The Maple Continuum

We are all familiar with maple syrup.  And most of us know that maple trees don’t exude syrup, but sap, which is thin, clear, and only faintly sweet.  The liquid must be reduced to become syrup, and in fact it can be further reduced to become pure crystalline maple sugar.  So while we are all acquainted with a certain concentration of maple syrup – the one on grocery store shelves and brunch tables – there is actually a broad spectrum of products that can be made with maple.

Let’s look at the two extremes of the maple continuum.

 

A glass of cool, raw maple sap.

Chilled Maple Sap

An amazing but subtle tasting experience, one that I appreciate more in lean years when the sap run is meagre.  Maple sap smells and tastes of green, nutty vegetables like pea shoots, with the sliest suggestion of sweetness.  It’s a fantastic way to begin a spring meal.  At left is a photo, not particularly inspiring because the sap looks just like water, but I think it gives some idea as to how elegant maple sap can be.

I’m still thinking about the best name for this.  Raw sap?  Green sap?

Best served at the same temperature at which it comes out of the tree, roughly 5-7°C.  Enjoy in the presence of pussy willows and other just-spring novelties.

On the opposite end of the spectrum is…

Maple Sugar

When all the water content has been driven off by boiling, we have maple sugar.  If you grew up down east you will know maple sugar candies: a firm, crystalline maple fudge pressed into the shape of a maple leaf.  They are ubiquitious at sugar shacks, tacky trading posts, and airport souvenir shops.

Below is a picture of some homemade maple sugar candy.  I’d feel guilty if I didn’t admit that I made this candy mostly by accident: I over-reduced a pot of syrup, noticed that it was starting to crystallize, then stirred in a bunch of butter and poured it into a tray to cool and solidify.

Maple sugar candy

Between these two extremes are infinite possible concentrations of maple syrup.  I’ve used different grades of maple syrup in cocktails, soups, dressings, meat glazes, sauces, and desserts.  The way that I process and consume the maple sap usually corresponds to the seasonal yield.  In lean years when I get little sap, I am more likely to consume it in its raw or lightly cooked forms.  Years with voluminous flow are more likely to be drastically reduced.

Why Maple Sap Runs (or doesn’t)

Checking the buckets for sap: no luck

No sap.  Not a drop.

Last week I tapped my maple trees, and since then I have collected about one teaspoon of sap from the buckets.

I recently learned that the spring sap run is a completely separate phenomenon from the normal transportation of water and nutrients through the stems of the maple during the growing season.  That transportation is going to happen no matter what.  The sap run, on the other hand, might not, as it requires a very specific set of circumstances, and is not a biologically necessary phenomenon from the tree’s standpoint.

Before we tackle the question of why sap runs, we need some background info on maples.

Why Maple Sap Contains Sucrose in the First Place

During the summer the leaves of the maple convert carbon dioxide and water into sucrose and oxygen in a process called photosynthesis.  Sucrose is very familiar to us.  You might have put sucrose in your coffee this morning.  The tree needs to save some of this sugar so that it can put out leaves the following year.  To this end in the fall the tree converts the sucrose to starch, which it stores in its roots and trunk.  In the spring the tree breaks the starch back down into sucrose.

Other types of trees do the same thing, but maples generally, and sugar maples specifically, have much higher concentrations of sucrose than most.

Sapwood and its Structure

There are two types of wood behind the bark of a maple.  The older wood, at the centre of the trunk, is called heartwood, and is more or less dead.  Closer to the bark we have the younger, living wood, called sapwood.

The main roll of sapwood is to convey nutrients and water between the roots below and the shoots above.  There are two types of transport tissue that do this: phloem transports nutrients, while xylem deals chiefly in water.  In the spring time, however, the water in the xylem also contains the abovementioned sucrose.  In scientific literature xylem tissue is often referred to as a conduit, or vessel, because of its role in transportation.

The second function of the sapwood is to provide structural support to the transport tissue as well as the branches high above.  One of these structural elements is wood fiber.  Fiber is made of single, elongated cells which, in maples, are filled with air.  The interior of a wood fiber cell is called the lumen (pl. lumina).  Gas-filled fibers are unique to maples and a few other plants such as walnut and butternut trees; in most other plants the fibers are filled with liquid.

 

So, Why Does Maple Sap Run?

The simple answer is positive stem pressure.  Sap runs when there is a buildup of pressure in the sapwood of the maple, a pressure so great that when we bore a hole into the sapwood, or if the sapwood is somehow damaged, sap is exuded from the wound.  (The technical term for sap flow is “sap exudation.”)

The real question here is why positive pressure develops in the sapwood.  We know from experience that freezing and thawing cycles build positive pressure in maples, but the truth is we don’t know exactly why this happens.  We have a general idea, of course, but physiological botanists are still proposing models.  I’d like to take some time to explain the most popular models of the last few decades.

The Physical Model of Sap Exudation (aka the Milburn-O’Malley Model)

The larger part of this model has been around for more than a hundred years, but it reached its current form in a 1984 study by Milburn and O’Malley.

When the temperature drops in the evening, the first parts of the tree to cool down are the smaller twigs and branches high in the tree.  As the temperature approaches freezing, the gas in the wood fibers contracts, while the sap in the conduits expands.  This creates a pressure differential across the fibers and the conduits.  Sap is thereby drawn into the lumen, where it freezes to the cell wall.  As it freezes it expands and further compresses the gas within the lumen.

As sap moves into the wood fibers, more sap is drawn from the lower parts of the tree into the upper branches, and the roots take in water from the ground.

When the temperature outside rises above freezing the following morning, the sap in the wood fibers melts, and is forced back into the conduits and down the tree by the pressurized gas and the force of gravity.  If we have tapped the sapwood, we have relieved some of this pressure.  The sap takes the path of least resistance, and much of it ends up in our bucket.

Problems with the Physical Model of Sap Exudation

While the physical model adequately explains the upward movement of sap and the intake of water by the roots during a cooling phase, it is not able to explain the hours of sustained positive pressure measured in maple stems.  For a number of reasons beyond my comprehension, botanists now know that pressurized air-bubbles in the lumina can not and do not account for the high pressure measured in maple stems.

Interestingly, it has been found that if maple sap is replaced with pure water and then run through maple branches exposed to freeze-thaw cycles, there is no buildup in stem pressure (Johnson et al. 1987).  The sucrose found in the sap is therefore playing some role in the process.  In fact, it has been found that stem pressure and volume of exudation are positively correlated to the sucrose concentration of the sap (ibid).

This last fact got some botanists thinking about osmosis, and osmotic pressure.

Osmosis

Osmosis is the diffusion of water through a semi-permeable membrane across which there is some kind of concentration gradient.  Imagine a cell.  The cell wall is our semi-permeable membrane.  On one side of the membrane is the interior of the cell, on the other the exterior environment.  If we surround this cell with brine, there is a very high concentration of sodium ions outside the membrane, and a relatively low concentration within.  Water will start to pass through the membrane from the cell to the environment, in an attempt to equalize the two concentrations of sodium.  This is osmosis.

 

The Osmotic Model of Sap Exudation (what I, but not the scientific community, call The Tyree Model)

The osmotic model of sap exudation assumes the existence of a membrane between the conduits and the surrounding fiber cells that is permeable to water, but impermeable to sucrose.  In other words, the sucrose is confined to the conduits, but water is able to move freely between the conduits and the surrounding fiber cells.

The ability of a high concentration solution to draw water through a semi-permeable membrane can be thought of as a kind of pressure, called osmotic pressure.

Take our example of the cell surrounded by brine.  Water flows through the cell membrane from the interior of the cell to the exterior.  Whenever water flows, it is moving from a point of high potential energy to a point of low potential energy.  In a river or waterfall the water starts with a relatively high gravitational potential energy and finishes with a lower gravitational potential energy.  At the start of a pipeline the water has a high potential energy because it is under pressure.  The end of the pipeline is under less pressure and has a lower potential energy, making the water flow from the start to the end.

Since water flows from the interior of the cell to the briny exterior, the high concentration of sodium ions is somehow lowering the potential energy of the surroundings.  This difference in potential energy across a semi-permeable membrane is called osmotic pressure.

As we discussed in the explanation of the physical model, as the branches cool, the gas in the wood fibers contracts, and sap in the conduits expands.  There is therefore a pressure differential, and because of the semi-permeable membrane, water (not sap) is drawn into the fibers, while the sucrose remains in the conduits.

We now have relatively pure water in the fibers, and a relatively concentrated solution of sucrose in the conduits.  This concentration gradient creates a high osmotic pressure.  Once the branches warm up, the physical pressures that separated the water from the sucrose are reversed, and the water in the fibers flows back into the conduits, down the tree, and out our tap.

This is the latest and most complete theory of sap exudation.  Some studies have been done that support the existance of a membrane that is permeable to water but impermeable to larger molecules like sucrose (Tyree 2008), but more work is needed to hash out the details of the osmotic model.

A Bunch of Caveats

The information I’ve presented here is hugely simplified.  There are entire issues that I have completely avoided (like air bubble formation and stability in the lumina), not because I don’t want to write about them, but because I don’t understand them at all.  I’m not a physiological botanist, and have probably misrepresented facts even in this abbreviated explanation.

Anyways.  Even if I’ve botched some of the details, the point is that sap flow is a completely separate mechanism from the usual transportation of sugar and water through the tree, and not one that is required for the plant to grow.  If the outside temperature does not cycle about 0°C, water will not be drawn into the surrounding fibers, and pressure will not build in the sapwood.  This has been an exceptionally early, warm spring, and I don’t think we’ll get the freeze-thaw cycles that we need for a good sap run.  But we’ll see.

Footprints

 

References

Johnson, R.W., M.T. Tyree and M.A. Dixon. 1987. A requirement for sucrose in xylem sap flow from dormant maple trees. Plant Physiol.  84:495–500

Milburn, J.A. and P.E.R O’Malley. 1984. Freeze-induced sap absorption in Acer pseudoplatanus: a possible mechanism. 62:2101–2106.

Tyree, M.T., Cirelli D., Jagels R. 2008.  Toward an improved model of maple sap exudation: the location and role of osmotic barriers in sugar maple, butternut and white birch.  Tree Physiol.  28(8):1145-55

Edmonton Maple Tap 2012

My copper-pipe method of tapping maplesLast year I tapped two maple trees in my backyard.  I got more than 40 L of sap, most of which was reduced to make about 1.5 L of syrup.  There’s a complete summary of the adventure here.

Last year my first day of sap collection was April 2.

Yesterday, March 7, I was in my backyard.  It was warm and sunny.  I had to squint because of the sunlight coming off the snow.  There was a steady stream of water rolling through my eavestroughs.  It felt exactly like April of last year, and it crossed my mind that the sap could be running at that very moment.

This morning I tapped my two trees, as well as a neighbour’s.  There haven’t been any signs of sap yet, but it was cold and overcast.

I’m expecting of lower yield than last year.  Ideal conditions for the run are a) lots of snow on the ground, which insulates the trees’ roots, and b) warm sunny days with freezing nights.  We had very little snowfall this year, and it looks as though we might go straight from freezing days and nights to very warm days and nights.  We’ll see.

Two buckets awaiting maple sap

Pancakes

Frying pancakes in bacon fatLast night was Pancake Tuesday, the appropriately subdued Canadian version of Mardi Gras, or Fat Tuesday.

I want to tell you about my pancakes.

 

The Recipe

Pancake styles occupy one point on a continuum between slack batters and stiff batters.  Slack, or high-liquid, batters make thin, soft, limp pancakes the size of dinner plates.  Stiff, or low-liquid, batters, yield thicker, cakey pancakes the size of tea saucers or smaller.  For home-cooking I favour the stiff variety, making a batter that is barely, barely pourable.  The resulting cakes are more dense, but still soft and moist.  They develop a delicate, crisp exterior during frying, something that the slack batters can’t do because of their high liquid content.

In the name of flavour, I make two substitutions to standard pancake recipes.  First, I convert half of the milk called for in the recipe to buttermilk.  I’ve experimented with all kinds of ratios, from no buttermilk, to all buttermilk.  The purpose of the buttermilk isn’t to make the pancakes sour, but to add a mild acidity that wakes up the palate.  Half buttermilk and half whole milk seems to be the right balance.

Second, I convert one quarter of the all-purpose flour called for in the recipe to whole wheat flour, which adds a bit of flavour, texture, and colour to the batter.

My full recipe is typed below.

 

The Cooking Procedure

The griddle is the supreme cooking vessel for pancakes, as the temperature is easy to control (375°F is the ideal setting) and the heat is uniformly distributed by the dense metal surface.  My griddle also has a trough around its perimeter that catches fat.  This is important.

Once my gridle is hot I fry an entire evening’s worth of bacon and sausage.  Fat renders from the meat and accumulates in the troughs.  I remove the meat to a tray and hold it in a 250°F oven.  Before cooking each batch of pancakes, I spoon some of the bacon fat from the trough over the surface of the griddle.  After the buttermilk and flour, this is the main source of flavour, and I think the key to superlative cakes.

Thanks to Andy and Vanessa for hosting dinner last night.  Sorry about the smoke.

The details:

 

Pancakes for Shrovetide
(buttermilk pancakes in bacon grease)

Ingredients

  • 1 pound quality bacon or sausage
  • 6 oz whole milk
  • 6 oz buttermilk
  • 2 large eggs
  • 2 oz unsalted butter, melted
  • 6 oz all-purpose flour
  • 2 oz whole wheat flour
  • 2 tbsp granulated sugar
  • 2 tsp baking powder
  • 1 tsp kosher salt

Procedure

  1. Fry the meat on a 375°F griddle until browned and rendered.  Remove to a tray and hold in a 250°F oven.
  2. Combine the milks, eggs, and melted butter in a large mixing bowl.  In a separate bowl, combine the flours, sugar, baking powder, and salt.  Whisk the dry ingredients into the wet until just combined.  Do not overmix.  The batter will still be a bit lumpy with unincorporated flour.
  3. Distribute the bacon fat evenly over the griddle.  Spoon the batter onto the griddle  in 2 oz rounds.  Fry until the bottoms are amber-gold, the edges of the pancake have set, and there are bubbles of air appearing on top.  Flip.  Again, once the bottom is amber-gold, the pancake is done.
  4. Enjoy with the bacon or sausage, and maple syrup.

Frying pancakes in the bacon fat

 

Birch Syrup

I just had my mind blown.

While Lisa and I were collecting sap from our maple trees, Judy was doing the same from a birch tree in her backyard in Spruce Grove.

She just brought over some of her syrup.  I had a spoonful.  I’m reeling.

I mentioned that our maple syrup has a distinct fruitiness that I’ve never come across in commercial syrup.  Judy’s birch syrup tastes like fruit juice – like pear juice, I would say – and it finishes with some of the green, nutty flavour of the fresh sap.

The birch syrup is very thin, nowhere near as thick and sticky as store-bought syrup.  The flavour is remarkable.  I don’t know exactly how I’ll use it in my kitchen.  I might just have a spoonful for breakfast every morning, until its gone.

Oat Cake in Maple Syrup

Oatcake in Maple Syrup

This is one of my favourite ways to showcase my maple syrup.  A simple oatcake is baked, then cut into squares and cooled.  The baking dish is then filled with hot maple syrup, which the cake soaks up like a sponge.  Essentially a lazy man’s pouding chômeur (a lazy man’s poor man’s pudding?)

Oatcake in Maple Syrup

Ingredients

  • 1 cup rolled oats
  • 1 1/4 cup boiling water
  • 1/2 cup unsalted butter
  • 1 cup packed dark brown sugar
  • 1 cup granulated sugar
  • 2 large eggs
  • 1 1/2 cups all-purpose flour
  • 1/2 tsp freshly ground cinnamon
  • 1/4 tsp freshly grated nutmeg
  • 1 tsp kosher salt
  • 1 tsp baking soda

For the soaking syrup:

  • 2 cups maple syrup
  • 2 cups water

Procedure

  1. Preheat oven to 350°F.  Grease and flour a 9″x14″ casserole.
  2. Combine the oats and water.  Set aside.
  3. In a stand mixer, cream the butter and sugars until light and fluffy, about 5 minutes.  Add the eggs one at a time until incorporated.
  4. Sift dry ingredients into a separate bowl.  Slowly add to butter mixture with mixer on lowest speed.  Scrape down the sides of the mixing bowl periodically.
  5. Fold in the oats.
  6. Pour the batter into the casserole.  Bake until a wooden skewer comes out clean, about 25-30 minutes.  Cool.
  7. Once the cake has cooled, cut into serving squares without removing from the casserole.  Heat the maple syrup mixture on the stove, then pour over the cake.  Let stand for several hours.  Gently warm in a low oven before serving.  Spoon any syrup left in the bottom of the casserole over the plated cake.  Serve with ice cream. 

Tapping Maples in Edmonton

Even though maple syrup is popularly described as a “Canadian” ingredient, I consider it a highly regional specialty within Canada, as it’s only made on a large scale in Eastern Ontario and Quebec. In contrast to the sugar maples that grow down east, the maple trees around Edmonton produce less, and less sweet, sap. Birch and elm can also be tapped for sap, but they have even lower yields.

These facts notwithstanding, I have a perverse obsession with maple syrup (one of my favourite desserts of all time is pouding chômeur) as well as an abstract, academic nostalgia for the ingredient. Granulated sugar is one of the few highly refined products that I use regularly, and I’m interested in finding ways to replace it with, say, honey and maple syrup. Consider this:

For the colonists, maple sugar was cheaper and more available than the heavily taxed cane sugar from the West Indies. Even after the Revolution, many Americans found a moral reason for preferring maple sugar to cane; cane sugar was produced largely with slave labor. Toward the end of the nineteenth century, cane and beet sugar became so cheap that the demand for maple sugar declined steeply.[1]

Besides all this, making maple syrup has an extremely low effort-to-benefit ratio: by drilling a hole in a tree and boiling the sap that leaks out, you can enjoy one of the great pleasures of the table.

In the spring of 2011, Lisa and I moved to a new house. The backyard of that new house was a bit like a wrapped birthday present. The wrapping was the three feet snow concealing the features of the yard. There were small tears in the wrapping, if you will: the tops of wooden stakes, promising some manner of garden; shrunken, frozen apples on one of the trees; and best of all, clinging to the topmost branches of a tall tree, those winged seed pods that fall to the ground spinning like propellers. Maple keys.

I resolved to tap this maple tree, even if it was of the low-sugar variety. I had only the most basic idea of how to do this.

 

Part One: Tapping

Some guidelines:

  • Any maple with a trunk wider than 10″ in diameter can be tapped without damage to the tree.
  • Tap the tree when the sap is running. The sap runs during the spring thaw, when the days are warm and nights are cold.
  • To tap the tree, drill a hole that is slightly smaller than the diameter of your spile (the metal spigot). The hole should go 2-3″ into the trunk, at a 10-20 degree incline, anywhere 2-6′ from the ground. Apparently south-facing holes have a higher yield in the earlier weeks of the sap run.
  • Lightly tap the spile into the hole and hang a bucket to collect the sap.

Some day in the future I may buy proper spiles and buckets. In the meantime I used some 1/2″ copper pipe from the plumbing section of the hardware store, and a plastic bucket supported by a wall hook. I covered the bucket with a plastic bag so that nothing could fall into the sap.

The first tree I tapped actually forks into two large trunks. Since both trunks are more than ten inches across, I put one tap in each.  Part way through the sap run, I realized that I have another maple tree on the other side of my yard, so in total I made three taps.

Here is the forked tree, on the east side of my backyard.

 

And here is the tree I missed at first, tucked away in the soggy southwest corner of the yard.

 

Part Two: Collecting

Some time on or around Saturday, April 2, 2011, the sap in my first maple tree started running.

Some observations:

  • the sap runs during the day, not so much at night
  • the sap looks pretty much like water
  • the specific gravity of the sap is 1.008 (small but detectable amounts of sugar)
  • the sap tastes every so slightly sweet, with some very pronounced flavours: woody, nutty, very “green”
  • I think I could drink a glass of the sap with breakfast every morning for the rest of my life

The buckets have to be emptied every day. Some days they would have overflowed if they hadn’t been emptied. Even during periods of low flow, it’s best to empty every day to maintain the freshness and cleanliness of the sap.

Sap flow started very high, then tapered quickly (daily sap quantities are listed below). The first full day of the run I got about 4 L from the eastern tree.

 

Part Three: Boiling

As I mentioned above, the sap itself is delicious, even without being reduced to syrup. If you are only tapping one tree, you might get more bang for your buck by drinking a few litres of sap instead of reducing to get a cup or so of syrup. Something to consider.

Either way, strain the sap to remove any debris and store in the fridge.

Once you’ve collected enough sap to fill a stock pot (10 L in my case), boil the sap over medium high heat. I was cautioned to do this outside, as allegedly a sticky residue can build up on surfaces if done indoors. I ignored this caution and boiled the sap on my stove with the vent hood running. No residue appeared, perhaps because I was processing a relatively small amount of sap.

As soon as the sap is brought to a simmer, it turns from clear to cloudy. If you continue to boil the sap down you get a very murky syrup that looks a lot like honey:

If you want clear syrup, you have to remove the “sugar sand,” the calcium compound that is clouding the liquid. Lisa and I simply stopped boiling partway through the reduction, let the sugar sand settle to the bottom of the pot, then decanted the syrup and continued boiling. My understanding is that the sugar sand is not harmful in any way; it’s removed simply to clarify the syrup. Here’s a picture of some of the sugar sand we filtered out:

If you’re diligent with your decanting and filtering, you will end with a more familiar looking, clear syrup.

I didn’t reduce my syrup to the same thickness as commercial syrup. Besides being less sweet, the flavour of my syrup is much different than store-bought: there is a pronounced fruitiness, one that I would associate with a fine honey.

 

Part Four: Analysis

Below are the quantities of sap I collected each day. Note how the flow starts very high, then tapers to almost nothing in a matter of days. At this point I thought that the run had ended, and I stopped checking my buckets. Then about a week later I was in my backyard and noticed that the buckets were overflowing. Some of the sap was lost, but I don’t know exactly how much. Also, I don’t know if the break in the flow had to do with the weather (it was very cold and overcast on those days), or if it is part of a regular cycle in the run. Interestingly, the second wave of the run produced noticeably sweeter sap.

The maple syrup article in On Food and Cooking says that sugar maple sap is typically reduced by about 40 times, and birch sap by about 100 times. Having a maple tree, but not a sugar maple, I was expecting to reduce somewhere between 40 and 100 times. I ended up reducing by only 29, though, as I mentioned above, my final product was not as thick and sweet as commercial syrup.

Tapping the trees took about ten minutes. Emptying and straining the sap took about ten minutes each day of the run. Boiling the sap down took a few hours every few days of the run, but obviously you don’t have to stand over the pot and watch the liquid reduce. In the end I got a litre and a half of syrup, which I suspect will amply garnish our pancakes for a year.

For the longest time I thought that there are few maples around Edmonton, and that the ones that are here are no good for syrup. I was wrong. Now as I walk through my back alley in McKernan, I see suckering maples everywhere. Most of them are too small to tap, but there is still a huge amount of “untapped’ sweetness in our city. Maples, like caragana, are much more common in the older communities of Edmonton than in the newer suburbs, as they are considered “messy” plants, what with all the suckering and keys…

Tapping maples in Edmonton is not only possible, it is worthwhile, and you should try it if you have the opportunity.  Next winter I’ll put a small ad in my community newspaper to see if there’s anyone interested in learning to do this. If you have a mature maple (or birch) tree in your yard, here are some resources for you.

  • A great website with lots of detailed information: http://www.tapmytrees.com/
  • Mack, Norman (ed.) Back to Basics: How to Learn and Enjoy Our Traditional Skills. ©1981. The Reader’s Digest Association (Canada) Ltd. Montreal, QB.
  • Or contact me through Button Soup

 

Reference

1. McGee, Harold. On Food and Cooking. ©2004 Scribner, New York. Page 668. I love this book.