All Things Evaporators: Part IV

In this final post of the evaporator series, we will examine a few remaining factors to account for when considering the boiling process for maple syrup. As you will see, a few instruments enable the necessary precision to ensure a high-quality batch of syrup every time.

In case you missed them, here is Part I, Part II, and Part III of the series.

Barometric Pressure Matters

One of the biggest factors influencing the boil in an evaporator is barometric pressure. Barometric pressure and weather fronts are frequently responsible for the day-to-day erratic behavior in the way sap boils. The boiling rate is directly associated with the barometric pressure on any given day. If you experience a high barometric pressure, sap boils faster; with low barometric pressure, the boil slows. Meteorological shifts can happen several times per day, and whenever pressure fluctuates the boiling point of water (212 F) will vary. Producers must adjust their thermometer to accurately produce syrup consistently at 219 F. Make sure you calibrate your thermometer in boiling water before the start of each boil and throughout the day as needed. A thermometer will give you a ballpark reading, but to get ultra-precise and guarantee 66 Brix syrup of the highest quality, additional instruments should be utilized.

Having the Right Instruments

You will choose one of three instruments to determine your syrup density coming off the evaporator.  Only one is the best and most accurate for reading syrup straight off the evaporator. As previously stated, syrup’s finishing point is 219 Fahrenheit, 7 degrees above the boiling point of water. Because barometric pressure influences boiling point, using only a thermometer can result in inconsistent finished syrup density. One better option is to use a refractometer, but the syrup sample has to be temperature-stable and filtered to get an accurate reading. For this reason, we do not recommend using a refractometer on syrup coming directly off the evaporator for obvious and practical reasons. (Refractometers are, however, the instrument of choice for measuring the density of cooled and filtered syrup during canning).

The most recommended instrument to determine the density of hot finished syrup is the hydrometer.  A hydrometer should be floated in a sample of finished syrup that is at least 211 degrees F. Hydrometers have two lines, one for cold and one for hot. You will use the hot line for your syrup density determination straight off the evaporator. Bring the instrument up to eye level or set it on a stable object close to eye level for the most accurate reading. The hydrometers red line should float even with syrup level in the container. Most hydrometers also have two scales, one for Brix and one for Baume (Baume measures specific gravity of a solution). The Brix scale is the most popular and frequently used today. Avoid letting scale build up on the outside of the glass as it will impact the density reading, and producers should regularly validate their hydrometers for accuracy. Once you confirm finished syrup of the proper density, you will filter your syrup for clarity and to remove niter. You can then use a color comparator to determine the grade of your syrup.


The evaporator has become the center piece of many maple operations. It is the first thing visitors see in your sugarhouse no matter what time of year they visit.  It is also one of the most essential pieces of equipment in your operation.  After all the process of making syrup requires that we must heat maple sap to 7 degrees above the boiling point of water to produce pure maple syrup. This results in the caramelization of maple sap into maple syrup. The addition of heat to maple sap results in the amber color we desire and the maple flavor we love.

Author: Les Ober, OSU Extension Geauga County

A Few Thoughts on Finishing Maple Syrup

Cold weather has set in and that has allowed me to scan the maple chat rooms. Many of the questions that keep popping up are about finishing maple syrup. Is it too thick or too thin? Should I use a thermometer, hydrometer, and/or refractometer? Here are some of my thoughts on the subject.

Most of these questions are coming from backyard producers with a relatively small number of taps. Making syrup on a flat pan or hobby rig is not an easy task. You deal with a lot more “what if’s” than you would on a big evaporator. The process is simple – build a fire under your pan and bring your sap to the boiling point of water. Use a thermometer to monitor the process. That thermometer reading will vary from day to day depending on the barometric pressure. When the temperature goes 2 degrees Fahrenheit above the boiling point of water, add more sap, preferably pre-heated sap. Continue the process until all your sap is in the pan and begins condensing down. At that point, stop boiling, take the liquid into the house to store, and finish the batch. Most hobbyists follow this procedure and it works well. The trouble starts when you have a rig that looks like a big evaporator but does not run like a big evaporator. Many hobby rigs have channels and a heater pan and that is good. Sap should come into the back channel and gradually work its way to the channel on the opposite side near the front. Higher density syrup should move ahead of the lower density syrup. The problem comes in when you have to decide how much sap to let in at any one time. It works okay as long as you can maintain a steady flow into the rig. You need to maintain a depth of 2-3 inches across the entire evaporator. Overflow the hobby rig with liquid, and you will kill the boil. Once this happens, the sap of lesser density intermingles with the heavier density syrup. Big problem! Despite the fact you have channels, you are now no better off than you would be with a flat pan. On commercial evaporators, we have a thing called a float that automatically maintains the level of sap moving across the rig. With a hobby evaporator, you are the float and maintaining the proper level takes time and experience.

A few words on syrup-testing instruments. As stated above, you absolutely must have a thermometer. Two other tools that I recommended are a hydrometer and a refractometer. The hydrometer is necessary and a refractometer is nice if it fits your budget. Others have mentioned the Murphy’s Compensation Cup. I have used one for the last three seasons, developed by Smokey Lake – the Murphy’s Cup is a very useful tool.

I have two ways of measuring density directly off the evaporator. Here is the formula I use. First, I draw a sample into a hydrometer cup once the temperature reaches 7 degrees above the boiling point of water. Remember thermometers need to be calibrated. With your hydrometer cup filled with hot syrup that is above 211 degree Fahrenheit, insert the hydrometer into the cup. When it hits the top red line, you have syrup. I check this several times. Once I have the syrup where I want it, I pour one of the samples into the Murphy Cup. This device has a dial with corresponding numbers to those on a hydrometer. You insert your hydrometer into the cup and let it settle for 3 to 5 minutes. When the reading on the dial and the hydrometer match, you are at the right density. After that, I can fine-tune my auto draw-off for subsequent runs. On the last run, we are hitting between 66.0 and 66.5 Brix with this system. Refractometers are available in digital and analog versions. The digital versions seem to be the most popular. They are very useful to check syrup prior to bottling. Do not use a refractometer at draw-off, a refractometer’s reading is only accurate on temperature-stable and filtered syrup. The only reason for us to have a refractometer in the sugarhouse is to check the sugar content of concentrate coming off your reverse osmosis unit.

Author: Les Ober, Geauga County OSU Extension

The Finishing Pan: Where The Art and Science of Making Maple Syrup Meet

The old timers called it an art, modern maple producers call it a science. In reality, it is probably a little of both. One thing is for certain, what happens in the finishing pan determines the success or the failure of a season. It is here that all of the maple syrup quality standards rise or fall. Here 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 has coalesced for the perfect draw-off.  The result is golden amber maple syrup with the perfect maple flavor.

There are several types of finishing pans on the market today. The reason for the difference is to manage niter or sugar sand. Niter is the mineral content in the sap that precipitates out in the boiling process. To manage niter, most front pans are designed to either change to a side or to a pan with a lower niter deposition. Reverse flow allows the operator to switch sides when niter builds. A variation on design is the one-sided draw-off which utilizes reverse flow and a series of valves to redirect the flow of sap from one side to the other. An example of this would be the Leader Revolution. The other style is the cross flow in which there are multiple front pans connected by stainless tubing. In this configuration, the pan closest to the draw-off point is rotated with a clean pan. The best policy is to start with a clean pan every day and change during the day when needed. Pans can be cleaned with white vinegar and hot water. This is a very effective way to clean pans with a minimal amount of elbow grease. The amount of niter present in sap varies from season to season and from woods to woods. If improperly controlled the result can be a scorched pan.

Once the sap, or in this case concentrated sap, reaches the front pan or finishing pan, the sap is approximately 19% sugar. This is sap that has not been run through a reverse osmosis unit. It has been concentrated by boiling only. RO concentrate enters the pan at a higher concentration. As the concentrated sap is crossing over into the front pan, it should reach 213 degrees at 29.9 inches barometric pressure. It is also at this temperature that the concentrated sap is not only becoming denser but also starts changing color. As the density increases, the sugars react with the heat to form the amber color we associate with pure maple syrup. It is also at this time when the bacteria in the sap can interact with the heat and the sugars to darken the syrup. All of this happens in the finishing pan and over a relatively short amount of time. This reaction can occur quickly and if the operator is not paying attention, syrup can actually burn or caramelize further darkening the color. In this case, the density often goes past 66 Brix as well resulting in a thick heavy syrup and potential profit loss.

To make sure we pull the syrup off at the right density, we can use a variety of instruments. The most common and least expensive are the thermometer and the hydrometer. Most evaporators come with a thermometer that is placed at the point of draw-off. Water boils at 212 degrees Fahrenheit at sea level and a barometric pressure of 29.9 inches of mercury (Hg). Because syrup is rarely produced in a location at sea level and the barometer is seldom at 29.9, producers must make some adjustments. We must boil water near the evaporator and reset the thermometer that is used to make syrup. This process of adjusting to match the barometric pressure must be done daily and whenever the barometer changes due to weather front movement. This can occur quite often during an average sugar season. This is why most producers prefer to use a hydrometer for the final test, and use the thermometer to give them an approximation of when to draw-off.

The hydrometer is the judge and the jury. There are two lines on a hydrometer.  The top line is for hot syrup and the bottom line for cold. Use the top line. Always use a hydrometer cup full of syrup that is at least 211 degrees Fahrenheit. Bring the instrument up to eye level or set it on a stable object close to eye level for the most accurate reading. Producers need to test their hydrometer annually against a calibrated refractometer. Hydrometers get jarred around and the paper containing the scale can move or become dirty and give a false reading. If the reading is inaccurate, replace it. A refractometer (the instrument used to verify hydrometers) is very accurate, the new ones are digital, and refractometers make temperature adjustments on the fly – however, they are also cost prohibitive for many.

The automatic draw-off is a great tool for any producer in any size operation. It makes drawing off syrup a lot easier, especially when boiling RO-concentrated sap. It is nothing more than a digital thermometer hooked to a valve that draws the syrup off at a very precise temperature. Everything I said about the syrup thermometer applies to automatic draw-offs. Most producers set there draw-offs with a hydrometer. During the sequence of opening and closing the auto draw-off, the syrup is actually being drawn off within a small band of temperatures. The thing to remember is that the draw-off will open at a very precise temperature, but if the flow is slowed by foam or a valve coming into the draw-off is restricted, the temperature will rise above the desired level resulting in denser syrup.  All automatic draw-offs should be installed with a valve between the pan and the draw-off. This allows the producer to adjust the flow of sap coming off the pan. Open the pan valve so a steady stream flows through the draw-off mechanism, and try to avoid a heavy stream that will result in a large batch. The draw-off should close and the temperature on the readout should drop 4 to 6 degrees and then quickly come back to the desired temperature. The result is a series of small batches coming off in a relatively short amount of time. The producer needs to check the final product in the bucket or tank when the auto valve closes and adjust the draw-off settings accordingly. It is very easy to get a denser product than desired if you are not making continual fine-tuning adjustments. It is not a set-and-forget instrument. Today there are newer auto draw-off that compensate for barometric pressure but again the cost may be prohibitive for smaller scale producers.

Another area to consider during the finishing process is foam control. You only control foam in the front pans at the point of draw-off and only if the flow out of the draw-off point is being held up by the foam. If this happens, a single drop of defoamer will reduce the foam to the point where the bubbles will decrease and flow will increase. Avoid using defoamer anywhere else as it causes the gradient to break down and syrup densities will intermingle. If you are foaming over in the front pan, it is usually because the foam is not properly controlled in the flue pan. Occasionally it may be necessary to knock this foam down but try to avoid this if possible.  If the foam is properly controlled in the flue pan there should be minimal problems in the front pan. The only exception would be coming into the first draw-off after a layoff. All types of sap will behave differently during the initial draw-off. Watch for increased bubbles and denser steam, this is a sign that you are making syrup across the front pan. In this case do not panic, just slow your boil down and stabilize the evaporator as quickly as possible. The result is usually one big batch of syrup followed by reduced boiling temperature. The next batch should be normal – if not, look for the problem.

Author: Les Ober, Geauga County OSU Extension