First: a measurement. Our sap is starting off at about 2.5 wt% sugar, according to our hydrometer. Not bad for a box elder, pretty much in line with a study from St. John's University in Minnesota. But more importantly, with that number, we can calculate all sorts of expectations for this batch of syrup. Allow us to momentarily digress!
|This is the sugar content of the box elder sap on the Balling scale, which is essentially the same as the Brix scale more commonly used for sap measurements. (Using the same hydrometer we use for brewing wine.)|
For example, knowing the wt% sugar, we can use the tables here to calculate the density of the sap and the resulting syrup (because syrup is normally 66 wt% sugar, which is very close to the solubility limit at room temperature), which will give us the volume of the sap and the resulting syrup, and our expected ratio. Those calculations are detailed in this spreadsheet, if you want to plug in the numbers from your own sap. For us, our 38.4 lbs of sap at 2.5 wt% sugar works out to about 4.6 gallons, from which we should be able to expect just over two cups of finished syrup. (But note the caveat below.)
|38.4 lbs of sap at 2.5 wt% sugar should yield about 2.1 cups of syrup, which would be a ratio of 34.6:1. That would be pretty good for a box elder.|
We can also use other tables in that document to calculate what the boiling point of the finished syrup should be. It would normally be 4 °C (7 °F) above the boiling point of pure water wherever we are, but that depends a little bit on what the boiling point of pure water is. We're in the suburban Denver, where water boils at about 95 °C (203 °F) due to about 18% lower absolute atmospheric pressure. (Don't worry, we used NIST data for the Antoine equation to calculate the vapor pressure of water at that temperature, and everything checks out. Well played, Universe.) As it turns out, the difference in boiling point increase for syrup at our elevation is within the precision of our thermocouple reader. Or, long story short, we're looking for a final temperature on our syrup of about 210 °F. Time to light the fire!
|We added a wind break to our Dakota Rocket Silo and started out using the widest-diameter pot we have.|
|With the volume decreasing by about half, some of the minerals ("sugar sand") started to precipitate out.|
|Eventually, the volume got low enough to combine everything into one pot. Time to make some pancakes! We ended up adding one more layer of blocks back onto the wind break.|
|Pancakes done, sap still boiling. We might have to modify our evaporator design for the next batch. In the meantime, let's make bacon, too!|
|Bacon done, sap at ~35% sugar. Daylight running out. Time to bring it inside to finish on the stove.|
|And.....the finished product in a pint jar. It's delicious.|
Ok, so the finished product looks much closer to one cup in volume than the two we expected. Why the difference? (Here's the caveat we mentioned above.) There are a number of potential contributions. The equations above are for solutions of sucrose in water, so the initial 2.5 wt% reading we took didn't account for other dissolved solids (e.g., the sugar sand that precipitated out during cooking). That is, we might have started out with lower than 2.5 wt% sugar. Also, there were some losses on transferring between buckets and pans, filtering, and what we took out for testing along the way. We were also physically unable to resist trying some of the 35% stuff on the pancakes, which probably accounted for at least 1/4 cup (don't judge!). Finally, we also ended up going a little bit over 66% sugar (closer to 70%, based on our honey refractometer's scale, which had store-bought maple syrup right at 66%), which would decrease the volume, in theory, by about another 1/6 cup. Overall, even if we got one cup syrup instead of two, our sap-to-syrup volume ratio was still a respectable 70:1. In general, it looks like the calculations above should get you to at least within a factor of two.
How is your syrup season going?