Using a refractometer
to measure the sugar
concentration of sap and syrup

Background
Have you ever looked at sunlight passing through a glass of clear water? What does it look like? Does it look different if you fill the same glass with iced tea? With orange juice? With chocolate milk? As you have probably noticed, sunlight passes through different solutions (liquids like water and milk) in different ways.

Now imagine two glasses of water. A pinch of sugar is placed in one glass, and a whole tablespoon of sugar is placed in the other. Both glasses are mixed until the sugar is completely dissolved, and then they are placed in a sunny windowsill.

While our eyes do not notice it, light passes through each glass in a slightly different manner - like what you noticed with the iced tea and milk. This is because the sugar concentration of the water causes light to "refract," or scatter when it passes through the water solution. The glass with more sugar would cause the sunlight to refract more than the glass with only a pinch of sugar.

Certain refractometers can determine sugar concentrations by measuring how light is refracted in a sugar solution. Maple sap and syrup sugar concentrations can be measured with these types of refractometers.

Using a refractometer
Because producers want to measure the sugar content of sap (1-6% sugar), syrup (66-67%), and concentrated sap (between 6 and 66%), refractometers are available with several different scales. Some refractometers can measure the low concentrations of sugar present in fresh sap, while others measure the higher concentrations found in syrup.

Typical refractometers scales are 1) 0-36%, 2) 28-55%, 3) 54-70%, and 4) 68-92%, but the scales may vary. A refractometer with the low scale would be useful for measuring sap, while a higher scale would be necessary for syrup measurements.

Once you have obtained a refractometer, you may be interested in measuring the sugar content of sap from maple trees in your yard or schoolyard, or you may want to measure the sugar content of maple syrup you have purchased or made yourself. You may even be interested in participating in the Cornell Sugar Maple Tree Improvement Program.

Obtaining a refractometer
Refractometers can be obtained from most maple syrup supply companies, and vary from $100 to over $300. Some companies to consider include Waterloo/Small or Dominion and Grimm.

Calibration
Think about the scale you might have at home in your bathroom. Before getting on it, do you check to see if it is pointing to zero? If it is not at zero (for example, if it is pointing at 4 pounds), you would probably turn the knob to adjust it. When you adjust a bathroom scale to read zero when nothing is on it, you are calibrating it. A refractometer needs to be calibrated in the same way.

To calibrate a refractometer, you will use a drop of water containing no sugar. As with the scale, you will adjust the refractometer to read 0 when it is measuring the water sample. The refractometer should read 0 for a water sample because there is no sugar in the sample - much like the scale should read 0 when nobody is standing on it.

To calibrate your refractometer, place a drop of water (preferably distilled) on the dark circular or rectangular area and close the cover. A shadow or dark area is visible on the scale inside the eyepiece. Turn the calibration screw until the shadow falls on the zero mark. Open the refractometer cover and dry the cover and glass prism using soft tissue paper or a cotton cloth.

The optical components of a refractometer change slightly at different temperatures, so it must be calibrated as temperatures change. You should check the calibration of your refractometer (see if it reads 0 for water) after every sap or syrup sample. Once it is stable (you have not needed to adjust it for several samples), you will only need to check the calibration after every fifteenth reading, or after each cleaning. If you are waiting a long time before making measurements (more than a couple of minutes), you should calibrate before and after each sample.

If in doubt, check the calibration. Your values will be meaningless if your refractometer is not properly calibrated.

Testing
You are now ready to measure the sugar concentration of a sap or syrup sample. Only one drop is needed for a sample, though it is important that the sample is very fresh. Old samples (for example, a droplet that has been hanging on a spile for several minutes) will have lost water to evaporation, so their refractometer readings will be inaccurate.

Place a drop of sap on the refractometer, close the cover, and quickly read the scale (the line at the top of the darker area). Readings should be taken to the nearest 0.1 percent. The refractometer should be dried with tissue paper and rinsed with water after each reading. If you are working rapidly, dry the refractometer after each measurement, and rinse and dry it after 10 to 15 measurements.

While many refractometers auto adjust for the sap or syrup temperature, some may come with a chart that requires mathematical adjustments. Check the documents that come with your refractometer to see whether you need to adjust for temperature.