I just finished making maple syrup for the season and thought I'd recap the process for anyone interested in how it works.
Maple syrup is made by boiling down the sap from one of a half dozen different species of maple trees, with sugar maple being the preferred. Depending on the sugar content of the sap, it can take anywhere from 30 to 40 gallons of sap to make one gallon of syrup. Given the amount of fuel required to boil the liquid down for that long, I find the rocket stove to be the ideal stove for small batch production.
I start off by freezing the sap as I collect it since sap, like milk, will spoil if not kept cool. Then the challenge is how to thaw it out and heat it up to add to the already boiling sap. So this year I built a second stove and documented the process here .
So I start the boiling process by heating up a pan of water on my new stove and setting a frozen gallon jug of sap in it to thaw out.
Once I have several gallons thawed out, I heat them up to the steaming point before adding it to the main pot:
I add the new sap to the existing pan via a smaller feeder can, since the boiling point of syrup is higher than that of sap (which is mostly water). Syrup is syrup when it boils at a temperature of 7 degrees F above the temperature that water boils at that location on that day. So the new sap is by definition at a slightly lower temp than what's in the pot and one does not want to "kill the boil." If you look closely you can see an egg in the pan that I use for a visual clue as to how much has boiled off. I try to keep the egg covered and add more hot sap when it begins to show itself. Plus when you're done you have a maple syrup flavored hard boiled egg.
Periodically, bubbles will form on the surface of the pan which insulates the top somewhat and doesn't allow the steam out. So that gets skimmed off:
Here's a shot of the main stove and pot supports which hold the pan slightly above the riser. I made them with the same mix that the stove body is made from. There is a small groove into which a piece or rebar can rest to support the pot while giving enough clearance for the exhaust gasses to ensure a clean burn:
The batch boils down to almost syrup and then gets finished inside on the stove where I have more control over the stove temps.
So, that's how I do it. If nothing else a great way to spend an early spring day when there is still snow on the ground and get something great in the process.