Maple Syrup: a sweet, delicious teacher of science for kids

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Springtime in rural Vermont is magical. It’s not just the melting snow with rivers of mud, it’s also magical because it’s maple syrup season. For a brief period each year the sap really flows and those lucky enough to have sugar maples can harvest the sweet nectar and boil it down into the delicious breakfast treat. For our budding family, “sugaring” has become a great outdoor adventure that helps combine some of the things we love: nature, science, and eating. Read on to learn how you can make your own maple syrup at home, and for ways you can use the experience to teach some science to your kids.

The maple sap harvesting season depends on the weather with ideal springtime conditions consisting of freezing nights and sunny, warm days. This is when sap begins to flow to prepare the tree for budding. Once the weather turns warmer, the trees bud and the sap season ends. In a good year, the season will last 4-6 weeks but it can be much shorter.

There are several different types of maple in North America and sap is traditionally collected from sugar maples, however red, silver, and black maples are all fine to tap. It’s pretty hard to figure out what type it is in the winter, so try to identify your maple trees in the summer.

During sugaring season, the sap flows along tracts in the tree, called xylem, to deliver water and nutrients from the roots to the distant branches. The movement of sap in trees is a scientific marvel of it’s own, but can be summarized to young children with a good talk on capillary action. In reality, it’s a complex interplay of capillary action, concentration gradients, and various forces driving pressure differences between the roots and the branches. A fun project on capillary action can be found here, from The STEM Laboratory.

But collecting the sap is only half the story, since it has a measly sugar content of around 2%. You need to concentrate that sap to 66% sugar to call it syrup. Depending on the exact sugar content of the collected sap, you might need to concentrate 40 gallons of sap to make just 1 gallon of syrup!

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Saying “thank you” to the tree with a hug!

The boiling step is perfect for a memorable science conversation with your child. It’s a simple but important lesson about solutions and concentrations, but it will stick with them because of the amazing, hands-on process they’ll be involved in. The sap looks like water but it’s actually a solution chalk full of things like sucrose, minerals, and organic & amino acids. As you boil, you lose some water but the majority of those solutes remain. The transition from clear fluid sap to viscous syrup is a great showcase of the physical property of concentration. See the bottom of this post for a review of this and other talking points about the science of syrup making for kids.

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Water molecules (tan Mickey Mouse ear things) are removed through evaporation as the sap is heated. When the sugar and minerals (brown hexagons) become concentrated enough, the solution becomes syrup!

 


Make your own Maple Syrup

Don’t feel daunted, this is incredibly easy. I’ve seen someone tapping their single maple tree in a suburb of Boston using just one tap and a recycled 3 liter Poland Spring water bottle.

    1. Wait for the season! There are online communities that discuss when the sap starts to flow in your specific area, but you can always just place a tap early in February to be the canary.
    2. Find a maple. This can be tough in winter but it’s super easy when there are leaves. Sugar maple leaves are distinct and easily identified (just bring a Canadian flag for reference). Make sure the tree is at least 10 inches in diameter so taking the sap won’t harm the tree.
    3. Drill hole and place a tap. There are a number of taps available with different diameters. (Here is a nice kit from Amazon). Drill to a depth of 2 inches with the appropriately sized bit and lightly hammer in the tap.IMG_0488
    4. Collect sap and boil. Depending on the number of taps, you could start boiling immediately or after you’ve collected several gallons. Remember, it’s quite a bit of boiling with an approximate 40:1 ratio of starting sap to finished syrup.
    5. Bottling. If you have a candy thermometer, you should boil the sap until it reaches 219°F. It should be bottled in sterile glass containers between 180 and 200°F to avoid microbe growth. If you’re going to eat your syrup right away, this doesn’t matter.
    6. Clean up. Pulling the taps is as easy as prying them out and letting the tree do the rest.

Talking Points with Your Kids (preferably with a mouthful of syrup-drenched waffle):

-What is maple syrup and why is it so sweet?

Maple syrup is mostly sucrose, the same disaccharide molecule as ordinary table sugar. There’s also water, otherwise it wouldn’t be liquid. A number of different molecules account for the maple flavor, and natural variation in these molecules can result in very different flavor profiles among syrup from different regions or time.

-What does boiling do?

When water starts boiling, it’s turning from a liquid (water) to a gas (water vapor). The water vapor floats away into the air leaving less water in the pot. This is called evaporation. Things that are dissolved in the water like salts, sugars, and other nutrients will stay in water and not evaporate. As more and more water leaves the pot, it concentrates the sugars and nutrients. The concentrated solution of sugar is called syrup.

-What would happen if we boil too much and the syrup becomes more concentrated? 

The solution would be super-saturated. When it cools, sugar crystals will fall from the solution (precipitate). You can eat these sugar crystals, just like rock candy or table sugar!

-What would happen if we boil too little and the syrup is less concentrated?

While the syrup would be less viscous and less sweet, an important concern is that it would be more hospitable to microbial growth. Sugar can be considered antimicrobial due to its high “osmolarity”, which pulls water from the innards of invading microbes. If the concentration is too low, bacteria and mold could find the solution a suitable breeding ground.

-Why is syrup so much thicker than water? What makes sugar crystals form?

Here you get to invoke the magical phrase of “intermolecular forces.” This is a big topic but one you shouldn’t shy away from. Almost everything is made up of molecules and different molecules stick to each other in varying degrees. As the solution becomes more concentrated, the sugar molecules have more opportunities to stick to one another. Eventually, they are so concentrated that these interactions become strong enough to trap the sugar as a solid and they precipitate as a sugar crystal.

-Why do you boil it till 219°F?

When you dissolve solutes like sucrose in water (or any liquid for that matter), they cause the boiling point to increase. This is called boiling point elevation. The more solutes there are, the more the boiling point is elevated. Water normally boils at 212°F, but when sucrose is present in the initial sap at 2%, it ever so slightly increases the boiling point. As you boil more and more water off, the sucrose is being concentrated, so it will elevate the boiling point even more. At a sucrose concentration of 66%, the boiling point of water will be elevated by around 7°F, which is 219°F. This is the recommended  point to stop boiling because of the antimicrobial properties of the syrup at this concentration, and because it isn’t concentrated enough to form sugar crystals when it cools.

-Why is the syrup brown?

The Maillard reaction is where a sugar reacts with an amine in the presence of heat. The product is usually brown. Sap has both sugars and various amine containing chemicals (like proteins and amino acids), so the molecular results of the Maillard reaction are responsible for a lot of the brown color of maple syrup. You can also see this reaction at work when browning meats or making toast! Source

-Why are some maple syrups darker than others?

Syrup is darker and has more maple flavor the later the sap is collected in the season. The majority of sugar in sap is sucrose which is two sugar molecule units, called glucose and fructose, stuck together. When you collect the syrup, naturally present microbes break down some of that sucrose into the individual sugars, which are much better at undergoing the Maillard reaction. As the sugaring season progresses, you have more microbes present which cause an increase in the sugars and therefore an increase in browning. Also as the season progresses, the nutrients in the sap shift to include more amino acids, which also contribute to increased browning. Source

Due to that nutrient profile changing to meet the needs of the tree preparing to bud, tons of other organic compounds are being pumped out which are thought to also react during processing and contribute to the distinctive maple taste that also gets stronger throughout the season.


This is a great project for some quality family time. We hope it also gives you the chance to explore some really interesting science with your kids!

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