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It’s still a mostly brown and grey world, with just a light dusting of snow covering the fields and forests. The grass hasn’t started to grow yet, the trees haven’t yet budded and people only come outside when they need to. Nature is waiting for spring to come. In these days, at the end of winter, where it still freezes at night, but gets above freezing during the day may seem very calm. However, there’s a select group of farmers for whom this is the busiest time of the year: the maple syrup farmers.
Whereas the ground is still too cold for ploughing or planting, the maple trees are starting to become active again. It’s the time when the sap starts to run and when the maple sap needs to be harvested.
As a result, in the middle of March, we sat in a car, driving through this still barren landscape, seemingly in the middle of nowhere, to visit a sugar house in Michigan (USA). We went to visit Maple Row Sugarhouse, where the yearly harvest of maple syrup is celebrated with a festival. We went to see how the maple sap is harvested and transformed into delicious, sweet, maple syrup (as well as maple cream!).
Maple syrup starts with a maple tree
We started our day out at the sugar house from which we took a bus to the maple tree forests, where it all starts. When we got close to the forest, our tour guide started pointing out blueish plastic tubes that ran throughout the whole forest. It’s a pretty amazing scene, that’s where maple syrup starts its life. Maple syrup is completely made by maple trees. All that humans do is tap it and concentrate the maple sap into a syrup.
Saps moves through a tree
A maple tree needs to transport moisture and mineral from its roots to its leaves. For doing this, maple trees, have ‘veins’ running through them through which the sap can run. In the winter, when temperatures drop below freezing and when the trees have lost their leaves, this transportation mechanism is nascent, not a lot of sap travels through the trees.
However, when spring arrives, the trees start to warm up again and the sap will start to move throughout the tree. The tree is preparing itself for spring, but at this point no buds will have formed. The sap really starts moving well when it still freezes during the night but thaws during the day.
If, at this point you drill a hole in the tree, just deep enough to reach those ‘veins’ some of that moving sap will come out of the hole you made. This is exactly what maple tree farmers do. They drill holes in the trees and catch the sap that leaves the tree.
You depend on the weather
Whether or not the sap runs is determined by the weather. The farmer can’t really influence the running of this sap. Our farmer told us that his ideal day during this time of year is a night with a few degrees of freezing followed by a warm day, above freezing with plenty of wind and rain. The wind moves away the cold air and together with the rain helps the tree to defrost. This is what makes the sap run best.
Since the running of sap depends on the weather, the season can also vary quite a bit in length. It is generally 4-8 weeks, however, that can vary quite a bit. If it stays cold for too long the season shortens, the same when it turns warm too quickly. Also, once a tree starts budding the season is over. The sap will start developing an off-flavour.
Sap can move fast
Under ideal weather conditions a single tree can give as much as 11 liters of sap. That is a lot of sap in one go which is why tapping systems have evolved considerably over the past decades.
The evolution of the tapping system
Tapping of these maple trees has been done for centuries. It’s a trade that was transferred from the native Americans to the European settlers. Over time the farmers have improved the system quite a bit and made it more efficient. Decades back, farmers might have had a few trees from which they tapped maple sap. However, over time, the farms have become larger and farmers have more trees to maintain at the same time. The harvesting system thus had to evolve.
From buckets …
They used to tap per individual tap. Below each tap you might have had a little bucket or vessel hanging onto the tap, to collect all the sap. That would require a farmer to come and check on the trees quite regularly. Especially on days that the sap flows really well, the buckets needed to be exchanged quite often. If you’ve got hundreds of trees that becomes a lot of work.
… to plastic tubing …
Therefore, nowadays, those separate buckets aren’t really used any more (except for demonstration & training purposes). Instead, farmers have complex networks of plastic tubing going throughout the forest! All these tubes connect all the trees together. The sap leaves the tree, straight into the tubes and then moves towards a central collection point. Instead of having 100s of buckets on all your trees, you only need one central collection point!
an even more advanced system will use a vacuum pump at the end of the tubing system. This pump won’t get the sap running, but it will help the sap move throughout the tubes more easily.
… to GPS & mobile tracking
Once you’ve only got a few larger collection vessels, you can upgrade the system even more. You can add level sensors to the vessels. Maple Row Sugarhouse, the farm we visited has these sensors in their main vessels so the farmer gets a message on his phone when the vessels starts getting full and needs to be emptied.
Maple tree forests
In order for maple tree tapping to become economically sustainable, you need quite a lot of trees together. Only then can a farmer be efficient enough to make that whole tubing network. Also, it becomes easier to keep an eye on the trees and ensure they are all tapped well.
If a maple tree would contain too many taps, it wouldn’t be able to survive and stay strong enough for the years to come. Therefore farmers need to know their trees and forest well enough to determine the right amount of taps on a tree. You want as many taps as possible, to maximize sap collection, without damaging the trees.
At Maple row Sugarhouse’s forests they don’t put more than 4 taps in one tree at a time. Only the older, larger trees get this many taps, the smaller a tree, the less taps can be placed on they tree.
A Northern American skill
Maple tree farming depends a lot on the weather. As a result, only in certain climates it’s possible to harvest maple sap from the trees. In other regions it stays either too warm or too cold for the harvest to make sense. You will mostly find maple syrup farms in the north (mid- and east-) of the USA and in the Canadian states above those.
Transforming maple sap into maple syrup
After we walked around in the maple tree forests and saw the harvesting system, it was time to hop back on the bus and drive back to the farm. There we headed into the Sugarhouse to have a look at what happens to the maple syrup next.
We got to taste some of the sap that leaves the trees. At this point, the sap doesn’t really taste sweet, it really just tastes like water. In order for maple sap to turn into maple syrup you have to get rid of a lot of that water to concentrate the flavour. Only when you concentrate the sugars will you be able to taste that distinct maple flavour. You tend to need as much as 50x the amount of sap to make an amount of syrup. So to make 1 liter of syrup, you’d need 50 liter of sap!
The old-fashioned way (>100 yrs back)
That means you need to boil off a lot of water from the sap! Over a century back in time they used to do that above a fire. They’d pour the sap into pots and boil them until enough water was evaporated. At the sugarhouse we saw a demonstration of that method.
… some major improvements …
Just like harvesting has improved over time though, the evaporation stage has as well. A major development was the creation of more efficient evaporation systems. They would be more continuous and not consist of pots boiling above a single fire. Instead, a larger fire (or other heat source) would heat a coherent system of vessels that are all connected through which the sap moves until it is concentrated enough at the end of the evaporation process.
.. nowadays: reverse osmosis & heat re-usage
But that system has improved even further. Using heat to evaporate water from a sap isn’t very efficient. It takes a lot of time and energy to boil the sap sap. Instead, major producers, including the farm we visited, now use reverse osmosis to get rid of the first 90% or so of their water.
In a reverse osmosis system the sap runs through a lot of small tubes lined with special membranes. Only water can move through those membranes, the sugars and minerals and other ‘impurities’ cannot. By increasing the pressure inside this system (300-500 psi/ 20-33 bar) the water moves through the membranes in an attempt to even out the pressure in the system. As a result, you end up with very pure water and more concentrated maple sap, exactly what you want!
Once the sap has run through the reverse osmosis system it will still go through an evaporator where it is boiled further down until enough water has left the sap. This evaporator through is a lot more efficient nowadays as well. For instance, they use the hot steam that evaporates from the sap to pre-heat the incoming sap. This sap will only be a few degrees above freezing when it enters the system. By pre-heating it with the steam (using a smart tubing system) it is considerably warmer (about 70C) when it enters the actual evaporator!
What to do with all that water?
They have to evaporate a lot of water from the ample sap. Some of this is used to clean the equipment in between shifts but the majority is not used for anything else and just leaves the property again (a lot of it in the form of steam). It’s very clean water though at this point!
Knowing when the syrup is finished boiling
As we discussed elsewhere on this website, the boiling point of a sugar + water solution depends on the concentration of sugar. The more sugar and the less water you have, the higher the boiling point. At sea level, pure water boils at 100C (212F). Once a lot of the water has boiled off, at a concentration of 85% sugar, the boiling temperature has risen to as high as 112C.
In maple syrup you will never go that high in sugar content (you will for maple cream though). Instead, maple sap officially turns into maple syrup once the sugar content is 66w% and it can go up to 69w%. There are several ways to determine whether the mapl syrup is done. First of all, you can check the boiling temperautre. Since the boiling temperature is intricately linked to the sugar content, it is a good measure. At Maple Row Sugarhouse they boil up to a temperature of about 102-104C (= 215-220F).
Once you’ve reached the right temperature, another way to check the content is to use a hydrometer. This is a smart device that uses density differences to indicate the sugar content of your syrup. You float the hydrometer in the syrup and how high it floats in the syrup determines how much sugar is in the syrup (see photo below). You actually use a very similar tool when brewing beer for determining alcohol & sugar content!
Last but not least, you can use a refractometer. None of them are highly complicated tools to use!
Why not continue cooking?
Maple syrup is quite liquid. It is a lot more liquid than a lot of ‘regular’ sugar syrups you may find in the supermarket (see also our post on rheology). So, you may wonder, why don’t they continue boiling somewhat longer?
Well, this is simply not possible. When you continue to boil the syrup it will become too concentrated (supersaturated) and actually starts to crystallize into maple sugar. When you make maple confections you actually use this phenomenon to make all sorts of candies. By boiling it to a specific temperature and pouring it into moulds, stirring it, etc. you can force crystallization to occur. This way you can make creams or harder candies. If you continue boiling for long enough and scrape the mixture down well, you can end up with maple sugar.
Supersaturation at the bottom of your bottle
If you’ve just finished a bottle of maple syrup, make sure to have a look at the bottom. You might just see why maple syrup shouldn’t be concentrated any further. Sugar crystals might have formed at the bottom of the bottle. While you stored it this sugar has started to crystallize, resulting in almost translucent crystals. These crystals are 100% sugar, during crystallization the other colored ingredients won’t form part of the crystal.
Filtering & packaging the syrup
Once the syrup has boiled down enough it leaves the evaporator. From here on, the sugar content will be checked once again and the syrup will be filtered. During filtered the last impurities are removed to make a nice clar syrup. As you can see on the photo below, filtering really drastically changes the appearance of the maple syrup.
Once the syrup is filtered it is ready for packaging. They do this at quite a high temperature to ensure that no unwanted micro organisms come in. Once packaged properly maple syrup is virtually good forever. Only once you open the bottle again can the syrup start spoiling again.
The colour & flavour of maple syrup
When sap comes out of a tree it is transparent, it looks just like water. When you start boiling it though it will turn anywhere from a light yellow colour to almost black. If you keep your system clean in between batches and control the process well the evaporation process does not influence the colour of the syrup. Instead, that colour is mostly determined by the tree. Generally, the start of a season will give lighter syrups whereas at the end of the season the syrup turns a darker colour. However, as we saw at the sugar house, the syrup can also start out darker and become lighter over time. The farmers don’t have a lot of control over this.
Since colour and flavour are highly related, the colour is used to determine the quality of a maple syrup. Lighter colours of maple syrup have a more subtle flavour, less off-flavours and are regarded as a higher quality one. You will use these to make maple candies for instance. Since you use a lot of maple syrup, you don’t want flavours to be too strong or it won’t taste good anymore.
Darker colours are considered a lower grade. Consumers tend to want slightly brown maple syrups. These are slightly less delicate, but perfect for using on a dessert for instance. The very dark colours can be sold to animal food manufacturers (where flavour is less important) or that can be used for applications where you don’t use a lot of maple syrup. For instance, if you’re making a maple syrup sausage you will only use a little maple syrup but you still want that maple flavour. Using a darker syrup is then a good choice.
Colour changes during the season
Later in the tapping season, when the plant starts preparing for budding of its leaves, the composition of the maple syrup will change. An important change is the ‘carbohydrate profile’ of the syrup, that is, which sugars are present. Early in the season there is barely any glucose or fructose in the sap, most of it is sucrose. However, later on the amount of glucose and fructose may increase.
This is part of the reason (there are a lot of other factors at play) why maple syrup later in the season tends to be darker. Sucrose does not take part in the Maillard reaction, which causes browning of the syrup when you heat it. Fructose and glucose on the other hand, do, since they are reducing sugars. As a result, the maple syrup will turn out browning after the boiling stage.
Maple syrup origins
After having seen where maple syrup comes from and how it’s made, our appreciation for the product (and its higher price) has definitely gone up. Next year, in those days where you’re just waiting for spring to arrive we’ll think about the maple farmers again. We’ll hope that it won’t heat up to quick, nor too slow, so they can have another good sugar season!
Childs, Stephen, Chemistry of maple syrup, Cornell maple bulletin 202 (2007), link
C.T. Ho, C.J. Mussinan, et. al., Recent advances in food and flavour chemistry, RSC Publishing, 2010, p.75, 978-1-84755-201-3, link
Roth sugarbush, temperature chart for correcting Brix values, link
Vermontville maple syrup festival, several maple candy recipes, link