How much sap does it really take to make one gallon of maple syrup?
I cringe a little when someone states matter-of-factly that it takes 40 gallons of sap to make one gallon of maple syrup. The answer to that question depends on the sugar content of the sap. The sap sugar content is impacted by many factors including tree health, leaf canopy size, soil conditions, climate patterns, water availability, and the time of the year. In reality, it can take anywhere from 20 to 100 gallons of raw maple sap to make one gallon of pure maple syrup.
Maple farmers have used a long-standing rule-of-thumb to estimate the projected yield of maple syrup based on the sugar content of the raw sap; however, that rule has limitations. Minor tweaks to the formula can improve the accuracy of the estimated yield, but even with those updates, the rule ignores a critical piece of the sugaring season: the sugar left in the evaporator at the end of the year known as the ‘sweet’. This can make the rule’s projections sorely inaccurate when we look at the season’s total sap collected divided by the total syrup produced.
In this article I discuss the Jones Rule of 86 and its limitations. Then I present a correction factor to account for unused sweet at the end of the year that can have surprisingly significant implications for how much sap it really takes to make one gallon of maple syrup.
The Jones Rule of 86
The rule-of-thumb known as the Jones Rule of 86 is used to estimate the number of gallons of raw sap needed to make one gallon of maple syrup. Once the sugar content of the sap has been measured, you simply divide 86 by the percentage of sugar. For example, if the sap is 2% sugar, then dividing 86 by 2 results in needing 43 gallons of sap, or a 43:1 ratio. If the sap was incredibly sweet at 4%, it would be halved again, and the raw sap to finished maple syrup would be about 22:1. In cases where the sap is as low as 1%, we need and 86 gallons of raw sap to make one gallon of maple syrup.
The Jones Rule of 86 has limitations. It was established back in 1946 by C.H. Jones at the University of Vermont. The legal sugar content for pure maple syrup back then was only 65°Brix, as opposed to the current requirements of 66°Brix in most states and Canada or 66.9°Brix in Vermont. (Yes – Vermont’s maple syrup is sweeter!) The extremely high sugar contents that are achievable by modern reverse osmosis machines also make the rule less accurate. Even with these caveats, the Jones Rule of 86 still works just fine as a quick estimate of the number of gallons of sap required to make one gallon of maple syrup.
Ten years ago, the team at UVM published some revised and more accurate formulas. The new formulas account for the higher sugar content in Vermont’s syrup and they also take into account the higher sugar concentrations from the use of reverse osmosis. Their report can be accessed here. For syrup at a density of 66°Brix, the adjusted formula divides the sugar content by 87.1 and then subtracts 0.32. For sweeter syrup at a density of 66.9°Brix here in Vermont, the adjusted formula divides the sugar content by 88.2 and then subtracts 0.32. These formulas improve the accuracy of the estimated gallons of sap to syrup by about 2%.
Given the inherent inaccuracies of measuring the sap sugar content, the sap volume, and the maple syrup sugar content, I stick with the Jones Rule of 86. It gets you in the right ballpark, the math is easier, and there’s something comforting about using the same rule-of-thumb that has been used for generations. But the rule does need a correction factor to accommodate for the sugar content left in the evaporator and I’ll outline that next.
When the Rule Does Not Work
I have a small sugaring operation with only 300 trees. It is larger than a typical hobby setup but tiny compared to the factory farms of today. My trees aren’t usually the sweetest, but this past year was surprisingly different, and my early season sap was in the 2.5% range, and even at the end of the season I was still measuring about 1.5% sap. If I assume an average sugar content of 2%, then by using the Jones Rule of 86, I would estimate a sap to syrup ratio of 43:1. My assumption might be that for every 43 gallons of sap collected during the season, that I would end up with one gallon of syrup. But we know what happens when we assume.
At the end of the season I tallied up my total estimated sap collected and divided by my total gallons of syrup produced; I calculated a 54:1 ratio. What happened?! My sap numbers were only estimates and I knew that some syrup was lost during the filtering process and multiple samples were enjoyed, but the yield was significantly lower than the rule had predicted.
This large discrepancy happens because the Jones Rule of 86 doesn’t take into account the significant amount of sugar that remains in the evaporator at the end of the season. At the start of the year, we need to sweeten the pans to build a concentration of the sugar in our continuous flow evaporator. There is a lot of sap boiled off before the first drops of syrup are even made. Read about sweetening the pans here. At the end of the year, that amount of sugar is still in the evaporator because the pans are full of ‘sweet’ that is often not used.
For a given day in the middle of the season, the Jones Rule of 86 is rather accurate. If you collect 43 gallons of 2% sap, then you will make about 1 gallon of maple syrup. But I like to know how much syrup I will produce based on how much sap I collect in a season, so I want to include what is left in the evaporator. I could try to get all of the sugar out, but I’m exhausted by April and working tirelessly to get out the late-season syrup just isn’t worth it for me.
The ‘Not-so-Sweet Factor’
There are ways to get the remaining sugar out of the sweet, but that requires more boiling which gets tricky as you try to replace the sap with water. More importantly, the flavor of the syrup at the end of the year is strong and can verge on off-flavors. So at the end of the season, many sugarmakers are just fine throwing in the towel and giving up on the sugar that remains in the evaporator. It’s just not worth the trouble to make not-so-good maple syrup.
For operations like mine, the sugar left in the pans can represent a significant portion of the sugar collected during the season. I typically produce 100 gallons of syrup. The evaporator holds roughly 50 gallons of semi-concentrated sap (the ‘sweet’). It is a continuous flow evaporator, so the sweet ranges from about 2% sugar to nearly fully sweet syrup. That means that on average, it is about half as sweet as syrup. This means that I can estimate that my evaporator still has 25 gallons worth of syrup in it. The problem is that I can’t, or don’t want to get the sugar out. Even though I make 100 gallons of syrup, I collect enough sap to make 125 gallons of syrup.
To account for the lost 25 gallons, I need to multiply my initial 43:1 estimate by the ratio of potential syrup to actual syrup. In my case that is 125/100, or 1.25. The result is that taking the whole year into account, my adjusted estimate including the sweet is now 43 * 1.25 = 54 gallons of sap to make one gallon of syrup. That is exactly the 54:1 that I calculated earlier.
I refer to this correction as the Not-so-Sweet Factor since it essentially represents a loss of sugar that is collected in the form of sap but never turned into maple syrup. The best Not-so-Sweet Factor is 1, where essentially all of the sugar from the sap is harvested and we maximize our yield. I can envision bad years too though. For example, if the pan gets sweetened, you make a little bit of syrup, and then the temperatures spike into the seventies and bring the season to an abrupt halt. (this happened about ten years ago) In this worst-case you could end up with a Not-so-Sweet Factor of 2 where you collect enough 2% sap to make 50 gallons of syrup, but you only draw off 25 gallons and the other 25 is left in the evaporator, perhaps with an off flavor. In that case, was it a 43:1 type of year or more like 86:1? I would say the latter since you had to collect all of that sap.
How to Yield More Syrup
No farmer wants to miss out on the maple sugars left in the evaporator at the end of the season. Ideally we want our Sweet Factor to be 1, so that we get all of the sugar out of the sap. In my case, I would make 125 gallons instead of 100. There are ways to get the sugar out of the sweet, but more boiling is required which requires more time and energy. My primary concern is the quality of the syrup because often the late-season syrup quality declines and off-flavors increase. If the syrup it sold to a bulk buyer, that is less of a concern, but still something to be aware of.
Less sweet in the pan is one way to increase your yield. There are various ways to reduce the amount of sweet and they may or may not work for a particular sugaring operation. A smaller finishing pan could be used to finish out the season. It is also possible to run the pans with less sap at the very end. Burning pans becomes a risk though, so you have to be careful about trying to squeeze out the last draw-off of syrup. There are also ways to backfill in with water as the sap boils. Whatever method is attempted, there is a balance between the level of effort required, the risk to equipment, and the resulting quality of the syrup.
Increasing the yield is another way to reduce the impact of the leftover sweet. For folks using reverse osmosis, the losses aren’t as bad because the evaporators are much smaller in comparison to the amount of syrup produced. Imagine if you concentrated with reverse somosis, and could make 500 gallons of syrup instead of 100 with an evaporator. Then accounting for the 25 gallons worth of syrup left in the pans, the correction factor would only be 525/500 which results in adding 5% from the ‘lost’ syrup. With 2% sap, the estimate of 43:1 would be reduced to a 45:1 ratio. That is significantly lower than 54:1 that is expected without reverse osmosis.
Another way to increase the amount of syrup from your sap is to increase the number of taps. In my case, moving from 300 to 400 taps might seem like I would make 33% more syrup. But once you roll in the reduced impact of the leftover sweet, the actual net gain in annual production will end up being about 36%. This of course needs to be balanced with how much the evaporator can handle in terms of numbers of taps.
The Bottom Line
I will keep using the Jones Rule of 86, but I will add 25% to the result based on my evaporator size and process. I will also keep using my old-school boiling process without reverse osmosis and I won’t try to squeeze out more syrup when the quality declines. I know that I am losing a lot of the sugar at the end of the year, but I enjoy traditionally crafting my maple syrup and don’t mind flushing the sweet back into the woods when the quality of the sap declines.