I noticed a long icicle hanging from the stub of a branch that had recently broken off our box elder tree in an ice storm. I plucked the icicle, and being both curious and thirsty, I licked it. To my surprise it tasted refreshingly sweet! After a little research, I learned that the box elder (also known as the ashleaf maple) is a member of the Maple Family (Aceraceae), as are the sugar maple, red maple, and black maple trees.

Thousands of years ago, perhaps in a similar way, indigenous peoples living in northeastern North America first discovered the value of maple sap as a drink, food and flavorant. When Europeans arrived, maple sap and syrup had become such treasured commodities that Native Americans celebrated the Sugar Moon—the first full moon of spring—with song and dance.

Today, most maple syrup comes from North America, with Canada by far the world’s largest producer.  No wonder the sugar maple leaf motif is found on the Canadian flag. To promote the delicious natural products derived from the sugar maple, Agriculture Canada developed a “flavour wheel” comprising 91 different flavors—grouped among 13 families such as vanilla, empyreumatic (burnt), milky, fruity, floral, and spicy—to highlight and categorize the subtle taste variations of maple syrup.

Although Canada ranks number one, maple syrup producers in the U.S. currently are tapping less than half of one percent of the 2 billion maple trees in America that are sufficiently large to yield economically viable quantities of sap. Surprisingly, West Virginia has many more maple trees than Vermont, a state famous for its maple syrup. Virginia is not far behind, with about two-thirds as many maple trees as the Green Mountain state. Clearly, sap and syrup from maple trees represent a potential agricultural growth market here in the Virginias. I suggest we intentionally look to purchase locally produced maple syrup and related products to encourage regional producers. 

The flow of sap suitable for making syrup is ephemeral, occurring for only four to eight weeks in late winter/early spring before the appearance of leaves on the maple trees.  It is also possible to tap a little sap in the fall when days are warm and nights are cold.

Typically, 40 to 50 gallons of sap are required to make 1 gallon of syrup. To transform maple sap—which is typically 98 percent water and only 2 percent sugar—into an energy-rich and flavorful syrup containing 66 percent sugar, it must be concentrated. One slow and laborious method Native Americans used to concentrate maple sap was by first pouring it into hollowed-out logs and then boiling it down by dropping hot rocks into the sap. The iron pots the colonists brought with them from Europe, which could be heated continuously by a fire, made the job of boiling sap down to syrup a lot easier.

Oral history tells us that Native Americans also concentrated sap in another, more energy-efficient way. They simply let the sap freeze overnight. (As water freezes, newly forming ice crystals naturally exclude impurities because most impurities do not fit well into the crystalline structure of ice.) By removing the purified layer of ice and repeating the freezing process on the remaining liquid, the sap concentrated into syrup!

Modern methods of concentrating sap into syrup often include a procedure called reverse osmosis.  Reverse osmosis is a more energy-efficient way to remove water than boiling sap for hours on end.  In this pressurized process, a membrane preferentially permits the passage of water molecules through it, while at the same time blocking the passage of key syrup components like sucrose, vanillin, furanone, and maltol.  The purified aqueous effluent that passes through the membrane is discarded and the original volume of sap is thus reduced by 80 percent or so. The concentrated liquid is then further concentrated in evaporators, filtered, and processed into the final product.

Why did I characterize maple syrup as a ‘sweet mystery’ in the title? To quote the University of Minnesota Extension Service, “the actual mechanism responsible for sap flow is still something of a botanical mystery. … The flow of maple sap is not related to the normal process by which water is transported in the xylem during the growing season. … Sap flow stops when the buds expand and the leaves develop. Sap flow also ends if the temperature is continuously above or continuously below freezing.”

Sucrose-rich sap exuded and collected from maple trees is not coming from the living tissue (phloem), that inner most layer of bark that normally conducts sugars and other metabolic products throughout the tree. Instead, the sap is coming from the woody portion of the tree (xylem) that normally transports water and inorganic ions, but not sugar!

During the sugar season when temperatures cycle back and forth above and below the freezing point of water, pressures inside a maple tree can rise to the level of a fully inflated automobile tire.  Experiments on maple trees have shown that formation of ice crystals in the xylem builds up pressure within frozen gas bubbles. Scientists hypothesize that as these ice crystals melt, they release the pressurized gas, causing sap to flow. This phenomenon does not occur in most other tree species such as oak or tulip poplar. “The critical factor appears to be related to the distribution of liquid and gas in the xylem.” But even this factor in combination with the cycle of freezing and thawing is not the whole story. Experiments show that no sap flows out of tapped maple trees unless sucrose also is present in the sap. Scientists do not yet fully understand this process of sap flow, and why it has evolved in only certain types of deciduous trees.

Wow! Maple syrup does not come from ordinary sap flow in the early spring as I once thought it did. There is an element of the miraculous about it. In fact, in all the world, the combination of the proper species of trees, their large abundance, and the necessary daily freeze/thaw conditions for large scale production of maple syrup is almost unique to northeast North America. Think about it.  Commercial maple syrup production like that found in Canada and the U.S. has no comparable counterpart anywhere in Europe, China or South America, although smaller operations are found in Japan and Korea.

But in the Far East, the emphasis is on the sap, not the syrup. Koreans have a long tradition of celebrating the arrival of spring by drinking maple sap—as it comes straight from the tree—in the warmth of a sauna. They call this drink of pure sap gorosoe (good for the bones). Chemical analysis shows that maple sap contains significant quantities of calcium, potassium, magnesium, and manganese.

Maple trees, along with honey, are among the few sources of sugar we can perennially harvest without harvesting the plant, necessitating replanting a new crop each year. Field studies have shown that tapped maple trees live just as long (upwards of three hundred years) and are just as healthy as their untapped cohorts. This is perhaps not surprising since the tap holes are small and can heal within months, and the collected sap removes only about 5 percent of the total stores of sugar within the tree.

Next sugar season, I am going to try tapping our maple trees here at Sweet Blue farm and see what happens. Like the Koreans, I plan to celebrate the arrival of spring with a drink of sap as a tonic for good health; and I’ll use the rest for making syrup. But that’s not all. Cornell’s Maple Program, run within the University’s College of Agricultural and Life Sciences, also suggests tapping birch, butternut, and black walnut trees. I’ll give them a try too!  

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