The sap of the maple tree is quite a complex mix of sucrose, water and a lot of compounds, including minerals like calcium. In the process of boiling sap we concentrate all these ingredients and as the sugar concentrations rise close to the levels of syrup, come of the calcium starts to come out of solution. Much the same as what you get in kettles or humidifiers. This is called nitre and is considered harmless.

The nitre can take two forms. It can either stick to the hot surfaces on the pans that makes a sugary scale that can scorch if it gets too hot, or it becomes very very fine granuals that stay in suspension. When the syrup first comes out of the evaporator we run it through what we call a pre-filter. This captures most of the fine granules. We get about 1 cup of this stuff per gallon of syrup and need to change these pre-filters frequently while we're boiling. The nitre particles are very small, in the range of 5 - 10 microns. Lots of it will pass through a regular coffee paper filter. (we tried that in the first year)

Many hobby and small syrup makers do their bottling at this point, while the syrup is still hot. If you put the syrup into clear bottles, after a few days there is a thin layer of the nitre will settle out and can be seen on the bottom of the jars. It is considered harmless, but not very appealing to the eye.

To avoid this, we have a micro filtration unit, seen below, that uses 5 micron filters and a high pressure pump.

But now comes the sticky part.

To safely bottle or can syrup, you need to seal the containers at 185 to 190 degrees, farenheit, just like strawberry preserves. But if you hit 190 degrees or more, the nitre starts to form and percipitate out. So the temperature must be controlled to a very fine degree. If you heat syrup in any normal kind of pan, the contact surface at the bottom will be too hot and cause nitre to percipitate. So we have a "canning tank" that has a double wall all around and on the bottom, with a heating element in it. The double wall is filled with water that stays quite stable at 185 degrees - safe for sterilizing and does not cause the nitre to percipitate.

So the work flow is

- take the syrup off the evaporator and run it through the pre-filters to remove 98% of the nitre, and store it in barrels until we are ready to bottle

- re-heat the syrup in a stainless pan on a propane "turkey frier" (or kitchen stove in the picture below)

- run the hot syrup through the micro-filter and into the water jacketed canning tank maintaining 185 degrees

- fill the bottles or cans

- lay the bottles or cans on their sides for about 5 minutes to sterilize the caps and seals.

- cool them and label them for color grade.