All Things Evaporators: Part II

Scroll down or click under the Evaporators and Finishing Archive tab to read Part I.

Managing Your Syrup Pan

What happens in the syrup (or front) pan determines the success or failure of every producer’s season. It is here that all the standards of maple syrup quality come together. Ideally, the right density meets the right color and the right flavor. The science is using instruments to determine the exact time to draw off the syrup. The art is that sixth sense of knowing when everything is moving toward the perfect draw-off. That sixth sense is something that requires experience and is often handed down generation to generation. If the science and art come together properly, the result is golden amber maple syrup with the perfect maple flavor.

When sap transitions from the flue pans to the syrup pans, many gallons of water have already been removed leaving a sap concentration of roughly 18-19 Brix. If syrup represents 1 gallon of the remaining liquid, approximately 9 more gallons of water still need to be evaporated in the syrup pans. The speed at which this happens is relative to the size of the evaporator you are running and the quality of your fuel source. In most rigs, the transition happens quickly, and operators must devote their undivided attention to avoid problems and ensure a quality product.

Unlike flue pans, the front or syrup pan is a flat bottom designed to create a surface with even heat exchange. There are several types of front pans on the market today. Traditional drop flue evaporators were equipped with a standard reverse flow pan which allowed the operator to change the side used to draw off when niter (sugar-sand) built up. Over the years, this style of evaporator has seen modifications.

One improvement included designing the pan so that the flow can be reversed while allowing the draw-off to remain on one side. This is accomplished with a series of valves and external plumbing directing the flow of sap from one side to the other. An example of this would be the “Leader Revolution Pan.” Producers found this improvement to be helpful to avoid the movement of draw-off equipment from one side of the rig to the other.


Cross flow design on a raised flue evaporator.  Electronic floats calibrate sap depth between the back and front pans.

Another front pan configuration is the cross-flow design. Cross-flow pans are installed setting across the arch hooked in series with the draw-off near the front of the last pan. There can be anywhere from two pans on a standard rig to four pans on bigger rigs designed to handle “High Brix Concentrate”.  Because niter tends to accumulate in the draw-off pan first, that pan needs to be switched out to avoid excessive niter build-up. Most producers using this system have one or two extra pans cleaned and ready if the draw-off pan needs to be switched.

The depth of the sap in the front pan is determined by the design of the evaporator. A drop flue rig will maintain the depth set by the flue pan float, but a raised flue rig allows producers to set a separate depth in the front plan. Producers should carry approximately 2 inches of liquid across the front pans allowing syrup to boil evenly to the draw-off point. As pointed out in the first evaporator post, if hot spots develop, that area of the pan will tend to boil faster increasing the risk of burning. The trouble usually occurs when you draw off large volumes of syrup at one time. This causes the liquid level to become very uneven, you might have 2 inches in one part of the pan and only a half-inch in another.  Removing small batches more often will prevent uneven syrup levels and ensure a steady even boil.


Site gauge for monitoring sap depth on a raised flue evaporator.

It bears repeating that producers should pay close attention to bubbles in the sap. As liquid temperatures go above 219 F, the liquid will gravitate toward the hot area, localized boiling becomes more intense over the hot spot, and steam and bubbling from the more intense boil becomes more concentrated and noticeable. The result is that the sap is becoming more concentrated in the hot zone. This means the sap is becoming more concentrated in that area. As concentration increases and sap thickens to syrup, the thicker liquid will not flow evenly toward the draw-off point, and you could be headed for trouble. At this point, you need to let more liquid into that portion of the pan to re-establish flow toward the draw-off. Maintaining a constant even flow of syrup in the form of low volume draw-offs stabilizes the process. Make all your adjustments in small increments and remember it takes time for that adjustment to affect the process.

Three important factors must be controlled to maintain a constant boil.

  • First, maintain a steady even fire in the firebox.
  • Second, control your foam in the flue pan.
  • Finally, control niter build-up.

In Part 3 of this series, the focus will be on controlling foam and niter build-up in the syrup-making process – come back next week!

Author: Les Ober, Geauga County OSU Extension

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